advent-of-code/advent-of-code.org

#+TITLE: Advent of Code
#+STARTUP: indent
#+OPTIONS: num:nil ^:nil d:nil
#+PROPERTY: header-args :cache yes
#+DRAWERS: HIDDEN

#+BEGIN_SRC emacs-lisp :exports results :results silent
  (require 'ert) ; Require ert for unit tests
#+END_SRC

* Day 1: Not Quite Lisp

Santa was hoping for a white Christmas, but his weather machine's
"snow" function is powered by stars, and he's fresh out! To save
Christmas, he needs you to collect *fifty stars* by December 25th.

Collect stars by helping Santa solve puzzles. Two puzzles will be made
available on each day in the advent calendar; the second puzzle is
unlocked when you complete the first. Each puzzle grants *one star*.
Good luck!

Here's an easy puzzle to warm you up.

Santa is trying to deliver presents in a large apartment building, but
he can't find the right floor - the directions he got are a little
confusing. He starts on the ground floor (floor =0=) and then follows
the instructions one character at a time.

An opening parenthesis, =(=, means he should go up one floor, and a
closing parenthesis, =)=, means he should go down one floor.

The apartment building is very tall, and the basement is very deep; he
will never find the top or bottom floors.

For example:

- =(())= and =()()= both result in floor 0.
- =(((= and =(()(()(= both result in floor 3.
- =))(((((= also results in floor 3.
- =())= and =))(= both result in floor -1 (the first basement level).
- =)))= and =)())())= both result in floor -3.

To what floor do the instructions take Santa?

----------------------------------------------------------------------

:HIDDEN:
#+name: 1-input
#+BEGIN_EXAMPLE
((((()(()(((((((()))(((()((((()())(())()(((()((((((()((()(()(((()(()((())))()((()()())))))))))()((((((())((()))(((((()(((((((((()()))((()(())()((())((()(()))((()))()))()(((((()(((()()))()())((()((((())()())()((((())()(()(()(((()(())(()(())(((((((())()()(((())(()(()(()(())))(()((((())((()))(((()(()()(((((()()(()(((()(((((())()))()((()(()))()((()((((())((((())(()(((())()()(()()()()()(())((((())((())(()()))()((((())))((((()())()((((())((()())((())(())(((((()((((()(((()((((())(()(((()()))()))((((((()((())()())))(((()(()))(()()(()(((()(()))((()()()())((()()()(((())())()())())())((()))(()(()))(((((()(()(())((()(())(())()((((()())()))((((())(())((())())((((()(((())(())((()()((((()((((((()(())()()(()(()()((((()))(())()())()))(())))(())))())()()(())(()))()((()(()(())()()))(()())))))(()))(()()))(())(((((()(()(()()((())()())))))((())())((())(()(())((()))(())(((()((((((((()()()(()))()()(((()))()((()()(())(())())()(()(())))(((((()(())(())(()))))())()))(()))()(()(((((((()((((())))())())())())()((((((((((((((()()((((((()()()())())()())())())(())(())))())((()())((()(()))))))()))))))))))))))))())((())((())()()))))))(((()((()(()()))((())(()()))()()())))(())))))))(()(((())))())()())))()()(())()))()(()))())((()()))))(()))))()))(()()(())))))))()(((()))))()(()))(())())))))()))((()))((()))())(())))))))))((((())()))()))()))())(())()()(())))())))(()())()))((()()(())))(())((((((()(())((()(((()(()()(())))()))))))()))()(()((()))()(()))(()(((())((((())())(())(()))))))))())))))))())())))))())))))()()(((())()(()))))))))())))))(())()()()))()))()))(()(())()()())())))))))())()(()(()))))()()()))))())(()))))()()))))()())))))(((())()()))(()))))))))))()()))))()()()))))(()())())()()())()(()))))()(()))(())))))))(((((())(())())()()))()()))(())))))()(()))))(())(()()))()())()))()))()))()))))())()()))())())))(()))(()))))))())()(((())()))))))))()))()())))())))())))()))))))))))()()))(()()))))))(())()(()))))())(()))))(()))))(()())))))())())()()))))())()))))))))(()))))()))))))()(()())))))))()))())))())))())))())))))))())(()()))))))(()())())))()())()))))))))))))))())))()(())))()))())()()(())(()()))(())))())()())(()(()(()))))())))))))))))())(()))()))()))))(())()())()())))))))))))()()))))))))))))())())))))(()())))))))))))())(())))()))))))))())())(()))()))(())))()))()()(())()))))))()((((())()))())())))))()))()))))((()())()))))())))(())))))))))))))))))()))))()()())()))()()))))())()))((()())))())))(()))(()())))))))()))()))))(())))))))(())))))())()()(()))())()))()()))))())()()))))())()))())))))))(()))))()())()))))))))(()))())))(()))()))))(())()))())())(())())())))))))((((())))))()))()))()())()(())))()))()))()())(()())()()(()())()))))())())))))(()))()))))())(()()(())))))(())()()((())())))))(())(())))))))())))))))))()(())))))))()())())())()(()))))))))(()))))))))())()()))()(()))))))()))))))())))))))(())))()()(())()())))))(((())))()((())()))())))(()()))())(())())))()(((()())))))()(()()())))()()(()()(()()))())()(()()()))())()()))()())(()))))())))))())))(())()()))))(()))))(())(()))(())))))()()))()))))())()))()()(())())))((()))())()))))))()()))))((()(()))))()()))))))())))))())(()((()())))))))))))()())())))()))(()))))))(()))(())()())))(()))))))))())()()()()))))(()())))))))((())))()))(()))(())(())()())()))))))))(())))())))(()))()()))(()()))(()))())))()(())))())((()((()(())))((())))()))))((((())())()())))(())))()))))))())(()()((())))())()(()())))))(()())()))())))))))((())())))))))(()(()))())()()(()()(((()(((()())))))()))))))()(())(()()((()()(())()()))())()())()))()())())())))))))(((())))))))()()))))))(((())()))(()()))(()()))))(()(()()((((())()())((()()))))(()(())))))()((()()()())()()((()((()()))(()))(((()()()))(((())))()(((())()))))))((()(())())))(()())(((((()(()))(()((()))(()())()))))(()(()))()(()))(())(((())(()()))))()()))(((()))))(()()()()))())))((()()()(())()))()))))()()))()))))))((((((()()()))))())((()()(((()))))(()(())(()()())())())))()(((()()))(())((())))(()))(()()()())((())())())(()))))()))()((()(())()(()()(())(()))(())()))(())(()))))(())(())())(()()(()((()()((())))((()))()((())))(((()()()()((((()))(()()))()()()(((())((())())(()()(()()()))()((())(())()))())(((()()(())))()((()
#+END_EXAMPLE
:END:

#+name: 1-solution
#+BEGIN_SRC emacs-lisp :var input=1-input :exports both
  (defun day1/parse-directions (directions)
    (seq-map
                   (lambda (char)
                     (cond ((eq char ?\() 1)
                           ((eq char ?\)) -1)
                           (t 0)))
                   directions))

  (seq-reduce #'+
              (day1/parse-directions input)
              0)
#+END_SRC

#+RESULTS[966acea0b884f9a83f09aae9b3c00803a516e3ca]: 1-solution
: 74

** Part 2
Now, given the same instructions, find the position of the first
character that causes him to enter the basement (floor =-1=). The
first character in the instructions has position =1=, the second
character has position =2=, and so on.

For example:

- =)= causes him to enter the basement at character position =1=.
- =()())= causes him to enter the basement at character position =5=.
What is the position of the character that causes Santa to first enter the basement?

----------------------------------------------------------------------

#+name: 1.2-solution
#+BEGIN_SRC emacs-lisp :var input=1-input :exports both
  (defun day1/steps-until (predicate reduce-fn sequence initial-value)
    (car (seq-reduce
          (lambda (acc next)
            (let ((counter (car acc))
                  (value (cdr acc)))
              (if (funcall predicate value) acc
                (cons (+ 1 counter)
                      (funcall reduce-fn value next)))))
          sequence
          (cons 0 initial-value))))

  (day1/steps-until
   (lambda (floor) (= floor -1))
   #'+
   (day1/parse-directions input)
   0)
#+END_SRC

#+RESULTS[048e5b88e35ca21b8be15e5cffaa117c4e1eca45]: 1.2-solution
: 1795

* Day 2: I Was Told There Would Be No Math

The elves are running low on wrapping paper, and so they need to
submit an order for more. They have a list of the dimensions (length
l, width w, and height h) of each present, and only want to order
exactly as much as they need.

Fortunately, every present is a box (a perfect [[https://en.wikipedia.org/wiki/Cuboid#Rectangular_cuboid][right rectangular
prism]]), which makes calculating the required wrapping paper for each
gift a little easier: find the surface area of the box, which is
=2*l*w + 2*w*h + 2*h*l=. The elves also need a little extra paper for
each present: the area of the smallest side.

For example:

- A present with dimensions 2x3x4 requires 2*6 + 2*12 + 2*8 = 52
  square feet of wrapping paper plus 6 square feet of slack, for a
  total of 58 square feet.
- A present with dimensions 1x1x10 requires 2*1 + 2*10 + 2*10 = 42
  square feet of wrapping paper plus 1 square foot of slack, for a
  total of 43 square feet.

All numbers in the elves' list are in feet. How many total *square
feet of wrapping paper* should they order?

----------------------------------------------------------------------

:HIDDEN:
#+name: 2-input
#+BEGIN_EXAMPLE
  20x3x11
  15x27x5
  6x29x7
  30x15x9
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  1x26x4
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  11x5x15
  23x10x15
  12x17x21
  27x15x1
  4x29x14
  5x24x25
  10x10x12
  18x12x9
  11x24x23
  24x23x3
  28x12x15
  29x9x14
  11x25x8
  5x12x2
  26x26x29
  9x21x2
  8x8x25
  1x16x30
  17x29x20
  9x22x13
  7x18x16
  3x3x23
  26x25x30
  15x23x24
  20x23x5
  20x16x10
  23x7x8
  20x18x26
  8x27x6
  30x23x23
  7x7x24
  21x11x15
  1x30x25
  26x27x22
  30x28x13
  20x13x13
  3x1x15
  16x7x1
  7x25x15
  12x7x18
  16x9x23
  16x12x18
  29x5x2
  17x7x7
  21x17x5
  9x9x17
  26x16x10
  29x29x23
  17x26x10
  5x19x17
  1x10x1
  14x21x20
  13x6x4
  13x13x3
  23x4x18
  4x16x3
  16x30x11
  2x11x2
  15x30x15
  20x30x22
  18x12x16
  23x5x16
  6x14x15
  9x4x11
  30x23x21
  20x7x12
  7x18x6
  15x6x5
  18x22x19
  16x10x22
  26x20x25
  9x25x25
  29x21x10
  9x21x24
  7x18x21
  14x3x15
  18x19x19
  4x29x17
  14x10x9
  2x26x14
  13x3x24
  4x4x17
  6x27x24
  2x18x3
  14x25x2
  30x14x17
  11x6x14
  4x10x18
  15x4x2
  27x7x10
  13x24x1
  7x12x6
  25x22x26
  19x2x18
  23x29x2
  2x15x4
  12x6x9
  16x14x29
  9x17x3
  21x9x12
  23x18x22
  10x8x4
  29x2x7
  19x27x15
  4x24x27
  25x20x14
  8x23x19
  1x24x19
  6x20x10
  15x8x5
  18x28x5
  17x23x22
  9x16x13
  30x24x4
  26x3x13
  12x22x18
  29x17x29
  26x4x16
  15x7x20
  9x15x30
  12x7x18
  28x19x18
  11x23x23
  24x20x1
  20x3x24
  1x26x1
  14x10x6
  5x27x24
  13x21x12
  20x20x5
  6x28x9
  11x26x11
  26x29x12
  21x4x11
  20x11x17
  22x27x20
  19x11x21
  2x11x11
  13x5x7
  12x10x25
  21x28x1
  15x30x17
  28x19x1
  4x19x12
  11x4x12
  4x10x30
  11x18x5
  22x20x12
  3x7x27
  20x26x4
  13x27x26
  23x14x13
  4x19x7
  26x27x16
  20x5x20
  18x5x8
  19x21x1
  22x8x1
  29x4x1
  24x10x15
  24x9x20
  10x3x8
  29x30x3
  2x8x24
  16x7x18
  2x11x23
  23x15x16
  21x12x6
  24x28x9
  6x1x13
  14x29x20
  27x24x13
  16x26x8
  5x6x17
  21x8x1
  28x19x21
  1x14x16
  18x2x9
  29x28x10
  22x26x27
  18x26x23
  22x24x2
  28x26x1
  27x29x12
  30x13x11
  1x25x5
  13x30x18
  3x13x22
  22x10x11
  2x7x7
  18x17x8
  9x22x26
  30x18x16
  10x2x3
  7x27x13
  3x20x16
  9x21x16
  1x18x15
  21x30x30
  4x25x23
  3x11x7
  5x6x12
  27x1x20
  13x15x24
  23x29x2
  13x5x24
  22x16x15
  28x14x3
  29x24x9
  2x20x4
  30x10x4
  23x7x20
  22x12x21
  3x19x11
  4x28x28
  5x4x7
  28x12x25
  2x16x26
  23x20x7
  5x21x29
  9x21x16
  9x6x10
  9x6x4
  24x14x29
  28x11x6
  10x22x1
  21x30x20
  13x17x8
  2x25x24
  19x21x3
  28x8x14
  6x29x28
  27x10x28
  30x11x12
  17x2x10
  14x19x17
  2x11x4
  26x1x2
  13x4x4
  23x20x18
  2x17x21
  28x7x15
  3x3x27
  24x17x30
  28x28x20
  21x5x29
  13x12x19
  24x29x29
  19x10x6
  19x12x14
  21x4x17
  27x16x1
  4x17x30
  23x23x18
  23x15x27
  26x2x11
  12x8x8
  15x23x26
  30x17x15
  17x17x15
  24x4x30
  9x9x10
  14x25x20
  25x11x19
  20x7x1
  9x21x3
  7x19x9
  10x6x19
  26x12x30
  21x9x20
  15x11x6
  30x21x9
  10x18x17
  22x9x8
  8x30x26
  28x12x27
  17x17x7
  11x13x8
  5x3x21
  24x1x29
  1x28x2
  18x28x10
  8x29x14
  26x26x27
  17x10x25
  22x30x3
  27x9x13
  21x21x4
  30x29x16
  22x7x20
  24x10x2
  16x29x17
  28x15x17
  19x19x22
  9x8x6
  26x23x24
  25x4x27
  16x12x2
  11x6x18
  19x14x8
  9x29x13
  23x30x19
  10x16x1
  4x21x28
  23x25x25
  19x9x16
  30x11x12
  24x3x9
  28x19x4
  18x12x9
  7x1x25
  28x7x1
  24x3x12
  30x24x22
  27x24x26
  9x30x30
  29x10x8
  4x6x18
  10x1x15
  10x4x26
  23x20x16
  6x3x14
  30x8x16
  25x14x20
  11x9x3
  15x23x25
  8x30x22
  22x19x18
  25x1x12
  27x25x7
  25x23x3
  13x20x8
  5x30x7
  18x19x27
  20x23x3
  1x17x21
  21x21x27
  13x1x24
  7x30x20
  21x9x18
  23x26x6
  22x9x29
  17x6x21
  28x28x29
  19x25x26
  9x27x21
  5x26x8
  11x19x1
  10x1x18
  29x4x8
  21x2x22
  14x12x8
#+END_EXAMPLE
:END:

