// Copyright 2021 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 site

import (
	"bytes"
	"regexp"
	"strings"

	"github.com/yuin/goldmark"
	"github.com/yuin/goldmark/ast"
	"github.com/yuin/goldmark/extension"
	"github.com/yuin/goldmark/parser"
	"github.com/yuin/goldmark/renderer/html"
	"github.com/yuin/goldmark/text"
	"github.com/yuin/goldmark/util"
	"golang.org/x/website/go.dev/cmd/internal/tmplfunc"
	"golang.org/x/website/internal/backport/html/template"
)

// markdownToHTML converts markdown to HTML using the renderer and settings that Hugo uses.
func markdownToHTML(markdown string) (template.HTML, error) {
	// 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(),
			parser.WithAutoHeadingID(),
			parser.WithASTTransformers(util.Prioritized(mdTransformFunc(mdLink), 1)),
		),
		goldmark.WithRendererOptions(html.WithUnsafe()),
		goldmark.WithExtensions(
			extension.NewTypographer(),
			extension.NewLinkify(
				extension.WithLinkifyAllowedProtocols([][]byte{[]byte("http"), []byte("https")}),
				extension.WithLinkifyEmailRegexp(regexp.MustCompile(`[^\x00-\x{10FFFF}]`)), // impossible
			),
		),
	)
	var buf bytes.Buffer
	if err := md.Convert([]byte(markdown), &buf); err != nil {
		return "", err
	}
	return template.HTML(buf.Bytes()), nil
}

// mdTransformFunc is a func implementing parser.ASTTransformer.
type mdTransformFunc func(*ast.Document, text.Reader, parser.Context)

func (f mdTransformFunc) Transform(node *ast.Document, reader text.Reader, pc parser.Context) {
	f(node, reader, pc)
}

// mdLink walks doc, adding rel=noreferrer target=_blank to non-relative links.
func mdLink(doc *ast.Document, _ text.Reader, _ parser.Context) {
	mdLinkWalk(doc)
}

func mdLinkWalk(n ast.Node) {
	switch n := n.(type) {
	case *ast.Link:
		dest := string(n.Destination)
		if strings.HasPrefix(dest, "https://") || strings.HasPrefix(dest, "http://") {
			n.SetAttributeString("rel", []byte("noreferrer"))
			n.SetAttributeString("target", []byte("_blank"))
		}
		return
	case *ast.AutoLink:
		// All autolinks are non-relative.
		n.SetAttributeString("rel", []byte("noreferrer"))
		n.SetAttributeString("target", []byte("_blank"))
		return
	}

	for child := n.FirstChild(); child != nil; child = child.NextSibling() {
		mdLinkWalk(child)
	}
}

// markdownTemplateToHTML converts a markdown template to HTML,
// first applying the template execution engine and then interpreting
// the result as markdown to be converted to HTML.
// This is the same logic used by the Go web site.
func (site *Site) markdownTemplateToHTML(markdown string, p *page) (template.HTML, error) {
	t := site.clone().New(p.file)
	if err := tmplfunc.Parse(t, string(p.data)); err != nil {
		return "", err
	}
	var buf bytes.Buffer
	if err := t.Execute(&buf, p.params); err != nil {
		return "", err
	}
	return markdownToHTML(buf.String())
}