// Copyright 2019 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 frontend

import (
	"bytes"
	"context"
	"fmt"
	"io"
	"net/url"
	"path"
	"path/filepath"
	"strings"

	"github.com/google/safehtml"
	"github.com/google/safehtml/template"
	"github.com/google/safehtml/uncheckedconversions"
	"github.com/microcosm-cc/bluemonday"
	"github.com/russross/blackfriday/v2"
	"golang.org/x/net/html"
	"golang.org/x/net/html/atom"
	"golang.org/x/pkgsite/internal"
	"golang.org/x/pkgsite/internal/derrors"
	"golang.org/x/pkgsite/internal/source"
)

func blackfridayReadmeHTML(readme *internal.Readme, mi *internal.ModuleInfo) (safehtml.HTML, error) {
	// blackfriday.Run() uses CommonHTMLFlags and CommonExtensions by default.
	renderer := blackfriday.NewHTMLRenderer(blackfriday.HTMLRendererParameters{Flags: blackfriday.CommonHTMLFlags})
	parser := blackfriday.New(blackfriday.WithExtensions(blackfriday.CommonExtensions | blackfriday.AutoHeadingIDs))

	// Render HTML similar to blackfriday.Run(), but here we implement a custom
	// Walk function in order to modify image paths in the rendered HTML.
	b := &bytes.Buffer{}
	contents := bytes.ReplaceAll([]byte(readme.Contents), []byte("\r"), nil)
	rootNode := parser.Parse(contents)
	var walkErr error
	rootNode.Walk(func(node *blackfriday.Node, entering bool) blackfriday.WalkStatus {
		switch node.Type {
		case blackfriday.Heading:
			if node.HeadingID != "" {
				// Prefix HeadingID with "readme-" on the unit page to prevent
				// a namespace clash with the documentation section.
				node.HeadingID = "readme-" + node.HeadingID
			}
		case blackfriday.Image, blackfriday.Link:
			useRaw := node.Type == blackfriday.Image
			if d := translateRelativeLink(string(node.LinkData.Destination), mi.SourceInfo, useRaw, readme); d != "" {
				node.LinkData.Destination = []byte(d)
			}
		case blackfriday.HTMLBlock, blackfriday.HTMLSpan:
			d, err := translateHTML(node.Literal, mi.SourceInfo, readme)
			if err != nil {
				walkErr = fmt.Errorf("couldn't transform html block(%s): %w", node.Literal, err)
				return blackfriday.Terminate
			}
			node.Literal = d
		}
		return renderer.RenderNode(b, node, entering)
	})
	if walkErr != nil {
		return safehtml.HTML{}, walkErr
	}
	return legacySanitizeHTML(b), nil
}

// LegacyReadmeHTML sanitizes readmeContents based on bluemondy.UGCPolicy and returns
// a safehtml.HTML. If readmeFilePath indicates that this is a markdown file,
// it will also render the markdown contents using blackfriday.
//
// This function is exported for use in an external tool that uses this package to
// compare readme files to see how changes in processing will affect them.
func LegacyReadmeHTML(ctx context.Context, mi *internal.ModuleInfo, readme *internal.Readme) (_ safehtml.HTML, err error) {
	defer derrors.Wrap(&err, "LegacyReadmeHTML(%s@%s)", mi.ModulePath, mi.Version)
	if readme == nil || readme.Contents == "" {
		return safehtml.HTML{}, nil
	}
	if !isMarkdown(readme.Filepath) {
		t := template.Must(template.New("").Parse(`<pre class="readme">{{.}}</pre>`))
		h, err := t.ExecuteToHTML(readme.Contents)
		if err != nil {
			return safehtml.HTML{}, err
		}
		return h, nil
	}

	return blackfridayReadmeHTML(readme, mi)
}

// legacySanitizeHTML reads HTML from r and sanitizes it to ensure it is safe.
func legacySanitizeHTML(r io.Reader) safehtml.HTML {
	// bluemonday.UGCPolicy allows a broad selection of HTML elements and
	// attributes that are safe for user generated content. This policy does
	// not allow iframes, object, embed, styles, script, etc.
	p := bluemonday.UGCPolicy()

