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renderer.go
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renderer.go
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// This file is part of gmnhg.
// gmnhg is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
// gmnhg is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
// You should have received a copy of the GNU General Public License
// along with gmnhg. If not, see <https://www.gnu.org/licenses/>.
// Package renderer contains an implementation of markdown => text/gemini
// renderer for github.com/gomarkdown/markdown.
package renderer
import (
"bytes"
"fmt"
"html"
"io"
"regexp"
"github.com/gomarkdown/markdown/ast"
"github.com/grokify/html-strip-tags-go"
"github.com/olekukonko/tablewriter"
)
var (
lineBreak = []byte{'\n'}
space = []byte{' '}
linkPrefix = []byte("=> ")
quoteBrPrefix = []byte("\n> ")
quotePrefix = []byte("> ")
itemPrefix = []byte("* ")
itemIndent = []byte{'\t'}
preformattedToggle = []byte("```")
codeDelimiter = []byte("`")
emphDelimiter = []byte("*")
strongDelimiter = []byte("**")
delDelimiter = []byte("~~")
horizontalRule = []byte("---")
subOpen = []byte("_{")
subClose = []byte("}")
supOpen = []byte("^(")
supClose = []byte(")")
)
// matches a FULL string that contains no non-whitespace characters
var emptyLineRegex = regexp.MustCompile(`\A[\s]*\z`)
// fairly tolerant to handle weird HTML
var tagPairRegexString = `<[\n\f ]*%s([\n\f ]+[^\n\f \/>"'=]+[\n\f ]*(=[\n\f ]*([a-zA-Z1-9\-]+|"[^\n\f"]+"|'[^\n\f']+'))?)*[\n\f ]*>.*?<[\n\f ]*/[\n\f ]*%s[\n\f ]*>`
// HTML block tags whose contents should not be rendered
var htmlNoRenderRegex = []*regexp.Regexp{
regexp.MustCompile(fmt.Sprintf(tagPairRegexString, "fieldset", "fieldset")),
regexp.MustCompile(fmt.Sprintf(tagPairRegexString, "form", "form")),
regexp.MustCompile(fmt.Sprintf(tagPairRegexString, "iframe", "iframe")),
regexp.MustCompile(fmt.Sprintf(tagPairRegexString, "script", "script")),
regexp.MustCompile(fmt.Sprintf(tagPairRegexString, "style", "style")),
regexp.MustCompile(fmt.Sprintf(tagPairRegexString, "canvas", "canvas")),
regexp.MustCompile(fmt.Sprintf(tagPairRegexString, "dialog", "dialog")),
regexp.MustCompile(fmt.Sprintf(tagPairRegexString, "progress", "progress")),
}
var lineBreakCharacters = regexp.MustCompile(`[\n\r]+`)
var hardBreakTag = regexp.MustCompile(`< *br */? *>`)
var escapedHtmlChar = regexp.MustCompile(`(?:^|[^\\\\])&[[:alnum:]]+;`)
// Renderer implements markdown.Renderer.
type Renderer struct{}
// NewRenderer returns a new Renderer.
func NewRenderer() Renderer {
return Renderer{}
}
func getNodeDelimiter(node ast.Node) []byte {
switch node.(type) {
case *ast.Code:
return codeDelimiter
case *ast.Emph:
return emphDelimiter
case *ast.Strong:
return strongDelimiter
case *ast.Del:
return delDelimiter
default:
return []byte{}
}
}
func (r Renderer) link(w io.Writer, node *ast.Link, entering bool) {
if entering {
if node.Footnote != nil {
fmt.Fprintf(w, "[^%d]: %s", node.NoteID, extractText(node.Footnote))
} else {
w.Write(linkPrefix)
w.Write(node.Destination)
w.Write(space)
r.text(w, node, true)
}
}
}
func (r Renderer) image(w io.Writer, node *ast.Image, entering bool) {
if entering {
w.Write(linkPrefix)
w.Write(node.Destination)
w.Write(space)
r.text(w, node, true)
}
}
func (r Renderer) blockquote(w io.Writer, node *ast.BlockQuote, entering bool) {
if entering {
if node := node.AsContainer(); node != nil {
for _, child := range node.Children {
w.Write(quotePrefix)
r.blockquoteText(w, child)
// double linebreak to ensure Gemini clients don't merge
// quotes; gomarkdown assumes separate blockquotes are
// paragraphs of the same blockquote while we don't
w.Write(lineBreak)
w.Write(lineBreak)
}
}
}
}
func (r Renderer) hr(w io.Writer, node *ast.HorizontalRule, entering bool) {
if entering {
w.Write(horizontalRule)
w.Write(lineBreak)
w.Write(lineBreak)
}
}
