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

internal/diagram/diagram.go

@@ -0,0 +1,65 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: MIT
+
+// Package diagram renders Mermaid diagrams to SVG with an engine of its
+// own, built on the standard library alone. The flowchart grammar,
+// including the historical "graph" spelling, and the sequenceDiagram
+// grammar are covered; every other diagram type is refused with an error
+// naming it.
+//
+// The output is deterministic: the same source always renders
+// byte-identical SVG, because every layout decision falls back to the
+// order of appearance and never to map iteration.
+package diagram
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Render renders the Mermaid diagram source to SVG. Statements are lines;
+// blank lines and lines opening with %% are comments. A diagram type the
+// engine does not carry is refused with an error naming the type, and a
+// statement the grammar does not know is refused with an error naming the
+// line.
+func Render(source []byte) ([]byte, error) {
+	lines := statementLines(source)
+	if len(lines) == 0 {
+		return nil, fmt.Errorf("diagram: empty source")
+	}
+	header := strings.Fields(lines[0])
+	switch strings.ToLower(header[0]) {
+	case "flowchart", "graph":
+		dir := "TB"
+		if len(header) > 1 {
+			dir = strings.ToUpper(header[1])
+			if !flowDirections[dir] {
+				return nil, fmt.Errorf("diagram: unknown flow direction %q", header[1])
+			}
+		}
+		return renderFlowchart(lines[1:], dir)
+	case "sequencediagram":
+		auto := len(header) > 1 && strings.ToLower(header[1]) == "autonumber"
+		return renderSequence(lines[1:], auto)
+	default:
+		return nil, fmt.Errorf("diagram: unsupported diagram type %q", header[0])
+	}
+}
+
+var flowDirections = map[string]bool{
+	"TB": true, "TD": true, "BT": true, "LR": true, "RL": true,
+}
+
+// statementLines splits the source into statements: one statement per
+// line, comments and blank lines dropped.
+func statementLines(source []byte) []string {
+	var lines []string
+	for raw := range strings.SplitSeq(string(source), "\n") {
+		line := strings.TrimSpace(raw)
+		if line == "" || strings.HasPrefix(line, "%%") {
+			continue
+		}
+		lines = append(lines, line)
+	}
+	return lines
+}