aoc2021/16.jl

using Pkg
Pkg.add("Match")
using Match

include("utils.jl")

function parse_input(filename)
    reverse(collect(collect(non_empty_lines(filename))[1]))
end

mutable struct BinaryReader
    input
    rest
    BinaryReader(input) = new(input, [])
end

function fill_rest(reader)
    char = pop!(reader.input)
    code = codepoint(char)
    number = code < 0x3A ? code - 0x30 : code - (0x41 - 10)
    reader.rest = reverse(parse.(Int, collect(string(number, base=2, pad=4))))
end

function read_bits(reader, count)
    result = []
    while length(result) < count
        if length(reader.rest) == 0
            fill_rest(reader)
        end
        push!(result, pop!(reader.rest))
    end
    result
end

function reader_length(reader)
    length(reader.rest) + length(reader.input) * 4
end

function to_int(bits)
    parse(Int, reduce(*, string.(bits)), base=2)
end

function parse_literal(reader, version)
    result = []
    cont = 1
    while cont == 1
        cont = read_bits(reader, 1)[1]
        result = [result; read_bits(reader, 4)]
    end
    (version, 4, to_int(result))
end

function parse_operator(reader, version, type)
    length_type = read_bits(reader, 1)[1]
    subpackets = []
    if length_type == 0
        subpacket_bits = to_int(read_bits(reader, 15))
        start_remaining = reader_length(reader)
        while reader_length(reader) + subpacket_bits > start_remaining
            push!(subpackets, parse_packet(reader))
        end
    else
        subpacket_count = to_int(read_bits(reader, 11))
        for _ in 1:subpacket_count
            push!(subpackets, parse_packet(reader))
        end
    end
    (version, type, subpackets)
end

function parse_packet(reader)
    version = to_int(read_bits(reader, 3))
    type = to_int(read_bits(reader, 3))
    if type == 4
        parse_literal(reader, version)
    else
        parse_operator(reader, version, type)
    end
end

function sum_versions(packet)
    result = packet[1]
    if packet[2] != 4
        result += reduce(+, sum_versions.(packet[3]))
    end
    result
end

function solution16_1()
    input = parse_input("16.data")
    reader = BinaryReader(input)
    packet = parse_packet(reader)
    sum_versions(packet)
end

function value(packet)
    @match packet[2] begin
        0 => reduce(+, value.(packet[3]))
        1 => reduce(*, value.(packet[3]))
        2 => reduce(min, value.(packet[3]))
        3 => reduce(max, value.(packet[3]))
        4 => packet[3]
        5 => value(packet[3][1]) > value(packet[3][2]) ? 1 : 0
        6 => value(packet[3][1]) < value(packet[3][2]) ? 1 : 0
        7 => value(packet[3][1]) == value(packet[3][2]) ? 1 : 0
    end
end

function solution16_2()
    input = parse_input("16.data")
    reader = BinaryReader(input)
    packet = parse_packet(reader)
    value(packet)
end

solution16_2()