Lua patient zero   bret mogilefsky (scea)
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Lua patient zero bret mogilefsky (scea)

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  • Without virtual memory and large caches, we’d have been screwed

Lua patient zero bret mogilefsky (scea) Presentation Transcript

  • 1. Lua Patient Zero: Grim Fandango
    Bret Mogilefsky
    Formerly of LucasArts, now SCEA
  • 2. Setting the stage
    One famous designer: Tim Schafer
    One venerable game engine: SCUMM
    Huge pedigree of beloved games includedManiac Mansion, Secret of Monkey Island, Sam and Max, Day of the Tentacle, Full Throttle
    One new game engine: Sith, for Jedi Knight
    One naïve coder: Me, fresh out of Cal!
    I stole from everyone!
  • 3. Scripting not a new idea atLucasArts
    SCUMM dated back to 1986
    Example:
    actor guybrush walk-to banana-tree
    wait-for-actoractor guybrush say-line “Mmm, bananas…”
    wait-for-actor
    actor guybrush face-camera
    actor guybrush say-line “Wish I had a banana-picker”
    Various attempts to unseat it had failed
    Nobody stopped me, though they predicted doom
  • 4. Breaking with the past
    SCUMM was long in the tooth and newb-hostile
    SCUMM the language was inseparable from SCUMM the engine
    I am as guilty of not-invented-here as anyone
    Got sets, characters, lighting, animation, and audio in no time, but no scripting
    Jedi Knight put me way ahead, but it wasn’t an adventure game
  • 5. Why Lua
    I was lucky!
    <Insert pic of Dr. Dobb’s Journal>
    It was small, elegant, and ready to go
    The manual was small enough that you could hand it to someone and expect them to read it
    It had no external dependencies
    The embedding API was ideal
    The code itself was approachable and readable
  • 6. Lua as I found it
    Started with Lua 2.5
    Switched to Lua 3.1 beta before shipping
    Very nice …but still lacked coroutines
    Any LEC adventure game required them!
  • 7. Why coroutines?
    Independent characters and objects get programmed with isolated scripts controlling their actions
    Think about what it would be like in C/C++!
    Threads were too heavy for undead pigeons, flaming beavers, driving demons, and Pentium 90MHz processors
    True concurrency wasn’t needed; just wanted to pick up where we left off last time each frame
  • 8. Lua modifications
    Addition of coroutines
    Made it re-entrant
    Flattened the Lua stack
    Implemented dump/restore of Lua state
  • 9. Lua re-entrancy
    Took state out of global variables to make it thread-safe
    First attempt was an array of Lua state structures looped over until a “yield()” was called, using a single thread
    But… DISASTER!
  • 10. Lua’s recursive stack
    Every Lua call also pushed a stack frame in C
    A yield from one Lua instance to resume another might happen at a different stack depth
    Welcome to Crash City
    <Sequence diagram: C->Lua->C->Lua->C->Lua>
  • 11. Flattening Lua’s stack
    Instead of recursing, return a yield code, push a structure, and start the next function
    <C->Lua->Lua->Lua>
    A couple years later, “Stackless Python” solved the same problem
    It still exists, whereas Lua incorporated coroutines as part of the language
    This would not have been possible if Lua wasn’t nice code to begin with
  • 12. Ways we used Lua
    Engine extension
    Command-line and console
    Debugger, including tasking extension
    Tool macro language
    • Patching
  • Engine extension
    All performance-critical code in C/C++
    Renderer, animation, lighting, sound, save/load
    All game-specific code in Lua
    Engine just calls “BOOT()”, then loops tasks
    Not just adventure gameplay
    Controls, menus, dialog trees, puzzle mosaic
    The scripting language is glue!
    120K lines of code
  • 13. Command-line and console
    Copy and paste code into game on-the-fly
    Iterate on a bugfix without restarting
    Trigger any condition as needed
    This kind of productivity was unprecedented!
    The compiler was small so we left it in
  • 14. Debugger
    Written by a scripter using the in-language line-hook and call-hook facility
    Added the task-hook for our coroutines
    Fancy features like evaluating expressions at each step were trivial due to compiler in engine
  • 15. Tool macro language
    Developed choreography rehearsal tool alongside game
    Stitched together assets into “costumes” and coordinated timing
    ChoreTool used Lua as a macro language
    Example: Tie real game trigger logic to a menu item
    Example: Automate insertion of keys at regular intervals
    Some macros became permanent parts of the tool, saving lots of programmer time
  • 16. Patching
    Blocker bugs in scripts were trivially replaced
    Saved enormous amounts of burn time
    Update to shipped game retro-patched save games as well
    Probably not an issue if we’d saved differently
  • 17. It wasn’t all roses
    High memory requirements
    Debugger went stale
    Heavyweight garbage collection
    So much flexibility it was easy to write bugs
    Typos in variable names not caught
    Lacked languageconventions
    No example code to examine
    Savegames steadily grew larger as you progress…
  • 18. How’d we do?
    Despite huge technical risks, we pulled it off
    Great reviews, and the game remains beloved
    Poor timing: We lost to Half-Life in “game of the year” categories over and over
    The engine went on to ship Escape from Monkey Island (with lots of mods)
    But really, the end of the adventure genre was upon us, and that was the end of the line for GrimE
  • 19. TheLua explosion
    Concurrent success of QuakeC and UnrealScript had made scripting languages HOT!
    GDC99 held a panel on them
    Rob Huebner on Java in Vampire: The Masquerade
    Kevin Bruner on writing your own for Obi-Wan
    Seamus McNally on not using one for Treadmarks
    200 miserable developers slumped in their chairs
    “Or you could just use Lua…”
    Furious scribbling followed by sudden influx on the mailing list
  • 20. Why Lua endures
    It’s small, simple, and proven
    Lua turned “scripting language” into a commodity feature
    You would be crazy to write your own and expect to do better