euler 27

euler/27.hs

@@ -0,0 +1,35 @@
+-- Considering quadratics of the form:
+-- 
+-- n² + an + b, where |a| < 1000 and |b| < 1000
+--
+-- Find the product of the coefficients, a and b, 
+-- for the quadratic expression that produces 
+-- the maximum number of primes for 
+-- consecutive values of n, starting with n = 0.
+
+import Data.List  (maximumBy)
+import Data.Ord   (comparing)
+import Data.Array (listArray)
+import Data.Maybe (Maybe (Nothing, Just))
+import Numerix    (primes, bisect)
+
+primeArr = listArray (0, len - 1) x
+	where
+		x   = takeWhile (< 10^4) primes
+		len = length x
+
+isPrime = toBool . bisect primeArr
+	where
+		toBool (Just _) = True
+		toBool Nothing  = False
+
+euler :: Int -> Int -> [Int]
+euler a b = map (\n -> n^2 + a*n + b) [0..]
+
+eulers = [(a * b, euler a b) | a <- [-999..999], b <- [-999..999]]
+
+match = length . takeWhile isPrime
+
+answer = maximumBy (comparing (match.snd)) eulers
+
+main = print $ fst answer