hugo/markup/pandoc/convert.go

// Copyright 2019 The Hugo Authors. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Package pandoc converts content to HTML using Pandoc as an external helper.
package pandoc

import (
	"bytes"

	"github.com/gohugoio/hugo/common/hexec"
	"github.com/gohugoio/hugo/htesting"
	"github.com/gohugoio/hugo/identity"
	"golang.org/x/net/html"

	"github.com/gohugoio/hugo/markup/converter"
	"github.com/gohugoio/hugo/markup/internal"
	"github.com/gohugoio/hugo/markup/tableofcontents"
)

// Provider is the package entry point.
var Provider converter.ProviderProvider = provider{}

type provider struct{}

func (p provider) New(cfg converter.ProviderConfig) (converter.Provider, error) {
	return converter.NewProvider("pandoc", func(ctx converter.DocumentContext) (converter.Converter, error) {
		return &pandocConverter{
			ctx: ctx,
			cfg: cfg,
		}, nil
	}), nil
}

type pandocResult struct {
	converter.ResultRender
	toc *tableofcontents.Fragments
}

func (r pandocResult) TableOfContents() *tableofcontents.Fragments {
	return r.toc
}

type pandocConverter struct {
	ctx converter.DocumentContext
	cfg converter.ProviderConfig
}

func (c *pandocConverter) Convert(ctx converter.RenderContext) (converter.ResultRender, error) {
	contentWithToc, err := c.getPandocContent(ctx.Src, c.ctx)
	if err != nil {
		return nil, err
	}
	content, toc, err := c.extractTOC(contentWithToc)
	if err != nil {
		return nil, err
	}
	return pandocResult{
		ResultRender: converter.Bytes(content),
		toc:          toc,
	}, nil
}

func (c *pandocConverter) Supports(feature identity.Identity) bool {
	return false
}

// getPandocContent calls pandoc as an external helper to convert pandoc markdown to HTML.
func (c *pandocConverter) getPandocContent(src []byte, ctx converter.DocumentContext) ([]byte, error) {
	logger := c.cfg.Logger
	binaryName := getPandocBinaryName()
	if binaryName == "" {
		logger.Println("pandoc not found in $PATH: Please install.\n",
			"                 Leaving pandoc content unrendered.")
		return src, nil
	}
	args := []string{"--mathjax", "--toc", "-s", "--metadata", "title=dummy"}
	return internal.ExternallyRenderContent(c.cfg, ctx, src, binaryName, args)
}

const pandocBinary = "pandoc"

func getPandocBinaryName() string {
	if hexec.InPath(pandocBinary) {
		return pandocBinary
	}
	return ""
}

// extractTOC extracts the toc from the given src html.
// It returns the html without the TOC, and the TOC data
func (a *pandocConverter) extractTOC(src []byte) ([]byte, *tableofcontents.Fragments, error) {
	var buf bytes.Buffer
	buf.Write(src)
	node, err := html.Parse(&buf)
	if err != nil {
		return nil, nil, err
	}

	var (
		f       func(*html.Node) bool
		body    *html.Node
		toc     *tableofcontents.Fragments
		toVisit []*html.Node
	)

	// find body
	f = func(n *html.Node) bool {
		if n.Type == html.ElementNode && n.Data == "body" {
			body = n
			return true
		}
		if n.FirstChild != nil {
			toVisit = append(toVisit, n.FirstChild)
		}
		if n.NextSibling != nil && f(n.NextSibling) {
			return true
		}
		for len(toVisit) > 0 {
			nv := toVisit[0]
			toVisit = toVisit[1:]
			if f(nv) {
				return true
			}
		}
		return false
	}
	if !f(node) {
		return nil, nil, err
	}

	// remove by pandoc generated title
	f = func(n *html.Node) bool {
		if n.Type == html.ElementNode && n.Data == "header" && attr(n, "id") == "title-block-header" {
			n.Parent.RemoveChild(n)
			return true
		}
		if n.FirstChild != nil {
			toVisit = append(toVisit, n.FirstChild)
		}
		if n.NextSibling != nil && f(n.NextSibling) {
			return true
		}
		for len(toVisit) > 0 {
			nv := toVisit[0]
			toVisit = toVisit[1:]
			if f(nv) {
				return true
			}
		}
		return false
	}
	f(body)

	// find toc
	f = func(n *html.Node) bool {
		if n.Type == html.ElementNode && n.Data == "nav" && attr(n, "id") == "TOC" {
			toc = parseTOC(n)
			if !a.cfg.MarkupConfig().Pandoc.PreserveTOC {
				n.Parent.RemoveChild(n)
			}
			return true
		}
		if n.FirstChild != nil {
			toVisit = append(toVisit, n.FirstChild)
		}
		if n.NextSibling != nil && f(n.NextSibling) {
			return true
		}
		for len(toVisit) > 0 {
			nv := toVisit[0]
			toVisit = toVisit[1:]
			if f(nv) {
				return true
			}
		}
		return false
	}
	f(body)
	if err != nil {
		return nil, nil, err
	}
	buf.Reset()
	err = html.Render(&buf, body)
	if err != nil {
		return nil, nil, err
	}
	// ltrim <html><head></head><body>\n\n and rtrim \n\n</body></html> which are added by html.Render
	res := buf.Bytes()[8:]
	res = res[:len(res)-9]
	return res, toc, nil
}

// parseTOC returns a TOC root from the given toc Node
func parseTOC(doc *html.Node) *tableofcontents.Fragments {
	var (
		toc tableofcontents.Builder
		f   func(*html.Node, int, int)
	)
	f = func(n *html.Node, row, level int) {
		if n.Type == html.ElementNode {
			switch n.Data {
			case "ul":
				if level == 0 {
					row++
				}
				level++
				f(n.FirstChild, row, level)
			case "li":
				for c := n.FirstChild; c != nil; c = c.NextSibling {
					if c.Type != html.ElementNode || c.Data != "a" {
						continue
					}
					href := attr(c, "href")[1:]
					toc.AddAt(&tableofcontents.Heading{
						Title: nodeContent(c),
						ID:    href,
					}, row, level)
				}
				f(n.FirstChild, row, level)
			}
		}
		if n.NextSibling != nil {
			f(n.NextSibling, row, level)
		}
	}
	f(doc.FirstChild, -1, 0)
	return toc.Build()
}

func attr(node *html.Node, key string) string {
	for _, a := range node.Attr {
		if a.Key == key {
			return a.Val
		}
	}
	return ""
}

func nodeContent(node *html.Node) string {
	var buf bytes.Buffer
	for c := node.FirstChild; c != nil; c = c.NextSibling {
		html.Render(&buf, c)
	}
	return buf.String()
}

// Supports returns whether Pandoc is installed on this computer.
func Supports() bool {
	hasBin := getPandocBinaryName() != ""
	if htesting.SupportsAll() {
		if !hasBin {
			panic("pandoc not installed")
		}
		return true
	}
	return hasBin
}