I think I just devised a solution that's easier on the client

build-tools/build-indexes

@@ -1770,41 +1770,61 @@ function buildModSprites() {
 					continue;
 				}
 				const ext = id.split(".")[1];
+				//These are our supported file extensions, if missing we skip
+				if (!["png","gif","mp3"].includes(ext)) continue;
 				id = toID(id.slice(0, id.length - 4));
-				modSprites[id] ||= {};
-				modSprites[id][modName] ||= [];
-				//TODO: Find a more space-efficient means of indicating which mons share certain resources in a given mod
-				modSprites[id][modName][subF] = null;
+				if (!modSprites[id]) modSprites[id] = {[modName]: [subF]};
+				else if (!modSprites[id][modName]) modSprites[id][modName] = [subF];
+				else modSprites[id][modName].push(subF);
 			}
+			//We're not adding the .JSON extension outright because the other files would attempt to copypaste it and it would cause problems
 			if (!inheritDataPresent) continue;
 			try {
-			//We're not outright giving this file the .json suffix because the other files would attempt to copypaste it and it would cause problems
 				const mappings = JSON.parse(fs.readFileSync(spritePath + "/reused"));
 				for (const mon in mappings) {
-					//null is our indicator for the presence of a custom sprite already being uploaded
+					let inherited = mappings[mon];
+					//null is not valid to inherit from
 					//Also you can't inherit from yourself
-					if (mappings[mon] === null || mappings[mon] === mon) continue;
-					modSprites[mon] ||= {};
-					modSprites[mon][modName] ||= {};
+					if (inherited === null || inherited === mon) continue;
 					//Skip if we already have custom data on this mon
-					if (modSprites[mon][modName].hasOwnProperty(subF)) continue;
-					let inherited = mappings[mon];
-					if (modSprites[inherited] && modSprites[inherited][modName] && modSprites[inherited][modName].hasOwnProperty(subF)) {
-						//If we already decided that the inheritor inherits in of itself we inherit from that
-						//(cycles result in neither inheriting)
-						modSprites[mon][modName][subF] = modSprites[inherited][modName][subF]; 
+					if (modSprites[mon] && modSprites[mon][modName] && modSprites[mon][modName].includes(subF)) continue;
+					if (!modSprites.ReusedResources[mon]) {
+						modSprites.ReusedResources[mon] = {[modName]: {}};
+					} else if (!modSprites.ReusedResources[mon][modName]) {
+						modSprites.ReusedResources[mon][modName] = {};
+					} else if (modSprites.ReusedResources[mon][modName].hasOwnProperty(subF)){
+						//We already handled this, it was presumably caught in an inherit chain
+						continue;
+					}
+					//If we already decided that the inheritor inherits in of itself we inherit from that
+					//(cycles result in neither inheriting)
+					if (modSprites.ReusedResources[inherited] && modSprites.ReusedResources[inherited][modName]
+						&& modSprites.ReusedResources[inherited][modName].hasOwnProperty(subF)) {
+						modSprites.ReusedResources[mon][modName][subF] = modSprites.ReusedResources[inherited][modName][subF]; 
 					} else {
 						//We don't have inherit data for this mapping, so let us inherit
-						while (mappings.hasOwnProperty(inherited) && inherited !== mon) {
-							//If there is a chain of inheritance track it to the source
+						const inheritChain = [mon];
+						//If there is a chain of inheritance track it to the source
+						//"inherited !== mon" breaks the loop if the inheritance is cyclic
+						while (mappings.hasOwnProperty(inherited) && inherited !== mon
+							//We're cutting off the inherit chain if we come across a preexisting element.
+							&& !(modSprites.hasOwnProperty(inherited) && modSprites[inherited].hasOwnProperty(modName)
+								&& modSprites[inherited][modName].contains(subF))) {
+							inheritChain.push(inherited);
 							inherited = mappings[inherited];
 						}
+						//If there's a cycle none of these guys are inheriting
 						if (inherited === mon) {
-							//CYCLE SPOTTED, DO NOT INHERIT
-							modSprites[mon][modName][subF] = null;
+							for (const chainInherit of inheritChain) {
+								modSprites[chainInherit] ||= {};
+								if (!modSprites[chainInherit].hasOwnProperty(modName)) modSprites[chainInherit][modName] = [subF];
+								else modSprites[chainInherit][modName].push(subF);
+							}
 						} else {
-							//Now we inherit
-							modSprites[mon][modName][subF] = inherited;
+							//We're inheriting here
+							for (const chainInherit of inheritChain) {
+								modSprites.ReusedResources[chainInherit][modName][subF] = inherited;
+							}
 						}
 					}
 				}