refactor(response): use strict Accept-Encoding parser

internal/http/response/builder.go

@@ -133,12 +133,16 @@ func (b *Builder) writeHeaders() {
 	b.w.WriteHeader(b.statusCode)
 }
 
+// values should be in sync with [Builder.compress] switch/case.
+var acceptEncoding = AcceptEncoding("br", "gzip", "deflate")
+
 func (b *Builder) compress(data []byte) {
 	if b.enableCompression && len(data) > compressionThreshold {
 		b.headers.Set("Vary", "Accept-Encoding")
-		acceptEncoding := b.r.Header.Get("Accept-Encoding")
-		switch {
-		case strings.Contains(acceptEncoding, "br"):
+
+		encoding := acceptEncoding.Parse(b.r.Header.Get("Accept-Encoding"))
+		switch encoding {
+		case "br":
 			b.headers.Set("Content-Encoding", "br")
 			b.writeHeaders()
 
@@ -146,7 +150,7 @@ func (b *Builder) compress(data []byte) {
 			brotliWriter.Write(data)
 			brotliWriter.Close()
 			return
-		case strings.Contains(acceptEncoding, "gzip"):
+		case "gzip":
 			b.headers.Set("Content-Encoding", "gzip")
 			b.writeHeaders()
 
@@ -154,7 +158,7 @@ func (b *Builder) compress(data []byte) {
 			gzipWriter.Write(data)
 			gzipWriter.Close()
 			return
-		case strings.Contains(acceptEncoding, "deflate"):
+		case "deflate":
 			b.headers.Set("Content-Encoding", "deflate")
 			b.writeHeaders()
 

internal/http/response/builder_test.go

@@ -358,7 +358,7 @@ func TestIfNoneMatch(t *testing.T) {
 func TestBuildResponseWithBrotliCompression(t *testing.T) {
 	body := strings.Repeat("a", compressionThreshold+1)
 	r, err := http.NewRequest("GET", "/", nil)
-	r.Header.Set("Accept-Encoding", "gzip, deflate, br")
+	r.Header.Set("Accept-Encoding", "br, gzip, deflate")
 	if err != nil {
 		t.Fatal(err)
 	}