From 99dcde4240c2f823af21d05985152fa58e84f9ed Mon Sep 17 00:00:00 2001
From: Julien Dessaux
Date: Fri, 24 Mar 2023 01:17:50 +0100
Subject: 2020-13 in haskell

---
 2020/13-Shuttle_Search/second.hs | 77 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 77 insertions(+)
 create mode 100644 2020/13-Shuttle_Search/second.hs

(limited to '2020/13-Shuttle_Search/second.hs')

diff --git a/2020/13-Shuttle_Search/second.hs b/2020/13-Shuttle_Search/second.hs
new file mode 100644
index 0000000..242824d
--- /dev/null
+++ b/2020/13-Shuttle_Search/second.hs
@@ -0,0 +1,77 @@
+-- requires cabal install --lib megaparsec parser-combinators
+module Main (main) where
+import Control.Monad (void, when, zipWithM)
+import Data.Either (fromRight)
+import Data.List (foldl')
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+import System.Exit (die)
+
+exampleExpectedOutput = 1068781
+
+type Input = [Maybe Int]
+
+type Parser = Parsec Void String
+
+parseShuttle :: Parser (Maybe Int)
+parseShuttle = do
+  num <- (Just . read <$> some digitChar) <|> (char 'x' *> return Nothing)
+  void . optional $ char ','
+  return $ num
+
+parseOps :: Parser Input
+parseOps = do
+  void $ some digitChar
+  void $ char '\n'
+  shuttles <- some parseShuttle
+  void $ char '\n'
+  void $ eof
+  return $ shuttles
+
+parseInput :: String -> IO Input
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseOps filename input of
+    Left bundle -> die $ errorBundlePretty bundle
+    Right ops -> return ops
+
+-- From rosetta code https://rosettacode.org/wiki/Chinese_remainder_theorem#Haskell
+egcd :: Int -> Int -> (Int, Int)
+egcd _ 0 = (1, 0)
+egcd a b = (t, s - q * t)
+  where
+    (s, t) = egcd b r
+    (q, r) = a `quotRem` b
+
+modInv :: Int -> Int -> Either String Int
+modInv a b =
+  case egcd a b of
+    (x, y)
+      | a * x + b * y == 1 -> Right x
+      | otherwise ->
+        Left $ "No modular inverse for " ++ show a ++ " and " ++ show b
+
+chineseRemainder :: [Int] -> [Int] -> Either String Int
+chineseRemainder residues modulii =
+  zipWithM modInv crtModulii modulii >>=
+  (Right . (`mod` modPI) . sum . zipWith (*) crtModulii . zipWith (*) residues)
+  where
+    modPI = product modulii
+    crtModulii = (modPI `div`) <$> modulii
+-- end of snippet from rosetta code
+
+compute :: Input -> Int
+compute input = fromRight 0 . uncurry chineseRemainder $ unzip $ fst $ foldl' accumul ([], 0) input
+  where
+    accumul :: ([(Int, Int)], Int) -> Maybe Int -> ([(Int, Int)], Int)
+    accumul (acc, i) Nothing = (acc, i+1)
+    accumul (acc, i) (Just v) = (acc ++ [(-i, v)], i+1)
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input
-- 
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