Excercise 003

e003.erl

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+% Find the largest prime factor of a composite number.
+
+% The prime factors of 13195 are 5, 7, 13 and 29.
+% What is the largest prime factor of the number 600851475143 ?
+
+-module(e003).
+-export([
+    main/1,
+    pfactor/1
+]).
+
+pfactor(N) ->
+    pfactor_(N, []).
+pfactor_(N, F) ->
+    Next = pfactor_next(N, 2),
+    if
+        Next == N ->
+            [N | F];
+        true ->
+            pfactor_(trunc(N / Next), [Next | F])
+    end.
+
+pfactor_next(N, Factor) ->
+    if
+        Factor == N ->
+            Factor;
+        N rem Factor == 0 ->
+            Factor;
+        true ->
+            pfactor_next(N, Factor + 1)
+    end.
+
+
+main(_) ->
+    io:format("Prime Factors of 13195: ~w~n", [pfactor(13195)]),
+    io:format("Prime Factors of 600851475143: ~w~n", [pfactor(600851475143)]).