solve: day 5

- solve: day 5, part 1
 - solve: day 5, part 2

2020/05/README.md

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+# Day 5
+
+## Part 1
+
+It's just binary where `F` and `L` are `0` and `B` and `R` are `1`.
+
+## Part 2
+
+At first I just looked at the difference between lists to get the answer. Since updated to find the first difference in the two lists (of all possible seats and the actual seats) after skipping the gap at the start.

2020/05/day5.hs

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+module Day5 (BoardingPass(BoardingPass), seatId) where
+
+data BoardingPass = BoardingPass { row :: Int, column :: Int }
+  deriving (Show)
+
+binaryStringToInt b = binaryStringToInt' (reverse b) 0 0
+  where
+    binaryStringToInt' [] _ acc = acc
+    binaryStringToInt' ('1':bs) m acc = binaryStringToInt' bs (m+1) (acc + 2^m)
+    binaryStringToInt' ('0':bs) m acc = binaryStringToInt' bs (m+1) acc
+
+charToBinaryDigit 'F' = '0'
+charToBinaryDigit 'L' = '0'
+charToBinaryDigit 'B' = '1'
+charToBinaryDigit 'R' = '1'
+
+stringToInt = binaryStringToInt . map charToBinaryDigit
+
+readBoardingPass str =
+    let (str', rest) = splitAt 10 str
+        (rowS, colS) = splitAt 7 str'
+    in
+        (BoardingPass (stringToInt rowS) (stringToInt colS), rest)
+
+instance Read BoardingPass where
+  readsPrec _ str = [readBoardingPass str]
+
+seatId (BoardingPass row column) = row * 8 + column

2020/05/part1.hs

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+#!/usr/bin/env runghc
+
+import Day5
+
+main = interact solve
+
+solve = show . maxSeatId . map (read :: String -> BoardingPass) . lines
+
+maxSeatId = maximum . map seatId

2020/05/part2.hs

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+#!/usr/bin/env runghc
+
+import Day5
+import Data.List
+
+main = interact solve
+
+solve = show . mySeat . map (seatId . read) . lines
+
+mySeat input = mySeat'
+  where
+    boardingPasses = sort input
+    start = head boardingPasses
+    mySeat' = fst . head . dropWhile (uncurry (==)) $ zip [start..] boardingPasses