AdventOfCode/2020/elixir/day10/day10part2-refactored.exs

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defmodule Day10Part2 do
def run do
File.read!("input")
|> String.split("\n", trim: true)
|> Enum.map(&String.to_integer/1)
|> count_arrangements()
|> IO.puts()
end
# Entry point. Start with one arrangement: the outlet.
def count_arrangements(list), do: count_arrangements(%{0 => 1}, [0 | Enum.sort(list)])
# Termination point. We've checked all the adapters. The number of arrangments
# to the last adapter is the answer.
def count_arrangements(arrangements, [last]), do: arrangements[last]
# Check each adapter in turn by comparing it to the next three adapters.
def count_arrangements(arrangements, [current | rest]) do
count_for_adapter(current, Enum.take(rest, 3), arrangements)
|> count_arrangements(rest)
end
# Compare the next three adapters to the current adapter.
# For each checked adapter, if the joltage difference is 3 or less, we can connect them,
# so add the number of arrangements up to the current adapter to that adapter.
def count_for_adapter(current, next_three, arrangements) do
Enum.reduce(next_three, arrangements, fn adapter, arrangements ->
if adapter - current <= 3 do
current_count = arrangements[current]
Map.update(arrangements, adapter, current_count, &(&1 + current_count))
else
arrangements
end
end)
end
end
Day10Part2.run()