#+name: 2-solution
#+BEGIN_SRC emacs-lisp :var input=2-input :exports both
  (defun day2/surface-area (w h l)
    "Calculate the surface area of a box."
    (+ (* 2 l w)
       (* 2 w h)
       (* 2 h l)))

  (defun day2/slack (w h l)
    "Find the area of the smallest side of a box.

  The smallest side is the side with the smallest area."
    (min (* l w)
         (* w h)
         (* h l)))

  (defun day2/wrapping-paper (w h l)
    "Find the area of wrapping paper needed to wrap a box."
    (+ (day2/surface-area w h l)
       (day2/slack w h l)))

  (ert-deftest day2/wrapping-paper ()
    (should (eq 58 (day2/wrapping-paper 2 3 4)))
    (should (eq 43 (day2/wrapping-paper 1 1 10))))

  (defun day2/parse-dimensions (dimension-string)
    "Parse a string representing the dimensions of a box separated
  by the character 'x' into a list of integers."
    (seq-map #'string-to-int
             (split-string dimension-string "x")))

  (defun day2/sum-list (input)
    (seq-reduce
     #'+
     (seq-map (lambda (dimension-string)
                (let ((dimensions (day2/parse-dimensions dimension-string)))
                  (eval (cons 'day2/wrapping-paper dimensions))))
              (split-string input))
     0))

  (ert-deftest day2/sum-list ()
    (let ((test-data (string-join '("2x3x4" "1x1x10") "\n")))
      (should (eq (+ 58 43)
                  (day2/sum-list test-data)))))

  (day2/sum-list input)
#+END_SRC

#+RESULTS[f343fb2890e11ccbcc408d25b441e9a808073e1f]: 2-solution
: 1606483

** Part 2
The elves are also running low on ribbon. Ribbon is all the same
width, so they only have to worry about the length they need to order,
which they would again like to be exact.

The ribbon required to wrap a present is the shortest distance around
its sides, or the smallest perimeter of any one face. Each present
also requires a bow made out of ribbon as well; the feet of ribbon
required for the perfect bow is equal to the cubic feet of volume of
the present. Don't ask how they tie the bow, though; they'll never
tell.

For example:

- A present with dimensions =2x3x4= requires =2+2+3+3 = 10= feet of
  ribbon to wrap the present plus =2*3*4 = 24= feet of ribbon for the
  bow, for a total of =34= feet.
- A present with dimensions =1x1x10= requires =1+1+1+1 = 4= feet of
  ribbon to wrap the present plus =1*1*10 = 10= feet of ribbon for the
  bow, for a total of =14= feet.

How many total feet of ribbon should they order?

----------------------------------------------------------------------

#+BEGIN_SRC emacs-lisp :var input=2-input :exports both
  (defun day2/smallest-perimeter (w h l)
    (min (* 2 (+ l w))
         (* 2 (+ w h))
         (* 2 (+ h l))))

  (defun day2/volume (w h l)
    (* w h l))

  (defun day2/ribbon (w h l)
    (+ (day2/smallest-perimeter w h l)
       (day2/volume w h l)))

  (ert-deftest day2/ribbon ()
    (should (eq 34 (day2/ribbon 2 3 4)))
    (should (eq 14 (day2/ribbon 1 1 10))))

  (defun day2/sum-ribbon (input)
    (seq-reduce
     #'+
     (seq-map (lambda (dimension-string)
                (let ((dimensions (day2/parse-dimensions dimension-string)))
                  (eval (cons 'day2/ribbon dimensions))))
              (split-string input))
     0))

  (day2/sum-ribbon input)
#+END_SRC

#+RESULTS[c13958a4756029347386f7b48c44959c7b8e3dea]:
: 3842356

* Day 3: Perfectly Spherical Houses in a Vacuum

Santa is delivering presents to an infinite two-dimensional grid of
houses.

He begins by delivering a present to the house at his starting
location, and then an elf at the North Pole calls him via radio and
tells him where to move next. Moves are always exactly one house to
the north (=^=), south (=v=), east (=>=), or west (=<=). After each
move, he delivers another present to the house at his new location.

However, the elf back at the north pole has had a little too much
eggnog, and so his directions are a little off, and Santa ends up
visiting some houses more than once. How many houses receive *at least
one present*?

For example:

- =>= delivers presents to =2= houses: one at the starting location,
  and one to the east.
- =^>v<= delivers presents to =4= houses in a square, including twice
  to the house at his starting/ending location.
- =^v^v^v^v^v= delivers a bunch of presents to some very lucky
  children at only =2= houses.

----------------------------------------------------------------------

:HIDDEN:
#+name: 3-input
#+BEGIN_EXAMPLE
  >^^v^<>v<<<v<v^>>v^^^<v<>^^><^<<^vv>>>^<<^>><vv<<v^<^^><>>><>v<><>^^<^^^<><>>vv>vv>v<<^>v<>^>v<v^<>v>><>^v<<<<v^vv^><v>v^>>>vv>v^^^<^^<>>v<^^v<>^<vv^^<^><<>^>><^<>>><><vv><>v<<<><><>v><<>^^^^v>>^>^<v<<vv^^<v<^<^>^^v^^^^^v<><^v><<><^v^>v<<>^<>^^v^<>v<v^>v>^^<vv^v><^<>^v<><^><v^><><><<<<>^vv^>^vvvvv><><^<vv^v^v>v<<^<^^v^<>^<vv><v<v^v<<v<<^^>>^^^v^>v<><^vv<<^<>v<v><><v^^><v<>^^>^^>v^>^<<<<v><v<<>v><^v>^>><v^^<^>v<vvvv<>>>>>^v^^>v<v<^<vv>^>^vv^>vv^^v<<^<^^<>v>vv^v>><>>>v^>^>^^v<>^<v<<>^vv>v^<<v>v<<><v>^vvv<v<vvv^v<vv<v^^^>v><<^<>><v^^>^v^>>^v<^<><v<>>v^<>>v<>>v^^^><^>>vvvv>^v<^><<>>^<>^>vv><v<<>>^^>v^^^><^<<^^v>v<^<<>v>^^vvv^v^>v^<>^^<>v^v>v>v<v^>vv>^^v<>v>>^<>><>v>v^<<vvvv<vvv><v^<^>^v<>>^><v>><>^<v>v<v>vv^>>vvv<>v>v<v^>>^>>v<<>^<>^<>>>^v<<<^<^v>vv^>><<><v^>^v^^^v<>^^vv><>><>>^>v^<v<>v<>>^<<^v>^^^<>^v^><>v<<v>vv^>vv<<>>><<^v^<>v<vv>>>^^<>^><<^>vv>>^<<v^^vv<>>><v>v><^<v<<>>>^^<>>^<^v><>vv^^^v>vvv>^><<>^^>^<<v^<v<^v<<>vvv<^<<>^>^v<vv<^>vvv>v>vv^<v^><>>^vv<^^^vv><^vv<v^<><v^vvv><<^>^^><v<<vv^>v<vv<v>^<>^v<<>v<v^v^>^>^>v<<^vvv<<<v>^^>^<<<<>vv>>^<>^>>>v<v>^^<v^<v<>>>vv>^^v<<>>>^^v><<<v<v<^v<>^^><v<^v<<v^><><^<><v<^^v>>><v^^v<<v^><^<><<v^>><^<>v>v^<><^<v>^v^>^>^vv^>^^<<vv^>vv<^vvv<>>^^<^>v^>^>^<v^><v<v>>>v<<<><^v<<><^<vv^v^^^>v<^^<v^vvv<v<><v<vv<^vv<>vv<v^<>>vvvvv<<>^v^v>vv>>>vvv^^<^<^<><>v<v>><^v><^<<<>><<<v>^>v<>^>^v>>^<>v^<^>><<>^<v>^>^^^>^^<v>>>><>^v^v><<<<vv^<vv<>vv>v<>v^<v^>v><>>>v^<><^vvv>vv^<^<<^<^^v>^>>>v<^<^v^^<^<^>>><v>vv>^<<><>^>>v>^<<>><^<>v<>vv^^>^>vvv^v<<^<^^<vv<>^vvv<^^v^vv^>>v<^>^^<v^<>v<^<^vv>v<<vv>vv>^>vvv>>>^^>v<>^v>v^<^>>v>^^v>>>>v^<v>v<^>v<v<<>>^v<^^<v><^<>>^<<vv^>>v<<v>^v<>><^>vv<v<^>>^^<vvvvvvvvv>>>v<v<>v^<>>^vv<v^^v<<^vvv^<<^><>vv<><<>>v>vv^><>>^^v^>>v^v^><<<>>^^<^v<<^<>>>>^<^>v^><<^>v<^v<^>>^^<<<<><^<^v^v<>>^v<^<<vv^<><^^vv><v^v^v>^>>^>^vv^>^v<v^v<<vvv^><>>^v^^><>v>vv><^>>vv<vvv<<<<^<>vvv^v<v>^<v<^>^<^<v<><>v^^^^<<vv<^^vv<v>><<v^><>>><v^>^v><^>^><vv^<><^<v>><<^vv<>>v^<<v<>v><v<><><vv>^>>v^<^<v>^><>>><^><v^v<>>>^^<^>v<v>vvv<>^<<><v^^>^>>v<^v>^>v>>>vv>v>>v^^^<^<vvv^<>^>^<v^<v^v>v>^>vv>vvv<>v<^>v>^^>>^<vv^^v>v^^^^^v^vv><^<><>^>vv<^>>^vvvv^^^>^<vv>^v<<^><^^>^<>^^>^<<v<^>>>^><<^^>v^v>>^>vvvv>^^v><v>>vv><<<vv<^>v>^^^<v>v^vvv<^><<^>^<>^><<<<<v^<<vv^v>^<>v<v>^>^>><>v^v<^vv^^>vv<<v^v>vv^vvv<<<<>^v<v^^v^v>v<<v>^^<>^vv^^>^>^v^vv^>>v^vv^^<vv><<v^v^^v><vv<^vvv<vv^^<<v>v^v^^^^v<^<^>v>^>v>^vv^v^^<v<^vvvv<<<>^<^^^<^^<>^<><vv<^^<<^>>><v^vvvv>^<>>^^>v^^v^<<v^^^<<<><^<v^v^^v<v^<>v><<v<>^v>v<^><^>vv^^<vvv<^v>>v>^<><v^><^^^<v^>>vv<<<<<^<>^v^v>^vv^<>v>v<^>vv<<^vv>vv<v<><>>v>><v<^<^^>><<v^v<<^><v<^<vv<v<<vv^>^<<><^^>^<^>>^<vv>><v<<vvv<^^v^>^^<^v>^v<v<>v><v^v^<<^<><<v<<^v>v<<>>^>v>>v>>v<^<<^<^>>>v>^^^v><^>^^>>v<<>^v><v>vvv^vv<<<>vvv<<>^>>>v<v<v^<^<^>^<^>v^^v<^^<v<>v<>>^^>^v^>v<<<<^<>v^><<<v>>>><<v^<^vv>v>><>>^<<<^<^^>v<>>v<>vv<<^<<><<^>v^^^vv^>vvvv>>v>v^><<v<>vv^<<><<vvv>^>>>^<<<^<^<<v>^>v<>>v>>vv^^><<<<^^^v>><<^><v><v^^><v<<v^^v^^v>>v<><><<>^><v><^<vv>><^v<>v<vvv<>^>><v>>v<^><<v>^<>^v><^><^^<v>^><^^v^<<><>>^>v^<^v^vv<><^>vv^>v^vvv^<>>^><^<^<>^<<v^v<^v><>^v<v>>^>>^v^vv>><vv><v^^<<^v^<>^v<<>^><^>><v>>v<<<v^^vv<>^^v>>><><><<v^<<<v^<^^><v^>v^^vv<v^<>>vv^<^v<>^v>>v^v>v<^^vv><>^v<<>v^<>v^>>v>vvv<^><><^^>^vv^>>v^>^<^^<><>><<>^^^><^v^v><<<><<^v^vv>v>><^>>><v^>v<v><><v^v<>v^^>>v<<>v>v<v<v<^^<><>v^^<>>v<^v<v>v<><v<v>^<<>v>vv^^<>>^^^<>^^>^v>v>>>^v^v><v^^<><v>^^v^v<^<^^><<v<^<^<>^<>><<>^>>^>^^><v><>v<><>><<<>>>>vv>>>^>>^v<^>v^^^v<<vv>><<<^<<<>>>>>^>vv<^v^<>^<v^>^v><v>vvv<>>>^v^^^v<<<<>>^^<vv<^<^^>^<>v<^<<<>><>>v<^<>^<vvv<^<>><><<v>^^^>^^<<v<v^>^^v^>><<^vv><v>^v>>^<v>v>^^>^v>^vvv<>v^v^^<><vv>vv^>>><>v<^><v<v^<><<<>^v>^v<<<^>^>^>v^v<<><vvv<<v^^<><v>^>>><vv>><v>>v^<vv>>vv<<^v^v<<><^v<vv>>>vv<>>>>^vv>v^<>vv>v^v<v^><v<^^^^^>vv<><<vvv^<v><^<vv><^^^vv^<>^^^^<^><^<>v^<v^v<<^v<<^^<>>^<v^^>>>vv<vvv<>v<<>><^vvv^<<^^<<>>>^<>>>v^^><>><<>><v^v>>>>>><>>><v^<<vvv^>v<>>v^<>vv<><^^^^v^<<^<v^vv><<^^>v<^vvv^v>>v>^>>v>^^><<v^<>v<>vv<^v^vv><v><<vv^v>>v^>>v<^^^>^><<v<>^><>v>>>vvv<v<vv<^>>^v<v>^<^^^^^v><>v><>v^v^v<v^vv^v>vvvv<>vv<<<vv<v<<>^<^>^^v^<<>^<v><^><v<v<><<>v^<<^<><vv>v<<^v>>^v<><v>^>>^^><>v^<^<vvv^>^>^<<<<>vv>^v^v<^^^<vv>><>^^<<v<^<^^>>>v^v<<^^^<v<v<
#+END_EXAMPLE
:END:

