# operationToImgproxy

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## Purpose

`operationToImgproxy` converts a resolved operation + concrete pixel
dimensions + gravity into the imgproxy processing-string segment that
sits between the `/<signature>/` prefix and the `/plain/s3://…@<fmt>`
suffix. It is the bridge between the compact grammar's abstract
operations and imgproxy's concrete URL DSL.

The five operations behave exactly as the upstream Go module: `cover`
and `crop` require a gravity prefix so anchoring takes effect during
cropping; `fit` ignores gravity (it always preserves aspect ratio);
`stretch` always emits `rs:force:W:H`; `resize` is the single-axis
variant of `fit`. Passthrough (`""`) and unknown operations emit an
empty string so the URL builder skips the processing segment entirely.

## Features

- Total function — every input returns a defined string, never
  throws.
- Mirrors the Go switch table byte-for-byte.
- Single-axis fallbacks (`g:<g>/w:W`, `w:W`, `h:H`) so callers can
  produce auto-axis resizes without re-implementing the table.
- Empty-string fallback drives the URL builder to skip the segment.

## Usage: Basic

```ts
import { operationToImgproxy } from "@suaveplan/caddy-imgproxy";

operationToImgproxy("cover", 1920, 1080, "t");
// → "g:no/rs:fill:1920:1080"

operationToImgproxy("fit", 800, 0, "c");
// → "w:800"
```

## Usage: Advanced

```ts
import {
    calculateDimensions,
    operationToImgproxy,
    parseImgproxyUrl,
} from "@suaveplan/caddy-imgproxy";

const parsed = parseImgproxyUrl("/ucbl4:3w1920/photo.png");
const { width, height } = calculateDimensions(parsed, 800);
operationToImgproxy(parsed.operation, width, height, parsed.gravity);
// → "g:sowe/c:1920:1440"
```

## API Reference

### `operationToImgproxy(op: ImgproxyOperation, width: number, height: number, gravity: string): string`

- **Parameters:**
    - `op` — one of `""|cover|crop|fit|stretch|resize`.
    - `width`, `height` — non-negative integers; either may be zero.
    - `gravity` — a grammar code accepted by `gravityToImgproxy`.
- **Returns:** the imgproxy processing string, or `""` when the
  operation cannot be expressed (e.g. `crop` without both dims).

## Implementation Notes

The switch is exhaustive over the `ImgproxyOperation` union; the
TypeScript compiler verifies no case escapes. Gravity is resolved
once at the top of the function so the `cover`/`crop` branches share
a single translation step. The empty-operation case returns `""`
explicitly rather than falling through so the exhaustiveness check
stays clean.