Move MixinWorldClient into 1.12.2 project

versions/1.12.2/src/main/java/com/replaymod/replay/mixin/MixinWorldClient.java

@@ -0,0 +1,31 @@
+package com.replaymod.replay.mixin;
+
+import net.minecraft.client.multiplayer.WorldClient;
+import net.minecraft.entity.Entity;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
+
+@Mixin(WorldClient.class)
+public abstract class MixinWorldClient {
+
+    /**
+     * Fixes a bug in vanilla Minecraft that leaves entities remaining in the entityList even after respawn.
+     * The entityList in WorldClient is a Set that assumes that the entity ID of its entities does not change.
+     * However in {@link WorldClient#addEntityToWorld(int, Entity)}, the entity passed in is first added to the
+     * set and subsequently its id is changed, leaving it stuck in he Set. That is the buggy method.
+     * This wouldn't be too much of a problem if the entity had the correct id to begin with, however the handler for
+     * the spawn player packet creates an EntityOtherPlayerMP which takes its id from a counter and then passes it to
+     * this method with the wrong id.
+     * This mixin fixes the id of the entity before it is added to the set instead of right after.
+     * The original id change right after is not changed, however it should not have any effect.
+     * @param entityId The id to be set for the entity
+     * @param entity The entity to be added
+     * @param ci Callback info
+     */
+    @Inject(method = "addEntityToWorld", at=@At("HEAD"))
+    public void replayModReplay_fix_addEntityToWorld(int entityId, Entity entity, CallbackInfo ci) {
+        entity.setEntityId(entityId);
+    }
+}