#+BEGIN_SRC emacs-lisp :var input=3-input :exports both
  (defun day3/make-coord (x y)
    (cons x y))

  (defun day3/coord-x (coord)
    (car coord))

  (defun day3/coord-y (coord)
    (cdr coord))

  (defun day3/add-coords (a b)
    (day3/make-coord
     (+ (day3/coord-x a)
        (day3/coord-x b))
     (+ (day3/coord-y a)
        (day3/coord-y b))))

  (defun day3/arrow-to-coord (arrow)
    (cond ((eq arrow ?>) (day3/make-coord 1 0))
          ((eq arrow ?<) (day3/make-coord -1 0))
          ((eq arrow ?^) (day3/make-coord 0 1))
          ((eq arrow ?v) (day3/make-coord 0 -1))
          (t (day3/make-coord 0 0))))

  (defun day3/make-map ()
    (make-hash-table :test #'equal))

  (defun day3/map-visit (map coord)
    (puthash coord
             (+ 1 (gethash coord map 0))
             map))

  (defun day3/deliver (directions)
    (let ((map (day3/make-map)))
      (day3/map-visit map (day3/make-coord 0 0))
      (cdr (seq-reduce
            (lambda (acc next)
              (let* ((loc (car acc))
                     (map (cdr acc))
                     (next-loc (day3/add-coords loc
                                                (day3/arrow-to-coord next))))
                (day3/map-visit map next-loc)
                (cons next-loc map)))
            directions
            (cons (day3/make-coord 0 0)
                  map)))))

  (defun day3/map-count-visited (map)
    (hash-table-count map))

  (ert-deftest day3/map-count-visited ()
    (should (eq 2 (day3/map-count-visited
                   (day3/deliver ">"))))
    (should (eq 4 (day3/map-count-visited
                   (day3/deliver "^>v<"))))
    (should (eq 2 (day3/map-count-visited
                   (day3/deliver "^v^v^v^v^v")))))

  (day3/map-count-visited
   (day3/deliver input))
#+END_SRC

#+RESULTS[de0154066fa58bb3f337d9fc6135a1e7620998c1]:
: 2592

** Part 2
The next year, to speed up the process, Santa creates a robot version
of himself, *Robo-Santa*, to deliver presents with him.

Santa and Robo-Santa start at the same location (delivering two
presents to the same starting house), then take turns moving based on
instructions from the elf, who is eggnoggedly reading from the same
script as the previous year.

This year, how many houses receive *at least one present*?

For example:

- =^v= delivers presents to =3= houses, because Santa goes north, and
  then Robo-Santa goes south.
- =^>v<= now delivers presents to =3= houses, and Santa and Robo-Santa
  end up back where they started.
- =^v^v^v^v^v= now delivers presents to =11= houses, with Santa going
  one direction and Robo-Santa going the other.

----------------------------------------------------------------------

#+BEGIN_SRC emacs-lisp :var input=3-input :exports both
  (require 'dash)

  (defun day3/alternate (count sequence)
    (->> (-zip (seq-into sequence 'list)
               (number-sequence 1 (seq-length sequence)))
         (-group-by (lambda (x) (mod (cdr x) count)))
         (-map (lambda (x) (-map #'car (cdr x))))))

  (defun day3/map-visit-n (map coord times)
    (puthash coord
             (+ times (gethash coord map 0))
             map))

  (defun day3/map-visits (map coord)
    (gethash coord map 0))

  (defun day3/map-locations (map)
    (hash-table-keys map))

  (defun day3/map-merge (a &rest maps)
    (-map (lambda (map)
            (-map (lambda (coord)
                    (day3/map-visit-n a coord (day3/map-visits map coord)))
                  (day3/map-locations map)))
          maps)
    a)

  (defun day3/deliver-with-robosanta (directions)
    (let* ((alternated (day3/alternate 2 (string-to-list directions)))
           (santa-directions (car alternated))
           (robot-directions (cadr alternated)))
      (day3/map-merge
       (day3/deliver santa-directions)
       (day3/deliver robot-directions))))

  (ert-deftest day3/deliver-with-robosanta ()
    (should (eq 3 (day3/map-count-visited
                   (day3/deliver-with-robosanta "^v"))))
    (should (eq 3 (day3/map-count-visited
                   (day3/deliver-with-robosanta "^>v<"))))
    (should (eq 11 (day3/map-count-visited
                    (day3/deliver-with-robosanta "^v^v^v^v^v")))))

  (day3/map-count-visited
   (day3/deliver-with-robosanta input))
#+END_SRC

#+RESULTS[25db54a0aebd3fd8cbb6d364a6a05908e4c1e208]:
: 2360

* Day 4: The Ideal Stocking Stuffer

Santa needs help [[https://en.wikipedia.org/wiki/Bitcoin#Mining][mining]] some AdventCoins (very similar to [[https://en.wikipedia.org/wiki/Bitcoin#Mining][bitcoins]]) to
use as gifts for all the economically forward-thinking little girls
and boys.

To do this, he needs to find [[https://en.wikipedia.org/wiki/MD5][MD5]] hashes which, in [[https://en.wikipedia.org/wiki/Hexadecimal][hexadecimal]], start
with at least *five zeroes*. The input to the MD5 hash is some secret
key (your puzzle input, given below) followed by a number in decimal.
To mine AdventCoins, you must find Santa the lowest positive number
(no leading zeroes: =1=, =2=, =3=, ...) that produces such a hash.

For example:

- If your secret key is =abcdef=, the answer is =609043=, because the
  MD5 hash of =abcdef609043= starts with five zeroes
  (=000001dbbfa...=), and it is the lowest such number to do so.
- If your secret key is =pqrstuv=, the lowest number it combines with
  to make an MD5 hash starting with five zeroes is =1048970=; that is,
  the MD5 hash of =pqrstuv1048970= looks like =000006136ef...=.

----------------------------------------------------------------------

:HIDDEN:
#+name: 4-input
#+BEGIN_EXAMPLE
  ckczppom
#+END_EXAMPLE
:END:

#+BEGIN_SRC emacs-lisp :var input=4-input :exports both
  (defun day4/mine-hash (key n)
    (md5 (format "%s%d" key n)))

  (defun day4/adventcoin-hash-p (hash)
    (string-equal
     "00000"
     (substring hash 0 5)))

  (defun day4/first-adventcoin (secret)
    (loop for x from 0
          while (not (day4/adventcoin-hash-p (day4/mine-hash secret x)))
          count x))

  (ert-deftest day4/first-adventcoin ()
    (should (eq 609043 (day4/first-adventcoin "abcdef")))
    (should (eq 1048970 (day4/first-adventcoin "pqrstuv"))))

  (day4/first-adventcoin (string-trim input))
#+END_SRC

#+RESULTS[461139efd69152ff32f7ba3996d41b323fd6c8bd]:
: 117946

** Part 2

Now find one that starts with *six zeroes*.

----------------------------------------------------------------------

#+BEGIN_SRC emacs-lisp :var input=4-input :exports both
  (defun day4/adventcoin-hash-6-p (n hash)
    (string-equal
     "000000"
     (substring hash 0 6)))

  (defun day4/first-adventcoin-n (n secret)
    (loop for x from 0
          while (not (day4/adventcoin-hash-6-p n (day4/mine-hash secret x)))
          count x))

  (day4/first-adventcoin-n 6 (string-trim input))
#+END_SRC

#+RESULTS[637ec8d92dd6b49b39e0eb247af8265cd6938e5a]:
: 3938038

* Day 5: Doesn't He Have Intern-Elves For This?

Santa needs help figuring out which strings in his text file are
naughty or nice.

A nice string is one with all of the following properties:

- It contains at least three vowels (=aeiou= only), like =aei=,
  =xazegov=, or =aeiouaeiouaeiou=.
- It contains at least one letter that appears twice in a row, like
  =xx=, =abcdde= (=dd=), or =aabbccdd= (=aa=, =bb=, =cc=, or =dd=).
- It does not contain the strings =ab=, =cd=, =pq=, or =xy=, even if
  they are part of one of the other requirements.

For example:

- =ugknbfddgicrmopn= is nice because it has at least three vowels
  (=u...i...o...=), a double letter (=...dd...=), and none of the
  disallowed substrings.
- =aaa= is nice because it has at least three vowels and a double
  letter, even though the letters used by different rules overlap.
- =jchzalrnumimnmhp= is naughty because it has no double letter.
- =haegwjzuvuyypxyu= is naughty because it contains the string =xy=.
- =dvszwmarrgswjxmb- is naughty because it contains only one vowel.

How many strings are nice?

----------------------------------------------------------------------

:HIDDEN:
#+name: 5-input
#+BEGIN_EXAMPLE
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#+END_EXAMPLE
:END:

#+BEGIN_SRC emacs-lisp :var input=5-input :exports both
  (defun day5/vowels (string)
    (seq-filter
     (apply-partially 'seq-contains "aeiou")
     string))

  (defun day5/has-doubles? (string)
    (string-match-p "\\([a-z]\\)\\1" string))

  (defun day5/naughty? (string)
       (-any
        (lambda (naughty-pattern)
          (s-contains? naughty-pattern string))
        '("ab" "cd" "pq" "xy")))

  (defun day5/nice? (string)
    (and (<= 3 (length (day5/vowels string)))
         (day5/has-doubles? string)
         (not (day5/naughty? string))))

  (ert-deftest day5/nice? ()
    (should (day5/nice? "ugknbfddgicrmopn"))
    (should (day5/nice? "aaa"))
    (should-not (day5/nice? "jchzalrnumimnmhp"))
    (should-not (day5/nice? "haegwjzuvuyypxyu"))
    (should-not (day5/nice? "dvszwmarrgswjxmb")))

  (length
   (-filter #'day5/nice?
            (split-string input)))
#+END_SRC

#+RESULTS[7332f2494e02290b4a00b217635390fb235e1de7]:
: 255

** Part 2

Realizing the error of his ways, Santa has switched to a better model
of determining whether a string is naughty or nice. None of the old
rules apply, as they are all clearly ridiculous.

Now, a nice string is one with all of the following properties:

- It contains a pair of any two letters that appears at least twice in
  the string without overlapping, like =xyxy= (=xy=) or =aabcdefgaa=
  (=aa=), but not like =aaa= (=aa=, but it overlaps).
- It contains at least one letter which repeats with exactly one
  letter between them, like =xyx=, =abcdefeghi= (=efe=), or even
  =aaa=.

For example:

- =qjhvhtzxzqqjkmpb= is nice because is has a pair that appears twice
  (=qj=) and a letter that repeats with exactly one letter between
  them (=zxz=).
- =xxyxx= is nice because it has a pair that appears twice and a
  letter that repeats with one between, even though the letters used
  by each rule overlap.
- =uurcxstgmygtbstg= is naughty because it has a pair (=tg=) but no
  repeat with a single letter between them.
- =ieodomkazucvgmuy= is naughty because it has a repeating letter with
  one between (=odo=), but no pair that appears twice.

How many strings are nice under these new rules?

