From 6125f7357d2e6f7637580a1037da90b8535f9eeb Mon Sep 17 00:00:00 2001
From: Julien Dessaux
Date: Thu, 16 Mar 2023 21:43:55 +0100
Subject: 2020-08 in haskell

---
 2020/08-Handheld_Halting/first.hs | 57 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)
 create mode 100644 2020/08-Handheld_Halting/first.hs

(limited to '2020/08-Handheld_Halting/first.hs')

diff --git a/2020/08-Handheld_Halting/first.hs b/2020/08-Handheld_Halting/first.hs
new file mode 100644
index 0000000..3d01106
--- /dev/null
+++ b/2020/08-Handheld_Halting/first.hs
@@ -0,0 +1,57 @@
+-- requires cabal install --lib megaparsec parser-combinators
+module Main (main) where
+import Control.Monad (void, when)
+import qualified Data.Set as S
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+import System.Exit (die)
+
+exampleExpectedOutput = 5
+
+data Op = Nop Int | Acc Int | Jmp Int
+
+type Parser = Parsec Void String
+
+parseOp :: Parser Op
+parseOp = do
+  op <- string "nop " <|> string "acc " <|> string "jmp "
+  sign <- (char '+' *> return 1) <|> (char '-' *> return (-1))
+  num <- some digitChar
+  void $ char '\n'
+  let n = sign * (read num)
+  return $ case op of
+    "nop " -> Nop n
+    "acc " -> Acc n
+    "jmp " -> Jmp n
+
+parseOps :: Parser [Op]
+parseOps = some parseOp <* eof
+
+parseInput :: String -> IO [Op]
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseOps filename input of
+    Left bundle -> die $ errorBundlePretty bundle
+    Right ops -> return ops
+
+compute :: [Op]-> Int
+compute ops = run 0 0 S.empty
+  where
+    run :: Int -> Int -> S.Set Int -> Int
+    run pointer acc visited
+      | S.member pointer visited = acc
+      | otherwise = case ops !! pointer of
+          Nop _ -> run (pointer + 1) acc visited'
+          Acc i -> run (pointer + 1) (acc + i) visited'
+          Jmp i -> run (pointer + i) acc visited'
+      where
+        visited' = S.insert pointer visited
+
+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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