day14.jl

const Platform = Matrix{Char}
const Vec2 = Tuple{Int64, Int64}

function read_platform(filename::AbstractString)
    open(filename) do file
        lines = collect.(collect(eachline(file)))
        permutedims(hcat(lines...))
    end
end

function Base.getindex(m::Platform, p::Vec2)
    (x,y) = p
    m[y, x]
end

function Base.setindex!(m::Platform, value::Char, p::Vec2)
    (x,y) = p
    m[y, x] = value
end

function in_bounds(pl::Platform, p::Vec2)
    (x,y) = p
    checkbounds(Bool, pl, y, x)
end

function tilt!(platform::Platform, v::Vec2)
    (height, width) = size(platform)
    (dx, dy) = v
    xs = (dx < 0) ? (2:width) : (dx > 0) ? (width - 1:-1:1) : (1:width)
    ys = (dy < 0) ? (2:height) : (dy > 0) ? (height - 1:-1:1) : (1:height)

    for y in ys
        for x in xs
            p = (x, y)
            if platform[p] == 'O'
                pp = p .+ v

                while in_bounds(platform, pp) && platform[pp] == '.'
                    pp = pp .+ v
                end

                platform[p] = '.'
                platform[pp .- v] = 'O'
            end
        end
    end
end

function cycle(platform::Platform)
    copy = deepcopy(platform)
    tilt!.((copy,), [(0, -1), (-1, 0), (0, 1), (1, 0)])
    copy
end

function load(platform::Platform)
    sum = 0
    (height, _) = size(platform)
    for (index, row) in enumerate(eachrow(platform))
        c = count(x -> x == 'O', row)
        multiplier = height - (index - 1)
        sum += c * multiplier
    end
    sum
end

function part1(platform::Platform)
    tilt!(platform, [0, -1])
    load(platform)
end

function part2(platform::Platform)
    seen_indices = Dict()
    seen = []
    current = platform
    i = 0
    while true
        prefix = get(seen_indices, current, -1)

        if prefix != -1
            period = i - prefix
            index = ((1_000_000_000 - prefix) % period) + prefix
            return load(seen[index + 1])
        else
            push!(seen, current)
            seen_indices[current] = i
        end

        current = cycle(current)
        i += 1
    end
end

test_data = read_platform("Day14_test.txt")
real_data = read_platform("Day14.txt")

@assert part1(test_data) == 136
@assert part2(test_data) == 64

println("part1 ", part1(real_data))
println("part2 ", part2(real_data))