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Task/Percolation-Mean-run-density/Haskell/percolation-mean-run-density.hs

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+import Control.Monad.Random
+import Control.Applicative
+import Text.Printf
+import Control.Monad
+import Data.Bits
+
+data OneRun = OutRun | InRun deriving (Eq, Show)
+
+randomList :: Int -> Double -> Rand StdGen [Int]
+randomList n p = take n . map f <$> getRandomRs (0,1)
+  where f n = if (n > p) then 0 else 1
+
+countRuns xs = fromIntegral . sum $
+               zipWith (\x y -> x .&. xor y 1) xs (tail xs ++ [0])
+
+calcK :: Int -> Double -> Rand StdGen Double
+calcK n p = (/ fromIntegral n) . countRuns <$> randomList n p
+
+printKs :: StdGen -> Double -> IO ()
+printKs g p = do
+  printf "p= %.1f, K(p)= %.3f\n" p (p * (1 - p))
+  forM_ [1..5] $ \n -> do
+    let est = evalRand (calcK (10^n) p) g
+    printf "n=%7d, estimated K(p)= %5.3f\n" (10^n::Int) est
+
+main = do
+  x <- newStdGen
+  forM_ [0.1,0.3,0.5,0.7,0.9] $ printKs x