const grid = [ collect(line) for line in readlines("input/6") ]

const size = length(grid)
const startY = findfirst(x -> '^' in values(x), grid)
const startX = findfirst(x -> x === '^', grid[startY])

const Coords = Tuple{Int,Int}

const index(pos::Coords)::Char = index(pos[1], pos[2])
const index(x::Int, y::Int)::Char = x > 0 && y > 0 && x <= size && y <= size ? grid[y][x] : '\0'

mutable struct Guard
	Guard() = new(( startX, startY ), ( 0, -1 ))
	pos::Coords
	vel::Coords
end
const rotate!(g::Guard) = g.vel = ( -g.vel[2], g.vel[1] )

begin
	local visited::Set{Coords} = Set()
	local guard = Guard()
	while index(guard.pos) !== '\0'
		push!(visited, guard.pos)
		while index(guard.pos .+ guard.vel) === '#'
			rotate!(guard)
		end
		guard.pos = guard.pos .+ guard.vel
	end

	println("Part one ", length(visited))
end

begin
	doesLoop = Threads.Atomic{Int}(0)
	Threads.@threads for y in 1:size
		for x in 1:size
			if index(x, y) !== '.' continue end
			local function tryPatrol()::Bool
				local visited::Set{Pair{Coords,Coords}} = Set()
				local guard = Guard()
				while true
					push!(visited, guard.pos => guard.vel)
					while index(guard.pos .+ guard.vel) === '#' || (guard.pos .+ guard.vel) === (x, y)
						rotate!(guard)
					end
					guard.pos = guard.pos .+ guard.vel
					if index(guard.pos) === '\0'           return false end
					if (guard.pos => guard.vel) in visited return true end
				end
			end
			if tryPatrol()
				Threads.atomic_add!(doesLoop, 1)
			end
		end
	end

	println("Part two ", doesLoop[])
end