# leaf-ffi
The C-ABI / UniFFI **Rust binding** for leaf: it wraps the filesystem-free
`leaf-core` `Doc` behind UniFFI so a native Apple app can drive the byte-offset
caret model and render the `VisualMap` as style runs. The native-Apple peer of
`leaf-wasm`.
This crate is only the Rust binding (`src/lib.rs` + the `uniffi-bindgen` bin).
The Swift side built on top of it lives elsewhere:
| Swift SDK | [`packages/leaf-swift`](../../packages/leaf-swift) | `Sources/LeafUI` (the AppKit/UIKit editor) + the committed `uniffi-generated/` Swift, exposed by the `Package.swift` at the repo root. The importable Swift package. |
| Demo app | [`apps/leaf-editor`](../../apps/leaf-editor) | The runnable cross-platform (macOS + iOS) example (`bootstrap.sh`, xcodegen `project.yml`). |
The Swift bindings are (re)generated from this crate by
`scripts/gen-bindings.sh` into `packages/leaf-swift/uniffi-generated/`, which is
**committed** — the Swift package is consumed by version from a bare git
checkout, so the binding must build as-is. CI diffs the committed binding
against this crate on every push; regenerate and commit after changing the FFI
surface.
## How long the calls take
`examples/bench.rs` times the calls a native frontend makes per interaction —
the frame, the offset lookups, a click, a keystroke, the counts — on a
generated 14,000-word document or one you name, and prints a table:
```sh
cargo run -p leaf-ffi --example bench # dev profile
cargo run -p leaf-ffi --example bench --release # what a shipping app sees
cargo run -p leaf-ffi --example bench -- path.md # your own document
cargo run -p leaf-ffi --example bench -- --dump path.md # write the generated one out
```
Run it in both profiles: the dev build is what the app is driven with while
developing, and it is ten times slower on exactly the scans the table shows.
A row that grows with the document is the finding; the Swift renderer's own
per-interaction cost sits above these numbers, not in them. The last rows
time the same gestures on a document answering with *changes* rather than
whole frames (`set_incremental_frames`), with the rows each lifts in the
label — the number that must not grow when the document is concatenated
with itself (`--dump` it, then `cat` two copies and name the result).