// Based on https://pages.uoregon.edu/ncp/Courses/MathInPlainTextEmail.html
func (r Renderer) subscript(w io.Writer, node *ast.Subscript, entering bool) {
if entering {
if node := node.AsLeaf(); node != nil {
w.Write(subOpen)
w.Write(bytes.ReplaceAll(node.Literal, lineBreak, space))
w.Write(subClose)
}
}
}
func (r Renderer) superscript(w io.Writer, node *ast.Superscript, entering bool) {
if entering {
if node := node.AsLeaf(); node != nil {
w.Write(supOpen)
w.Write(bytes.ReplaceAll(node.Literal, lineBreak, space))
w.Write(supClose)
}
}
}
func (r Renderer) heading(w io.Writer, node *ast.Heading, entering bool) {
if entering {
// pad headings with the relevant number of #-s; Gemini spec
// used to allow 3 at maximum before a space
bufLength := node.Level + 1
heading := make([]byte, bufLength)
heading[len(heading)-1] = ' '
for i := 0; i < len(heading)-1; i++ {
heading[i] = '#'
}
w.Write(heading)
r.text(w, node, true)
} else {
w.Write(lineBreak)
}
}
func extractLinks(node ast.Node) (stack []ast.Node) {
if node := node.AsContainer(); node != nil {
for _, subnode := range node.Children {
stack = append(stack, extractLinks(subnode)...)
}
}
switch node := node.(type) {
case *ast.Image:
stack = append(stack, node)
case *ast.Link:
stack = append(stack, node)
// footnotes are represented as links which embed an extra node
// containing footnote text; the link itself is not considered a
// container
if node.Footnote != nil {
stack = append(stack, extractLinks(node.Footnote)...)
}
}
return stack
}
func (r Renderer) renderLinks(w io.Writer, links []ast.Node) (count uint) {
for _, link := range links {
if link, ok := link.(*ast.Link); ok && link.Footnote == nil {
r.link(w, link, true)
w.Write(lineBreak)
count++
}
}
return
}
func (r Renderer) renderFootnotes(w io.Writer, links []ast.Node) (count uint) {
for _, link := range links {
if link, ok := link.(*ast.Link); ok && link.Footnote != nil {
r.link(w, link, true)
w.Write(lineBreak)
count++
}
}
return
}
func (r Renderer) renderImages(w io.Writer, links []ast.Node) (count uint) {
for _, link := range links {
if link, ok := link.(*ast.Image); ok {
r.image(w, link, true)
w.Write(lineBreak)
count++
}
}
return
}
func (r Renderer) linksList(w io.Writer, links []ast.Node) {
for _, renderer := range []func(Renderer, io.Writer, []ast.Node) uint{
Renderer.renderFootnotes,
Renderer.renderImages,
Renderer.renderLinks,
} {
linksRendered := renderer(r, w, links)
// ensure breaks between link blocks of the same type
if linksRendered > 0 {
w.Write(lineBreak)
}
}
}
func isLinksOnlyParagraph(node *ast.Paragraph) bool {
for _, child := range node.Children {
switch child := child.(type) {
case *ast.Text:
if emptyLineRegex.Find(child.Literal) != nil {
continue
}
case *ast.Link, *ast.Image:
continue
}
return false
}
return true
}
func isLinksOnlyList(node *ast.List) bool {
for _, child := range node.Children {
child, ok := child.(*ast.ListItem)
if !ok {
return false // should never happen
}
for _, liChild := range child.Children {
liChild, ok := liChild.(*ast.Paragraph)
if !ok {
return false // sublist, etc
}
if !isLinksOnlyParagraph(liChild) {
return false
}
}
}
return true
}
func (r Renderer) paragraph(w io.Writer, node *ast.Paragraph, entering bool) (noNewLine bool) {
linksOnly := isLinksOnlyParagraph(node)
noNewLine = linksOnly
if entering {
children := node.Children
// current version of gomarkdown/markdown finds an empty
// *ast.Text element before links/images, breaking the heuristic
if len(children) >= 2 {
firstChild, elementIsText := children[0].(*ast.Text)
if elementIsText && len(firstChild.Literal) == 0 {
children = children[1:]
}
}
if !linksOnly {
for _, child := range children {
// only render links text in the paragraph if they're
// combined with some other text on page
switch child := child.(type) {
case *ast.Text, *ast.Emph, *ast.Strong, *ast.Del, *ast.Link, *ast.Image:
r.text(w, child, true)
case *ast.Code:
r.text(w, child, false)
