go/doc/comment: add Printer and basic comment printing

[This CL is part of a sequence implementing the proposal #51082.
The design doc is at https://go.dev/s/godocfmt-design.]

Implement printing of plain text doc paragraphs.

For #51082.


Change-Id: Ieff0af64a900f566bfc833c3b5706488f1444798
Reviewed-on: https://go-review.googlesource.com/c/go/+/397279
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

api/next/51082.txt

@@ -1,6 +1,10 @@
 pkg go/doc/comment, method (*List) BlankBefore() bool #51082
 pkg go/doc/comment, method (*List) BlankBetween() bool #51082
 pkg go/doc/comment, method (*Parser) Parse(string) *Doc #51082
+pkg go/doc/comment, method (*Printer) Comment(*Doc) []uint8 #51082
+pkg go/doc/comment, method (*Printer) HTML(*Doc) []uint8 #51082
+pkg go/doc/comment, method (*Printer) Markdown(*Doc) []uint8 #51082
+pkg go/doc/comment, method (*Printer) Text(*Doc) []uint8 #51082
 pkg go/doc/comment, type Block interface, unexported methods #51082
 pkg go/doc/comment, type Code struct #51082
 pkg go/doc/comment, type Code struct, Text string #51082
@@ -37,4 +41,12 @@ pkg go/doc/comment, type Parser struct, LookupPackage func(string) (string, bool
 pkg go/doc/comment, type Parser struct, LookupSym func(string, string) bool #51082
 pkg go/doc/comment, type Parser struct, Words map[string]string #51082
 pkg go/doc/comment, type Plain string #51082
+pkg go/doc/comment, type Printer struct #51082
+pkg go/doc/comment, type Printer struct, DocLinkBaseURL string #51082
+pkg go/doc/comment, type Printer struct, DocLinkURL func(*DocLink) string #51082
+pkg go/doc/comment, type Printer struct, HeadingID func(*Heading) string #51082
+pkg go/doc/comment, type Printer struct, HeadingLevel int #51082
+pkg go/doc/comment, type Printer struct, TextCodePrefix string #51082
+pkg go/doc/comment, type Printer struct, TextPrefix string #51082
+pkg go/doc/comment, type Printer struct, TextWidth int #51082
 pkg go/doc/comment, type Text interface, unexported methods #51082

src/go/doc/comment/html.go

@@ -0,0 +1,81 @@
+// 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"
+)
+
+// An htmlPrinter holds the state needed for printing a Doc as HTML.
+type htmlPrinter struct {
+	*Printer
+}
+
+// HTML returns an HTML formatting of the Doc.
+// See the [Printer] documentation for ways to customize the HTML output.
+func (p *Printer) HTML(d *Doc) []byte {
+	hp := &htmlPrinter{Printer: p}
+	var out bytes.Buffer
+	for _, x := range d.Content {
+		hp.block(&out, x)
+	}
+	return out.Bytes()
+}
+
+// block prints the block x to out.
+func (p *htmlPrinter) block(out *bytes.Buffer, x Block) {
+	switch x := x.(type) {
+	default:
+		fmt.Fprintf(out, "?%T", x)
+
+	case *Paragraph:
+		out.WriteString("<p>")
+		p.text(out, x.Text)
+		out.WriteString("\n")
+	}
+}
+
+// text prints the text sequence x to out.
+func (p *htmlPrinter) text(out *bytes.Buffer, x []Text) {
+	for _, t := range x {
+		switch t := t.(type) {
+		case Plain:
+			p.escape(out, string(t))
+		}
+	}
+}
+
+// escape prints s to out as plain text,
+// escaping < & " ' and > to avoid being misinterpreted
+// in larger HTML constructs.
+func (p *htmlPrinter) escape(out *bytes.Buffer, s string) {
+	start := 0
+	for i := 0; i < len(s); i++ {
+		switch s[i] {
+		case '<':
+			out.WriteString(s[start:i])
+			out.WriteString("&lt;")
+			start = i + 1
+		case '&':
+			out.WriteString(s[start:i])
+			out.WriteString("&amp;")
+			start = i + 1
+		case '"':
+			out.WriteString(s[start:i])
+			out.WriteString("&quot;")
+			start = i + 1
+		case '\'':
+			out.WriteString(s[start:i])
+			out.WriteString("&apos;")
+			start = i + 1
+		case '>':
+			out.WriteString(s[start:i])
+			out.WriteString("&gt;")
+			start = i + 1
+		}
+	}
+	out.WriteString(s[start:])
+}

