From 2eee738a9fd30ec77bbe5bdb3b905997571ac845 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tam=C3=A1s=20B=C3=A1lint=20Misius?= <lbphacker@gmail.com>
Date: Tue, 12 Dec 2023 22:08:31 +0100
Subject: [PATCH] Fix another extremely rare pmap corruption

The real fix was made in Simulation::kill_part, where photons[y][x] wasn't cleared when i = 0 because the pmap branch triggered instead. Broken since ff7428fc7038, which only partially fixed some related bug. I decided to also fix every other case, even if they are not strictly necessary because they write 0 to pmap/photons anyway.

Reproduce with

	sim.loadSave(3062273, 1)
	sim.ensureDeterminism(true)
	tpt.setfpscap(2)
	local function F()
		print(sim.framerender(), sim.randomseed())
		if sim.framerender() == 0 then
			local a = 3498763327
			print(sim.hash(), a)
			sim.framerender(1200)
			sim.reloadSave()
			sim.randomseed(a, a + 1, a + 2, a + 3)
			tpt.set_pause(0)
		end
	end
	event.register(event.tick, F)
---
 src/simulation/Simulation.cpp    | 22 +++++++++++-----------
 src/simulation/elements/BANG.cpp |  2 +-
 2 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/src/simulation/Simulation.cpp b/src/simulation/Simulation.cpp
index b1040f7d9..2fb58ae58 100644
--- a/src/simulation/Simulation.cpp
+++ b/src/simulation/Simulation.cpp
@@ -1411,7 +1411,7 @@ int Simulation::try_move(int i, int x, int y, int nx, int ny)
 			return 1;
 		}
 
-		if (ID(pmap[ny][nx]) == ri)
+		if (pmap[ny][nx] && ID(pmap[ny][nx]) == ri)
 			pmap[ny][nx] = 0;
 		parts[ri].x += float(x - nx);
 		parts[ri].y += float(y - ny);
@@ -1469,9 +1469,9 @@ bool Simulation::move(int i, int x, int y, float nxf, float nyf)
 	parts[i].y = nyf;
 	if (ny != y || nx != x)
 	{
-		if (ID(pmap[y][x]) == i)
+		if (pmap[y][x] && ID(pmap[y][x]) == i)
 			pmap[y][x] = 0;
-		if (ID(photons[y][x]) == i)
+		if (photons[y][x] && ID(photons[y][x]) == i)
 			photons[y][x] = 0;
 		// kill_part if particle is out of bounds
 		if (nx < CELL || nx >= XRES - CELL || ny < CELL || ny >= YRES - CELL)
@@ -1676,9 +1676,9 @@ void Simulation::kill_part(int i)//kills particle number i
 
 	if (x >= 0 && y >= 0 && x < XRES && y < YRES)
 	{
-		if (ID(pmap[y][x]) == i)
+		if (pmap[y][x] && ID(pmap[y][x]) == i)
 			pmap[y][x] = 0;
-		else if (ID(photons[y][x]) == i)
+		else if (photons[y][x] && ID(photons[y][x]) == i)
 			photons[y][x] = 0;
 	}
 
@@ -1726,13 +1726,13 @@ bool Simulation::part_change_type(int i, int x, int y, int t)
 	if (elements[t].Properties & TYPE_ENERGY)
 	{
 		photons[y][x] = PMAP(i, t);
-		if (ID(pmap[y][x]) == i)
+		if (pmap[y][x] && ID(pmap[y][x]) == i)
 			pmap[y][x] = 0;
 	}
 	else
 	{
 		pmap[y][x] = PMAP(i, t);
-		if (ID(photons[y][x]) == i)
+		if (photons[y][x] && ID(photons[y][x]) == i)
 			photons[y][x] = 0;
 	}
 	return false;
@@ -1828,9 +1828,9 @@ int Simulation::create_part(int p, int x, int y, int t, int v)
 	{
 		int oldX = (int)(parts[p].x + 0.5f);
 		int oldY = (int)(parts[p].y + 0.5f);
-		if (ID(pmap[oldY][oldX]) == p)
+		if (pmap[oldY][oldX] && ID(pmap[oldY][oldX]) == p)
 			pmap[oldY][oldX] = 0;
-		if (ID(photons[oldY][oldX]) == p)
+		if (photons[oldY][oldX] && ID(photons[oldY][oldX]) == p)
 			photons[oldY][oldX] = 0;
 
 		oldType = parts[p].type;
@@ -2889,9 +2889,9 @@ killed:
 				}
 				if (ny!=y || nx!=x)
 				{
-					if (ID(pmap[y][x]) == i)
+					if (pmap[y][x] && ID(pmap[y][x]) == i)
 						pmap[y][x] = 0;
-					else if (ID(photons[y][x]) == i)
+					else if (photons[y][x] && ID(photons[y][x]) == i)
 						photons[y][x] = 0;
 					if (nx<CELL || nx>=XRES-CELL || ny<CELL || ny>=YRES-CELL)
 					{
diff --git a/src/simulation/elements/BANG.cpp b/src/simulation/elements/BANG.cpp
index 3e9db3f81..e53c8e164 100644
--- a/src/simulation/elements/BANG.cpp
+++ b/src/simulation/elements/BANG.cpp
@@ -73,7 +73,7 @@ static int update(UPDATE_FUNC_ARGS)
 	}
 	else if(parts[i].tmp==1)
 	{
-		if ((ID(pmap[y][x]) == i))
+		if (pmap[y][x] && ID(pmap[y][x]) == i)
 		{
 			sim->flood_prop(x, y, Particle::GetProperties()[FIELD_TMP], 2);
 		}