Replace the MD5 hashing of images with xxHash

Note that we only use this for change detection.

The previous implementation invoked `MD5FromReaderFast` that created a MD5 has from 8 64 bytes chunks in the file, which is obviously very fast. The new implementation creates the hash from the entire file and ... seems to be even more effective:

```
name          old time/op    new time/op    delta
HashImage-10    9.45µs ±21%   10.89µs ± 1%     ~     (p=0.343 n=4+4)

name          old alloc/op   new alloc/op   delta
HashImage-10      144B ± 0%        8B ± 0%  -94.44%  (p=0.029 n=4+4)

name          old allocs/op  new allocs/op  delta
HashImage-10      4.00 ± 0%      1.00 ± 0%  -75.00%  (p=0.029 n=4+4)
```

common/hashing/hashing.go

@@ -0,0 +1,86 @@
+// Copyright 2024 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package hashing provides common hashing utilities.
+package hashing
+
+import (
+	"encoding/hex"
+	"io"
+	"sync"
+
+	"github.com/cespare/xxhash/v2"
+)
+
+// XXHashFromReader calculates the xxHash for the given reader.
+func XXHashFromReader(r io.ReadSeeker) (uint64, int64, error) {
+	h := getXxHashReadFrom()
+	defer putXxHashReadFrom(h)
+
+	size, err := io.Copy(h, r)
+	if err != nil {
+		return 0, 0, err
+	}
+	return h.Sum64(), size, nil
+}
+
+// XXHashFromString calculates the xxHash for the given string.
+func XXHashFromString(s string) (uint64, error) {
+	h := xxhash.New()
+	h.WriteString(s)
+	return h.Sum64(), nil
+}
+
+// XxHashFromStringHexEncoded calculates the xxHash for the given string
+// and returns the hash as a hex encoded string.
+func XxHashFromStringHexEncoded(f string) string {
+	h := xxhash.New()
+	h.WriteString(f)
+	hash := h.Sum(nil)
+	return hex.EncodeToString(hash)
+}
+
+type xxhashReadFrom struct {
+	buff []byte
+	*xxhash.Digest
+}
+
+func (x *xxhashReadFrom) ReadFrom(r io.Reader) (int64, error) {
+	for {
+		n, err := r.Read(x.buff)
+		if n > 0 {
+			x.Digest.Write(x.buff[:n])
+		}
+		if err != nil {
+			if err == io.EOF {
+				err = nil
+			}
+			return int64(n), err
+		}
+	}
+}
+
+var xXhashReadFromPool = sync.Pool{
+	New: func() any {
+		return &xxhashReadFrom{Digest: xxhash.New(), buff: make([]byte, 48*1024)}
+	},
+}
+
+func getXxHashReadFrom() *xxhashReadFrom {
+	return xXhashReadFromPool.Get().(*xxhashReadFrom)
+}
+
+func putXxHashReadFrom(h *xxhashReadFrom) {
+	h.Reset()
+	xXhashReadFromPool.Put(h)
+}

common/hashing/hashing_test.go

@@ -0,0 +1,79 @@
+// Copyright 2024 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package hashing
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/cespare/xxhash/v2"
+	qt "github.com/frankban/quicktest"
+)
+
+func TestXxHashFromReader(t *testing.T) {
+	c := qt.New(t)
+	s := "Hello World"
+	r := strings.NewReader(s)
+	got, size, err := XXHashFromReader(r)
+	c.Assert(err, qt.IsNil)
+	c.Assert(size, qt.Equals, int64(len(s)))
+	c.Assert(got, qt.Equals, uint64(7148569436472236994))
+}
+
+func TestXxHashFromString(t *testing.T) {
+	c := qt.New(t)
+	s := "Hello World"
+	got, err := XXHashFromString(s)
+	c.Assert(err, qt.IsNil)
+	c.Assert(got, qt.Equals, uint64(7148569436472236994))
+}
+
+func TestXxHashFromStringHexEncoded(t *testing.T) {
+	c := qt.New(t)
+	s := "The quick brown fox jumps over the lazy dog"
+	got := XxHashFromStringHexEncoded(s)
+	// Facit: https://asecuritysite.com/encryption/xxhash?val=The%20quick%20brown%20fox%20jumps%20over%20the%20lazy%20dog
+	c.Assert(got, qt.Equals, "0b242d361fda71bc")
+}
+
+func BenchmarkXXHashFromReader(b *testing.B) {
+	r := strings.NewReader("Hello World")
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		XXHashFromReader(r)
+		r.Seek(0, 0)
+	}
+}
+
+func BenchmarkXXHashFromString(b *testing.B) {
+	s := "Hello World"
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		XXHashFromString(s)
+	}
+}
+
+func BenchmarkXXHashFromStringHexEncoded(b *testing.B) {
+	s := "The quick brown fox jumps over the lazy dog"
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		XxHashFromStringHexEncoded(s)
+	}
+}
+
+func xxHashFromString(f string) uint64 {
+	h := xxhash.New()
+	h.WriteString(f)
+	return h.Sum64()
+}

helpers/general.go

@@ -27,12 +27,11 @@ import (
 	"unicode"
 	"unicode/utf8"
 
