Day 8

8.jl

@@ -0,0 +1,42 @@
+const grid = [ collect(line) for line in readlines("input/8") ]
+const size = length(grid)
+
+const Coords = Tuple{Int,Int}
+
+ants::Dict{Char,Vector{Coords}} = Dict()
+for (y,row) in enumerate(grid)
+	for (x,c) in enumerate(row)
+		if c != '.'
+			get!(ants, c, []) .= push!(ants[c], (x, y))
+		end
+	end
+end
+
+isOnMap(n) = 1 <= n[1] <= size && 1 <= n[2] <= size
+partOneGrid = deepcopy(grid)
+
+partOneSet, partTwoSet = Set(), Set()
+
+for (freq,coords) in ants
+	for (a,b) in Iterators.filter((x) -> x[1] != x[2], ( sort([a, b]) for a in coords, b in coords ))
+		local diff = a .- b
+		for n in (a .+ diff, b .- diff)
+			if isOnMap(n)
+				push!(partOneSet, n)
+			end
+		end
+		for (n,fn) in ((a,.-), (b,.+))
+			while isOnMap(n)
+				n = fn(n, diff)
+				if isOnMap(n)
+					push!(partTwoSet, n)
+				end
+			end
+		end
+	end
+end
+
+const partOne, partTwo = length(partOneSet), length(partTwoSet)
+
+@show partOne, partTwo
+