internal/web/markdown.go

// Copyright 2020 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 web

import (
	"bytes"
	"unicode/utf8"

	"github.com/yuin/goldmark"
	"github.com/yuin/goldmark/parser"
	"github.com/yuin/goldmark/renderer/html"
)

// renderMarkdown converts a limited and opinionated flavor of Markdown (compliant with
// CommonMark 0.29) to HTML for the purposes of Go websites.
//
// The Markdown source may contain raw HTML,
// but Go templates have already been processed.
func renderMarkdown(src []byte) ([]byte, error) {
	src = replaceTabs(src)
	// parser.WithHeadingAttribute allows custom ids on headings.
	// html.WithUnsafe allows use of raw HTML, which we need for tables.
	md := goldmark.New(
		goldmark.WithParserOptions(parser.WithHeadingAttribute()),
		goldmark.WithRendererOptions(html.WithUnsafe()))
	var buf bytes.Buffer
	if err := md.Convert(src, &buf); err != nil {
		return nil, err
	}
	return buf.Bytes(), nil
}

// replaceTabs replaces all tabs in text with spaces up to a 4-space tab stop.
//
// In Markdown, tabs used for indentation are required to be interpreted as
// 4-space tab stops. See https://spec.commonmark.org/0.30/#tabs.
// Go also renders nicely and more compactly on the screen with 4-space
// tab stops, while browsers often use 8-space.
// And Goldmark crashes in some inputs that mix spaces and tabs.
// Fix the crashes and make the Go code consistently compact across browsers,
// all while staying Markdown-compatible, by expanding to 4-space tab stops.
//
// This function does not handle multi-codepoint Unicode sequences correctly.
func replaceTabs(text []byte) []byte {
	var buf bytes.Buffer
	col := 0
	for len(text) > 0 {
		r, size := utf8.DecodeRune(text)
		text = text[size:]

		switch r {
		case '\n':
			buf.WriteByte('\n')
			col = 0

		case '\t':
			buf.WriteByte(' ')
			col++
			for col%4 != 0 {
				buf.WriteByte(' ')
				col++
			}

		default:
			buf.WriteRune(r)
			col++
		}
	}
	return buf.Bytes()
}