feat(day2): Implement part 1

day2/Main.hs

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+module Main where
+
+import System.Environment (getArgs)
+
+import Text.Parsec (parse, many1, char, sepBy, endBy)
+import Text.Parsec.Char (endOfLine)
+import Text.ParserCombinators.Parsec (GenParser)
+import Text.ParserCombinators.Parsec.Number (decimal)
+
+type Report = [Level]
+type Level = Int
+
+reports :: GenParser Char st [Report]
+reports = endBy report endOfLine
+
+report :: GenParser Char st Report
+report = sepBy level (many1 (char ' '))
+
+level :: GenParser Char st Level
+level = decimal
+
+pairUp :: [a] -> [(a, a)]
+pairUp (a:b:tl) = (a, b) : pairUp (b:tl)
+pairUp _ = []
+
+allIncOrDec :: Report -> Bool
+allIncOrDec (a:b:tl)
+    | a /= b = all (uncurry (if a < b then (<) else (>))) $ pairUp $ b:tl
+    | otherwise = False
+allIncOrDec _ = True
+
+notTooDifferent :: Report -> Bool
+notTooDifferent (a:b:tl) = let dif = abs (a - b)
+                           in (1 <= dif) && (dif <= 3) && notTooDifferent (b:tl)
+notTooDifferent _ = True
+
+part1 :: [Report] -> Int
+part1 = length . filter allIncOrDec . filter notTooDifferent
+
+main :: IO ()
+main = do
+    args <- getArgs
+    (fp:_) <- return args
+    content <- readFile fp
+    case parse reports fp content of
+        Left e -> putStrLn ("Input file is incorrectly formatted : " ++ show e)
+        Right r -> do
+            putStrLn ("Solution (Part 1) : " ++ show (part1 r))
+            -- putStrLn ("Solution (Part 2) : " ++ show (part2 r))

day2/test_input

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+7 6 4 2 1
+1 2 7 8 9
+9 7 6 2 1
+1 3 2 4 5
+8 6 4 4 1
+1 3 6 7 9