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day09.ex
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day09.ex
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defmodule Y2023.Day09 do
use Advent.Day, no: 09
def part1(input) do
input
|> Enum.map(&next_value_in_sequence/1)
|> Enum.sum()
end
def part2(input) do
input
|> Enum.map(&previous_value_in_sequence/1)
|> Enum.sum()
end
@doc """
iex> Day09.next_value_in_sequence([0, 3, 6, 9, 12, 15])
18
iex> Day09.next_value_in_sequence([1, 3, 6, 10, 15, 21])
28
iex> Day09.next_value_in_sequence([10, 13, 16, 21, 30, 45])
68
"""
def next_value_in_sequence(list, acc \\ 0, acc_fn \\ &(&1 + &2)) do
acc = acc_fn.(acc, List.last(list))
if Enum.all?(list, &(&1 == 0)) do
acc
else
new_list = differences_in_sequence(list)
next_value_in_sequence(new_list, acc)
end
end
@doc """
iex> Day09.previous_value_in_sequence([10, 13, 16, 21, 30, 45])
5
"""
def previous_value_in_sequence(list) do
list
|> Enum.reverse()
|> next_value_in_sequence(0, &(&2 - &1))
end
defp differences_in_sequence(list, new_list \\ [])
defp differences_in_sequence([_item], list), do: Enum.reverse(list)
defp differences_in_sequence([a, b | rest], list) do
differences_in_sequence([b | rest], [b - a | list])
end
def parse_input(input) do
for row <- String.split(input, "\n", trim: true) do
row
|> String.split(" ")
|> Enum.map(&String.to_integer/1)
end
end
def part1_verify, do: input() |> parse_input() |> part1()
def part2_verify, do: input() |> parse_input() |> part2()
end