1 files changed, 188 insertions, 0 deletions
diff --git a/src/go/doc/comment/markdown.go b/src/go/doc/comment/markdown.go
new file mode 100644
index 0000000..d8550f2
--- /dev/null
+++ b/src/go/doc/comment/markdown.go
@@ -0,0 +1,188 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package comment
+
+import (
+	"bytes"
+	"fmt"
+	"strings"
+)
+
+// An mdPrinter holds the state needed for printing a Doc as Markdown.
+type mdPrinter struct {
+	*Printer
+	headingPrefix string
+	raw           bytes.Buffer
+}
+
+// Markdown returns a Markdown formatting of the Doc.
+// See the [Printer] documentation for ways to customize the Markdown output.
+func (p *Printer) Markdown(d *Doc) []byte {
+	mp := &mdPrinter{
+		Printer:       p,
+		headingPrefix: strings.Repeat("#", p.headingLevel()) + " ",
+	}
+
+	var out bytes.Buffer
+	for i, x := range d.Content {
+		if i > 0 {
+			out.WriteByte('\n')
+		}
+		mp.block(&out, x)
+	}
+	return out.Bytes()
+}
+
+// block prints the block x to out.
+func (p *mdPrinter) block(out *bytes.Buffer, x Block) {
+	switch x := x.(type) {
+	default:
+		fmt.Fprintf(out, "?%T", x)
+
+	case *Paragraph:
+		p.text(out, x.Text)
+		out.WriteString("\n")
+
+	case *Heading:
+		out.WriteString(p.headingPrefix)
+		p.text(out, x.Text)
+		if id := p.headingID(x); id != "" {
+			out.WriteString(" {#")
+			out.WriteString(id)
+			out.WriteString("}")
+		}
+		out.WriteString("\n")
+
+	case *Code:
+		md := x.Text
+		for md != "" {
+			var line string
+			line, md, _ = strings.Cut(md, "\n")
+			if line != "" {
+				out.WriteString("\t")
+				out.WriteString(line)
+			}
+			out.WriteString("\n")
+		}
+
+	case *List:
+		loose := x.BlankBetween()
+		for i, item := range x.Items {
+			if i > 0 && loose {
+				out.WriteString("\n")
+			}
+			if n := item.Number; n != "" {
+				out.WriteString(" ")
+				out.WriteString(n)
+				out.WriteString(". ")
+			} else {
+				out.WriteString("  - ") // SP SP - SP
+			}
+			for i, blk := range item.Content {
+				const fourSpace = "    "
+				if i > 0 {
+					out.WriteString("\n" + fourSpace)
+				}
+				p.text(out, blk.(*Paragraph).Text)
+				out.WriteString("\n")
+			}
+		}
+	}
+}
+
+// text prints the text sequence x to out.
+func (p *mdPrinter) text(out *bytes.Buffer, x []Text) {
+	p.raw.Reset()
+	p.rawText(&p.raw, x)
+	line := bytes.TrimSpace(p.raw.Bytes())
+	if len(line) == 0 {
+		return
+	}
+	switch line[0] {
+	case '+', '-', '*', '#':
+		// Escape what would be the start of an unordered list or heading.
+		out.WriteByte('\\')
+	case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
+		i := 1
+		for i < len(line) && '0' <= line[i] && line[i] <= '9' {
+			i++
+		}
+		if i < len(line) && (line[i] == '.' || line[i] == ')') {
+			// Escape what would be the start of an ordered list.
+			out.Write(line[:i])
+			out.WriteByte('\\')
+			line = line[i:]
+		}
+	}
+	out.Write(line)
+}
+
+// rawText prints the text sequence x to out,
+// without worrying about escaping characters
+// that have special meaning at the start of a Markdown line.
+func (p *mdPrinter) rawText(out *bytes.Buffer, x []Text) {
+	for _, t := range x {
+		switch t := t.(type) {
+		case Plain:
+			p.escape(out, string(t))
+		case Italic:
+			out.WriteString("*")
+			p.escape(out, string(t))
+			out.WriteString("*")
+		case *Link:
+			out.WriteString("[")
+			p.rawText(out, t.Text)
+			out.WriteString("](")
+			out.WriteString(t.URL)
+			out.WriteString(")")
+		case *DocLink:
+			url := p.docLinkURL(t)
+			if url != "" {
+				out.WriteString("[")
+			}
+			p.rawText(out, t.Text)
+			if url != "" {
+				out.WriteString("](")
+				url = strings.ReplaceAll(url, "(", "%28")
+				url = strings.ReplaceAll(url, ")", "%29")
+				out.WriteString(url)
+				out.WriteString(")")
+			}
+		}
+	}
+}
+
+// escape prints s to out as plain text,
+// escaping special characters to avoid being misinterpreted
+// as Markdown markup sequences.
+func (p *mdPrinter) escape(out *bytes.Buffer, s string) {
+	start := 0
+	for i := 0; i < len(s); i++ {
+		switch s[i] {
+		case '\n':
+			// Turn all \n into spaces, for a few reasons:
+			//   - Avoid introducing paragraph breaks accidentally.
+			//   - Avoid the need to reindent after the newline.
+			//   - Avoid problems with Markdown renderers treating
+			//     every mid-paragraph newline as a <br>.
+			out.WriteString(s[start:i])
+			out.WriteByte(' ')
+			start = i + 1
+			continue
+		case '`', '_', '*', '[', '<', '\\':
+			// Not all of these need to be escaped all the time,
+			// but is valid and easy to do so.
+			// We assume the Markdown is being passed to a
+			// Markdown renderer, not edited by a person,
+			// so it's fine to have escapes that are not strictly
+			// necessary in some cases.
+			out.WriteString(s[start:i])
+			out.WriteByte('\\')
+			out.WriteByte(s[i])
+			start = i + 1
+		}
+	}
+	out.WriteString(s[start:])
+}