dnf: day 9

2020/09/README.md

@@ -0,0 +1 @@
+# Day 9

2020/09/day9.hs

@@ -0,0 +1,4 @@
+module Day9 where
+
+parse :: String -> [Int]
+parse = map read . lines

2020/09/part1.hs

@@ -0,0 +1,22 @@
+#!/usr/bin/env runghc
+
+import qualified Data.Set as S
+import Data.List (subsequences)
+import Data.Either (fromLeft, lefts)
+import Day9
+
+main = interact (show . solve . parse)
+
+solve = head . lefts . verify 25
+
+verify :: Int -> [Int] -> [Either Int Int]
+verify len xs
+  | length xs < len + 1 = []
+  | otherwise = verified:(verify len $ tail xs)
+    where
+      (preamble, [x]) = splitAt len $ take (len + 1) xs
+      pairs = combinations 2 preamble
+      sums = S.fromList $ map sum pairs
+      verified = if S.member x sums then (Right x) else (Left x)
+
+combinations n xs = filter ((n==) . length) $ subsequences xs

2020/09/small.txt

@@ -0,0 +1,20 @@
+35
+20
+15
+25
+47
+40
+62
+55
+65
+95
+102
+117
+150
+182
+127
+219
+299
+277
+309
+576

2020/README.md

@@ -15,7 +15,7 @@ asdf install
 |  S  |  M  |  T  |  W  |  T  |  F  |  S  |
 | :-: | :-: | :-: | :-: | :-: | :-: | :-: |
 |     |     | [1] | [2] | [3] | [4] | [5] |
-| [6] | [7] | [8] |  9  | 10  | 11  | 12  |
+| [6] | [7] | [8] |  [9]  | 10  | 11  | 12  |
 | 13  | 14  | 15  | 16  | 17  | 18  | 19  |
 | 21  | 22  | 23  | 24  | 25  |     |     |
 
@@ -29,3 +29,4 @@ asdf install
 [6]: ./06
 [7]: ./07
 [8]: ./08
+[9]: ./09