add flood fill utility to api.

add api function for walking through the nodes surrounding a given
position.

mods/bg_api/util_node.lua

@@ -48,3 +48,37 @@ function blockgame.random_walk (data)
 	end
 	return pos
 end
+
+
+
+function blockgame.flood_fill (start, step, max_range)
+	max_range = max_range or 4
+
+	local queue = { { pos = start, range = 0 } }
+	local visited = {}
+	local matches = {}
+	local rejected = {}
+
+	while #queue > 0 do
+		local current = table.remove(queue, 1)
+		local pos = current.pos
+		local range = current.range
+
+		local hash = minetest.hash_node_position(pos)
+
+		if not visited[hash] and range <= max_range and step(pos, range) then
+			table.insert(matches, pos)
+
+			local neighbors = blockgame.vector.get_neighbors(pos)
+			for _, p in pairs(neighbors) do
+				table.insert(queue, { pos = p, range = range + 1 })
+			end
+		else
+			table.insert(rejected, pos)
+		end
+
+		visited[hash] = true
+	end
+
+	return matches, rejected
+end