package classes

import (
	"bytes"
	"strings"

	xhtml "golang.org/x/net/html"
)

// SetImageAlt sets the alt attribute of every img in html whose src
// attribute (unescaped) equals src, adding it when missing. It returns the
// new HTML, the number of matching images and the alt the first match had
// (nil when it had none). It works on the x/net/html tokenizer: a matching
// img tag is written again from its token, so every attribute value,
// alt included, is escaped by the tokenizer and never concatenated; every
// other byte of html is copied as it is. An empty alt marks a decorative
// image (alt="").
func SetImageAlt(html, src, alt string) (string, int, *string) {
	if src == "" || !containsFold(html, "<img") {
		return html, 0, nil
	}
	z := xhtml.NewTokenizer(strings.NewReader(html))
	var b bytes.Buffer
	b.Grow(len(html) + len(alt))
	matches := 0
	var old *string
	for {
		tt := z.Next()
		if tt == xhtml.ErrorToken {
			break
		}
		raw := z.Raw()
		if tt == xhtml.StartTagToken || tt == xhtml.SelfClosingTagToken {
			// Raw is copied before Token, which reads the tag; Token is
			// the only reader of the tag (TagName would consume it).
			kept := append([]byte(nil), raw...)
			tok := z.Token()
			if tok.Data == "img" && imgSrc(tok) == src {
				prev := setAlt(&tok, alt)
				if matches == 0 {
					old = prev
				}
				matches++
				b.WriteString(tok.String())
				continue
			}
			b.Write(kept)
			continue
		}
		b.Write(raw)
	}
	if matches == 0 {
		return html, 0, nil
	}
	return b.String(), matches, old
}

func imgSrc(tok xhtml.Token) string {
	for _, a := range tok.Attr {
		if a.Namespace == "" && a.Key == "src" {
			return a.Val
		}
	}
	return ""
}

// setAlt sets tok's alt to alt and returns the previous value (nil when
// there was none). Repeated alt attributes collapse to one.
func setAlt(tok *xhtml.Token, alt string) *string {
	var prev *string
	attrs := tok.Attr[:0]
	set := false
	for _, a := range tok.Attr {
		if a.Namespace == "" && a.Key == "alt" {
			if prev == nil {
				v := a.Val
				prev = &v
			}
			if set {
				continue
			}
			a.Val = alt
			set = true
		}
		attrs = append(attrs, a)
	}
	if !set {
		attrs = append(attrs, xhtml.Attribute{Key: "alt", Val: alt})
	}
	tok.Attr = attrs
	return prev
}