Compression class final

Compresses responses a client said it can decompress.

final app = Router()
  ..layer(const Compression())
  ..merge(routes);

Only gzip: it is the encoding every client has supported for two decades, and brotli would mean a dependency for a few percent. A client that asks for nothing, or asks only for something else, gets the body uncompressed.

A response is left alone when any of these is true, because compressing it would cost more than it saves or would break something:

• the client did not offer gzip;
• the body is already encoded — re-encoding is how a double-gzipped response happens, which no client will unwrap twice;
• the type is not in types, so it is probably compressed already;
• a known length is below minimumBytes, where the gzip header alone can make the answer bigger.

Vary: Accept-Encoding is always added, compressed or not: without it a shared cache can hand a gzipped body to a client that never asked for one.