----------------------------------------------------------------------

#+BEGIN_SRC emacs-lisp :var input=5-input :exports both
  (defun day5/double-pair? (string)
    (string-match-p "\\([a-z][a-z]\\).*\\1"
                    string))

  (ert-deftest day5/double-pair? ()
    (should (day5/double-pair? "xyxy"))
    (should (day5/double-pair? "aabcdefgaa"))
    (should-not (day5/double-pair? "aaa")))

  (defun day5/letter-sandwich? (string)
    (string-match-p "\\([a-z]\\)[a-z]\\1"
                    string))

  (ert-deftest day5/letter-sandwich? ()
    (should (day5/letter-sandwich? "xyx"))
    (should (day5/letter-sandwich? "abcdefeghi"))
    (should (day5/letter-sandwich? "aaa")))

  (defun day5/nice?? (string)
    (and (day5/double-pair? string)
         (day5/letter-sandwich? string)))

  (ert-deftest day5/nice?? ()
    (should (day5/nice?? "qjhvhtzxzqqjkmpb"))
    (should (day5/nice?? "xxyxx"))
    (should-not (day5/nice?? "uurcxstgmygtbstg"))
    (should-not (day5/nice?? "ieodomkazucvgmuy")))

  (length
   (-filter #'day5/nice??
            (split-string input)))
#+END_SRC

#+RESULTS[714e53b40cf4a27435a59eafbd8c488cb05bd539]:
: 55

* Day 6: Probably a Fire Hazard

Because your neighbors keep defeating you in the holiday house
decorating contest year after year, you've decided to deploy one
million lights in a 1000x1000 grid.

Furthermore, because you've been especially nice this year, Santa has
mailed you instructions on how to display the ideal lighting
configuration.

Lights in your grid are numbered from 0 to 999 in each direction; the
lights at each corner are at =0,0=, =0,999=, =999,999=, and =999,0=.
The instructions include whether to =turn on=, =turn off=, or =toggle=
various inclusive ranges given as coordinate pairs. Each coordinate
pair represents opposite corners of a rectangle, inclusive; a
coordinate pair like =0,0= through =2,2= therefore refers to 9 lights
in a 3x3 square. The lights all start turned off.

To defeat your neighbors this year, all you have to do is set up your
lights by doing the instructions Santa sent you in order.

For example:

- =turn on 0,0 through 999,999= would turn on (or leave on) every
  light.
- =toggle 0,0 through 999,0= would toggle the first line of 1000
  lights, turning off the ones that were on, and turning on the ones
  that were off.
- =turn off 499,499 through 500,500= would turn off (or leave off) the
  middle four lights.

After following the instructions, how many lights are lit?

----------------------------------------------------------------------

:HIDDEN:
#+name: 6-input
#+BEGIN_EXAMPLE
  turn on 887,9 through 959,629
  turn on 454,398 through 844,448
  turn off 539,243 through 559,965
  turn off 370,819 through 676,868
  turn off 145,40 through 370,997
  turn off 301,3 through 808,453
  turn on 351,678 through 951,908
  toggle 720,196 through 897,994
  toggle 831,394 through 904,860
  toggle 753,664 through 970,926
  turn off 150,300 through 213,740
  turn on 141,242 through 932,871
  toggle 294,259 through 474,326
  toggle 678,333 through 752,957
  toggle 393,804 through 510,976
  turn off 6,964 through 411,976
  turn off 33,572 through 978,590
  turn on 579,693 through 650,978
  turn on 150,20 through 652,719
  turn off 782,143 through 808,802
  turn off 240,377 through 761,468
  turn off 899,828 through 958,967
  turn on 613,565 through 952,659
  turn on 295,36 through 964,978
  toggle 846,296 through 969,528
  turn off 211,254 through 529,491
  turn off 231,594 through 406,794
  turn off 169,791 through 758,942
  turn on 955,440 through 980,477
  toggle 944,498 through 995,928
  turn on 519,391 through 605,718
  toggle 521,303 through 617,366
  turn off 524,349 through 694,791
  toggle 391,87 through 499,792
  toggle 562,527 through 668,935
  turn off 68,358 through 857,453
  toggle 815,811 through 889,828
  turn off 666,61 through 768,87
  turn on 27,501 through 921,952
  turn on 953,102 through 983,471
  turn on 277,552 through 451,723
  turn off 64,253 through 655,960
  turn on 47,485 through 734,977
  turn off 59,119 through 699,734
  toggle 407,898 through 493,955
  toggle 912,966 through 949,991
  turn on 479,990 through 895,990
  toggle 390,589 through 869,766
  toggle 593,903 through 926,943
  toggle 358,439 through 870,528
  turn off 649,410 through 652,875
  turn on 629,834 through 712,895
  toggle 254,555 through 770,901
  toggle 641,832 through 947,850
  turn on 268,448 through 743,777
  turn off 512,123 through 625,874
  turn off 498,262 through 930,811
  turn off 835,158 through 886,242
  toggle 546,310 through 607,773
  turn on 501,505 through 896,909
  turn off 666,796 through 817,924
  toggle 987,789 through 993,809
  toggle 745,8 through 860,693
  toggle 181,983 through 731,988
  turn on 826,174 through 924,883
  turn on 239,228 through 843,993
  turn on 205,613 through 891,667
  toggle 867,873 through 984,896
  turn on 628,251 through 677,681
  toggle 276,956 through 631,964
  turn on 78,358 through 974,713
  turn on 521,360 through 773,597
  turn off 963,52 through 979,502
  turn on 117,151 through 934,622
  toggle 237,91 through 528,164
  turn on 944,269 through 975,453
  toggle 979,460 through 988,964
  turn off 440,254 through 681,507
  toggle 347,100 through 896,785
  turn off 329,592 through 369,985
  turn on 931,960 through 979,985
  toggle 703,3 through 776,36
  toggle 798,120 through 908,550
  turn off 186,605 through 914,709
  turn off 921,725 through 979,956
  toggle 167,34 through 735,249
  turn on 726,781 through 987,936
  toggle 720,336 through 847,756
  turn on 171,630 through 656,769
  turn off 417,276 through 751,500
  toggle 559,485 through 584,534
  turn on 568,629 through 690,873
  toggle 248,712 through 277,988
  toggle 345,594 through 812,723
  turn off 800,108 through 834,618
  turn off 967,439 through 986,869
  turn on 842,209 through 955,529
  turn on 132,653 through 357,696
  turn on 817,38 through 973,662
  turn off 569,816 through 721,861
  turn on 568,429 through 945,724
  turn on 77,458 through 844,685
  turn off 138,78 through 498,851
  turn on 136,21 through 252,986
  turn off 2,460 through 863,472
  turn on 172,81 through 839,332
  turn on 123,216 through 703,384
  turn off 879,644 through 944,887
  toggle 227,491 through 504,793
  toggle 580,418 through 741,479
  toggle 65,276 through 414,299
  toggle 482,486 through 838,931
  turn off 557,768 through 950,927
  turn off 615,617 through 955,864
  turn on 859,886 through 923,919
  turn on 391,330 through 499,971
  toggle 521,835 through 613,847
  turn on 822,787 through 989,847
  turn on 192,142 through 357,846
  turn off 564,945 through 985,945
  turn off 479,361 through 703,799
  toggle 56,481 through 489,978
  turn off 632,991 through 774,998
  toggle 723,526 through 945,792
  turn on 344,149 through 441,640
  toggle 568,927 through 624,952
  turn on 621,784 through 970,788
  toggle 665,783 through 795,981
  toggle 386,610 through 817,730
  toggle 440,399 through 734,417
  toggle 939,201 through 978,803
  turn off 395,883 through 554,929
  turn on 340,309 through 637,561
  turn off 875,147 through 946,481
  turn off 945,837 through 957,922
  turn off 429,982 through 691,991
  toggle 227,137 through 439,822
  toggle 4,848 through 7,932
  turn off 545,146 through 756,943
  turn on 763,863 through 937,994
  turn on 232,94 through 404,502
  turn off 742,254 through 930,512
  turn on 91,931 through 101,942
  toggle 585,106 through 651,425
  turn on 506,700 through 567,960
  turn off 548,44 through 718,352
  turn off 194,827 through 673,859
  turn off 6,645 through 509,764
  turn off 13,230 through 821,361
  turn on 734,629 through 919,631
  toggle 788,552 through 957,972
  toggle 244,747 through 849,773
  turn off 162,553 through 276,887
  turn off 569,577 through 587,604
  turn off 799,482 through 854,956
  turn on 744,535 through 909,802
  toggle 330,641 through 396,986
  turn off 927,458 through 966,564
  toggle 984,486 through 986,913
  toggle 519,682 through 632,708
  turn on 984,977 through 989,986
  toggle 766,423 through 934,495
  turn on 17,509 through 947,718
  turn on 413,783 through 631,903
  turn on 482,370 through 493,688
  turn on 433,859 through 628,938
  turn off 769,549 through 945,810
  turn on 178,853 through 539,941
  turn off 203,251 through 692,433
  turn off 525,638 through 955,794
  turn on 169,70 through 764,939
  toggle 59,352 through 896,404
  toggle 143,245 through 707,320
  turn off 103,35 through 160,949
  toggle 496,24 through 669,507
  turn off 581,847 through 847,903
  turn on 689,153 through 733,562
  turn on 821,487 through 839,699
  turn on 837,627 through 978,723
  toggle 96,748 through 973,753
  toggle 99,818 through 609,995
  turn on 731,193 through 756,509
  turn off 622,55 through 813,365
  turn on 456,490 through 576,548
  turn on 48,421 through 163,674
  turn off 853,861 through 924,964
  turn off 59,963 through 556,987
  turn on 458,710 through 688,847
  toggle 12,484 through 878,562
  turn off 241,964 through 799,983
  turn off 434,299 through 845,772
  toggle 896,725 through 956,847
  turn on 740,289 through 784,345
  turn off 395,840 through 822,845
  turn on 955,224 through 996,953
  turn off 710,186 through 957,722
  turn off 485,949 through 869,985
  turn on 848,209 through 975,376
  toggle 221,241 through 906,384
  turn on 588,49 through 927,496
  turn on 273,332 through 735,725
  turn on 505,962 through 895,962
  toggle 820,112 through 923,143
  turn on 919,792 through 978,982
  toggle 489,461 through 910,737
  turn off 202,642 through 638,940
  turn off 708,953 through 970,960
  toggle 437,291 through 546,381
  turn on 409,358 through 837,479
  turn off 756,279 through 870,943
  turn off 154,657 through 375,703
  turn off 524,622 through 995,779
  toggle 514,221 through 651,850
  toggle 808,464 through 886,646
  toggle 483,537 through 739,840
  toggle 654,769 through 831,825
  turn off 326,37 through 631,69
  turn off 590,570 through 926,656
  turn off 881,913 through 911,998
  turn on 996,102 through 998,616
  turn off 677,503 through 828,563
  turn on 860,251 through 877,441
  turn off 964,100 through 982,377
  toggle 888,403 through 961,597
  turn off 632,240 through 938,968
  toggle 731,176 through 932,413
  turn on 5,498 through 203,835
  turn on 819,352 through 929,855
  toggle 393,813 through 832,816
  toggle 725,689 through 967,888
  turn on 968,950 through 969,983
  turn off 152,628 through 582,896
  turn off 165,844 through 459,935
  turn off 882,741 through 974,786
  turn off 283,179 through 731,899
  toggle 197,366 through 682,445
  turn on 106,309 through 120,813
  toggle 950,387 through 967,782
  turn off 274,603 through 383,759
  turn off 155,665 through 284,787
  toggle 551,871 through 860,962
  turn off 30,826 through 598,892
  toggle 76,552 through 977,888
  turn on 938,180 through 994,997
  toggle 62,381 through 993,656
  toggle 625,861 through 921,941
  turn on 685,311 through 872,521
  turn on 124,934 through 530,962
  turn on 606,379 through 961,867
  turn off 792,735 through 946,783
  turn on 417,480 through 860,598
  toggle 178,91 through 481,887
  turn off 23,935 through 833,962
  toggle 317,14 through 793,425
  turn on 986,89 through 999,613
  turn off 359,201 through 560,554
  turn off 729,494 through 942,626
  turn on 204,143 through 876,610
  toggle 474,97 through 636,542
  turn off 902,924 through 976,973
  turn off 389,442 through 824,638
  turn off 622,863 through 798,863
  turn on 840,622 through 978,920
  toggle 567,374 through 925,439
  turn off 643,319 through 935,662
  toggle 185,42 through 294,810
  turn on 47,124 through 598,880
  toggle 828,303 through 979,770
  turn off 174,272 through 280,311
  turn off 540,50 through 880,212
  turn on 141,994 through 221,998
  turn on 476,695 through 483,901
  turn on 960,216 through 972,502
  toggle 752,335 through 957,733
  turn off 419,713 through 537,998
  toggle 772,846 through 994,888
  turn on 881,159 through 902,312
  turn off 537,651 through 641,816
  toggle 561,947 through 638,965
  turn on 368,458 through 437,612
  turn on 290,149 through 705,919
  turn on 711,918 through 974,945
  toggle 916,242 through 926,786
  toggle 522,272 through 773,314
  turn on 432,897 through 440,954
  turn off 132,169 through 775,380
  toggle 52,205 through 693,747
  toggle 926,309 through 976,669
  turn off 838,342 through 938,444
  turn on 144,431 through 260,951
  toggle 780,318 through 975,495
  turn off 185,412 through 796,541
  turn on 879,548 through 892,860
  turn on 294,132 through 460,338
  turn on 823,500 through 899,529
  turn off 225,603 through 483,920
  toggle 717,493 through 930,875
  toggle 534,948 through 599,968
  turn on 522,730 through 968,950
  turn off 102,229 through 674,529
#+END_EXAMPLE
:END:

#+BEGIN_SRC emacs-lisp :var input=6-input :exports both
  (defun day6/make-light ()
    nil)

