AOC.jl/2020/src/day_13.jl

# https://adventofcode.com/2020/day/13
using AdventOfCode
using Mods

t_str, buses_str = readlines("2020/data/day_13.txt")
time = parse(Int, t_str)
buses = map(x->x=="x" ? nothing : parse(Int, x), split(buses_str, ','))

function part_1()
    wait, bus = min(map(x->(x*ceil(time / x), x), filter(!isnothing, buses))...)
    (wait - time) * bus
end
@info part_1()

function part_2()
    mods = map(k->Mod{buses[k]}(-(k-1)), filter(k->!isnothing(buses[k]), keys(buses)))
    CRT(mods...).val
end
@info part_2()