From 32b3333bfbb9186b2faae2b25a845e7cba6fb8cf Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Miko=C5=82aj=20Pir=C3=B3g?=
 <69601940+aetn23@users.noreply.github.com>
Date: Sun, 28 Aug 2022 17:35:58 +0200
Subject: [PATCH] Exp overflow fix (#5070)

---
 Source/loadsave.cpp |  5 +++--
 Source/monster.cpp  |  8 +-------
 Source/monster.h    | 25 ++++++++++++++++++++++++-
 3 files changed, 28 insertions(+), 10 deletions(-)

diff --git a/Source/loadsave.cpp b/Source/loadsave.cpp
index 69da533b1..6ca3babf4 100644
--- a/Source/loadsave.cpp
+++ b/Source/loadsave.cpp
@@ -27,6 +27,7 @@
 #include "lighting.h"
 #include "menu.h"
 #include "missiles.h"
+#include "monster.h"
 #include "mpq/mpq_writer.hpp"
 #include "pfile.h"
 #include "qol/stash.h"
@@ -632,7 +633,7 @@ void LoadMonster(LoadHelper *file, Monster &monster)
 	monster.whoHit = file->NextLE<int8_t>();
 	monster.level = file->NextLE<int8_t>();
 	file->Skip(1); // Alignment
-	monster.exp = file->NextLE<uint16_t>();
+	file->Skip(2); // Skip exp - now calculated from monstdat when the monster dies
 
 	if (monster.isPlayerMinion()) // Don't skip for golems
 		monster.toHit = file->NextLE<uint8_t>();
@@ -1398,7 +1399,7 @@ void SaveMonster(SaveHelper *file, Monster &monster)
 	file->WriteLE<int8_t>(monster.whoHit);
 	file->WriteLE<int8_t>(monster.level);
 	file->Skip(1); // Alignment
-	file->WriteLE<uint16_t>(monster.exp);
+	file->WriteLE<uint16_t>(static_cast<uint16_t>(std::min<unsigned>(std::numeric_limits<uint16_t>::max(), monster.exp(sgGameInitInfo.nDifficulty))));
 
 	file->WriteLE<uint8_t>(static_cast<uint8_t>(std::min<uint16_t>(monster.toHit, std::numeric_limits<uint8_t>::max()))); // For backwards compatibility
 	file->WriteLE<uint8_t>(monster.minDamage);
diff --git a/Source/monster.cpp b/Source/monster.cpp
index 6edfa73d4..10a44b1a8 100644
--- a/Source/monster.cpp
+++ b/Source/monster.cpp
@@ -156,7 +156,6 @@ void InitMonster(Monster &monster, Direction rd, size_t typeIndex, Point positio
 	monster.rndItemSeed = AdvanceRndSeed();
 	monster.aiSeed = AdvanceRndSeed();
 	monster.whoHit = 0;
-	monster.exp = monster.data().exp;
 	monster.toHit = monster.data().toHit;
 	monster.minDamage = monster.data().minDamage;
 	monster.maxDamage = monster.data().maxDamage;
@@ -185,7 +184,6 @@ void InitMonster(Monster &monster, Direction rd, size_t typeIndex, Point positio
 			monster.maxHitPoints += 64;
 		monster.hitPoints = monster.maxHitPoints;
 		monster.level += 15;
-		monster.exp = 2 * (monster.exp + 1000);
 		monster.toHit += NightmareToHitBonus;
 		monster.minDamage = 2 * (monster.minDamage + 2);
 		monster.maxDamage = 2 * (monster.maxDamage + 2);
@@ -201,7 +199,6 @@ void InitMonster(Monster &monster, Direction rd, size_t typeIndex, Point positio
 			monster.maxHitPoints += 192;
 		monster.hitPoints = monster.maxHitPoints;
 		monster.level += 30;
-		monster.exp = 4 * (monster.exp + 1000);
 		monster.toHit += HellToHitBonus;
 		monster.minDamage = 4 * monster.minDamage + 6;
 		monster.maxDamage = 4 * monster.maxDamage + 6;
@@ -3108,7 +3105,6 @@ void PrepareUniqueMonst(Monster &monster, UniqueMonsterType monsterType, size_t
 		monster.level = monster.data().level + 5;
 	}
 
-	monster.exp *= 2;
 	monster.maxHitPoints = uniqueMonsterData.mmaxhp << 6;
 
 	if (!gbIsMultiplayer)
@@ -3148,7 +3144,6 @@ void PrepareUniqueMonst(Monster &monster, UniqueMonsterType monsterType, size_t
 			monster.maxHitPoints += 64;
 		monster.level += 15;
 		monster.hitPoints = monster.maxHitPoints;
-		monster.exp = 2 * (monster.exp + 1000);
 		monster.minDamage = 2 * (monster.minDamage + 2);
 		monster.maxDamage = 2 * (monster.maxDamage + 2);
 		monster.minDamageSpecial = 2 * (monster.minDamageSpecial + 2);
@@ -3161,7 +3156,6 @@ void PrepareUniqueMonst(Monster &monster, UniqueMonsterType monsterType, size_t
 			monster.maxHitPoints += 192;
 		monster.level += 30;
 		monster.hitPoints = monster.maxHitPoints;
-		monster.exp = 4 * (monster.exp + 1000);
 		monster.minDamage = 4 * monster.minDamage + 6;
 		monster.maxDamage = 4 * monster.maxDamage + 6;
 		monster.minDamageSpecial = 4 * monster.minDamageSpecial + 6;
@@ -3663,7 +3657,7 @@ void M_StartHit(Monster &monster, const Player &player, int dam)
 void MonsterDeath(Monster &monster, Direction md, bool sendmsg)
 {
 	if (!monster.isPlayerMinion())
-		AddPlrMonstExper(monster.level, monster.exp, monster.whoHit);
+		AddPlrMonstExper(monster.level, monster.exp(sgGameInitInfo.nDifficulty), monster.whoHit);
 
 	MonsterKillCounts[monster.type().type]++;
 	monster.hitPoints = 0;
diff --git a/Source/monster.h b/Source/monster.h
index cbc47f007..4d0115467 100644
--- a/Source/monster.h
+++ b/Source/monster.h
@@ -195,7 +195,7 @@ struct Monster { // note: missing field _mAFNum
 	uint32_t rndItemSeed;
 	/** Seed used to determine AI behaviour/sync sounds in multiplayer games? */
 	uint32_t aiSeed;
-	uint16_t exp;
+
 	uint16_t toHit;
 	uint16_t toHitSpecial;
 	uint16_t resistance;
@@ -317,6 +317,29 @@ struct Monster { // note: missing field _mAFNum
 		return pgettext("monster", data().name);
 	}
 
+	/**
+	 * @brief Calculates monster's experience points.
+	 * Fetches base exp value from @p MonstersData array.
+	 * @param difficulty - difficulty on which calculation is performed
+	 * @return Monster's experience points, including bonuses from difficulty and monster being unique
+	 */
+	unsigned int exp(_difficulty difficulty) const
+	{
+		unsigned int monsterExp = data().exp;
+
+		if (difficulty == DIFF_NIGHTMARE) {
+			monsterExp = 2 * (monsterExp + 1000);
+		} else if (difficulty == DIFF_HELL) {
+			monsterExp = 4 * (monsterExp + 1000);
+		}
+
+		if (isUnique()) {
+			monsterExp *= 2;
+		}
+
+		return monsterExp;
+	}
+
 	/**
 	 * @brief Returns the network identifier for this monster
 	 *