2020 day10/haskell: add solution

2020/README.md

@@ -15,6 +15,7 @@ https://adventofcode.com/2020/
 | 7 |  `**`  |         |      |
 | 8 |        |  `**`   |      |
 | 9 |        |  `**`   |      |
+|10 |        |  `**`   |      |
 |11 |  `**`  |         |      |
 
 `test.sh` can be used to run all solutions and automatically compares them to (my) puzzle inputs and the expected outputs.

2020/data/day10.expected

@@ -0,0 +1,2 @@
+1885
+2024782584832

2020/data/day10.input

@@ -0,0 +1,93 @@
+80
+87
+10
+122
+57
+142
+134
+59
+113
+139
+101
+41
+138
+112
+46
+96
+43
+125
+36
+54
+133
+17
+42
+98
+7
+114
+78
+67
+77
+28
+149
+58
+20
+105
+31
+19
+18
+27
+40
+71
+117
+66
+21
+72
+146
+90
+97
+94
+123
+1
+119
+30
+84
+61
+91
+118
+2
+29
+104
+73
+13
+76
+24
+148
+68
+111
+131
+83
+49
+8
+132
+9
+64
+79
+124
+95
+88
+135
+3
+51
+39
+6
+60
+108
+14
+35
+147
+89
+34
+65
+50
+145
+128

2020/day10/day10.hs

@@ -0,0 +1,17 @@
+import AoC
+
+import Data.List
+
+pairs :: [a] -> [(a, a)]
+pairs xs = zipWith (,) xs (tail xs)
+
+part1 :: [Int] -> Int
+part1 = product . map length . group . sort . map (uncurry $ flip (-)) . pairs
+
+numArrangements :: [Int] -> Int
+numArrangements = snd . head . go
+    where go xs = foldr (\x acc -> (x, sum . map snd . takeWhile (\(y, _) -> x-y <= 3) $ acc) : acc) [(last xs, 1)] $ init xs
+
+main = runAoC (addPhonePort . sortReverse . (0:) . map read . lines) part1 numArrangements
+    where addPhonePort (x:xs) = (x+3):x:xs
+          sortReverse = sortBy (flip compare)