package classes

import (
	"bytes"
	"errors"
	"fmt"
	"io"
	"regexp"
	"slices"
	"strings"

	"github.com/microcosm-cc/bluemonday"
	"github.com/yuin/goldmark"
	"github.com/yuin/goldmark/extension"
	"github.com/yuin/goldmark/parser"
	"github.com/yuin/goldmark/renderer/html"
	xhtml "golang.org/x/net/html"
)

var (
	markdownEngine = goldmark.New(
		goldmark.WithExtensions(extension.Footnote, extension.Table),
		goldmark.WithParserOptions(parser.WithAttribute()),
		// Winter's Markdown::parse passes raw HTML through, so imported HTML
		// bodies round-trip (D-03, PHP parity); htmlPolicy then sanitizes the
		// rendered result on every write path.
		goldmark.WithRendererOptions(html.WithUnsafe()),
	)

	// htmlPolicy is the allow-list for stored journal HTML.
	htmlPolicy = newHTMLPolicy()
)

// newHTMLPolicy extends bluemonday's UGC policy only with what goldmark
// renders and what the imported posts and categories use.
func newHTMLPolicy() *bluemonday.Policy {
	p := bluemonday.UGCPolicy()
	// Winter neither adds nor requires rel="nofollow", so links keep their
	// stored form.
	p.RequireNoFollowOnLinks(false)
	// Imported author footers: <footer class=... itemscope itemtype=...>.
	p.AllowElements("footer")
	p.AllowAttrs("class").Matching(bluemonday.SpaceSeparatedTokens).Globally()
	p.AllowAttrs("itemscope").Matching(regexp.MustCompile(`^(|itemscope)$`)).Globally()
	p.AllowAttrs("itemtype").Matching(regexp.MustCompile(`^https?://schema\.org/[A-Za-z]+$`)).Globally()
	p.AllowAttrs("itemprop").Matching(bluemonday.SpaceSeparatedTokens).Globally()
	// Inline styles in imported footers and goldmark's table cell alignment.
	// bluemonday validates each property value.
	p.AllowStyles("color", "font-size", "font-weight", "text-align").Globally()
	// goldmark's footnote markup.
	p.AllowAttrs("role").Matching(regexp.MustCompile(`^doc-(noteref|endnotes|backlink)$`)).OnElements("a", "div")
	// Pasted chat transcripts in imported category bodies: dir="auto" (the
	// UGC policy allows only ltr and rtl) and two data attributes.
	p.AllowAttrs("dir").Matching(regexp.MustCompile(`(?i)^(ltr|rtl|auto)$`)).Globally()
	p.AllowAttrs("data-message-author-role", "data-message-id").
		Matching(regexp.MustCompile(`^[A-Za-z0-9_-]+$`)).OnElements("div")
	return p
}

// FormatHTML converts journal post markdown to stored HTML using goldmark
// footnote, table, and attribute extensions. Raw HTML in the source passes
// through the renderer, as it does in Winter's Markdown::parse. The result is
// then sanitized with a bluemonday allow-list that strips, as Winter's
// Html::clean does, what it does not allow: script, style, iframe, object,
// embed, form, base and meta elements, event handler attributes, and URLs
// other than http, https, mailto and relative ones. HTML that the policy
// keeps whole is returned byte for byte, so imported bodies round-trip. It
// does not call cabana.RenderMarkdown, whose admin preview stays without
// unsafe HTML.
func FormatHTML(src string) (string, error) {
	var buf bytes.Buffer
	if err := markdownEngine.Convert([]byte(strings.TrimSpace(src)), &buf); err != nil {
		return "", fmt.Errorf("journal: markdown: %w", err)
	}
	out, _ := sanitizeHTML(buf.String())
	return out, nil
}

// CheckHTML gates HTML that is stored as is, such as category content. It
// uses the FormatHTML allow-list but refuses instead of stripping: doc is
// refused when the policy would remove or change any element, attribute or
// text in it.
func CheckHTML(doc string) error {
	if _, changed := sanitizeHTML(doc); changed {
		return errors.New("journal: HTML contains elements, attributes or URLs that are not allowed")
	}
	return nil
}

// sanitizeHTML returns doc unchanged when htmlPolicy keeps all of it, and the
// policy's output with changed set otherwise. Keeping all of it means doc and
// the policy output tokenize to the same HTML tokens, so an output that only
// re-escapes text or attribute values is not a change.
func sanitizeHTML(doc string) (string, bool) {
	clean := htmlPolicy.Sanitize(doc)
	if clean == doc || sameTokens(doc, clean) {
		return doc, false
	}
	return clean, true
}

// sameTokens reports whether a and b tokenize to the same sequence of HTML
// tokens: type, tag name or decoded text, and the set of decoded attributes.
func sameTokens(a, b string) bool {
	ta, oka := htmlTokens(a)
	tb, okb := htmlTokens(b)
	return oka && okb && slices.Equal(ta, tb)
}

func htmlTokens(doc string) ([]string, bool) {
	var out []string
	z := xhtml.NewTokenizer(strings.NewReader(doc))
	for {
		tt := z.Next()
		if tt == xhtml.ErrorToken {
			return out, errors.Is(z.Err(), io.EOF)
		}
		t := z.Token()
		attrs := make([]string, 0, len(t.Attr))
		for _, a := range t.Attr {
			attrs = append(attrs, a.Namespace+":"+a.Key+"="+a.Val)
		}
		slices.Sort(attrs)
		out = append(out, fmt.Sprintf("%d|%s|%q", tt, t.Data, attrs))
	}
}