solve: day 10

- solve: day 10, part 1
  - solve: day 10, part 2

2020/10/day10.hs

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+module Day10 where
+
+parse :: String -> [Int]
+parse = map read . lines

2020/10/part1.hs

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+#!/usr/bin/env runghc
+
+import Day10
+import Data.List
+
+main = interact (show . solve . parse)
+
+solve xs = ones * threes
+  where
+    sorted = 0:(sort xs)
+    differences = zipWith (flip (-)) sorted (tail sorted)
+    ones = count 1 differences
+    threes = count 3 differences + 1
+
+count x = length . filter (==x)

2020/10/part2.hs

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+#!/usr/bin/env runghc
+
+import Day10
+import Data.Array
+import Data.IntSet (IntSet)
+import qualified Data.IntSet as IntSet
+
+main = interact (show . solve . parse)
+
+countPaths inp = pathsFrom start
+  where
+    start = IntSet.findMax inpSet + 3
+
+    pathsFrom 0 = 1
+    pathsFrom x
+      | x < 0 = 0
+      | IntSet.member x allowed =
+          sum $ map ((!) memo) $ filter (>= 0) $ map ((-) x) [1, 2, 3]
+      | otherwise = 0
+
+    inpSet = IntSet.fromList inp
+    allowed = IntSet.insert start inpSet
+
+    memo = listArray bnds
+      [ pathsFrom i | i <- range bnds ]
+    bnds = (0, start)
+
+solve = countPaths