markup/goldmark: support latex fences $ and $$

Patched and updated from
janhuenermann@ad38246

markup/goldmark/convert.go

@@ -23,6 +23,7 @@ import (
 	"github.com/gohugoio/hugo/markup/goldmark/goldmark_config"
 	"github.com/gohugoio/hugo/markup/goldmark/images"
 	"github.com/gohugoio/hugo/markup/goldmark/internal/extensions/attributes"
+	"github.com/gohugoio/hugo/markup/goldmark/internal/extensions/latex"
 	"github.com/gohugoio/hugo/markup/goldmark/internal/render"
 
 	"github.com/gohugoio/hugo/markup/converter"
@@ -154,6 +155,10 @@ func newMarkdown(pcfg converter.ProviderConfig) goldmark.Markdown {
 		extensions = append(extensions, c)
 	}
 
+	if cfg.Extensions.Latex {
+		extensions = append(extensions, &latex.LatexAsPlainTextExtension{})
+	}
+
 	if pcfg.Conf.EnableEmoji() {
 		extensions = append(extensions, emoji.Emoji)
 	}

markup/goldmark/goldmark_config/config.go

@@ -38,6 +38,7 @@ var Default = Config{
 		},
 		Footnote:        true,
 		DefinitionList:  true,
+		Latex:           false,
 		Table:           true,
 		Strikethrough:   true,
 		Linkify:         true,
@@ -75,6 +76,7 @@ type Extensions struct {
 	Typographer    Typographer
 	Footnote       bool
 	DefinitionList bool
+	Latex          bool
 
 	// GitHub flavored markdown
 	Table           bool

markup/goldmark/internal/extensions/latex/latex.go

@@ -0,0 +1,107 @@
+package latex
+
+import (
+	"github.com/yuin/goldmark"
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/parser"
+	"github.com/yuin/goldmark/text"
+	"github.com/yuin/goldmark/util"
+)
+
+const LatexTrigger = byte('$')
+
+type LatexParser struct {
+}
+
+func (s *LatexParser) Trigger() []byte {
+	return []byte{LatexTrigger}
+}
+
+func (s *LatexParser) Parse(parent ast.Node, block text.Reader, pc parser.Context) ast.Node {
+	// Finds any LaTeX inlines/blocks and returns an AST text node, ensuring
+	// that other parsers don't pick up the LaTeX syntax.
+	// Source: https://github.com/FurqanSoftware/goldmark-katex/blob/0bf781ec55b4dcac337505863ecccf093e4ee967/parser.go
+
+	buf := block.Source()
+	ln, pos := block.Position()
+
+	lstart := pos.Start
+	lend := pos.Stop
+	line := buf[lstart:lend]
+
+	var start, end, advance int
+
+	if len(line) > 1 && line[1] == LatexTrigger {
+		// Block LaTeX
+		start = lstart + 2
+		offset := 2
+
+	L:
+		for x := 0; x < 5; x++ {
+			for j := offset; j < len(line); j++ {
+				if len(line) > j+1 && line[j] == LatexTrigger && line[j+1] == LatexTrigger {
+					end = lstart + j
+					advance = 2
+					break L
+				}
+			}
+			if lend == len(buf) {
+				break
+			}
+			if end == 0 {
+				rest := buf[lend:]
+				j := 1
+				for j < len(rest) && rest[j] != '\n' {
+					j++
+				}
+				lstart = lend
+				lend += j
+				line = buf[lstart:lend]
+				ln++
+				offset = 0
+			}
+		}
+
+	} else {
+		// Inline LaTeX
+		start = lstart + 1
+
+		for i := 1; i < len(line); i++ {
+			c := line[i]
+			if c == '\\' {
+				i++
+				continue
+			}
+			if c == LatexTrigger {
+				end = lstart + i
+				advance = 1
+				break
+			}
+		}
+		if end >= len(buf) || buf[end] != LatexTrigger {
+			return nil
+		}
+	}
+
+	if start >= end {
+		return nil
+	}
+
+	newpos := end + advance
+	if newpos < lend {
+		block.SetPosition(ln, text.NewSegment(newpos, lend))
+	} else {
+		block.Advance(newpos)
+	}
+
+	return ast.NewTextSegment(text.NewSegment(lstart, newpos))
+}
+
+type LatexAsPlainTextExtension struct {
+}
+
+func (e *LatexAsPlainTextExtension) Extend(m goldmark.Markdown) {
+	m.Parser().AddOptions(parser.WithInlineParsers(
+		util.Prioritized(&LatexParser{}, 0),
+	))
+}