src/go/doc/comment/markdown.go

@@ -0,0 +1,115 @@
+// 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"
+)
+
+// An mdPrinter holds the state needed for printing a Doc as Markdown.
+type mdPrinter struct {
+	*Printer
+	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}
+
+	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")
+	}
+}
+
+// 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))
+		}
+	}
+}
+
+// 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:])
+}

src/go/doc/comment/print.go

@@ -0,0 +1,122 @@
+// 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"
+)
+
+// A Printer is a doc comment printer.
+// The fields in the struct can be filled in before calling
+// any of the printing methods
+// in order to customize the details of the printing process.
+type Printer struct {
+	// HeadingLevel is the nesting level used for
+	// HTML and Markdown headings.
+	// If HeadingLevel is zero, it defaults to level 3,
+	// meaning to use <h3> and ###.
+	HeadingLevel int
+
+	// HeadingID is a function that computes the heading ID
+	// (anchor tag) to use for the heading h when generating
+	// HTML and Markdown. If HeadingID returns an empty string,
+	// then the heading ID is omitted.
+	// If HeadingID is nil, h.DefaultID is used.
+	HeadingID func(h *Heading) string
+
+	// DocLinkURL is a function that computes the URL for the given DocLink.
+	// If DocLinkURL is nil, then link.DefaultURL(p.DocLinkBaseURL) is used.
+	DocLinkURL func(link *DocLink) string
+
+	// DocLinkBaseURL is used when DocLinkURL is nil,
+	// passed to [DocLink.DefaultURL] to construct a DocLink's URL.
+	// See that method's documentation for details.
+	DocLinkBaseURL string
+
+	// TextPrefix is a prefix to print at the start of every line
+	// when generating text output using the Text method.
+	TextPrefix string
+
+	// TextCodePrefix is the prefix to print at the start of each
+	// preformatted (code block) line when generating text output,
+	// instead of (not in addition to) TextPrefix.
+	// If TextCodePrefix is the empty string, it defaults to TextPrefix+"\t".
+	TextCodePrefix string
+
+	// TextWidth is the maximum width text line to generate,
+	// measured in Unicode code points,
+	// excluding TextPrefix and the newline character.
+	// If TextWidth is zero, it defaults to 80 minus the number of code points in TextPrefix.
+	// If TextWidth is negative, there is no limit.
+	TextWidth int
+}
+
+type commentPrinter struct {
+	*Printer
+	headingPrefix string
+	needDoc       map[string]bool
+}
+
+// Comment returns the standard Go formatting of the Doc,
+// without any comment markers.
+func (p *Printer) Comment(d *Doc) []byte {
+	cp := &commentPrinter{Printer: p}
+	var out bytes.Buffer
+	for i, x := range d.Content {
+		if i > 0 && blankBefore(x) {
+			out.WriteString("\n")
+		}
+		cp.block(&out, x)
+	}
+
+	return out.Bytes()
+}
+
+// blankBefore reports whether the block x requires a blank line before it.
+// All blocks do, except for Lists that return false from x.BlankBefore().
+func blankBefore(x Block) bool {
+	if x, ok := x.(*List); ok {
+		return x.BlankBefore()
+	}
+	return true
+}
+
+// block prints the block x to out.
+func (p *commentPrinter) 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")
+	}
+}
+
+// text prints the text sequence x to out.
+func (p *commentPrinter) text(out *bytes.Buffer, indent string, x []Text) {
+	for _, t := range x {
+		switch t := t.(type) {
+		case Plain:
+			p.indent(out, indent, string(t))
+		}
+	}
+}
+
+// indent prints s to out, indenting with the indent string
+// after each newline in s.
+func (p *commentPrinter) indent(out *bytes.Buffer, indent, s string) {
+	for s != "" {
+		line, rest, ok := strings.Cut(s, "\n")
+		out.WriteString(line)
+		if ok {
+			out.WriteString("\n")
+			out.WriteString(indent)
+		}
+		s = rest
+	}
+}

src/go/doc/comment/testdata/blank.txt

@@ -0,0 +1,12 @@
+-- input --
+	$
+	Blank line at start and end.
+	$
+-- gofmt --
+Blank line at start and end.
+-- text --
+Blank line at start and end.
+-- markdown --
+Blank line at start and end.
+-- html --
+<p>Blank line at start and end.