Add HTTPCompressor for HTTP Content-Encoding negotiation.

go.mod

@@ -0,0 +1,5 @@
+module github.com/andybalholm/brotli
+
+go 1.12
+
+require github.com/golang/gddo v0.0.0-20190419222130-af0f2af80721

go.sum

@@ -0,0 +1,2 @@
+github.com/golang/gddo v0.0.0-20190419222130-af0f2af80721 h1:KRMr9A3qfbVM7iV/WcLY/rL5LICqwMHLhwRXKu99fXw=
+github.com/golang/gddo v0.0.0-20190419222130-af0f2af80721/go.mod h1:xEhNfoBDX1hzLm2Nf80qUvZ2sVwoMZ8d6IE2SrsQfh4=

writer.go

@@ -1,8 +1,12 @@
 package brotli
 
 import (
+	"compress/gzip"
 	"errors"
 	"io"
+	"net/http"
+
+	"github.com/golang/gddo/httputil"
 )
 
 const (
@@ -114,3 +118,38 @@ func (w *Writer) Close() error {
 func (w *Writer) Write(p []byte) (n int, err error) {
 	return w.writeChunk(p, operationProcess)
 }
+
+type nopCloser struct {
+	io.Writer
+}
+
+func (nopCloser) Close() error { return nil }
+
+// HTTPCompressor chooses a compression method (brotli, gzip, or none) based on
+// the Accept-Encoding header, sets the Content-Encoding header, and returns a
+// WriteCloser that implements that compression. The Close method must be called
+// before the current HTTP handler returns.
+//
+// Due to https://github.com/golang/go/issues/31753, the response will not be
+// compressed unless you set a Content-Type header before you call
+// HTTPCompressor.
+func HTTPCompressor(w http.ResponseWriter, r *http.Request) io.WriteCloser {
+	if w.Header().Get("Content-Type") == "" {
+		return nopCloser{w}
+	}
+
+	if w.Header().Get("Vary") == "" {
+		w.Header().Set("Vary", "Accept-Encoding")
+	}
+
+	encoding := httputil.NegotiateContentEncoding(r, []string{"br", "gzip"})
+	switch encoding {
+	case "br":
+		w.Header().Set("Content-Encoding", "br")
+		return NewWriter(w)
+	case "gzip":
+		w.Header().Set("Content-Encoding", "gzip")
+		return gzip.NewWriter(w)
+	}
+	return nopCloser{w}
+}