  (defun day6/turn-on-light (light)
    t)

  (defun day6/turn-off-light (light)
    nil)

  (defun day6/toggle-light (light)
    (not light))

  (defun day6/lit? (light)
    light)

  (defun day6/make-grid (size)
    (let ((table (make-hash-table :size (* size size))))
      (-each (number-sequence 0 (- (* size size) 1))
        (lambda (pos) (puthash pos (day6/make-light) table)))
      (cons size
            table)))

  (defun day6/grid-size (grid)
    (car grid))

  (defun day6/grid-table (grid)
    (cdr grid))

  (defun day6/grid-lights (grid)
    (hash-table-values (day6/grid-table grid)))

  (defun day6/grid-location (grid x y)
    (+ x (* y (day6/grid-size grid))))

  (ert-deftest day6/grid-location ()
    (should (eq 99 (let ((grid (day6/make-grid 100)))
                     (day6/grid-location grid 99 0))))
    (should (eq 100 (let ((grid (day6/make-grid 100)))
                      (day6/grid-location grid 0 1)))))

  (defun day6/grid-light (grid location)
    (gethash location (day6/grid-table grid)))

  (defun day6/grid-do (grid fun location)
    (puthash location
             (funcall fun (day6/grid-light grid location))
             (day6/grid-table grid)))

  (defun day6/change-range (grid light-fun x1 y1 x2 y2)
    (loop for x from x1 to x2
          do (loop for y from y1 to y2
                   do (let* ((location (day6/grid-location grid x y)))
                        (day6/grid-do grid light-fun location)))))

  (defun day6/lit-count (grid)
    (length (-filter #'day6/lit? (day6/grid-lights grid))))

  (ert-deftest day6/lit-count ()
    (should (eq 1000000
                (let ((grid (day6/make-grid 1000)))
                  (day6/change-range grid #'day6/turn-on-light
                                     0 0 999 999)
                  (day6/lit-count grid))))
    (should (eq 4
                (let ((grid (day6/make-grid 1000)))
                  (day6/change-range grid #'day6/turn-on-light
                                     499 499 500 500)
                  (day6/lit-count grid)))))

  (defun day6/parse-instruction (instruction)
    (if (string-match (concat "\\(turn \\(on\\|off\\)\\|toggle\\) "
                              "\\([0-9]+\\),\\([0-9]+\\)"
                              " through "
                              "\\([0-9]+\\),\\([0-9]+\\)")
                      instruction)
        (list (cond ((string-equal "turn on" (match-string 1 instruction))
                     #'day6/turn-on-light)
                    ((string-equal "turn off" (match-string 1 instruction))
                     #'day6/turn-off-light)
                    ((string-equal "toggle" (match-string 1 instruction))
                     #'day6/toggle-light))
              (string-to-int (match-string 3 instruction))
              (string-to-int (match-string 4 instruction))
              (string-to-int (match-string 5 instruction))
              (string-to-int (match-string 6 instruction)))))

  (ert-deftest day6/parse-instruction ()
    (should (equal '(day6/turn-on-light 887 9 959 629)
                   (day6/parse-instruction "turn on 887,9 through 959,629")))
    (should (equal '(day6/turn-off-light 539 243 559 965)
                   (day6/parse-instruction "turn off 539,243 through 559,965")))
    (should (equal '(day6/toggle-light 720 196 897 994)
                   (day6/parse-instruction "toggle 720,196 through 897,994"))))

  (defun day6/run-instruction (grid instruction)
    (let ((parsed-instruction (day6/parse-instruction instruction)))
      (eval (append '(day6/change-range grid)
                    `(',(first parsed-instruction))
                    (rest parsed-instruction)))))

  (let ((grid (day6/make-grid 1000)))
    (-each (split-string input "\n" t)
      (apply-partially 'day6/run-instruction grid))
    (day6/lit-count grid))
#+END_SRC

#+RESULTS[2e528f53a45a3c7e0dcc549d5ac69918287ddd6e]:
: 377891

** Part 2

You just finish implementing your winning light pattern when you
realize you mistranslated Santa's message from Ancient Nordic Elvish.

The light grid you bought actually has individual brightness controls;
each light can have a brightness of zero or more. The lights all start
at zero.

The phrase =turn on= actually means that you should increase the
brightness of those lights by =1=.

The phrase =turn off= actually means that you should decrease the
brightness of those lights by =1=, to a minimum of zero.

The phrase =toggle= actually means that you should increase the
brightness of those lights by =2=.

What is the total brightness of all lights combined after following
Santa's instructions?

For example:

- =turn on 0,0 through 0,0= would increase the total brightness by
  =1=.
- =toggle 0,0 through 999,999= would increase the total brightness by
  =2000000=.

----------------------------------------------------------------------

#+BEGIN_SRC emacs-lisp :var input=6-input :exports both
  (defun day6.2/make-light ()
    0)

  (defun day6.2/turn-on-light (light)
    (+ 1 light))

  (defun day6.2/turn-off-light (light)
    (if (>= 0 light) 0
      (- light 1)))

  (defun day6.2/toggle-light (light)
    (+ 2 light))

  (defun day6.2/parse-instruction (instruction)
    (if (string-match (concat "\\(turn \\(on\\|off\\)\\|toggle\\) "
                              "\\([0-9]+\\),\\([0-9]+\\)"
                              " through "
                              "\\([0-9]+\\),\\([0-9]+\\)")
                      instruction)
        (list (cond ((string-equal "turn on" (match-string 1 instruction))
                     #'day6.2/turn-on-light)
                    ((string-equal "turn off" (match-string 1 instruction))
                     #'day6.2/turn-off-light)
                    ((string-equal "toggle" (match-string 1 instruction))
                     #'day6.2/toggle-light))
              (string-to-int (match-string 3 instruction))
              (string-to-int (match-string 4 instruction))
              (string-to-int (match-string 5 instruction))
              (string-to-int (match-string 6 instruction)))))

  (defun day6.2/run-instruction (grid instruction)
    (let ((parsed-instruction (day6.2/parse-instruction instruction)))
      (eval (append '(day6/change-range grid)
                    `(',(first parsed-instruction))
                    (rest parsed-instruction)))))

  (defun day6.2/make-grid (size)
    (let ((table (make-hash-table :size (* size size))))
      (-each (number-sequence 0 (- (* size size) 1))
        (lambda (pos) (puthash pos (day6.2/make-light) table)))
      (cons size
            table)))

  (defun day6.2/total-brightness (grid)
    (-reduce #'+
             (day6/grid-lights grid)))

  (let ((grid (day6.2/make-grid 1000)))
    (-each (split-string input "\n" t)
      (apply-partially 'day6.2/run-instruction grid))
    (day6.2/total-brightness grid))
#+END_SRC

#+RESULTS[ceaf378dab9cbe4211307b258b4fe66d2a04482b]:
: 14110788

* Day 7: Some Assembly Required
This year, Santa brought little Bobby Tables a set of wires and
[[https://en.wikipedia.org/wiki/Bitwise_operation][bitwise logic gates]]! Unfortunately, little Bobby is a little under the
recommended age range, and he needs help assembling the circuit.

Each wire has an identifier (some lowercase letters) and can carry a
[[https://en.wikipedia.org/wiki/16-bit][16-bit signal]] (a number from =0= to =65535=). A signal is provided to
each wire by a gate, another wire, or some specific value. Each wire
can only get a signal from one source, but can provide its signal to
multiple destinations. A gate provides no signal until all of its
inputs have a signal.

The included instructions booklet describes how to connect the parts
together: =x AND y -> z= means to connect wires =x= and =y= to an
=AND= gate, and then connect its output to wire =z=.

For example:

- =123 -> x= means that the signal =123= is provided to wire =x=.
- =x AND y -> z= means that the [[https://en.wikipedia.org/wiki/Bitwise_operation#AND][bitwise AND]] of wire =x= and wire =y=
  is provided to wire =z=.
- =p LSHIFT 2 -> q= means that the value from wire =p= is [[https://en.wikipedia.org/wiki/Logical_shift][left-shifted]]
  by =2= and then provided to wire =q=.
- =NOT e -> f= means that the =bitwise complement= of the value from
  wire =e= is provided to wire =f=.

Other possible gates include =OR= ([[https://en.wikipedia.org/wiki/Bitwise_operation#OR][bitwise OR]]) and =RSHIFT=
([[https://en.wikipedia.org/wiki/Logical_shift][right-shift]]). If, for some reason, you'd like to *emulate* the
circuit instead, almost all programming languages (for example, [[https://en.wikipedia.org/wiki/Bitwise_operations_in_C][C]],
[[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators][JavaScript]], or [[https://wiki.python.org/moin/BitwiseOperators][Python]]) provide operators for these gates.

For example, here is a simple circuit:

#+BEGIN_EXAMPLE
123 -> x
456 -> y
x AND y -> d
x OR y -> e
x LSHIFT 2 -> f
y RSHIFT 2 -> g
NOT x -> h
NOT y -> i
#+END_EXAMPLE

After it is run, these are the signals on the wires:

#+BEGIN_EXAMPLE
d: 72
e: 507
f: 492
g: 114
h: 65412
i: 65079
x: 123
y: 456
#+END_EXAMPLE

In little Bobby's kit's instructions booklet (provided as your puzzle
input), what signal is ultimately provided to *wire a*?

