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

internal/diagram/svg.go

@@ -0,0 +1,145 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: MIT
+
+package diagram
+
+import (
+	"fmt"
+	"strings"
+)
+
+// point is a coordinate in the layout. Everything is an integer, so the
+// same layout always writes the same bytes.
+type point struct {
+	x, y int
+}
+
+// svgBuilder writes SVG elements in the order they are given.
+type svgBuilder struct {
+	b      strings.Builder
+	width  int
+	height int
+}
+
+func newSVGBuilder(width, height int) *svgBuilder {
+	s := &svgBuilder{width: width, height: height}
+	fmt.Fprintf(&s.b, `<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 %d %d" font-family="sans-serif">`, width, height)
+	return s
+}
+
+// rect draws a rectangle, rounded when rx is positive.
+func (s *svgBuilder) rect(x, y, w, h, rx int, attrs string) {
+	if w < 0 || h < 0 {
+		return
+	}
+	if rx > 0 {
+		fmt.Fprintf(&s.b, `<rect x="%d" y="%d" width="%d" height="%d" rx="%d"%s/>`, x, y, w, h, rx, attrs)
+	} else {
+		fmt.Fprintf(&s.b, `<rect x="%d" y="%d" width="%d" height="%d"%s/>`, x, y, w, h, attrs)
+	}
+}
+
+func (s *svgBuilder) line(x1, y1, x2, y2 int, attrs string) {
+	fmt.Fprintf(&s.b, `<line x1="%d" y1="%d" x2="%d" y2="%d"%s/>`, x1, y1, x2, y2, attrs)
+}
+
+func (s *svgBuilder) path(d, attrs string) {
+	s.b.WriteString(`<path d="`)
+	s.b.WriteString(d)
+	s.b.WriteString(`"`)
+	s.b.WriteString(attrs)
+	s.b.WriteString(`/>`)
+}
+
+// text writes one line of text. The anchor is middle, start or end.
+func (s *svgBuilder) text(p point, str, anchor, attrs string) {
+	s.b.WriteString(`<text x="`)
+	fmt.Fprintf(&s.b, `%d`, p.x)
+	s.b.WriteString(`" y="`)
+	fmt.Fprintf(&s.b, `%d`, p.y)
+	if anchor != "" {
+		s.b.WriteString(`" text-anchor="`)
+		s.b.WriteString(anchor)
+	}
+	s.b.WriteString(`"`)
+	s.b.WriteString(attrs)
+	s.b.WriteString(`>`)
+	s.b.WriteString(escapeXML(str))
+	s.b.WriteString(`</text>`)
+}
+
+func (s *svgBuilder) polygon(pts []point, attrs string) {
+	if len(pts) < 3 {
+		return
+	}
+	s.b.WriteString(`<polygon points="`)
+	for i, p := range pts {
+		if i > 0 {
+			s.b.WriteString(" ")
+		}
+		fmt.Fprintf(&s.b, `%d,%d`, p.x, p.y)
+	}
+	s.b.WriteString(`"`)
+	s.b.WriteString(attrs)
+	s.b.WriteString(`/>`)
+}
+
+func (s *svgBuilder) finish() []byte {
+	s.b.WriteString("</svg>")
+	return []byte(s.b.String())
+}
+
+var xmlEscaper = strings.NewReplacer("&", "&amp;", "<", "&lt;", ">", "&gt;", `"`, "&quot;")
+
+func escapeXML(s string) string {
+	return xmlEscaper.Replace(s)
+}
+
+// textWidth estimates the width a string needs at the given font size.
+// Characters outside the basic plane are wide: the estimate stays on the
+// generous side so labels keep their padding.
+func textWidth(s string, size int) int {
+	w := 0
+	for _, r := range s {
+		if r > 0x2E7F {
+			w += size
+		} else {
+			w += (size*6 + 4) / 10
+		}
+	}
+	return w
+}
+
+// labelLines splits a label into its lines.
+func labelLines(s string) []string {
+	if s == "" {
+		return []string{""}
+	}
+	normalised := strings.ReplaceAll(s, "<br/>", "<br>")
+	normalised = strings.ReplaceAll(normalised, "<br />", "<br>")
+	lines := strings.Split(normalised, "<br>")
+	if len(lines) == 0 {
+		return []string{""}
+	}
+	return lines
+}
+
+// styleString joins style pairs into a style attribute, declaration order
+// preserved.
+func styleString(pairs []stylePair) string {
+	if len(pairs) == 0 {
+		return ""
+	}
+	var b strings.Builder
+	b.WriteString(` style="`)
+	for i, p := range pairs {
+		if i > 0 {
+			b.WriteString(";")
+		}
+		b.WriteString(p.key)
+		b.WriteString(":")
+		b.WriteString(p.value)
+	}
+	b.WriteString(`"`)
+	return b.String()
+}