package mogrify

import (
	"fmt"
	"regexp"
	"strings"
)

// Outgoing HTML filters for responses (experimental).

var reImgTag = regexp.MustCompile(`<img ([^>]+)>`)

// LazyLoadImages modifies <img> tags to add loading="lazy" attribute to them.
// Ignores image tags that already include the attribute.
func LazyLoadImages(input string) string {
	m := reImgTag.FindAllStringSubmatch(input, -1)

	if m == nil {
		return input
	}

	for _, match := range m {
		var (
			tag     = match[0]
			attrs   = match[1]
			replace = fmt.Sprintf(`<img %s loading="lazy">`, attrs)
		)
		if strings.Contains(attrs, "loading=") {
			continue
		}
		input = strings.Replace(input, tag, replace, -1)
	}

	return input
}