package attach

import (
	"path"
	"strings"
)

// VariantKey is the blob key of a thumbnail's copy in another image format,
// stored next to the thumbnail: thumbKey with its extension replaced by
// format, so thumb_42_480_0_0_0_auto.jpg becomes
// thumb_42_480_0_0_0_auto.webp. When the thumbnail already carries that
// extension (a thumbnail of a .webp original holds JPEG bytes under the
// original's name, see ThumbKey), format is appended instead
// (thumb_42_480_0_0_0_auto.webp.webp), so the variant never overwrites the
// thumbnail. format follows the token rule of ThumbFilename ([a-z0-9]+,
// lower-cased first); an empty thumbKey or a format outside the rule
// answers "".
func VariantKey(thumbKey, format string) string {
	format = strings.ToLower(format)
	if thumbKey == "" || !thumbToken.MatchString(format) {
		return ""
	}
	ext := path.Ext(thumbKey)
	if strings.EqualFold(strings.TrimPrefix(ext, "."), format) {
		return thumbKey + "." + format
	}
	return strings.TrimSuffix(thumbKey, ext) + "." + format
}