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day15.ex
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day15.ex
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defmodule Y2023.Day15 do
use Advent.Day, no: 15
@doc """
iex> Day15.part1(~W(rn=1 cm- qp=3 cm=2 qp- pc=4 ot=9 ab=5 pc- pc=6 ot=7))
1320
"""
def part1(input) do
input
|> Enum.map(&hash/1)
|> Enum.sum()
end
@doc """
iex> Day15.part2(~W(rn=1 cm- qp=3 cm=2 qp- pc=4 ot=9 ab=5 pc- pc=6 ot=7))
145
"""
def part2(input) do
input
|> Enum.reduce(%{}, &place_lens/2)
|> to_focussing_power()
end
defp place_lens(lens, map) do
{label, op} = parse_lens(lens)
box_no = hash(label)
case op do
"-" ->
Map.update(map, box_no, [], fn box ->
Enum.reject(box, fn {x, _} -> x == label end)
end)
{"=", focal_length} ->
entry = {label, focal_length}
Map.update(map, box_no, [entry], fn list ->
index = Enum.find_index(list, fn {x, _} -> x == label end)
if index do
List.replace_at(list, index, entry)
else
list ++ [entry]
end
end)
end
end
defp parse_lens(string) do
if String.contains?(string, "-") do
label = String.split(string, "-") |> hd()
{label, "-"}
else
[label, num] = String.split(string, "=", parts: 2)
{label, {"=", String.to_integer(num)}}
end
end
defp to_focussing_power(map) do
Enum.reduce(map, 0, fn {box_no, lenses}, acc ->
Enum.reduce(lenses, {1, acc}, fn {_label, length}, {index, acc2} ->
{index + 1, acc2 + (box_no + 1) * index * length}
end)
|> elem(1)
end)
end
@doc """
iex> Day15.hash("rn=1")
30
iex> Day15.hash("cm-")
253
iex> Day15.hash("qp=3")
97
"""
def hash(string) do
string
|> String.to_charlist()
|> Enum.reduce(0, fn char, acc ->
rem((acc + char) * 17, 256)
end)
end
@doc """
iex> Day15.parse_input("rn=1,cm-,qp=3,cm=2,qp-,pc=4,ot=9,ab=5,pc-,pc=6,ot=7")
~W(rn=1 cm- qp=3 cm=2 qp- pc=4 ot=9 ab=5 pc- pc=6 ot=7)
"""
def parse_input(input) do
input
|> String.split(",", trim: true)
end
def part1_verify, do: input() |> parse_input() |> part1()
def part2_verify, do: input() |> parse_input() |> part2()
end