Designing SDKs

When designing a new SDK for Firefly Zero, we follow these principles:

  1. Reduce type convertions. Try to use the same integer type everywhere. WebAssembly is 32-bit system, so in Rust it will be i32. In Go, len and slice indexing use int, so that’s the best “default” type. If the language supports generic constructors, consider using it to automatically cast 8-bit and 16-bit integers to the default integer type. Using 8-bit or 16-bit integer in some types, like Pad, would potentially reduce the size of heap allocations, but most of the time these definitions never leave the stack, and in WebAssembly on the stack everything takes at least 32 bit.
  2. Provide type casts. If the language provides custom type casting, define it for converting Pad to Point (and back), Pad to DPad, Canvas to Image, Style to LineStyle (and back), etc.
  3. Provide custom types. While load_file can return a raw byte array and draw_image can accept any byte array, it’s good to provide custom File and Image types (and other custom types too whenever is possible). Even if the language doesn’t provide nominal type safety, a definition like enemy: Image is much more descriptive than Enemy: []byte.
  4. Host calls are functions. You can optionally provide Line type with draw method for the user’s convenience. However, there must be always a draw_line function, like for any other WebAssembly host call. The idea is that host calls are side-effects and that’s how we make it explicit.
  5. Keep the host functions names. If the host function is called draw_line, the SDK function wrapping it must be called draw_line (or drawLine, or DrawLine) too. Don’t call it render_line or anything else. The called host function names are visible in the runtime error messages and in firefly_cli inspect, so it should be easy for the user to map it to the SDK function calls in their code.
  6. Keep consistent type names. Use the following type names:
    1. Grpahics: Angle, Canvas, Color, Font, Image, LineStyle, Point, RGB, Size, Style, SubImage.
    2. FS: File.
    3. Input: Pad, DPad, Buttons.
    4. Net: Peer, Peers, Stash.
    5. Stats: Badge, Board, Progress.
    6. Shapes: Shape, Line, Rect, RoundedRect, Circle, Ellipse, Triangle, Arc, Sector.
  7. Follow the language style guides. Follow the naming convention and all the best practice. If there are several competing standards, pick the most commonly used one. Use all possible linters and code formatters.
  8. Keep modules flat. There should be only 2 public modules: firefly.audio with everything related to audio and firefly with everything else (and sudo if your SDK provides sudo functions). Even though the host runtime defines modules like graphics, misc, and more, the SDKs’ public API shouldn’t reflect that structure. Make it fast and easy for users to find and import all they need.
  9. Avoid allocations. It should be possible to use SDK to write allocation-free apps. For example, in Go, ReadPad returns (Pad, bool) instead of *Pad so it doesn’t escape on the heap. In case of reading a file, most users will want it the file be automatically allocated with the correct size, so you should make both possible. For example, the Go version of LoadFile accept an optional buffer and if it is nil, a new one is automatically allocated. Or in Rust version, there are two implementations: load_file and load_file_buf.
  10. Optimize (for size). You should provide a simple to use API but it must never sacrifice binary size or performance. If someone wants to push the limits of what Firefly Zero can do, they should be able to do that without bypassing the official SDK. However, in many cases, the compiler is smart enough to optimize out unused code branches, so always validate your assumptions.
  11. Keep the arguments order. The host functions follow strict rules for arguments order. In particular, drawing functions always accept Point first and Style last. Your wrappers should follow the same order.
  12. Use 32-bit float. 32-bit float arithmetic is much faster on the device than 64-bit one.
  13. Provide helpers. Don’t stop just at the host function wrappers. Provide everything that a typical game might need. For example, arithmetic operations for Point, radial coordinates for Pad, or optional integration with a popular 2D math library.
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