day_1.exs

defmodule Day1 do
  def find_pair(list) do
    Enum.reduce list, [], fn x, acc ->
      case inner_pair(x, list) do
        y_list when y_list != [] -> [x, hd(y_list)]
        _ -> acc
      end
    end
  end

  def find_three_numbers(list) do
    Enum.reduce list, [], fn x, acc ->
      Enum.reduce list, acc, fn y, acc ->
        case inner_pair(x, y, list) do
          z_list when z_list != [] -> [x, y, hd(z_list)]
          _ -> acc
        end
      end
    end
  end

  def multiply_pair(list) do
    list |> find_pair |> Enum.reduce(1, fn x, acc -> x * acc end)
  end

  def multiply_three_numbers(list) do
    list |> find_three_numbers |> Enum.reduce(1, fn x, acc -> x * acc end)
  end

  defp inner_pair(x, list) do
    Enum.filter list, fn y ->
      x + y == 2020
    end
  end

  defp inner_pair(x, y, list), do: inner_pair(x + y, list)
end

case System.argv() do
  ["--test"] ->
    ExUnit.start()

    defmodule Day1Test do
      use ExUnit.Case

      import Day1

      test "Find a pair which sum of them will be 2020" do
        numbers = [1721, 979, 366, 299, 675, 1456]
        assert Enum.sum(find_pair(numbers)) == 2020
      end

      test "Find three number which sum of them will be 2020" do
        numbers = [1721, 979, 366, 299, 675, 1456]
        assert Enum.sum(find_three_numbers(numbers)) == 2020
      end

      test "Multiply a pair which sum of them will be 2020" do
        numbers = [1721, 979, 366, 299, 675, 1456]
        assert multiply_pair(numbers) == 514579
      end

      test "Multiply three numbers which sum of them will be 2020" do
        numbers = [1721, 979, 366, 299, 675, 1456]
        assert multiply_three_numbers(numbers) == 241861950
      end
    end

  _ ->

    numbers = [1597,1857,1703,1956,1809,1683,1629,230,1699,1875,1564,1700,1911,1776,1955,1585,1858,1725,1813,1568,1962,1535,487,1621,1620,1573,1918,1794,2003,1957,1840,1936,285,1630,1753,1649,1951,1968,1916,1694,1593,1980,1806,1779,1637,1674,1842,1659,1553,1846,1677,1944,1811,1645,1784,1791,1988,1864,1596,1773,1132,1715,1938,1901,1617,1892,1708,1788,1765,1684,1595,1971,1798,1543,507,1772,1757,1950,1844,1898,1671,1602,1599,1524,2005,1855,1624,1884,1990,1604,1873,1736,1747,1900,1534,1713,1690,1525,1838,587,74,1979,1635,1896,1580,1727,1994,1940,1819,1758,1852,1639,1754,1559,1919,1879,1874,1921,1575,1693,1710,1949,1719,1933,1673,1909,1989,1897,909,1889,1641,1658,1530,1541,1952,1627,1839,1803,833,1527,1756,2009,1605,1548,1660,1966,1770,1552,1939,1726,382,1665,1960,1733,1983,1544,1974,1985,1625,609,1931,1749,1975,1562,1563,1922,2008,2010,1704,1545,1636,1762,1841,1717,622,2007,1670,1771,1910,1978,1615,1805,1999,1643,1748,1531,1539,1787,1722,1111,1774,1540,1607,1902,1991,1763,1691,1812,1783,1579]

    IO.inspect(Day1.multiply_pair(numbers), label: "part 1")
    IO.inspect(Day1.multiply_three_numbers(numbers), label: "part 2")
end