-	"github.com/cespare/xxhash/v2"
+	bp "github.com/gohugoio/hugo/bufferpool"
+
 	"github.com/spf13/afero"
 
 	"github.com/jdkato/prose/transform"
-
-	bp "github.com/gohugoio/hugo/bufferpool"
 )
 
 // FilePathSeparator as defined by os.Separator.
@@ -258,13 +257,7 @@ func SliceToLower(s []string) []string {
 	return l
 }
 
-// XxHashString takes a string and returns its xxHash hash.
-func XxHashString(f string) string {
-	h := xxhash.New()
-	h.WriteString(f)
-	hash := h.Sum(nil)
-	return hex.EncodeToString(hash)
-}
+// XXHashFromReader creates a xxHash hash from the given reader.
 
 // MD5String takes a string and returns its MD5 hash.
 func MD5String(f string) string {

resources/image.go

@@ -493,7 +493,7 @@ func (i *imageResource) relTargetPathFromConfig(conf images.ImageConfig) interna
 	}
 
 	h := i.hash()
-	idStr := fmt.Sprintf("_hu%s_%d", h, i.size())
+	idStr := fmt.Sprintf("_hu%d_%d", h, i.size())
 
 	// Do not change for no good reason.
 	const md5Threshold = 100

resources/resource.go

@@ -26,6 +26,7 @@ import (
 	"github.com/gohugoio/hugo/identity"
 	"github.com/gohugoio/hugo/resources/internal"
 
+	"github.com/gohugoio/hugo/common/hashing"
 	"github.com/gohugoio/hugo/common/herrors"
 	"github.com/gohugoio/hugo/common/paths"
 
@@ -307,7 +308,7 @@ type fileInfo interface {
 }
 
 type hashProvider interface {
-	hash() string
+	hash() uint64
 }
 
 var _ resource.StaleInfo = (*StaleValue[any])(nil)
@@ -403,7 +404,7 @@ func (l *genericResource) size() int64 {
 	return l.h.size
 }
 
-func (l *genericResource) hash() string {
+func (l *genericResource) hash() uint64 {
 	if err := l.h.init(l); err != nil {
 		panic(err)
 	}
@@ -628,15 +629,15 @@ type targetPather interface {
 }
 
 type resourceHash struct {
-	value    string
+	value    uint64
 	size     int64
 	initOnce sync.Once
 }
 
 func (r *resourceHash) init(l hugio.ReadSeekCloserProvider) error {
 	var initErr error
 	r.initOnce.Do(func() {
-		var hash string
+		var hash uint64
 		var size int64
 		f, err := l.ReadSeekCloser()
 		if err != nil {
@@ -656,6 +657,6 @@ func (r *resourceHash) init(l hugio.ReadSeekCloserProvider) error {
 	return initErr
 }
 
-func hashImage(r io.ReadSeeker) (string, int64, error) {
-	return helpers.MD5FromReaderFast(r)
+func hashImage(r io.ReadSeeker) (uint64, int64, error) {
+	return hashing.XXHashFromReader(r)
 }

tpl/hash/hash.go

@@ -16,10 +16,9 @@ package hash
 
 import (
 	"context"
-	"encoding/hex"
 	"hash/fnv"
 
-	"github.com/cespare/xxhash/v2"
+	"github.com/gohugoio/hugo/common/hashing"
 	"github.com/gohugoio/hugo/deps"
 	"github.com/gohugoio/hugo/tpl/internal"
 	"github.com/spf13/cast"
@@ -51,14 +50,7 @@ func (ns *Namespace) XxHash(v any) (string, error) {
 		return "", err
 	}
 
-	hasher := xxhash.New()
-
-	_, err = hasher.WriteString(conv)
-	if err != nil {
-		return "", err
-	}
-	hash := hasher.Sum(nil)
-	return hex.EncodeToString(hash), nil
+	return hashing.XxHashFromStringHexEncoded(conv), nil
 }
 
 const name = "hash"

tpl/tplimpl/template_ast_transformers.go

@@ -18,12 +18,12 @@ import (
 	"fmt"
 	"strings"
 
-	"github.com/gohugoio/hugo/helpers"
 	htmltemplate "github.com/gohugoio/hugo/tpl/internal/go_templates/htmltemplate"
 	texttemplate "github.com/gohugoio/hugo/tpl/internal/go_templates/texttemplate"
 
 	"github.com/gohugoio/hugo/tpl/internal/go_templates/texttemplate/parse"
 
+	"github.com/gohugoio/hugo/common/hashing"
 	"github.com/gohugoio/hugo/common/maps"
 	"github.com/gohugoio/hugo/tpl"
 	"github.com/mitchellh/mapstructure"
@@ -254,7 +254,7 @@ func (c *templateContext) handleDefer(withNode *parse.WithNode) {
 		c.err = errors.New("resources.PostProcess cannot be used in a deferred template")
 		return
 	}
-	innerHash := helpers.XxHashString(s)
+	innerHash := hashing.XxHashFromStringHexEncoded(s)
 	deferredID := tpl.HugoDeferredTemplatePrefix + innerHash
 
 	c.deferNodes[deferredID] = inner