----------------------------------------------------------------------

:HIDDEN:
#+name: 7-input
#+BEGIN_EXAMPLE
  af AND ah -> ai
  NOT lk -> ll
  hz RSHIFT 1 -> is
  NOT go -> gp
  du OR dt -> dv
  x RSHIFT 5 -> aa
  at OR az -> ba
  eo LSHIFT 15 -> es
  ci OR ct -> cu
  b RSHIFT 5 -> f
  fm OR fn -> fo
  NOT ag -> ah
  v OR w -> x
  g AND i -> j
  an LSHIFT 15 -> ar
  1 AND cx -> cy
  jq AND jw -> jy
  iu RSHIFT 5 -> ix
  gl AND gm -> go
  NOT bw -> bx
  jp RSHIFT 3 -> jr
  hg AND hh -> hj
  bv AND bx -> by
  er OR es -> et
  kl OR kr -> ks
  et RSHIFT 1 -> fm
  e AND f -> h
  u LSHIFT 1 -> ao
  he RSHIFT 1 -> hx
  eg AND ei -> ej
  bo AND bu -> bw
  dz OR ef -> eg
  dy RSHIFT 3 -> ea
  gl OR gm -> gn
  da LSHIFT 1 -> du
  au OR av -> aw
  gj OR gu -> gv
  eu OR fa -> fb
  lg OR lm -> ln
  e OR f -> g
  NOT dm -> dn
  NOT l -> m
  aq OR ar -> as
  gj RSHIFT 5 -> gm
  hm AND ho -> hp
  ge LSHIFT 15 -> gi
  jp RSHIFT 1 -> ki
  hg OR hh -> hi
  lc LSHIFT 1 -> lw
  km OR kn -> ko
  eq LSHIFT 1 -> fk
  1 AND am -> an
  gj RSHIFT 1 -> hc
  aj AND al -> am
  gj AND gu -> gw
  ko AND kq -> kr
  ha OR gz -> hb
  bn OR by -> bz
  iv OR jb -> jc
  NOT ac -> ad
  bo OR bu -> bv
  d AND j -> l
  bk LSHIFT 1 -> ce
  de OR dk -> dl
  dd RSHIFT 1 -> dw
  hz AND ik -> im
  NOT jd -> je
  fo RSHIFT 2 -> fp
  hb LSHIFT 1 -> hv
  lf RSHIFT 2 -> lg
  gj RSHIFT 3 -> gl
  ki OR kj -> kk
  NOT ak -> al
  ld OR le -> lf
  ci RSHIFT 3 -> ck
  1 AND cc -> cd
  NOT kx -> ky
  fp OR fv -> fw
  ev AND ew -> ey
  dt LSHIFT 15 -> dx
  NOT ax -> ay
  bp AND bq -> bs
  NOT ii -> ij
  ci AND ct -> cv
  iq OR ip -> ir
  x RSHIFT 2 -> y
  fq OR fr -> fs
  bn RSHIFT 5 -> bq
  0 -> c
  14146 -> b
  d OR j -> k
  z OR aa -> ab
  gf OR ge -> gg
  df OR dg -> dh
  NOT hj -> hk
  NOT di -> dj
  fj LSHIFT 15 -> fn
  lf RSHIFT 1 -> ly
  b AND n -> p
  jq OR jw -> jx
  gn AND gp -> gq
  x RSHIFT 1 -> aq
  ex AND ez -> fa
  NOT fc -> fd
  bj OR bi -> bk
  as RSHIFT 5 -> av
  hu LSHIFT 15 -> hy
  NOT gs -> gt
  fs AND fu -> fv
  dh AND dj -> dk
  bz AND cb -> cc
  dy RSHIFT 1 -> er
  hc OR hd -> he
  fo OR fz -> ga
  t OR s -> u
  b RSHIFT 2 -> d
  NOT jy -> jz
  hz RSHIFT 2 -> ia
  kk AND kv -> kx
  ga AND gc -> gd
  fl LSHIFT 1 -> gf
  bn AND by -> ca
  NOT hr -> hs
  NOT bs -> bt
  lf RSHIFT 3 -> lh
  au AND av -> ax
  1 AND gd -> ge
  jr OR js -> jt
  fw AND fy -> fz
  NOT iz -> ja
  c LSHIFT 1 -> t
  dy RSHIFT 5 -> eb
  bp OR bq -> br
  NOT h -> i
  1 AND ds -> dt
  ab AND ad -> ae
  ap LSHIFT 1 -> bj
  br AND bt -> bu
  NOT ca -> cb
  NOT el -> em
  s LSHIFT 15 -> w
  gk OR gq -> gr
  ff AND fh -> fi
  kf LSHIFT 15 -> kj
  fp AND fv -> fx
  lh OR li -> lj
  bn RSHIFT 3 -> bp
  jp OR ka -> kb
  lw OR lv -> lx
  iy AND ja -> jb
  dy OR ej -> ek
  1 AND bh -> bi
  NOT kt -> ku
  ao OR an -> ap
  ia AND ig -> ii
  NOT ey -> ez
  bn RSHIFT 1 -> cg
  fk OR fj -> fl
  ce OR cd -> cf
  eu AND fa -> fc
  kg OR kf -> kh
  jr AND js -> ju
  iu RSHIFT 3 -> iw
  df AND dg -> di
  dl AND dn -> do
  la LSHIFT 15 -> le
  fo RSHIFT 1 -> gh
  NOT gw -> gx
  NOT gb -> gc
  ir LSHIFT 1 -> jl
  x AND ai -> ak
  he RSHIFT 5 -> hh
  1 AND lu -> lv
  NOT ft -> fu
  gh OR gi -> gj
  lf RSHIFT 5 -> li
  x RSHIFT 3 -> z
  b RSHIFT 3 -> e
  he RSHIFT 2 -> hf
  NOT fx -> fy
  jt AND jv -> jw
  hx OR hy -> hz
  jp AND ka -> kc
  fb AND fd -> fe
  hz OR ik -> il
  ci RSHIFT 1 -> db
  fo AND fz -> gb
  fq AND fr -> ft
  gj RSHIFT 2 -> gk
  cg OR ch -> ci
  cd LSHIFT 15 -> ch
  jm LSHIFT 1 -> kg
  ih AND ij -> ik
  fo RSHIFT 3 -> fq
  fo RSHIFT 5 -> fr
  1 AND fi -> fj
  1 AND kz -> la
  iu AND jf -> jh
  cq AND cs -> ct
  dv LSHIFT 1 -> ep
  hf OR hl -> hm
  km AND kn -> kp
  de AND dk -> dm
  dd RSHIFT 5 -> dg
  NOT lo -> lp
  NOT ju -> jv
  NOT fg -> fh
  cm AND co -> cp
  ea AND eb -> ed
  dd RSHIFT 3 -> df
  gr AND gt -> gu
  ep OR eo -> eq
  cj AND cp -> cr
  lf OR lq -> lr
  gg LSHIFT 1 -> ha
  et RSHIFT 2 -> eu
  NOT jh -> ji
  ek AND em -> en
  jk LSHIFT 15 -> jo
  ia OR ig -> ih
  gv AND gx -> gy
  et AND fe -> fg
  lh AND li -> lk
  1 AND io -> ip
  kb AND kd -> ke
  kk RSHIFT 5 -> kn
  id AND if -> ig
  NOT ls -> lt
  dw OR dx -> dy
  dd AND do -> dq
  lf AND lq -> ls
  NOT kc -> kd
  dy AND ej -> el
  1 AND ke -> kf
  et OR fe -> ff
  hz RSHIFT 5 -> ic
  dd OR do -> dp
  cj OR cp -> cq
  NOT dq -> dr
  kk RSHIFT 1 -> ld
  jg AND ji -> jj
  he OR hp -> hq
  hi AND hk -> hl
  dp AND dr -> ds
  dz AND ef -> eh
  hz RSHIFT 3 -> ib
  db OR dc -> dd
  hw LSHIFT 1 -> iq
  he AND hp -> hr
  NOT cr -> cs
  lg AND lm -> lo
  hv OR hu -> hw
  il AND in -> io
  NOT eh -> ei
  gz LSHIFT 15 -> hd
  gk AND gq -> gs
  1 AND en -> eo
  NOT kp -> kq
  et RSHIFT 5 -> ew
  lj AND ll -> lm
  he RSHIFT 3 -> hg
  et RSHIFT 3 -> ev
  as AND bd -> bf
  cu AND cw -> cx
  jx AND jz -> ka
  b OR n -> o
  be AND bg -> bh
  1 AND ht -> hu
  1 AND gy -> gz
  NOT hn -> ho
  ck OR cl -> cm
  ec AND ee -> ef
  lv LSHIFT 15 -> lz
  ks AND ku -> kv
  NOT ie -> if
  hf AND hl -> hn
  1 AND r -> s
  ib AND ic -> ie
  hq AND hs -> ht
  y AND ae -> ag
  NOT ed -> ee
  bi LSHIFT 15 -> bm
  dy RSHIFT 2 -> dz
  ci RSHIFT 2 -> cj
  NOT bf -> bg
  NOT im -> in
  ev OR ew -> ex
  ib OR ic -> id
  bn RSHIFT 2 -> bo
  dd RSHIFT 2 -> de
  bl OR bm -> bn
  as RSHIFT 1 -> bl
  ea OR eb -> ec
  ln AND lp -> lq
  kk RSHIFT 3 -> km
  is OR it -> iu
  iu RSHIFT 2 -> iv
  as OR bd -> be
  ip LSHIFT 15 -> it
  iw OR ix -> iy
  kk RSHIFT 2 -> kl
  NOT bb -> bc
  ci RSHIFT 5 -> cl
  ly OR lz -> ma
  z AND aa -> ac
  iu RSHIFT 1 -> jn
  cy LSHIFT 15 -> dc
  cf LSHIFT 1 -> cz
  as RSHIFT 3 -> au
  cz OR cy -> da
  kw AND ky -> kz
  lx -> a
  iw AND ix -> iz
  lr AND lt -> lu
  jp RSHIFT 5 -> js
  aw AND ay -> az
  jc AND je -> jf
  lb OR la -> lc
  NOT cn -> co
  kh LSHIFT 1 -> lb
  1 AND jj -> jk
  y OR ae -> af
  ck AND cl -> cn
  kk OR kv -> kw
  NOT cv -> cw
  kl AND kr -> kt
  iu OR jf -> jg
  at AND az -> bb
  jp RSHIFT 2 -> jq
  iv AND jb -> jd
  jn OR jo -> jp
  x OR ai -> aj
  ba AND bc -> bd
  jl OR jk -> jm
  b RSHIFT 1 -> v
  o AND q -> r
  NOT p -> q
  k AND m -> n
  as RSHIFT 2 -> at
#+END_EXAMPLE
:END:

#+BEGIN_SRC emacs-lisp :var input=7-input :exports both
  (defun day7/parse-expression (expression-string)
    (cond ((s-numeric? expression-string)
           (string-to-int expression-string))
          ((s-contains? " " expression-string)
           (let ((expression (-map #'day7/parse-expression
                                   (split-string expression-string))))
             (cond ((= 1 (length expression)) ;; single term
                    (car expression))
                   ((= 3 (length expression)) ;; re-order infix operation
                    (list (cadr expression)
                          (car expression)
                          (caddr expression)))
                   (t expression))))
          (t (intern (downcase expression-string)))))

  (ert-deftest day7/parse-expression ()
    (should (equal 123
                   (day7/parse-expression "123")))
    (should (equal 'x
                   (day7/parse-expression "x")))
    (should (equal 'ab
                   (day7/parse-expression "ab")))
    (should (equal '(and x y)
                   (day7/parse-expression "x AND y"))))

  (defun day7/parse-connection (connection-string)
    (string-match "\\(.*?\\) -> \\(.*\\)"
                  connection-string)
    (cons (day7/parse-expression (match-string 2 connection-string))
          (day7/parse-expression (match-string 1 connection-string))))

  (ert-deftest day7/parse-connection ()
    (should (equal '(x . 123)
                   (day7/parse-connection "123 -> x")))
    (should (equal '(d . (and x y))
                   (day7/parse-connection "x AND y -> d"))))

  (defun day7/make-circuit (connection-strings)
    (let ((expressions (-map #'day7/parse-connection
                             connection-strings))
          (circuit (make-hash-table :size (length connection-strings))))
      (-each expressions
        (lambda (expr) (puthash (car expr)
                                (cdr expr)
                                circuit)))
      circuit))

  (defun day7/circuit-get (circuit wire)
    (or (gethash wire circuit)
        (error (format "Unknown wire: %s" wire))))

  (defun day7/circuit-put (circuit wire value)
    (puthash wire value circuit))

  (defun day7/truncate-to-16bit (num)
    (string-to-number
     (substring (format "%04x" num) -4)
     16))

  (defun day7/and (a b)
    (logand a b))

  (defun day7/or (a b)
    (logior a b))

  (defun day7/not (n)
    (day7/truncate-to-16bit (lognot n)))

  (defun day7/rshift (n count)
    (lsh n (- count)))

  (defun day7/lshift (n count)
    (day7/truncate-to-16bit (lsh n count)))

  (defun day7/evaluate (circuit expr)
    (cond ((numberp expr) (day7/truncate-to-16bit expr))
          ((symbolp expr)
           ;; Evaluate and store the result in the circuit table for
           ;; future lookups
           (let ((result (day7/evaluate circuit
                                        (day7/circuit-get circuit expr))))
             (day7/circuit-put circuit expr result)
             result))
          ((listp expr)
           (let ((op (cond ((eq 'and (car expr)) #'day7/and)
                           ((eq 'or (car expr)) #'day7/or)
                           ((eq 'not (car expr)) #'day7/not)
                           ((eq 'lshift (car expr)) #'day7/lshift)
                           ((eq 'rshift (car expr)) #'day7/rshift)))
                 (args (-map (apply-partially 'day7/evaluate circuit)
                             (cdr expr))))
             (apply op args)))
          (t expr))
    )

  (ert-deftest day7/evaluate ()
    (let* ((sample-input (list "123 -> x"
                               "456 -> y"
                               "x AND y -> d"
                               "x OR y -> e"
                               "x LSHIFT 2 -> f"
                               "y RSHIFT 2 -> g"
                               "NOT x -> h"
                               "NOT y -> i"))
           (circuit (day7/make-circuit sample-input)))
      (should (eq 72 (day7/evaluate circuit 'd)))
      (should (eq 507 (day7/evaluate circuit 'e)))
      (should (eq 492 (day7/evaluate circuit 'f)))
      (should (eq 114 (day7/evaluate circuit 'g)))
      (should (eq 65412 (day7/evaluate circuit 'h)))
      (should (eq 65079 (day7/evaluate circuit 'i)))
      (should (eq 123 (day7/evaluate circuit 'x)))
      (should (eq 456 (day7/evaluate circuit 'y)))))

  (let* ((connection-strings (split-string input "\n" t))
         (circuit (day7/make-circuit connection-strings)))
    (day7/evaluate circuit 'a))
#+END_SRC

#+RESULTS[0e096ea6071516679cc36a305883d2bdd808c930]:
: 956

** Part 2
Now, take the signal you got on wire =a=, override wire =b= to that
signal, and reset the other wires (including wire =a=). What new
signal is ultimately provided to wire =a=?

----------------------------------------------------------------------

#+BEGIN_SRC emacs-lisp :var input=7-input :exports both
  (let* ((connection-strings (split-string input "\n" t))
         (circuit (day7/make-circuit connection-strings)))
    (day7/circuit-put circuit 'b 956)
    (day7/evaluate circuit 'a))
#+END_SRC

#+RESULTS[61e8544d7d4f32d111e9a666de842ecc4248e390]:
: 40149

* Day 8: Matchsticks
Space on the sleigh is limited this year, and so Santa will be
bringing his list as a digital copy. He needs to know how much space
it will take up when stored.

It is common in many programming languages to provide a way to escape
special characters in strings. For example, C, JavaScript, Perl,
Python, and even PHP handle special characters in very similar ways.

However, it is important to realize the difference between the number
of characters *in the code representation of the string literal* and
the number of characters *in the in-memory string itself*.

For example:

- =""= is =2= characters of code (the two double quotes), but the
  string contains zero characters.
- ="abc"= is =5= characters of code, but 3 characters in the string
  data.
- ="aaa\"aaa"= is =10= characters of code, but the string itself
  contains six ="a"= characters and a single, escaped quote character,
  for a total of =7= characters in the string data.
- ="\x27"= is =6= characters of code, but the string itself contains
  just one - an apostrophe (='=), escaped using hexadecimal notation.

Santa's list is a file that contains many double-quoted string
literals, one on each line. The only escape sequences used are =\\=
(which represents a single backslash), =\"= (which represents a lone
double-quote character), and =\x= plus two hexadecimal characters
(which represents a single character with that ASCII code).

Disregarding the whitespace in the file, what is *the number of
characters of code for string literals* minus *the number of
characters in memory for the values of the strings* in total for the
entire file?

For example, given the four strings above, the total number of
characters of string code =(2 + 5 + 10 + 6 = 23)= minus the total
number of characters in memory for string values =(0 + 3 + 7 + 1 =
11)= is =23 - 11 = 12=.

