Started with brute force. Added memoization when part 2 didn’t finish.
defmodule Day03 do
def part1(input) do
solve(input, 2)
end
def part2(input) do
solve(input, 12)
end
defp solve(input, num_batteries) do
parse(input)
|> Enum.map(fn bank ->
power = Integer.pow(10, num_batteries-1)
{result, _} = find_max(bank, power, %{})
result
end)
|> Enum.sum
end
defp find_max(_, 0, memo), do: {0, memo}
defp find_max([], _mul, memo), do: {-10_000_000_000_000, memo}
defp find_max([j | rest], mul, memo) do
{amount1, memo} = memo_find_max(rest, div(mul, 10), memo)
amount1 = j * mul + amount1
{amount2, memo} = memo_find_max(rest, mul, memo)
{max(amount1, amount2), memo}
end
defp memo_find_max(rest, mul, memo) do
key = {mul,rest}
case memo do
%{^key => result} ->
{result, memo}
%{} ->
{result, memo} = find_max(rest, mul, memo)
memo = Map.put(memo, key, result)
{result, memo}
end
end
defp parse(input) do
input
|> Enum.map(fn line ->
String.to_charlist(line)
|> Enum.map(&(&1 - ?0))
end)
end
end
