[Scummvm-tracker] [ScummVM :: Bugs] #10270: SCI: LSL7: Ocean outside portholes jumps

[ScummVM :: Bugs]

#10270: SCI: LSL7: Ocean outside portholes jumps
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Reporter:  DanielSWolf  |       Owner:  sluicebox
    Type:  defect       |      Status:  closed
Priority:  normal       |   Component:  Engine: SCI
 Version:               |  Resolution:  fixed
Keywords:  sci32        |        Game:  Leisure Suit Larry 7
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Comment (by sluicebox):

 You're welcome! Thank you for your kind words -- I am highly susceptible
 to compliments.

 This was a hard one, and highly intimidating since the uncanny csnover
 took an in depth look and didn't figure it out. I have different skills
 and many limitations so I had to think for a long time about how I could
 ever approach this. (I refuse to swim in disassembled exe's for more than
 a few minutes and have forgotten 100% of the trigonometry I've ever
 learned.) But it's a visually obvious bug, and reported by the honorable
 LarryScale author, so even though I don't know (care?) about LSL7 myself,
 I've really wanted to take this one down for years.

 But I'm good with SCI scripts. (I suspect I'm the only person to have ever
 decompiled this game.) So for this, I modded the LSL7 ocean script to log
 the output of every motion to a text file. Wow that sentence makes it
 sound easy. It meant identifying a large swath of code I could safely
 overwrite and then manually writing this entire feature (with full file
 i/o, string formatting, etc) in byte code (pen and paper and hex editor)
 within those bytes plus a bunch of SCI3-specific patches to get the
 modified strings working (relocations!) and then, finally, patching
 scummvm to allow any of this arbitrary file access. But by the end of the
 day I had a modded script patch file I could drop in the game directory
 and run in dosbox and scummvm. I could then diff the log files to see
 where things really went significantly wrong. That narrowed this down to
 the exact state before the big problem. Further tweaks to my patch logged
 additional state and that let me somewhat tediously bisect further until I
 hit the mod instruction. At that point I had the answer, and while it was
 difficult, it didn't involve any x86 so I threw a party. At that point, I
 could quickly confirm my findings against many exe's to get the version
 check right.

 I've never had to touch the core VM before, so this was a lot of fun.
 "SCI3 mod opcode is broken in 2022" is not what anyone would have expected
 from this.
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Ticket URL: <https://bugs.scummvm.org/ticket/10270#comment:8>
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