From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Bjarne Koll <git@lynxplay.dev>
Date: Mon, 11 Nov 2024 21:35:22 +0100
Subject: [PATCH] Fix incorrect invulnerability damage reduction

Fixes incorrect spigot handling of the invulnerability damage
reduction applied when an already invulnerable entity is damaged with a
larger damage amount than the initial damage.
Vanilla still damages entities even if invulnerable if the damage to be
applied is larger than the previous damage taken. In that case, vanilla
applies the difference between the previous damage taken and the
proposed damage.

Spigot's damage modifier API takes over the computation of damage
reducing effects, however spigot invokes this handling with the initial
damage before computing the difference to the previous damage amount.
This leads to the reduction values to generally be larger than expected,
as they are computed on the not-yet-reduced value.
Spigot applies these reductions after calling the EntityDamageEvent and
*then* subtracts the previous damage point, leading to the final damage
amount being smaller than expected.

This patch cannot simply call the EntityDamageEvent with the reduced
damage, as that would lead to EntityDamageEvent#getDamage() returning
the already reduced damage, which breaks its method contract.
Instead, this patch makes use of the DamageModifier API, implementing
the last-damage-reduction as a DamageModifier.

diff --git a/src/main/java/org/bukkit/event/entity/EntityDamageEvent.java b/src/main/java/org/bukkit/event/entity/EntityDamageEvent.java
index 4e6b1c263904c9871b2c15c3c9cbdc4a1fa4a285..d1a5424ff3b289f1c82449ef6d88eb52665df41b 100644
--- a/src/main/java/org/bukkit/event/entity/EntityDamageEvent.java
+++ b/src/main/java/org/bukkit/event/entity/EntityDamageEvent.java
@@ -247,6 +247,7 @@ public class EntityDamageEvent extends EntityEvent implements Cancellable {
          * raw {@link EntityDamageEvent#getDamage()}.
          */
         BASE,
+        INVULNERABILITY_REDUCTION, // Paper - fix invulnerability reduction in EntityDamageEvent - needs to be right under BASE as its the first reduction all others are based on
         /**
          * This represents the damage increased by freezing status.
          */