115 lines
2.4 KiB
Elixir
115 lines
2.4 KiB
Elixir
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defmodule Day9 do
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@example """
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35
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20
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15
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25
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47
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40
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62
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55
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65
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95
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102
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117
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150
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182
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127
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219
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299
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277
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309
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576
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"""
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@example_premable_length 5
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def part1(example \\ false) do
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input = if example, do: @example, else: File.read!("lib/day9/input.txt")
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length = if example, do: @example_premable_length, else: 25
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input
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|> parse_input()
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|> find_first_non_sum(length)
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end
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def part2(example \\ false) do
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input = if example, do: @example, else: File.read!("lib/day9/input.txt")
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length = if example, do: @example_premable_length, else: 25
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input = parse_input(input)
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non_sum = find_first_non_sum(input, length)
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{start, length} = find_range_that_sums_to(input, non_sum)
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{_, after_start} = Enum.split(input, start)
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{range, _} = Enum.split(after_start, length)
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Enum.min(range) + Enum.max(range)
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end
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def parse_input(input) do
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input
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|> String.trim()
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|> String.split("\n")
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|> Enum.map(&String.to_integer/1)
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end
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def find_first_non_sum([_ | rest] = input, length) do
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if is_number_sum_of_pairs_first(input, length) do
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find_first_non_sum(rest, length)
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else
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{_, [non_sum | _]} = Enum.split(input, length)
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non_sum
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end
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end
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def is_number_sum_of_pairs_first(inputs, count) do
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{before, [first | _]} = Enum.split(inputs, count)
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before
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|> all_pairs()
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|> Enum.any?(fn {a, b} ->
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# IO.puts("check if #{a} + #{b} == #{first}")
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a + b == first
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end)
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end
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def all_pairs(inputs) do
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inputs
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|> Enum.drop(-1)
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|> Enum.with_index()
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|> Enum.flat_map(fn {a, a_index} ->
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{_, rest} = Enum.split(inputs, a_index + 1)
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Enum.map(rest, fn b ->
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{a, b}
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end)
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end)
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end
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def find_range_that_sums_to([_ | rest] = numbers, target, start \\ 0) do
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case do_find_leading_range_that_sums_to(numbers, target) do
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false ->
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find_range_that_sums_to(rest, target, start + 1)
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count when is_integer(count) ->
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{start, count}
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end
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end
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def do_find_leading_range_that_sums_to(numbers, remaining, count \\ 0)
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def do_find_leading_range_that_sums_to(_, 0, count) do
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count
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end
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def do_find_leading_range_that_sums_to([head | rest], remaining, count) do
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# IO.puts("trying start: #{start} count: #{count}, target: #{target}")
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if head <= remaining do
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do_find_leading_range_that_sums_to(rest, remaining - head, count + 1)
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else
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false
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end
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end
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end
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