Solve day 1, part 1

AdventOfCode2020.cabal

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+cabal-version:       2.4
+-- Initial package description 'AdventOfCode2020.cabal' generated by 'cabal
+--  init'.  For further documentation, see
+-- http://haskell.org/cabal/users-guide/
+
+name:                AdventOfCode2020
+version:             0.1.0.0
+-- synopsis:
+-- description:
+-- bug-reports:
+-- license:
+license-file:        LICENSE
+author:              Gaël Depreeuw
+maintainer:          gael@depreeuw.dev
+-- copyright:
+-- category:
+extra-source-files:  CHANGELOG.md, README.md
+
+library
+  exposed-modules:     Day1
+  -- other-modules:
+  -- other-extensions:
+  build-depends:       base ^>=4.13.0.0
+                     , containers ^>=0.6.4.1
+  hs-source-dirs:      src
+  default-language:    Haskell2010
+
+executable AdventOfCode2020
+  main-is:             Main.hs
+  -- other-modules:
+  -- other-extensions:
+  build-depends:       base ^>=4.13.0.0, AdventOfCode2020
+  -- hs-source-dirs:
+  default-language:    Haskell2010

CHANGELOG.md

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+# Revision history for AdventOfCode2020
+
+## 0.1.0.0 -- YYYY-mm-dd
+
+* First version. Released on an unsuspecting world.

Main.hs

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+module Main where
+
+import qualified Day1 (fixExpenseReport)
+
+day1 :: IO ()
+day1 = do
+  r <- readFile "./input/day1"
+  putStr "[Day 1-1] fix: "
+  print . Day1.fixExpenseReport . fmap read . lines $ r
+
+
+main :: IO ()
+main = do
+  day1

Setup.hs

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+import Distribution.Simple
+main = defaultMain

hie.yaml

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+cradle:
+  cabal:
+    - path: "src"
+      component: "lib:AdventOfCode2020"
+
+    - path: "./Main.hs"
+      component: "AdventOfCode2020:exe:AdventOfCode2020"

input/day1

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+1834
+1546
+1119
+1870
+1193
+1198
+1542
+1944
+1817
+1249
+1361
+1856
+1258
+1425
+1835
+1520
+1792
+1130
+2004
+1366
+1549
+1347
+1507
+1699
+1491
+1557
+1865
+1948
+1199
+1229
+1598
+1756
+1643
+1306
+1838
+1157
+1745
+1603
+1972
+1123
+1963
+1759
+1118
+1526
+1695
+1661
+1262
+1117
+1844
+1922
+1997
+1630
+1337
+1721
+1147
+1848
+1476
+1975
+1942
+1569
+1126
+1313
+1449
+1206
+1722
+1534
+1706
+1596
+1700
+1811
+906
+1666
+1945
+1271
+1629
+1456
+1316
+1636
+1884
+1556
+1317
+1393
+1953
+1658
+2005
+1252
+1878
+1691
+60
+1872
+386
+1369
+1739
+1460
+1267
+1935
+1992
+1310
+1818
+1320
+1437
+1486
+1205
+1286
+1670
+1577
+1237
+1558
+1937
+1938
+1656
+1220
+1732
+1647
+1857
+1446
+1516
+1450
+1860
+1625
+1377
+1312
+1588
+1895
+1967
+1567
+1582
+1428
+1415
+1731
+1919
+1651
+1597
+1982
+1576
+1172
+1568
+1867
+1660
+1754
+1227
+1121
+1733
+537
+1809
+1322
+1876
+1665
+1124
+1461
+1888
+1368
+1235
+1479
+1529
+1148
+1996
+1939
+1340
+1531
+1438
+1897
+1152
+1321
+1770
+897
+1750
+1111
+1772
+1615
+1798
+1359
+1470
+1610
+1362
+1973
+1892
+1830
+599
+1341
+1681
+1572
+1873
+42
+1246
+1447
+1800
+1524
+1214
+1784
+1664
+1882
+1989
+1797
+1211
+1170
+1854
+1287
+1641
+1760

src/Day1.hs

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+module Day1 (fixExpenseReport) where
+
+import qualified Data.IntSet as DIS
+import Data.Maybe (mapMaybe)
+
+yearToFind :: Int
+yearToFind = 2020
+
+-- Turn the list into an IntSet. For each element in the set, look up the
+-- number in the set to make it 2020. If found, return the multiplication of
+-- the two numbers.
+fixExpenseReport :: [Int] -> Int
+fixExpenseReport expenses =
+    let s = DIS.fromList expenses in
+        head $ mapMaybe (`findOther` s) expenses
+
+findOther :: Int -> DIS.IntSet -> Maybe Int
+findOther n s = let n' = yearToFind - n in
+    if DIS.member n' s then Just (n*n') else Nothing