case *ast.Hardbreak:
w.Write(lineBreak)
case *ast.HTMLSpan:
if hardBreakTag.Match(child.AsLeaf().Literal) {
w.Write(lineBreak)
}
case *ast.Subscript:
r.subscript(w, child, true)
case *ast.Superscript:
r.superscript(w, child, true)
}
}
w.Write(lineBreak)
}
}
return
}
func (r Renderer) code(w io.Writer, node *ast.CodeBlock) {
w.Write(preformattedToggle)
if node.IsFenced {
w.Write(node.Info)
}
w.Write(lineBreak)
w.Write(node.Literal)
w.Write(preformattedToggle)
w.Write(lineBreak)
}
func (r Renderer) list(w io.Writer, node *ast.List, level int) {
// the text/gemini spec included with the current Gemini spec does
// not specify anything about the formatting of lists of level >= 2,
// as of now this will just render them like in Markdown
isNumbered := (node.ListFlags & ast.ListTypeOrdered) != 0
for number, item := range node.Children {
item, ok := item.(*ast.ListItem)
if !ok {
panic("rendering anything but list items is not supported")
}
isTerm := (item.ListFlags & ast.ListTypeTerm) == ast.ListTypeTerm
if l := len(item.Children); l >= 1 {
// add extra line break to split up definitions
if isTerm && number > 0 {
w.Write(lineBreak)
}
for i := 0; i < level; i++ {
w.Write(itemIndent)
}
if isNumbered {
w.Write([]byte(fmt.Sprintf("%d. ", number+1)))
} else if !isTerm {
w.Write(itemPrefix)
}
r.text(w, item, true)
w.Write(lineBreak)
if l >= 2 {
if list, ok := item.Children[1].(*ast.List); ok {
r.list(w, list, level+1)
}
}
}
}
}
func textWithNewlineReplacement(node ast.Node, replacement []byte, unescapeHtml bool) []byte {
buf := bytes.Buffer{}
delimiter := getNodeDelimiter(node)
// special case for footnotes: we want them in the text
if node, ok := node.(*ast.Link); ok && node.Footnote != nil {
fmt.Fprintf(&buf, "[^%d]", node.NoteID)
}
if leaf := node.AsLeaf(); leaf != nil {
// replace all newlines in text with preferred symbols; this may
// be spaces for general text, allowing for soft wrapping, which
// is recommended as per Gemini spec p. 5.4.1, or line breaks
// with a blockquote symbols for blockquotes, or just nothing
buf.Write(delimiter)
switch node := node.(type) {
case *ast.Hardbreak:
buf.Write(lineBreak)
// If the blockquote ends with a double space, the parser will
// not create a Hardbreak at the end, so this works.
if _, ok := leaf.Parent.(*ast.BlockQuote); !ok {
buf.Write(quotePrefix)
}
case *ast.HTMLSpan:
if hardBreakTag.Match(leaf.Literal) {
buf.Write(lineBreak)
}
buf.Write(leaf.Content)
case *ast.HTMLBlock:
buf.Write([]byte(extractHtml(node, quotePrefix)))
default:
textWithoutBreaks := lineBreakCharacters.ReplaceAll(leaf.Literal, replacement)
if unescapeHtml {
unescapedText := escapedHtmlChar.ReplaceAll(textWithoutBreaks, []byte(html.UnescapeString(string(textWithoutBreaks))))
buf.Write(unescapedText)
} else {
buf.Write(textWithoutBreaks)
}
}
buf.Write(delimiter)
}
if node := node.AsContainer(); node != nil {
buf.Write(delimiter)
for _, child := range node.Children {
// skip non-text child elements from rendering
switch child := child.(type) {
case *ast.List:
default:
buf.Write(textWithNewlineReplacement(child, replacement, unescapeHtml))
}
}
buf.Write(delimiter)
}
return buf.Bytes()
}
func (r Renderer) text(w io.Writer, node ast.Node, unescapeHtml bool) {
w.Write(textWithNewlineReplacement(node, space, unescapeHtml))
}
func (r Renderer) blockquoteText(w io.Writer, node ast.Node) {
w.Write(textWithNewlineReplacement(node, quoteBrPrefix, true))
}
func extractText(node ast.Node) string {
return string(textWithNewlineReplacement(node, space, true))
}
func extractHtml(node *ast.HTMLBlock, linePrefix []byte) string {
// Only render contents of allowed tags
literal := node.Literal
for _, re := range htmlNoRenderRegex {
literal = re.ReplaceAllLiteral(literal, []byte{})
}
if len(literal) > 0 {
literalWithBreaks := hardBreakTag.ReplaceAll(lineBreakCharacters.ReplaceAll(literal, space), append([]byte(lineBreak), linePrefix...))
