feat: just work

Author: RyosukeDTomita

math/eratosthenes/eratosthenesMonad.hs

@@ -0,0 +1,38 @@
+import Control.Monad (forM_, when)
+import Data.Vector.Unboxed qualified as VU
+import Data.Vector.Unboxed.Mutable qualified as VUM
+
+type PrimeTable = VU.Vector Bool
+
+primeTableToList :: PrimeTable -> [Int]
+primeTableToList t =
+  [ i
+    | i <- [0 .. VU.length t - 1],
+      t VU.! i
+  ]
+
+isInPrimeTable :: PrimeTable -> Int -> Bool
+isInPrimeTable t n
+  | 0 <= n && n < VU.length t = t VU.! n
+  | otherwise = error "out of range"
+
+sieve :: Int -> PrimeTable
+sieve n = VU.create $ do
+  vec <- VUM.replicate (n + 1) True
+  VUM.write vec 0 False
+  VUM.write vec 1 False
+  forM_ [2 .. n] $ \i -> do
+    b <- VUM.read vec i
+    when b $ do
+      forM_ [2 * i, 3 * i .. n] $ \j -> do
+        VUM.write vec j False
+  return vec
+
+main = do
+  let t = sieve 100
+  print $ primeTableToList t
+
+-- print $ take 20 primes
+-- print $ 7 `elem` primes
+
+-- print $ 102 `elem` primes -- 素数でない数を探索すると止まらない