add `blockgame.score_nearby_nodes` utility to API.

expose public function that walks through nodes near a given position,
counting up a score based on their groups.

mods/bg_api/util_node.lua

@@ -82,3 +82,38 @@ function blockgame.flood_fill (start, step, max_range)
 
 	return matches, rejected
 end
+
+
+
+function blockgame.score_nearby_nodes (pos, max_distance, group_scores, score_fn)
+	local score = 0
+	local source_pos = pos
+
+	blockgame.flood_fill(pos, function (pos, distance)
+		if pos == source_pos then return true end
+
+		local name = minetest.get_node(pos).name
+
+		local gain = 0
+		-- TODO: add support for individual nodes and not just groups.
+		for group, value in pairs(group_scores) do
+			local group = minetest.get_item_group(name, group)
+			if group > 0 then
+				gain = math.max(gain, value * group)
+			end
+		end
+
+		gain = score_fn(gain, distance, name, pos)
+
+		-- TODO: add support for optionally counting negative scores.
+		if gain > 0 then
+			score = score + gain
+			return true
+		end
+
+		-- TODO: add support for optionally going through non-scored nodes.
+		return false
+	end, max_distance)
+
+	return score
+end