	// Allow width and align attributes on img, div, and p tags.
	// This is used to center elements in a readme as well as to size it
	// images appropriately where used, like the gin-gonic/logo/color.png
	// image in the github.com/gin-gonic/gin README.
	p.AllowAttrs("width", "align").OnElements("img")
	p.AllowAttrs("width", "align").OnElements("div")
	p.AllowAttrs("width", "align").OnElements("p")
	s := p.SanitizeReader(r).String()
	// Trust that bluemonday properly sanitizes the HTML.
	return uncheckedconversions.HTMLFromStringKnownToSatisfyTypeContract(s)
}

// isMarkdown reports whether filename says that the file contains markdown.
func isMarkdown(filename string) bool {
	ext := strings.ToLower(filepath.Ext(filename))
	// https://tools.ietf.org/html/rfc7763 mentions both extensions.
	return ext == ".md" || ext == ".markdown"
}

// translateRelativeLink converts relative image paths to absolute paths.
//
// README files sometimes use relative image paths to image files inside the
// repository. As the discovery site doesn't host the full repository content,
// in order for the image to render, we need to convert the relative path to an
// absolute URL to a hosted image.
func translateRelativeLink(dest string, info *source.Info, useRaw bool, readme *internal.Readme) string {
	destURL, err := url.Parse(dest)
	if err != nil || destURL.IsAbs() {
		return ""
	}
	if destURL.Path == "" {
		// This is a fragment; leave it.
		return "#readme-" + destURL.Fragment
	}
	// Paths are relative to the README location.
	destPath := path.Join(path.Dir(readme.Filepath), path.Clean(trimmedEscapedPath(destURL)))
	if useRaw {
		return info.RawURL(destPath)
	}
	return info.FileURL(destPath)
}

// trimmedEscapedPath trims surrounding whitespace from u's path, then returns it escaped.
func trimmedEscapedPath(u *url.URL) string {
	u.Path = strings.TrimSpace(u.Path)
	return u.EscapedPath()
}

// translateHTML parses html text into parsed html nodes. It then
// iterates through the nodes and replaces the src key with a value
// that properly represents the source of the image from the repo.
func translateHTML(htmlText []byte, info *source.Info, readme *internal.Readme) (_ []byte, err error) {
	defer derrors.Wrap(&err, "translateHTML(readme.Filepath=%s)", readme.Filepath)

	r := bytes.NewReader(htmlText)
	nodes, err := html.ParseFragment(r, nil)
	if err != nil {
		return nil, err
	}
	var buf bytes.Buffer
	changed := false
	for _, n := range nodes {
		// We expect every parsed node to begin with <html><head></head><body>.
		if n.DataAtom != atom.Html {
			return nil, fmt.Errorf("top-level node is %q, expected 'html'", n.DataAtom)
		}
		// When the parsed html nodes don't have a valid structure
		// (i.e: an html comment), then just return the original text.
		if n.FirstChild == nil || n.FirstChild.NextSibling == nil || n.FirstChild.NextSibling.DataAtom != atom.Body {
			return htmlText, nil
		}
		n = n.FirstChild.NextSibling
		// n is now the body node. Walk all its children.
		for c := n.FirstChild; c != nil; c = c.NextSibling {
			if walkHTML(c, info, readme) {
				changed = true
			}
			if err := html.Render(&buf, c); err != nil {
				return nil, err
			}
		}
	}
	if changed {
		return buf.Bytes(), nil
	}
	// If there were no changes, return the original.
	return htmlText, nil
}

// walkHTML crawls through an html node and replaces the src
// tag link with a link that properly represents the image
// from the repo source.
// It reports whether it made a change.
func walkHTML(n *html.Node, info *source.Info, readme *internal.Readme) bool {
	changed := false
	if n.Type == html.ElementNode && n.DataAtom == atom.Img {
		var attrs []html.Attribute
		for _, a := range n.Attr {
			if a.Key == "src" {
				if v := translateRelativeLink(a.Val, info, true, readme); v != "" {
					a.Val = v
					changed = true
				}
			}
			attrs = append(attrs, a)
		}
		n.Attr = attrs
	}
	for c := n.FirstChild; c != nil; c = c.NextSibling {
		if walkHTML(c, info, readme) {
			changed = true
		}
	}
	return changed
}