feat: render Markdown, mathematics and Mermaid diagrams server-side

internal/mathml/mathml.go

@@ -0,0 +1,97 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: MIT
+
+// Package mathml converts TeX mathematics to MathML Core with an engine of
+// its own, built on the standard library alone. The grammar covers the
+// standard command surface that maps to MathML: the complete symbol tables,
+// fractions, scripts, operators with movable limits, stretchy delimiters,
+// the amsmath environments and bounded macros.
+//
+// A construct outside the mappable surface is neither dropped nor
+// mistranslated: it stays in the output as its verbatim source inside an
+// merror element, so the author sees exactly what was not understood.
+package mathml
+
+import "strings"
+
+// Render converts the TeX source to a MathML Core math element. The block
+// form sets display="block". The same source always produces
+// byte-identical output.
+func Render(source []byte, display bool) []byte {
+	p := newParser(source, display)
+	body := p.parseAll()
+	var b strings.Builder
+	b.WriteString(`<math xmlns="http://www.w3.org/1998/Math/MathML"`)
+	if display {
+		b.WriteString(` display="block"`)
+	}
+	b.WriteString(">")
+	writeNodes(&b, body)
+	b.WriteString("</math>")
+	return []byte(b.String())
+}
+
+// node is one element of the MathML tree. Attributes keep the order the
+// parser gave them, which keeps the output deterministic.
+type node struct {
+	kind     string
+	text     string
+	attrs    []attribute
+	children []*node
+}
+
+type attribute struct {
+	key   string
+	value string
+}
+
+func el(kind string, children ...*node) *node {
+	return &node{kind: kind, children: children}
+}
+
+// elA builds an element that carries attributes.
+func elA(kind string, attrs []attribute, children ...*node) *node {
+	return &node{kind: kind, attrs: attrs, children: children}
+}
+
+func text(kind, text string, attrs ...attribute) *node {
+	return &node{kind: kind, text: text, attrs: attrs}
+}
+
+// errorNode degrades a span of source to its verbatim text in a marked
+// element.
+func errorNode(source string) *node {
+	return &node{kind: "merror", children: []*node{{kind: "mtext", text: source}}}
+}
+
+// writeNodes renders nodes without indentation; the output is one line,
+// which keeps it a single inline unit in the surrounding HTML.
+func writeNodes(b *strings.Builder, nodes []*node) {
+	for _, n := range nodes {
+		writeNode(b, n)
+	}
+}
+
+func writeNode(b *strings.Builder, n *node) {
+	b.WriteString("<")
+	b.WriteString(n.kind)
+	for _, a := range n.attrs {
+		b.WriteString(" ")
+		b.WriteString(a.key)
+		b.WriteString(`="`)
+		escapeXML(b, a.value)
+		b.WriteString(`"`)
+	}
+	b.WriteString(">")
+	escapeXML(b, n.text)
+	writeNodes(b, n.children)
+	b.WriteString("</")
+	b.WriteString(n.kind)
+	b.WriteString(">")
+}
+
+var xmlEscaper = strings.NewReplacer("&", "&amp;", "<", "&lt;", ">", "&gt;", `"`, "&quot;")
+
+func escapeXML(b *strings.Builder, s string) {
+	xmlEscaper.WriteString(b, s)
+}