@@ -2,7 +2,12 @@ package storage
 
 import (
 	"io"
+	"strings"
 	"time"
+
+	"gitlab.com/tozd/go/errors"
+	"go.alanpearce.eu/homestead/internal/buffer"
+	"go.alanpearce.eu/homestead/internal/builder/template"
 )
 
 type File struct {
@@ -10,7 +15,8 @@ Path         string
 	ContentType  string
 	LastModified time.Time
 	Etag         string
-	Encodings    map[string]io.ReadSeeker
+	StyleHash    string
+	Encodings    map[string]*buffer.Buffer
 }
 
 func (f *File) AvailableEncodings() []string {
@@ -21,3 +27,31 @@ }
 
 	return encs
 }
+
+func (f *File) CalculateStyleHash() (err error) {
+	buf := f.Encodings["identity"]
+	if buf == nil {
+		return errors.New("buffer not initialised")
+	}
+	_, err = buf.Seek(0, io.SeekStart)
+	if err != nil {
+		return errors.WithMessage(err, "could not seek buffer")
+	}
+
+	mime, _, _ := strings.Cut(f.ContentType, ";")
+	switch mime {
+	case "text/html":
+		f.StyleHash, err = template.GetHTMLStyleHash(buf)
+		if err != nil {
+			return errors.WithMessage(err, "could not calculate HTML style hash")
+		}
+	default:
+		return
+	}
+	_, err = buf.Seek(0, io.SeekStart)
+	if err != nil {
+		return errors.WithMessage(err, "could not seek buffer")
+	}
+
+	return
+}