Haskell 003

haskell/e003.hs

@@ -0,0 +1,27 @@
+{-
+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 ?
+-}
+
+import Text.Printf
+
+pfactor :: (Integral a) => a -> [a]
+pfactor n = pfactor_ n []
+
+pfactor_ :: (Integral a) => a -> [a] -> [a]
+pfactor_ n factors = do
+    let next = pfactor_next n 2
+    if next == n
+        then factors ++ [n]
+        else pfactor_ (n `div` next) (factors ++ [next])
+
+pfactor_next :: (Integral a) => a -> a -> a
+pfactor_next n factor
+    | n == factor = factor
+    | n `rem` factor == 0 = factor
+    | otherwise = pfactor_next n (factor + 1)
+
+main = do
+    printf "Largest prime factor of 600851475143: %d\n" (last (pfactor 600851475143) :: Int)