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Fix for Deltarune crash bug in Rudinn + Hathy battle
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| In Deltarune, the following steps crash the game. I didn't find this bug, but I know how to fix it. | |
| 1. Recruit Susie permanently. | |
| 2. Go to the location of the save point that is called "Field - Maze of Death". | |
| 3. Walk left to the next screen. | |
| 4. Walk past the puzzle and start an encounter with the Rudinn walking around. This starts a battle with the Rudinn and a Hathy. | |
| 5. Highlight Act and press Z. | |
| 6. Highlight Hathy and press Z. | |
| 7. Highlight S-Flatter and press X to cancel. | |
| 8. Highlight Act and press Z again. | |
| 9. Highlight Rudinn and press Z. | |
| The game will crash with a negative subscript error in obj_battlecontroller_Draw_0. The heart cursor got pointed to a negative index. | |
| In this battle, Rudinn has 3 Act possibilities (Check, Convince, Lecture), while Hathy has 4 (Check, Flatter, X-Flatter, S-Flatter). | |
| Because the heart cursor remembers where it was pointed, the steps above cause the cursor to point to a position that no longer exists. | |
| While initializing the Act list for the targeted monster, obj_battlecontroller_Step_0 tries to detect this situation and correct it, | |
| but the code to correct it has a bug. | |
| The broken code looks like the following. This came from UndertaleModTool, based on a disassembly. Naturally, I don't have the | |
| source code, so this isn't exactly what the real code looks like. | |
| if (global.bmenuno == 11) // battle menu ID of monster target select | |
| { | |
| global.bmenuno = 9; // battle menu ID of ACT select | |
| self.actcoord = global.bmenucoord[((9 * 32000) + global.charturn)]; | |
| self.thisenemy = global.bmenucoord[((11 * 32000) + global.charturn)]; | |
| self.i = 0; | |
| for (self.i = 0; i < 6; i += 1) | |
| { | |
| if (global.canact[((self.thisenemy * 32000) + self.actcoord)] == 0) | |
| { | |
| if (self.actcoord > 0) | |
| { | |
| global.bmenucoord[((9 * 32000) + global.charturn)] -= 1; | |
| } | |
| } | |
| } | |
| self.onebuffer = 1; | |
| } | |
| The bug is on the innermost "if" statement. The write to global.bmenucoord[((9 * 32000) + global.charturn)] is updating the value that | |
| self.actcoord came from, but not self.actcoord itself! This means that 6 will be subtracted from the cursor location, which causes it | |
| to go negative, and hence the crash later in Draw. | |
| The fix is to make that innermost "if" statement like this: | |
| if (self.actcoord > 0) | |
| { | |
| global.bmenucoord[((9 * 32000) + global.charturn)] -= 1; | |
| self.actcoord -= 1; | |
| } | |
| Then it'll only subtract the necessary number of times to get the cursor to a safe location, fixing the bug with the intended behavior. | |
| To fix this, you can use UndertaleModTool. Go to the disassembly tab for script gml_Object_obj_battlecontroller_Step_0. Find the line: | |
| 04234: pop.i.v [array]bmenucoord | |
| Immediately after that line, add the following four lines (with no number before them): | |
| push.v self.actcoord | |
| pushi.e 1 | |
| sub.i.v | |
| pop.v.v self.actcoord | |
| When you then click the Decompiled tab, you'll see that the innermost "if" statement has been corrected. Save data.win, and the | |
| bug will be fixed. | |
| -- Myria 2018/11/11 |
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