Day 6

6.jl

@@ -0,0 +1,60 @@
+
+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
+