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-- requires cabal install --lib megaparsec parser-combinators heap vector
module Main (main) where
import Control.Monad (void, when)
import Data.Either
import qualified Data.Matrix as MTX
import Data.Ratio
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
exampleExpectedOutput = 480
type Pair = (Rational, Rational) -- X Y
type Entry = (Pair, Pair, Pair) -- ButtonA ButtonB Prize
type Input = [Entry]
type Parser = Parsec Void String
parseNumber :: Parser Rational
parseNumber = fromInteger . read <$> some digitChar
parseButton :: Parser Pair
parseButton = (,) <$> (string "Button " *> letterChar *> string ": X+" *> parseNumber)
<*> (string ", Y+" *> parseNumber <* eol)
parsePrize :: Parser Pair
parsePrize = (,) <$> (string "Prize: X=" *> parseNumber)
<*> (string ", Y=" *> parseNumber <* eol)
parseEntry :: Parser Entry
parseEntry = (,,) <$> parseButton
<*> parseButton
<*> parsePrize
parseInput' :: Parser Input
parseInput' = some (parseEntry <* optional eol) <* eof
parseInput :: String -> IO Input
parseInput filename = do
input <- readFile filename
case runParser parseInput' filename input of
Left bundle -> error $ errorBundlePretty bundle
Right input' -> return input'
-- ax + bx = c
-- ay + by = d
compute :: Input -> Rational
compute = sum . map compute'
where
compute' ((ax, ay), (bx, by), (px, py)) = let (Right sol) = MTX.rref $ MTX.fromList 2 3 [ ax, bx, px
, ay, by, py ]
x = sol MTX.! (1, 3)
y = sol MTX.! (2, 3)
in if (denominator x == 1 && denominator y == 1) then 3 * x + y else 0
main :: IO ()
main = do
example <- parseInput "example"
let exampleOutput = compute example
when (exampleOutput /= exampleExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
input <- parseInput "input"
print $ compute input
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