map identity by checksum

lb_common.lua

@@ -44,3 +44,23 @@ rawset(_G, "lb_comp", function(a, b)
 	local s = (a.flags & (F_SPBEXP | F_SPBBIG)) - (b.flags & (F_SPBEXP | F_SPBBIG))
 	return s > 0 or not(s < 0 or a.time >= b.time)
 end)
+
+local function djb2(message)
+	local digest = 5381
+	for c in message:gmatch(".") do
+		digest = (($ << 5) + $) + string.byte(c)
+	end
+
+	return digest
+end
+
+-- Produce a checksum by using the maps title, subtitle and zone
+rawset(_G, "lb_map_checksum", function(mapnum)
+	local mh = mapheaderinfo[mapnum]
+	if not mh then
+		return nil
+	end
+
+	local digest = string.format("%04x", djb2(mh.lvlttl..mh.subttl..mh.zonttl))
+	return string.sub(digest, #digest - 3)
+end)