datascape/embed.go

package main

import (
	"bytes"
	"path"
	"strconv"
	"strings"

	"github.com/yuin/goldmark"
	"github.com/yuin/goldmark/ast"
	"github.com/yuin/goldmark/parser"
	"github.com/yuin/goldmark/renderer"
	"github.com/yuin/goldmark/text"
	"github.com/yuin/goldmark/util"
)

// Embeds are ordinary Markdown images: ![caption](photo.jpg "left").
//
// Destinations are relative to the page they sit on. Pages always render at a
// URL ending in "/" (ServeHTTP redirects the slashless form), so the browser
// resolves the destination and nothing here builds a URL or touches the
// filesystem. Goldmark's own image parser produces the nodes; only rendering
// is overridden. The title slot carries the float alignment.
const (
	alignLeft   = "left"
	alignRight  = "right"
	alignCenter = "center"

	// embedThumbW is the width embeds request through the ?w= form of the
	// file's own URL, which keeps the <img src> relative.
	embedThumbW = 300
)

// embedAlign maps an image title to its float class. A title that is not an
// alignment keyword is ignored and the embed floats right.
func embedAlign(title []byte) string {
	switch strings.TrimSpace(string(title)) {
	case alignLeft:
		return alignLeft
	case alignCenter:
		return alignCenter
	}
	return alignRight
}

// embedIsImage reports whether a destination names an image, i.e. whether it
// renders as a <figure> rather than degrading to a link. Extension-only — no
// stat, so a destination gone stale simply 404s when clicked.
func embedIsImage(dest []byte) bool {
	return isImageFile(path.Base(string(dest)))
}

type embedRenderer struct{}

func (r *embedRenderer) RegisterFuncs(reg renderer.NodeRendererFuncRegisterer) {
	reg.Register(ast.KindImage, r.render)
}

// render writes a <figure> holding a thumbnail that links to the full file,
// with the image's inline children as <figcaption>. Children are rendered by
// goldmark — WalkContinue, not WalkSkipChildren — so a caption may carry
// emphasis or a link. A non-image destination renders as a plain anchor
// labelled with the caption, or with the file name when there is none.
func (r *embedRenderer) render(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
	n := node.(*ast.Image)
	name := path.Base(string(n.Destination))
	dest := util.EscapeHTML(util.URLEscape(n.Destination, true))
	captioned := n.FirstChild() != nil

	if !embedIsImage(n.Destination) {
		if entering {
			w.WriteString(`<a href="`)
			w.Write(dest)
			w.WriteString(`">`)
			if !captioned {
				w.Write(util.EscapeHTML([]byte(name)))
			}
		} else {
			w.WriteString(`</a>`)
		}
		return ast.WalkContinue, nil
	}

	if entering {
		w.WriteString(`<figure class="embed embed-`)
		w.WriteString(embedAlign(n.Title))
		w.WriteString(`"><a class="embed-link" href="`)
		w.Write(dest)
		w.WriteString(`"><img src="`)
		w.Write(dest)
		w.WriteString(`?w=`)
		w.WriteString(strconv.Itoa(embedThumbW))
		w.WriteString(`" alt="`)
		w.Write(util.EscapeHTML([]byte(name)))
		w.WriteString(`"></a>`)
		if captioned {
			w.WriteString(`<figcaption>`)
		}
		return ast.WalkContinue, nil
	}
	if captioned {
		w.WriteString(`</figcaption>`)
	}
	w.WriteString(`</figure>`)
	return ast.WalkContinue, nil
}

// embedBlockTransformer lifts an embed that sits alone in a paragraph up to
// block level. Goldmark wraps inline content in <p>, but the embed renders a
// <figure> (block), and a <figure> inside a <p> is auto-closed by the browser —
// which strands empty <p> elements (they still carry .content paragraph margins)
// and breaks the float layout once several embeds share a page. Dissolving the
// wrapping paragraph makes each embed render as a clean block sibling with no
// stray <p>.
type embedBlockTransformer struct{}

func (t embedBlockTransformer) Transform(doc *ast.Document, reader text.Reader, pc parser.Context) {
	source := reader.Source()
	var paras []*ast.Paragraph
	ast.Walk(doc, func(n ast.Node, entering bool) (ast.WalkStatus, error) {
		if entering {
			if p, ok := n.(*ast.Paragraph); ok && t.paragraphFiguresOnly(p, source) {
				paras = append(paras, p)
			}
		}
		return ast.WalkContinue, nil
	})
	for _, p := range paras {
		parent := p.Parent()
		if parent == nil {
			continue
		}
		// InsertBefore isolates the embed from p first, so hoist each embed to a
		// block sibling ahead of the (soon removed) paragraph, then drop p.
		for c := p.FirstChild(); c != nil; {
			next := c.NextSibling()
			if _, ok := c.(*ast.Image); ok {
				parent.InsertBefore(parent, p, c)
			}
			c = next
		}
		parent.RemoveChild(parent, p)
	}
}

// paragraphFiguresOnly reports whether p holds at least one figure-rendering
// image and nothing else visible — only such images and whitespace/line-break
// text. Only these are safe to dissolve: a paragraph carrying prose, or an
// image that degrades to an inline link (non-image destination), stays wrapped
// so the fallback anchor keeps its paragraph.
func (t embedBlockTransformer) paragraphFiguresOnly(p *ast.Paragraph, source []byte) bool {
	hasFigure := false
	for c := p.FirstChild(); c != nil; c = c.NextSibling() {
		switch n := c.(type) {
		case *ast.Image:
			if !embedIsImage(n.Destination) {
				return false
			}
			hasFigure = true
		case *ast.Text:
			if len(bytes.TrimSpace(n.Segment.Value(source))) != 0 {
				return false
			}
		default:
			return false
		}
	}
	return hasFigure
}

type embedExt struct{}

// newEmbedExt returns a goldmark extension that renders Markdown images as
// wiki embeds. It adds no parser of its own: goldmark parses the images, and
// the renderer registered at 500 wins over the default HTML renderer
// (registered at 1000 — lowest priority number is applied last and wins).
func newEmbedExt() goldmark.Extender { return &embedExt{} }

func (e *embedExt) Extend(m goldmark.Markdown) {
	m.Parser().AddOptions(parser.WithASTTransformers(
		util.Prioritized(embedBlockTransformer{}, 100),
	))
	m.Renderer().AddOptions(renderer.WithNodeRenderers(
		util.Prioritized(&embedRenderer{}, 500),
	))
}