2020 Day 1 Part 1

2020/day1/day1.ex

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+defmodule Day1 do
+  def all_pairs(list) do
+    Enum.reduce(0..(length(list) - 1), [], fn idx, pairs ->
+      item = Enum.at(list, idx)
+      other_items = Enum.slice(list, (idx + 1)..(length(list) - 1))
+      new_pairs = Enum.map(other_items, fn elt -> {item, elt} end)
+      pairs ++ new_pairs
+    end)
+  end
+
+  def part1(input) do
+    lines = String.split(input)
+    ints = Enum.map(lines, fn line -> String.to_integer(line) end)
+    pairs = Day1.all_pairs(ints)
+    [{a, b}] = Enum.filter(pairs, fn {a, b} -> a + b == 2020 end)
+    a * b
+  end
+end
+
+input = File.read!("input.txt")
+IO.inspect(Day1.part1(input))

2020/day1/input.txt

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+1895
+1732
+1660
+1658
+1878
+367
+2010
+1989
+431
+1946
+1614
+2003
+945
+1856
+1934
+1937
+1781
+1947
+1991
+1917
+1604
+1707
+1966
+1959
+1182
+1828
+1880
+1908
+1942
+1687
+1611
+1922
+1913
+1803
+1976
+1718
+1885
+1971
+2000
+1912
+1981
+1776
+1901
+1941
+1935
+1977
+1907
+1893
+1898
+1975
+2001
+1833
+1951
+1939
+1988
+1870
+1985
+1932
+1930
+1938
+1926
+1931
+1982
+76
+1979
+657
+1872
+1933
+1961
+1987
+1998
+1994
+418
+1914
+1929
+1810
+2009
+1712
+830
+1990
+1900
+1876
+1753
+1859
+1965
+1963
+1905
+1921
+1685
+1694
+697
+1899
+1997
+1964
+1927
+1952
+1894
+1960
+1986
+1883
+1616
+1993
+1892
+1943
+2005
+1995
+1915
+1663
+1954
+1902
+1191
+1948
+1875
+1850
+1955
+1962
+1984
+1957
+1969
+1887
+1953
+1786
+1638
+1909
+1881
+603
+1973
+1784
+1869
+1925
+1968
+1737
+1807
+1950
+1992
+1936
+1918
+1891
+1897
+1940
+1919
+1910
+1862
+1958
+1832
+1904
+1791
+1920
+1874
+1729
+1643
+2007
+1871
+1999
+1584
+1890
+1924
+1974
+1701
+1906
+143
+1725
+1945
+1783
+1873
+1903
+167
+1855
+1633
+1956
+1996
+1808
+1884
+1916
+829
+2002
+1852
+1835
+1889
+1983
+1949
+1970
+1774
+1764
+1609
+1882
+1857
+2004
+1911
+1896
+1980
+2006
+1967
+2008
+1972
+1648
+1923
+1978
+1675
+1831