package nostr

import "unicode/utf8"

// AppendJSONString appends s as a JSON string literal to dst using the same
// escaping rules as JavaScript's JSON.stringify:
//
//   - quote and backslash are escaped;
//   - \b \f \n \r \t use their short escapes;
//   - other C0 control bytes become \u00XX with lowercase hex digits;
//   - everything else, including DEL, U+2028 and U+2029, is emitted as UTF-8.
//
// Invalid UTF-8 is replaced with U+FFFD, matching what a JSON parser would
// produce for such input in a JS engine. This is what makes the event ID stable
// across implementations: NIP-01 defines the serialization in terms of
// JSON.stringify, not in terms of Go's encoder.
func AppendJSONString(dst []byte, s string) []byte {
	dst = append(dst, '"')
	start := 0
	for i := 0; i < len(s); {
		c := s[i]
		if c < utf8.RuneSelf {
			if c >= 0x20 && c != '"' && c != '\\' {
				i++
				continue
			}
			dst = append(dst, s[start:i]...)
			switch c {
			case '"':
				dst = append(dst, '\\', '"')
			case '\\':
				dst = append(dst, '\\', '\\')
			case '\b':
				dst = append(dst, '\\', 'b')
			case '\f':
				dst = append(dst, '\\', 'f')
			case '\n':
				dst = append(dst, '\\', 'n')
			case '\r':
				dst = append(dst, '\\', 'r')
			case '\t':
				dst = append(dst, '\\', 't')
			default:
				const hexDigits = "0123456789abcdef"
				dst = append(dst, '\\', 'u', '0', '0', hexDigits[c>>4], hexDigits[c&0x0f])
			}
			i++
			start = i
			continue
		}
		r, size := utf8.DecodeRuneInString(s[i:])
		if r == utf8.RuneError && size == 1 {
			dst = append(dst, s[start:i]...)
			dst = append(dst, "\uFFFD"...)
			i++
			start = i
			continue
		}
		i += size
	}
	dst = append(dst, s[start:]...)
	return append(dst, '"')
}