# `local-backend` — Default in-process `FingerprintBackend`

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_(no submodules)_
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## Purpose

`LocalBackend` is the default implementation of the `FingerprintBackend`
contract that hooks and components depend on. It composes `AudioFingerprinter`
and `FingerprintMatcher` so callers get a single facade with the same shape
the optional `PythonBackend` exposes from its gated subpath. The class owns
no state apart from its two collaborator instances.

## Features

- Single facade for `fingerprint(doc, opts?)` and `match(query, corpus)`.
- `AbortSignal` and progress passthrough to the underlying fingerprinter.
- Pure in-process: no I/O, no fetch, no server dependency.
- Matches the `FingerprintBackend` interface byte-for-byte so it can be
  swapped at runtime via a hook prop.
- Constructor takes no arguments — defaults rule.

## Usage: Basic Example

```ts
import { LocalBackend } from "@suaveplan/react-audio-fingerprint/backends/local-backend";

const backend = new LocalBackend();
const fp = await backend.fingerprint(doc);
```

## Usage: Advanced Example

Inject the backend into the hook explicitly to keep the default exporting tree
shaken when a custom backend is in use:

```ts
import { LocalBackend } from "@suaveplan/react-audio-fingerprint/backends/local-backend";

const backend = new LocalBackend();
const controller = new AbortController();
const fp = await backend.fingerprint(doc, {
    signal: controller.signal,
    onProgress: ({ fraction }) => console.log(fraction),
});
const results = await backend.match(fp, corpus);
```

## API Reference

- `class LocalBackend implements FingerprintBackend`.
- `fingerprint(doc, opts?): Promise<Fingerprint>`.
- `match(query, corpus): Promise<MatchResult[]>`.

## Implementation Notes

The class deliberately avoids exposing the underlying `AudioFingerprinter` and
`FingerprintMatcher` instances so consumers cannot mutate them after
construction — this preserves determinism and keeps the public surface stable
across versions. There is no caching layer here; consumers wishing to cache
fingerprint results between renders should do so at the hook layer, where the
relationship to the React commit phase is explicit. Errors from the
underlying components propagate unchanged; the backend deliberately does NOT
re-wrap them so the original `ReactAudioFingerprintError` code surfaces to the
caller.