literalStripped := strip.StripTags(string(literalWithBreaks))
return html.UnescapeString(literalStripped)
}
return ""
}
func (r Renderer) tableHead(t *tablewriter.Table, node *ast.TableHeader) {
if node := node.AsContainer(); node != nil {
// should always have a single row consisting of at least one
// cell but worth checking nonetheless; tablewriter only
// supports a single header row as of now therefore ignore
// second row and the rest
if len(node.Children) > 0 {
if row := node.Children[0].AsContainer(); row != nil {
cells := make([]string, len(row.Children))
for i, cell := range row.Children {
cells[i] = extractText(cell)
}
t.SetHeader(cells)
}
}
}
}
func (r Renderer) tableBody(t *tablewriter.Table, node *ast.TableBody) {
if node := node.AsContainer(); node != nil {
for _, row := range node.Children {
if row := row.AsContainer(); row != nil {
cells := make([]string, len(row.Children))
for i, cell := range row.Children {
cells[i] = extractText(cell)
}
t.Append(cells)
}
}
}
}
func (r Renderer) table(w io.Writer, node *ast.Table, entering bool) {
if entering {
w.Write(preformattedToggle)
w.Write(lineBreak)
// gomarkdown appears to only parse headings consisting of a
// single line and always have a TableBody preceded by a single
// TableHeader but we're better off not relying on it
t := tablewriter.NewWriter(w)
t.SetAutoFormatHeaders(false) // TODO: tablewriter options should probably be configurable
if node := node.AsContainer(); node != nil {
for _, child := range node.Children {
switch child := child.(type) {
case *ast.TableHeader:
r.tableHead(t, child)
case *ast.TableBody:
r.tableBody(t, child)
}
}
}
t.Render()
} else {
w.Write(preformattedToggle)
w.Write(lineBreak)
}
}
func (r Renderer) htmlBlock(w io.Writer, node *ast.HTMLBlock, entering bool) {
if entering {
htmlString := extractHtml(node, []byte{})
if len(htmlString) > 0 {
w.Write([]byte(htmlString))
w.Write(lineBreak)
w.Write(lineBreak)
}
}
}
// RenderNode implements Renderer.RenderNode().
func (r Renderer) RenderNode(w io.Writer, node ast.Node, entering bool) ast.WalkStatus {
// entering in gomarkdown was made to have elements of type switch
// to enclose themselves within the second pass with entering =
// false, as Markdown is quite similar to HTML in its structure.
// As Gemtext is line-oriented, and not tag-oriented, most of
// container subroutines have to handle their subelements on
// themselves.
noNewLine := true
fetchLinks := false
switch node := node.(type) {
case *ast.BlockQuote:
r.blockquote(w, node, entering)
fetchLinks = true
case *ast.HorizontalRule:
r.hr(w, node, entering)
case *ast.Heading:
r.heading(w, node, entering)
noNewLine = false
case *ast.Paragraph:
switch node.Parent.(type) {
// these (should) handle underlying paragraphs themselves
case *ast.BlockQuote, *ast.ListItem, *ast.Footnotes:
default:
noNewLine = r.paragraph(w, node, entering)
fetchLinks = true
}
case *ast.CodeBlock:
r.code(w, node)
// code block is not considered a wrapping element
w.Write(lineBreak)
case *ast.List:
// lists of level >= 2 are rendered recursively along with the
// first level; the list is a container
_, parentIsDocument := node.Parent.(*ast.Document)
// footnotes are rendered as links after the parent paragraph
if !node.IsFootnotesList && parentIsDocument && !entering {
if !isLinksOnlyList(node) {
r.list(w, node, 0)
noNewLine = false
}
fetchLinks = true
}
case *ast.Table:
r.table(w, node, entering)
noNewLine = false
fetchLinks = true
case *ast.HTMLBlock:
// Do not render if already rendered as part of a blockquote
if _, ok := node.Parent.(*ast.BlockQuote); !ok {
r.htmlBlock(w, node, entering)
}
}
if !noNewLine && !entering {
w.Write(lineBreak)
}
if fetchLinks && !entering {
links := extractLinks(node)
if len(links) > 0 {
r.linksList(w, links)
}
}
return ast.GoToNext
}
// RenderHeader implements Renderer.RenderHeader().
func (r Renderer) RenderHeader(w io.Writer, node ast.Node) {}
// RenderFooter implements Renderer.RenderFooter().
func (r Renderer) RenderFooter(w io.Writer, node ast.Node) {}