diff --git a/binaries/data/mods/public/simulation/ai/petra/buildManager.js b/binaries/data/mods/public/simulation/ai/petra/buildManager.js
index 801dec9e01..9e87bcf199 100644
--- a/binaries/data/mods/public/simulation/ai/petra/buildManager.js
+++ b/binaries/data/mods/public/simulation/ai/petra/buildManager.js
@@ -10,40 +10,38 @@ import { aiWarn } from "simulation/ai/common-api/utils.js";
 export function BuildManager()
 {
 	// List of buildings we have builders for, with number of possible builders.
-	this.builderCounters = new Map();
+	this.builders = new Map();
 	// List of buildings we can't currently build (because no room, no builder or whatever),
 	// with time we should wait before trying again to build it.
 	this.unbuildables = new Map();
 }
 
+function addBuilder(builders, civ, entity)
+{
+	for (const buildable of entity.buildableEntities(civ))
+	{
+		if (!builders.has(buildable))
+			builders.set(buildable, []);
+		builders.get(buildable).push(entity.id());
+	}
+}
+
+function removeBuilder(builders, entityId)
+{
+	for (const entities of builders.values())
+	{
+		const entityIndex = entities.indexOf(entityId);
+		if (entityIndex !== -1)
+			entities.splice(entityIndex, 1);
+	}
+}
+
 /** Initialization at start of game */
 BuildManager.prototype.init = function(gameState)
 {
 	const civ = gameState.getPlayerCiv();
 	for (const ent of gameState.getOwnUnits().values())
-		this.incrementBuilderCounters(civ, ent, 1);
-};
-
-BuildManager.prototype.incrementBuilderCounters = function(civ, ent, increment)
-{
-	for (const buildable of ent.buildableEntities(civ))
-	{
-		if (this.builderCounters.has(buildable))
-		{
-			const count = this.builderCounters.get(buildable) + increment;
-			if (count < 0)
-			{
-				aiWarn(" Petra error in incrementBuilderCounters for " + buildable +
-					" with count < 0");
-				continue;
-			}
-			this.builderCounters.set(buildable, count);
-		}
-		else if (increment > 0)
-			this.builderCounters.set(buildable, increment);
-		else
-			aiWarn(" Petra error in incrementBuilderCounters for " + buildable + " not yet set");
-	}
+		addBuilder(this.builders, civ, ent);
 };
 
 /** Update the builders counters */
@@ -58,30 +56,25 @@ BuildManager.prototype.checkEvents = function(gameState, events)
 			continue;
 		const ent = gameState.getEntityById(evt.entity);
 		if (ent && ent.isOwn(PlayerID) && ent.hasClass("Unit"))
-			this.incrementBuilderCounters(civ, ent, 1);
+			addBuilder(this.builders, civ, ent);
 	}
 
 	for (const evt of events.Destroy)
-	{
-		if (events.Create.some(e => e.entity == evt.entity) || !evt.entityObj)
-			continue;
-		const ent = evt.entityObj;
-		if (ent && ent.isOwn(PlayerID) && ent.hasClass("Unit"))
-			this.incrementBuilderCounters(civ, ent, -1);
-	}
+		removeBuilder(this.builders, evt.entity);
 
 	for (const evt of events.OwnershipChanged)   // capture events
 	{
-		let increment;
 		if (evt.from == PlayerID)
-			increment = -1;
-		else if (evt.to == PlayerID)
-			increment = 1;
-		else
+		{
+			removeBuilder(this.builders, evt.entity);
 			continue;
+		}
+		else if (evt.to != PlayerID)
+			continue;
+
 		const ent = gameState.getEntityById(evt.entity);
-		if (ent && ent.hasClass("Unit"))
-			this.incrementBuilderCounters(civ, ent, increment);
+		if (ent?.hasClass("Unit"))
+			addBuilder(this.builders, civ, ent);
 	}
 
 	for (const evt of events.ValueModification)
@@ -92,10 +85,10 @@ BuildManager.prototype.checkEvents = function(gameState, events)
 
 		// Unfortunately there really is not an easy way to determine the changes
 		// at this stage, so we simply have to dump the cache.
-		this.builderCounters = new Map();
+		this.builders = new Map();
 
 		for (const ent of gameState.getOwnUnits().values())
-			this.incrementBuilderCounters(civ, ent, 1);
+			addBuilder(this.builders, civ, ent);
 	}
 };
 
@@ -106,9 +99,9 @@ BuildManager.prototype.checkEvents = function(gameState, events)
 BuildManager.prototype.findStructuresByFilter = function(gameState, filter)
 {
 	const result = [];
-	for (const [templateName, count] of this.builderCounters)
+	for (const [templateName, entities] of this.builders)
 	{
-		if (!count || gameState.isTemplateDisabled(templateName))
+		if (!entities.length || gameState.isTemplateDisabled(templateName))
 			continue;
 		const template = gameState.getTemplate(templateName);
 		if (!template || !template.available(gameState))
@@ -130,8 +123,8 @@ BuildManager.prototype.findStructureWithClass = function(gameState, classes)
 
 BuildManager.prototype.hasBuilder = function(template)
 {
-	const numBuilders = this.builderCounters.get(template);
-	return numBuilders && numBuilders > 0;
+	const numBuilders = this.builders.get(template);
+	return numBuilders && numBuilders.length > 0;
 };
 
 BuildManager.prototype.isUnbuildable = function(gameState, template)
@@ -182,7 +175,7 @@ BuildManager.prototype.resetMissingRoom = function(gameState)
 BuildManager.prototype.Serialize = function()
 {
 	return {
-		"builderCounters": this.builderCounters,
+		"builders": this.builders,
 		"unbuildables": this.unbuildables
 	};
 };
@@ -192,3 +185,4 @@ BuildManager.prototype.Deserialize = function(data)
 	for (const key in data)
 		this[key] = data[key];
 };
+