----------------------------------------------------------------------

:HIDDEN:
#+name: 8-input
#+BEGIN_EXAMPLE
  "qxfcsmh"
  "ffsfyxbyuhqkpwatkjgudo"
  "byc\x9dyxuafof\\\xa6uf\\axfozomj\\olh\x6a"
  "jtqvz"
  "uzezxa\"jgbmojtwyfbfguz"
  "vqsremfk\x8fxiknektafj"
  "wzntebpxnnt\"vqndz\"i\x47vvjqo\""
  "higvez\"k\"riewqk"
  "dlkrbhbrlfrp\\damiauyucwhty"
  "d\""
  "qlz"
  "ku"
  "yy\"\"uoao\"uripabop"
  "saduyrntuswlnlkuppdro\\sicxosted"
  "tj"
  "zzphopswlwdhebwkxeurvizdv"
  "xfoheirjoakrpofles\"nfu"
  "q\xb7oh\"p\xce\"n"
  "qeendp\"ercwgywdjeylxcv"
  "dcmem"
  "\"i\x13r\"l"
  "ikso\xdcbvqnbrjduh\"uqudzki\xderwk"
  "wfdsn"
  "pwynglklryhtsqbno"
  "hcoj\x63iccz\"v\"ttr"
  "zf\x23\\hlj\\kkce\\d\\asy\"yyfestwcdxyfj"
  "xs"
  "m\"tvltapxdvtrxiy"
  "bmud"
  "k\"a"
  "b\"oas"
  "\"yexnjjupoqsxyqnquy\"uzfdvetqrc"
  "vdw\xe3olxfgujaj"
  "qomcxdnd\"\\cfoe\""
  "fpul"
  "m\"avamefphkpv"
  "vvdnb\\x\\uhnxfw\"dpubfkxfmeuhnxisd"
  "hey\\"
  "ldaeigghlfey"
  "eure\"hoy\xa5iezjp\\tm"
  "yygb\"twbj\\r\"\x10gmxuhmp\""
  "weirebp\x39mqonbtmfmd"
  "ltuz\\hs\"e"
  "ysvmpc"
  "g\x8amjtt\"megl\"omsaihifwa"
  "yimmm"
  "iiyqfalh"
  "cwknlaaf"
  "q\x37feg\xc6s\"xx"
  "uayrgeurgyp\\oi"
  "xhug\"pt\"axugllbdiggzhvy"
  "kdaarqmsjfx\xc3d"
  "\"vkwla"
  "d\""
  "tmroz\"bvfinxoe\\mum\"wmm"
  "\"n\"bbswxne\\p\\yr\"qhwpdd"
  "skzlkietklkqovjhvj\xfe"
  "pbg\\pab\"bubqaf\"obzcwxwywbs\\dhtq"
  "xxjidvqh\"lx\\wu\"ij"
  "daef\x5fe\x5b\\kbeeb\x13qnydtboof"
  "ogvazaqy\"j\x73"
  "y"
  "n\"tibetedldy\\gsamm\"nwu"
  "wldkvgdtqulwkad"
  "dpmxnj"
  "twybw\"cdvf\"mjdajurokbce"
  "ru\"\\lasij\"i"
  "roc\\vra\\lhrm"
  "pbkt\x60booz\"fjlkc"
  "j\x4dytvjwrzt"
  "\\uiwjkniumxcs"
  "cbhm\"nexccior\"v\"j\"nazxilmfp\x47"
  "qdxngevzrlgoq"
  "\"lrzxftytpobsdfyrtdqpjbpuwmm\x9e"
  "mdag\x0asnck\xc2ggj\"slb\"fjy"
  "wyqkhjuazdtcgkcxvjkpnjdae"
  "aixfk\xc0iom\x21vueob"
  "dkiiakyjpkffqlluhaetires"
  "ysspv\"lysgkvnmwbbsy"
  "gy\"ryexcjjxdm\"xswssgtr"
  "s"
  "ddxv"
  "qwt\"\x27puilb\"pslmbrsxhrz"
  "qdg\xc9e\\qwtknlvkol\x54oqvmchn\\"
  "lvo"
  "b"
  "fk\"aa\"\"yenwch\\\\on"
  "srig\x63hpwaavs\\\x80qzk\"xa\"\xe6u\\wr"
  "yxjxuj\"ghyhhxfj\"\xa6qvatre"
  "yoktqxjxkzrklkoeroil"
  "\"jfmik\""
  "smgseztzdwldikbqrh\""
  "jftahgctf\"hoqy"
  "tcnhicr\"znpgckt\"ble"
  "vqktnkodh\"lo\"a\\bkmdjqqnsqr"
  "ztnirfzqq"
  "s"
  "xx"
  "iqj\"y\\hqgzflwrdsusasekyrxbp\\ad"
  "\\xzjhlaiynkioz\"\"bxepzimvgwt"
  "s\x36rbw"
  "mniieztwrisvdx"
  "atyfxioy\x2b\\"
  "irde\x85\x5cvbah\\jekw\"ia"
  "bdmftlhkwrprmpat\"prfaocvp"
  "w\\k"
  "umbpausy"
  "zfauhpsangy"
  "p\"zqyw"
  "wtztypyqvnnxzvlvipnq\"zu"
  "deicgwq\\oqvajpbov\\or\"kgplwu"
  "mbzlfgpi\\\\zqcidjpzqdzxityxa"
  "lfkxvhma"
  "\xf2yduqzqr\"\\fak\"p\"n"
  "mpajacfuxotonpadvng"
  "anb\\telzvcdu\\a\xf2flfq"
  "lrs\"ebethwpmuuc\"\x86ygr"
  "qmvdbhtumzc\"ci"
  "meet"
  "yopg\x0fdxdq\"h\\ugsu\xffmolxjv"
  "uhy"
  "fzgidrtzycsireghazscvmwcfmw\\t"
  "cqohkhpgvpru"
  "bihyigtnvmevx\"xx"
  "xz"
  "zofomwotzuxsjk\"q\"mc\"js\"dnmalhxd"
  "\\ktnddux\\fqvt\"ibnjntjcbn"
  "ia"
  "htjadnefwetyp\xd5kbrwfycbyy"
  "\"\\hkuxqddnao"
  "meqqsz\x83luecpgaem"
  "cvks\x87frvxo\"svqivqsdpgwhukmju"
  "sgmxiai\\o\"riufxwjfigr\xdf"
  "fgywdfecqufccpcdn"
  "faghjoq\x28abxnpxj"
  "zuppgzcfb\"dctvp\"elup\"zxkopx"
  "xqs\x45xxdqcihbwghmzoa"
  "anbnlp\\cgcvm\"hc"
  "xf\"fgrngwzys"
  "nrxsjduedcy\x24"
  "\x71sxl\"gj\"sds\"ulcruguz\\t\\ssvjcwhi"
  "jhj\"msch"
  "qpovolktfwyiuyicbfeeju\x01"
  "nkyxmb\"qyqultgt\"nmvzvvnxnb"
  "ycsrkbstgzqb\"uv\\cisn"
  "s"
  "ueptjnn\"\"sh"
  "lp\"z\"d\"mxtxiy"
  "yzjtvockdnvbubqabjourf\"k\"uoxwle"
  "\x82\"wqm\""
  "\xb5cwtuks\x5fpgh"
  "wd"
  "tbvf"
  "ttbmzdgn"
  "vfpiyfdejyrlbgcdtwzbnm"
  "uc"
  "otdcmhpjagqix"
  "\\\xb1qso\"s"
  "scowax"
  "behpstjdh\xccqlgnqjyz\"eesn"
  "r\xe1cbnjwzveoomkzlo\\kxlfouhm"
  "jgrl"
  "kzqs\\r"
  "ctscb\x7fthwkdyko\"\x62pkf\"d\xe6knmhurg"
  "tc\"kw\x3ftt"
  "bxb\x5ccl"
  "jyrmfbphsldwpq"
  "jylpvysl\"\"juducjg"
  "en\\m\"kxpq\"wpb\\\""
  "madouht\"bmdwvnyqvpnawiphgac\""
  "vuxpk\"ltucrw"
  "aae\x60arr"
  "ttitnne\"kilkrgssnr\xfdurzh"
  "oalw"
  "pc\"\"gktkdykzbdpkwigucqni\"nxiqx"
  "dbrsaj"
  "bgzsowyxcbrvhtvekhsh\"qgd"
  "kudfemvk\"\"\"hkbrbil\"chkqoa"
  "zjzgj\\ekbhyfzufy"
  "\\acos\"fqekuxqzxbmkbnn\x1ejzwrm"
  "elxahvudn\"txtmomotgw"
  "\x2eoxmwdhelpr\"cgi\xf7pzvb"
  "eapheklx"
  "hfvma\"mietvc\"tszbbm\"czex"
  "h\"iiockj\\\xc1et"
  "d\"rmjjftm"
  "qlvhdcbqtyrhlc\\"
  "yy\"rsucjtulm\"coryri\"eqjlbmk"
  "tv"
  "r\"bfuht\\jjgujp\""
  "kukxvuauamtdosngdjlkauylttaokaj"
  "srgost\"\"rbkcqtlccu\x65ohjptstrjkzy"
  "yxwxl\\yjilwwxffrjjuazmzjs"
  "dxlw\\fkstu\"hjrtiafhyuoh\"sewabne"
  "\x88sj\"v"
  "rfzprz\xec\"oxqclu\"krzefp\\q"
  "cfmhdbjuhrcymgxpylllyvpni"
  "ucrmjvmimmcq\x88\xd9\"lz"
  "lujtt\""
  "gvbqoixn\"pmledpjmo\"flydnwkfxllf"
  "dvxqlbshhmelsk\x8big\"l"
  "mx\x54lma\x8bbguxejg"
  "\x66jdati\xeceieo"
  "\"iyyupixei\x54ff"
  "xohzf\"rbxsoksxamiu"
  "vlhthspeshzbppa\x4drhqnohjop\"\"mfjd"
  "f\"tvxxla\"vurian\"\"idjq\x3aptm\xc3olep"
  "gzqz"
  "kbq\\wogye\\altvi\\hbvmodny"
  "j\xd8"
  "ofjozdhkblvndl"
  "hbitoupimbawimxlxqze"
  "ypeleimnme"
  "xfwdrzsc\\oxqamawyizvi\\y"
  "enoikppx\xa1ixe\"yo\"gumye"
  "fb"
  "vzf"
  "zxidr"
  "cu\x31beirsywtskq"
  "lxpjbvqzztafwezd"
  "\\jyxeuo\x18bv"
  "b\"vawc\"p\\\\giern\"b"
  "odizunx\"\"t\\yicdn\"x\"sdiz"
  "\"\"tebrtsi"
  "ctyzsxv\xa6pegfkwsi\"tgyltaakytccb"
  "htxwbofchvmzbppycccliyik\xe5a"
  "ggsslefamsklezqkrd"
  "rcep\"fnimwvvdx\"l"
  "zyrzlqmd\x12egvqs\\llqyie"
  "\x07gsqyrr\\rcyhyspsvn"
  "butg\""
  "gb"
  "gywkoxf\"jsg\\wtopxvumirqxlwz"
  "rj\"ir\"wldwveair\x2es\"dhjrdehbqnzl"
  "ru\"elktnsbxufk\\ejufjfjlevt\\lrzd"
  "\"widsvok"
  "oy\"\x81nuesvw"
  "ay"
  "syticfac\x1cfjsivwlmy\"pumsqlqqzx"
  "m"
  "rjjkfh\x78cf\x2brgceg\"jmdyas\"\\xlv\xb6p"
  "tmuvo\"\x3ffdqdovjmdmkgpstotojkv\"as"
  "jd\\ojvynhxllfzzxvbn\"wrpphcvx"
  "pz"
  "\"twr"
  "n\\hdzmxe\"mzjjeadlz"
  "fb\"rprxuagvahjnri"
  "rfmexmjjgh\\xrnmyvnatrvfruflaqjnd"
  "obbbde\"co\"qr\"qpiwjgqahqm\\jjp\""
  "vpbq\"\"y\"czk\\b\x52ed\"lnzepobp"
  "syzeajzfarplydipny\"y\"\xe8ad"
  "mpyodwb"
  "\x47rakphlqqptd"
  "wa\"oj\"aiy"
  "a"
  "ropozx"
  "q\x51nbtlwa"
  "etukvgx\\jqxlkq"
  "\"tp\"rah\"pg\"s\"bpdtes\\tkasdhqd"
  "dn\"qqpkikadowssb\xcah\"dzpsf\\ect\"jdh"
  "pxunovbbrrn\\vullyn\"bno\"\"\"myfxlp\""
  "qaixyazuryvkmoulhcqaotegfj\\mpzm"
  "bvfrbicutzbjwn\\oml\"cf\"d\"ezcpv\"j"
  "rmbrdtneudemigdhelmb"
  "aq\\aurmbhy"
  "wujqvzw"
  "gf\"tssmvm\"gm\"hu\x9a\xb7yjawsa"
  "hrhqqxow\xe2gsydtdspcfqy\"zw\\ou"
  "ianwwf\\yko\\tdujhhqdi"
  "xylz\"zpvpab"
  "lwuopbeeegp"
  "aoop\x49jhhcexdmdtun"
  "\\\\mouqqcsgmz"
  "tltuvwhveau\x43b\"ymxjlcgiymcynwt"
  "gsugerumpyuhtjljbhrdyoj"
  "lnjm\xb8wg\"ajh"
  "zmspue\"nfttdon\\b\"eww"
  "\"w\x67jwaq\x7ernmyvs\\rmdsuwydsd\"th"
  "ogtgvtlmcvgllyv"
  "z\"fqi\"rvddoehrciyl"
  "yustxxtot\"muec\"xvfdbzunzvveq"
  "mqslw"
  "txqnyvzmibqgjs\xb6xy\x86nfalfyx"
  "kzhehlmkholov"
  "plpmywcnirrjutjguosh\\"
  "pydbnqofv\"dn\\m"
  "aegqof"
  "eambmxt\\dxagoogl\\zapfwwlmk"
  "afbmqitxxqhddlozuxcpjxgh"
  "vgts"
  "bfdpqtoxzzhmzcilehnflna"
  "s\"idpz"
  "\xcfhgly\"nlmztwybx\"ecezmsxaqw"
  "aackfgndqcqiy"
  "\x22unqdlsrvgzfaohoffgxzfpir\"s"
  "abh\"ydv\"kbpdhrerl"
  "bdzpg"
  "ekwgkywtmzp"
  "wtoodejqmrrgslhvnk\"pi\"ldnogpth"
  "njro\x68qgbx\xe4af\"\\suan"
#+END_EXAMPLE
:END:

#+BEGIN_SRC emacs-lisp :var input=8-input :exports both
  (defun day8/hex-replacements (string-literal)
    (-map (lambda (escaped-hex)
            (let ((hex-code (string-to-number (substring escaped-hex 2) 16)))
              (cons escaped-hex (char-to-string hex-code))))
          (-map #'car (s-match-strings-all "\\\\x[0-9,a-f][0-9,a-f]" string-literal))))

  (defun day8/parse-string (string-literal)
    (let ((contents (substring string-literal 1 -1)))
      (-reduce-from
       (lambda (acc replacement)
         (s-replace (car replacement)
                    (cdr replacement)
                    acc))
       contents
       (append '(("\\\"" . "\"")
                 ("\\\\" . "\\"))
               (day8/hex-replacements contents)))))

  (ert-deftest day8/parse-string ()
    (should (string-equal ""
                          (day8/parse-string "\"\"")))
    (should (string-equal "abc"
                          (day8/parse-string "\"abc\"")))
    (should (string-equal "aaa\"aaa"
                          (day8/parse-string "\"aaa\\\"aaa\"")))
    (should (string-equal "'"
                          (day8/parse-string "\"\\x27\""))))


  (let* ((code-strings (split-string input "\n" t))
         (string-values (-map #'day8/parse-string code-strings)))
    (- (-sum (-map #'length code-strings))
       (-sum (-map #'length string-values))))
#+END_SRC

#+RESULTS[acaa7a9432ace3b411d0bfcd4c61620b43ce21c5]:
: 1350

** Part 2
Now, let's go the other way. In addition to finding the number of
characters of code, you should now *encode each code representation as
a new string* and find the number of characters of the new encoded
representation, including the surrounding double quotes.

