markup/goldmark: Make auto IDs GitHub compatible

You can turn off this behaviour:

```toml
[markup]
  [markup.goldmark]
    [markup.goldmark.parser]
      autoHeadingIDAsciiOnly = true
```
Note that the `anchorize` now adapts its behaviour depending on the default Markdown handler.

Fixes #6616

markup/goldmark/autoid.go

@@ -0,0 +1,125 @@
+// Copyright 2019 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 goldmark
+
+import (
+	"bytes"
+	"strconv"
+	"unicode"
+	"unicode/utf8"
+
+	"github.com/gohugoio/hugo/common/text"
+
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/parser"
+	"github.com/yuin/goldmark/util"
+
+	bp "github.com/gohugoio/hugo/bufferpool"
+)
+
+func sanitizeAnchorNameString(s string, asciiOnly bool) string {
+	return string(sanitizeAnchorName([]byte(s), asciiOnly))
+}
+
+func sanitizeAnchorName(b []byte, asciiOnly bool) []byte {
+	return sanitizeAnchorNameWithHook(b, asciiOnly, nil)
+}
+
+func sanitizeAnchorNameWithHook(b []byte, asciiOnly bool, hook func(buf *bytes.Buffer)) []byte {
+	buf := bp.GetBuffer()
+
+	if asciiOnly {
+		// Normalize it to preserve accents if possible.
+		b = text.RemoveAccents(b)
+	}
+
+	for len(b) > 0 {
+		r, size := utf8.DecodeRune(b)
+		switch {
+		case asciiOnly && size != 1:
+		case isSpace(r):
+			buf.WriteString("-")
+		case r == '-' || isAlphaNumeric(r):
+			buf.WriteRune(unicode.ToLower(r))
+		default:
+		}
+
+		b = b[size:]
+	}
+
+	if hook != nil {
+		hook(buf)
+	}
+
+	result := make([]byte, buf.Len())
+	copy(result, buf.Bytes())
+
+	bp.PutBuffer(buf)
+
+	return result
+}
+
+func isAlphaNumeric(r rune) bool {
+	return r == '_' || unicode.IsLetter(r) || unicode.IsDigit(r)
+}
+
+func isSpace(r rune) bool {
+	return r == ' ' || r == '\t'
+}
+
+var _ parser.IDs = (*idFactory)(nil)
+
+type idFactory struct {
+	asciiOnly bool
+	vals      map[string]struct{}
+}
+
+func newIDFactory(asciiOnly bool) *idFactory {
+	return &idFactory{
+		vals:      make(map[string]struct{}),
+		asciiOnly: asciiOnly,
+	}
+}
+
+func (ids *idFactory) Generate(value []byte, kind ast.NodeKind) []byte {
+	return sanitizeAnchorNameWithHook(value, ids.asciiOnly, func(buf *bytes.Buffer) {
+		if buf.Len() == 0 {
+			if kind == ast.KindHeading {
+				buf.WriteString("heading")
+			} else {
+				buf.WriteString("id")
+			}
+		}
+
+		if _, found := ids.vals[util.BytesToReadOnlyString(buf.Bytes())]; found {
+			// Append a hypen and a number, starting with 1.
+			buf.WriteRune('-')
+			pos := buf.Len()
+			for i := 1; ; i++ {
+				buf.WriteString(strconv.Itoa(i))
+				if _, found := ids.vals[util.BytesToReadOnlyString(buf.Bytes())]; !found {
+					break
+				}
+				buf.Truncate(pos)
+			}
+		}
+
+		ids.vals[buf.String()] = struct{}{}
+
+	})
+}
+
+func (ids *idFactory) Put(value []byte) {
+	ids.vals[util.BytesToReadOnlyString(value)] = struct{}{}
+}