Ugly, but it worked. I didn’t consider @mexicat’s throw/catch idea – that makes it a lot cleaner.
defmodule Day01 do
def readinput() do
File.stream!("1.input.txt")
|> Enum.map(fn numstr -> numstr |> String.trim() |> String.to_integer() end)
end
def part1(numbers \\ readinput()) do
find2020_2(numbers, tl(numbers))
end
def part2(numbers \\ readinput()) do
find2020_3(numbers, tl(numbers), tl(tl(numbers)))
end
defp find2020_2(numbers, []) when tl(numbers) == [], do: :barf
defp find2020_2(numbers, []), do: find2020_2(tl(numbers), tl(tl(numbers)))
defp find2020_2([n1 | _nums1] = numbers, [n2 | nums2]) do
if n1 + n2 == 2020, do: n1 * n2, else: find2020_2(numbers, nums2)
end
defp find2020_3(numbers, [], []) when length(numbers) < 3, do: :barf
defp find2020_3(numbers, [], []),
do: find2020_3(tl(numbers), tl(tl(numbers)), tl(tl(tl(numbers))))
defp find2020_3(numbers1, numbers2, []) when tl(numbers2) == [], do: find2020_3(numbers1, [], [])
defp find2020_3(numbers1, numbers2, []), do: find2020_3(numbers1, tl(numbers2), tl(tl(numbers2)))
defp find2020_3([n1 | _nums1] = numbers1, [n2 | _num2] = numbers2, [n3 | nums3]) do
if n1 + n2 + n3 == 2020, do: n1 * n2 * n3, else: find2020_3(numbers1, numbers2, nums3)
end
end
I later tried these libraries:
{:combination, "~> 0.0.3"},
{:math_combinations, "~> 0.2.0"},
{:comb, git: "https://github.com/tallakt/comb.git", tag: "master"}
They were all very slow.