For example:

- =""= encodes to ="\"\""=, an increase from =2= characters to =6=.
- ="abc"= encodes to ="\"abc\""=, an increase from =5= characters to
  =9=.
- ="aaa\"aaa"= encodes to ="\"aaa\\\"aaa\""=, an increase from =10=
  characters to =16=.
- ="\x27"= encodes to ="\"\\x27\""=, an increase from =6= characters
  to =11=.

Your task is to find *the total number of characters to represent the
newly encoded strings* minus *the number of characters of code in each
original string literal*. For example, for the strings above, the
total encoded length (=6 + 9 + 16 + 11 = 42=) minus the characters in
the original code representation (=23=, just like in the first part of
this puzzle) is =42 - 23 = 19=.

----------------------------------------------------------------------

#+BEGIN_SRC emacs-lisp :var input=8-input :exports both
  (defun day8/encode-string (string-literal)
    (format "\"%s\""
            (-reduce-from
             (lambda (acc replacement)
               (s-replace (car replacement)
                          (cdr replacement)
                          acc))
             string-literal
             '(("\\" . "\\\\")
               ("\"" . "\\\"")))))

  (ert-deftest day8/encode-string ()
    (should (string-equal "\"\\\"\\\"\""
                          (day8/encode-string "\"\"")))
    (should (string-equal "\"\\\"abc\\\"\""
                          (day8/encode-string "\"abc\"")))
    (should (string-equal "\"\\\"aaa\\\\\\\"aaa\\\"\""
                          (day8/encode-string "\"aaa\\\"aaa\"")))
    (should (string-equal "\"\\\"\\\\x27\\\"\""
                          (day8/encode-string "\"\\x27\""))))

  (let* ((code-strings (split-string input "\n" t))
         (encoded-values (-map #'day8/encode-string code-strings)))
    (- (-sum (-map #'length encoded-values))
       (-sum (-map #'length code-strings))))
#+END_SRC

#+RESULTS[bb50e3c25f6d0d1247acdefae2e5c755c90fe7a7]:
: 2085

* Day 9: All in a Single Night

Every year, Santa manages to deliver all of his presents in a single
night.

This year, however, he has some new locations to visit; his elves have
provided him the distances between every pair of locations. He can
start and end at any two (different) locations he wants, but he must
visit each location exactly once. What is the *shortest distance* he
can travel to achieve this?

For example, given the following distances:

#+name: 9-example
#+BEGIN_EXAMPLE
London to Dublin = 464
London to Belfast = 518
Dublin to Belfast = 141
#+END_EXAMPLE

The possible routes are therefore:

#+BEGIN_EXAMPLE
Dublin -> London -> Belfast = 982
London -> Dublin -> Belfast = 605
London -> Belfast -> Dublin = 659
Dublin -> Belfast -> London = 659
Belfast -> Dublin -> London = 605
Belfast -> London -> Dublin = 982
#+END_EXAMPLE

The shortest of these is =London -> Dublin -> Belfast = 605=, and so
the answer is =605= in this example.

What is the distance of the shortest route?

----------------------------------------------------------------------

:HIDDEN:
#+name: 9-input
#+BEGIN_EXAMPLE
  Tristram to AlphaCentauri = 34
  Tristram to Snowdin = 100
  Tristram to Tambi = 63
  Tristram to Faerun = 108
  Tristram to Norrath = 111
  Tristram to Straylight = 89
  Tristram to Arbre = 132
  AlphaCentauri to Snowdin = 4
  AlphaCentauri to Tambi = 79
  AlphaCentauri to Faerun = 44
  AlphaCentauri to Norrath = 147
  AlphaCentauri to Straylight = 133
  AlphaCentauri to Arbre = 74
  Snowdin to Tambi = 105
  Snowdin to Faerun = 95
  Snowdin to Norrath = 48
  Snowdin to Straylight = 88
  Snowdin to Arbre = 7
  Tambi to Faerun = 68
  Tambi to Norrath = 134
  Tambi to Straylight = 107
  Tambi to Arbre = 40
  Faerun to Norrath = 11
  Faerun to Straylight = 66
  Faerun to Arbre = 144
  Norrath to Straylight = 115
  Norrath to Arbre = 135
  Straylight to Arbre = 127
#+END_EXAMPLE
:END:

#+BEGIN_SRC emacs-lisp :var example=9-example :var input=9-input :noweb yes
  (defun day9/make-map ()
    (make-hash-table :test #'equal))

  (defun day9/store-distance (map a b distance)
    (puthash (cons a b) distance map))

  (defun day9/distance (map a b)
    (gethash (cons a b) map
             (gethash (cons b a) map 0)))

  (defun day9/parse-distance (distance-string)
    (if (string-match "\\(.*?\\) to \\(.*?\\) = \\([0-9]+\\)"
                      distance-string)
        (cons (cons (match-string-no-properties 1 distance-string)
                    (match-string-no-properties 2 distance-string))
              (string-to-number (match-string-no-properties 3 distance-string)))))

  (ert-deftest day9/parse-distance ()
    (should (equal '(("London" . "Dublin") . 464)
                   (day9/parse-distance "London to Dublin = 464"))))

  (defun day9/build-map (input)
    (let ((map (day9/make-map)))
      (-each (-map #'day9/parse-distance
                   (split-string input "\n" t))
        (lambda (distance)
          (day9/store-distance map
                               (caar distance)
                               (cdar distance)
                               (cdr distance))))
      map))

  (defun day9/trip-distance (map trip)
    (let ((start (first trip))
          (steps (rest trip)))
      (if (null steps) 0
        (+ (day9/distance map start (first steps))
           (day9/trip-distance map steps)))))

  (defun day9/trip-distance< (min current map trip)
    (let ((start (first trip))
          (steps (rest trip)))
      (cond ((and min (> current min)) nil)
            ((null steps) current)
            (t (let ((distance (day9/distance map start (first steps))))
                 (day9/trip-distance< min (+ distance current) map
                                      steps))))))

  (lexical-let ((example example))
    (ert-deftest day9/trip-distance ()
      (let ((map (day9/build-map example)))
        (should (eq 982 (day9/trip-distance map
                                            '("Dublin"
                                              "London"
                                              "Belfast")))))))

  (defun day9/permutations (lst)
    (if (not lst) '(nil)
      (mapcan
       (lambda (e)
         (mapcar (lambda (perm) (cons e perm))
                 (day9/permutations (-remove-item e lst))))
       lst)))

  (defun day9/trips (map)
    "Don't include reversed location permutations"
    (cl-loop for perm in (day9/permutations (day9/locations map))
              with unique-perms = (make-hash-table :test 'equal)
              unless (gethash (reverse perm) unique-perms)
              do (puthash perm t unique-perms)
              finally return (hash-table-keys unique-perms)))

  (defun day9/locations (map)
    (-distinct
     (append (-map #'car (hash-table-keys map))
             (-map #'cdr (hash-table-keys map)))))

  (lexical-let ((example example))
    (ert-deftest day9/locations ()
      (let ((map (day9/build-map example)))
        (should (seq-empty-p
                 (-difference '("Dublin" "London" "Belfast")
                              (day9/locations map)))))))

  (defun day9/min-trip-distance (map)
    (cl-loop for path in (day9/trips map)
             with min-distance
             do (let ((distance (day9/trip-distance< min-distance 0 map path)))
                  (if (and (null min-distance) distance)
                      (setf min-distance distance))
                  (if (and min-distance distance
                           (< distance min-distance))
                      (setf min-distance distance)))
             finally return min-distance))

  (day9/min-trip-distance
   (day9/build-map input))
#+END_SRC

#+RESULTS[052969aa6870de45711d74f55a9ef7dbd058a865]:
: 251

** Part 2
The next year, just to show off, Santa decides to take the route with
the *longest distance* instead.

He can still start and end at any two (different) locations he wants,
and he still must visit each location exactly once.

For example, given the distances above, the longest route would be
=982= via (for example) =Dublin -> London -> Belfast=.

What is the distance of the longest route?

----------------------------------------------------------------------

#+BEGIN_SRC emacs-lisp :var input=9-input :exports both
  (defun day9/trip-distance> (max current map trip)
    (let ((start (first trip))
          (steps (rest trip)))
      (cond ((null steps) current)
            (t (let ((distance (day9/distance map start (first steps))))
                 (day9/trip-distance> max (+ distance current) map
                                      steps))))))

  (defun day9/max-trip-distance (map)
    (cl-loop for path in (day9/trips map)
             with max-distance
             do (let ((distance (day9/trip-distance> max-distance 0 map path)))
                  (if (and (null max-distance) distance)
                      (setf max-distance distance))
                  (if (and max-distance distance
                           (< max-distance distance))
                      (setf max-distance distance)))
             finally return max-distance))

  (day9/max-trip-distance
   (day9/build-map input))
#+END_SRC

#+RESULTS[60abd7588f60147684eafda03eae00b226d39891]:
: 898

* Day 10: Elves Look, Elves Say

Today, the Elves are playing a game called look-and-say. They take
turns making sequences by reading aloud the previous sequence and
using that reading as the next sequence. For example, =211= is read as
"one two, two ones", which becomes =1221= (=1= =2=, =2= =1=s).

Look-and-say sequences are generated iteratively, using the previous
value as input for the next step. For each step, take the previous
value, and replace each run of digits (like =111=) with the number of
digits (=3=) followed by the digit itself (=1=).

For example:

- =1= becomes =11= (=1= copy of digit =1=).
- =11= becomes =21= (=2= copies of digit =1=).
- =21= becomes =1211= (one =2= followed by one =1=).
- =1211= becomes =111221= (one =1=, one =2=, and two =1=s).
- =111221= becomes =312211= (three =1=s, two =2=s, and one =1=).

Starting with the digits in your puzzle input, apply this process 40
times. What is *the length of the result*?

----------------------------------------------------------------------

:HIDDEN:
#+name: 10-input
#+BEGIN_EXAMPLE
3113322113
#+END_EXAMPLE
:END:

#+BEGIN_SRC emacs-lisp :var input=10-input :exports both
  (defun day10/say-sequence (seq)
    (let ((partitioned ))
      (mapconcat
       (lambda (chars)
         (format "%d%s" (length chars) (char-to-string (car chars))))
       (-partition-by #'identity (seq-into seq 'list))
       "")))

  (ert-deftest day10/say-sequence ()
    (should (string-equal "11" (day10/say-sequence "1")))
    (should (string-equal "21" (day10/say-sequence "11")))
    (should (string-equal "1211" (day10/say-sequence "21")))
    (should (string-equal "111221" (day10/say-sequence "1211")))
    (should (string-equal "312211" (day10/say-sequence "111221"))))

  (defun day10/say-sequence-n (times seq)
    (-reduce-from
     (lambda (acc next)
       (day10/say-sequence acc))
     seq
     (number-sequence 1 times)))

  (ert-deftest day10/say-sequence-n ()
    (should (string-equal "312211" (day10/say-sequence-n 5 "1"))))

  (length (day10/say-sequence-n 40 (string-trim input)))
#+END_SRC

#+RESULTS[a92ffee565329427383332212debc42eac7b4af2]:
: 329356

** Part 2
Neat, right? You might also enjoy hearing [[https://www.youtube.com/watch?v=ea7lJkEhytA][John Conway talking about
this sequence]] (that's Conway of *Conway's Game of Life* fame).

Now, starting again with the digits in your puzzle input, apply this
process *50* times. What is *the length of the new result*?

----------------------------------------------------------------------

#+BEGIN_SRC emacs-lisp :var input=10-input :exports both
  (length (day10/say-sequence-n 50 (string-trim input)))
#+END_SRC

#+RESULTS[cc7bbc9c6f3abcff179e832a1ed20371b788b059]:
: 4666278