Add untested efficient flood fill algorithm

http://www.adammil.net/blog/v126_A_More_Efficient_Flood_Fill.html

adammil_flood_fill.lua

@@ -0,0 +1,116 @@
+-- http://www.adammil.net/blog/v126_A_More_Efficient_Flood_Fill.html
+
+local can_go
+local marked_places
+local function calc_2d_index(x, y)
+	return (y + 32768) * 65536 + x + 32768
+end
+local function mark(x, y)
+	marked_places[calc_2d_index(x, y)] = true
+end
+
+local _fill
+local function fill(x, y)
+	if can_go(x, y) then
+		_fill(x, y)
+	end
+end
+
+local corefill
+function _fill(x, y)
+	while true do
+		local ox = x
+		local oy = y
+		while can_go(x, y-1) do
+			y = y-1
+		end
+		while can_go(x-1, y) do
+			x = x-1
+		end
+		if x == ox
+		and y == oy then
+			break
+		end
+	end
+	corefill(x, y)
+end
+
+function corefill(x, y)
+	local lastcnt = 0
+	repeat
+		local cnt = 0
+		local sx = x
+		if lastcnt ~= 0
+		and not can_go(y, x) then
+			-- go right to find the x start
+			repeat
+				lastcnt = lastcnt-1
+				if lastcnt == 0 then
+					return
+				end
+				x = x+1
+			until can_go(x, y)
+			sx = x
+		else
+			-- go left if possible, and mark and _fill above
+			while can_go(x-1, y) do
+				x = x-1
+				mark(x, y)
+				if can_go(x, y-1) then
+					_fill(x, y-1)
+				end
+				cnt = cnt+1
+				lastcnt = lastcnt+1
+			end
+		end
+
+		-- go right if possible, and mark
+		while can_go(sx, y) do
+			mark(sx, y)
+			cnt = cnt+1
+			sx = sx+1
+		end
+
+		if cnt < lastcnt then
+			local e = x + lastcnt
+			sx = sx+1
+			while sx < e do
+				if can_go(sx, y) then
+					corefill(sx, y)
+				end
+				sx = sx+1
+			end
+		elseif cnt > lastcnt then
+			local ux = x + lastcnt + 1
+			while ux < sx do
+				if can_go(ux, y-1) then
+					_fill(ux, y-1)
+				end
+				ux = ux+1
+			end
+		end
+		lastcnt = cnt
+		y = y+1
+	until lastcnt == 0
+end
+
+local function apply_fill(go_test, x0, y0, allow_revisit)
+	if allow_revisit then
+		can_go = go_test
+	else
+		local visited = {}
+		can_go = function(x, y)
+			local vi = calc_2d_index(x, y)
+			if visited[vi] then
+				return false
+			end
+			visited[vi] = true
+			return go_test(x, y)
+		end
+	end
+	marked_places = {}
+	fill(x0, y0)
+	return marked_places
+end
+
+return apply_fill