Something something etag middleware

internal/middleware/etag_cache.go

@@ -0,0 +1,66 @@
+package middleware
+
+import (
+	"fmt"
+	"hash/crc64"
+	"log"
+	"net/http"
+	"strings"
+	"time"
+)
+
+type ETag struct {
+	Name       string
+	Value      string
+	Expiration time.Duration
+	HashTime   time.Duration
+}
+
+var etags = make([]*ETag, 0)
+
+func (etag *ETag) Header() string {
+	return fmt.Sprintf("\"pb-%s-%s\"", etag.Name, etag.Value)
+}
+
+func (etag *ETag) CacheControlHeader() string {
+	return fmt.Sprintf("max-age=%d", int(etag.Expiration.Seconds()))
+}
+
+func StaticEtagMiddleware(etag ETag, next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		cc := etag.CacheControlHeader()
+		w.Header().Set("ETag", etag.Header())
+		w.Header().Set("Cache-Control", cc)
+
+		log.Printf("Returned etag with Cache-Control '%s'", cc)
+
+		if match := r.Header.Get("If-None-Match"); match != "" {
+			if strings.Contains(match, etag.Value) {
+				log.Print("ETag cache hit")
+				w.WriteHeader(http.StatusNotModified)
+				return
+			}
+		}
+		next.ServeHTTP(w, r)
+	})
+}
+
+// GenerateETagFromBuffer calculates etag value from one or more buffers (e.g. embedded files)
+func GenerateETagFromBuffer(name string, expiration time.Duration, buffers ...[]byte) (*ETag, error) {
+	start := time.Now()
+	hash := crc64.New(crc64.MakeTable(crc64.ECMA))
+	for _, buffer := range buffers {
+		_, err := hash.Write(buffer)
+		if err != nil {
+			return nil, fmt.Errorf("unable to generate etag from buffer: %w", err)
+		}
+	}
+	etag := &ETag{
+		Name:       name,
+		Expiration: expiration,
+		Value:      fmt.Sprintf("%x", hash.Sum64()),
+		HashTime:   time.Since(start),
+	}
+	etags = append(etags, etag)
+	return etag, nil
+}