Add solution for problem 64.

Problem064.hs

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+-- For how many N <= 10000 does sqrt(N) have an odd period when written as a continued fraction?
+import Euler
+
+-- Term r c === r*sqrt(n)+c
+data Term = Term (Ratio Integer) (Ratio Integer) deriving (Eq, Show)
+
+--   1/(r*sqrt(n) + c)
+-- = (r*sqrt(n) - c) / (r^2*n - c^2)
+-- = (r/(r^2*n - c^2))*sqrt(n) - c/(r^2*n - c^2)
+termRecip :: Term -> Integer -> Term
+termRecip (Term r c) n = Term (r / denom) (-c / denom)
+  where denom = r^2 * (fromIntegral n) - c^2
+
+-- sqrt(n) = rem0
+--         = a0 + 1/rem1
+--         = a0 + 1/(a1 + 1/rem2)
+--         = ...
+--         = a0 + 1/(a1 + 1/(a2 + 1/(a3 + ...)))
+--
+-- rem0 = sqrt(n),       a0 = floor(rem0)
+-- rem1 = 1/(rem0 - a0), a1 = floor(rem1)
+-- rem2 = 1/(rem1 - a1), a2 = floor(rem2)
+--
+toFraction :: Integer -> ([Integer], [Integer])
+toFraction n = let sqrtN = sqrt $ fromIntegral n in
+  if floor sqrtN ^ 2 == n then ([floor sqrtN], [])
+  else flip unfoldPeriodicState (Term 1 0) $ state $ \(Term r c) ->
+    let a = floor (fromRational r * sqrtN + fromRational c) :: Integer in
+    (Just a, termRecip (Term r (c - fromIntegral a)) n)
+
+main = print $ length $ filter (odd . length . snd . toFraction) [1..10000]