// Phase 258 P0: index_of_string minimal test (実物と同形)
// Target: StringUtils.index_of_string(s, substr)
//
// Loop form (matches actual implementation):
//   local i = 0
//   loop(i <= s.length() - substr.length()) {
//       if s.substring(i, i + substr.length()) == substr { return i }
//       i = i + 1
//   }
//   return -1
//
// Expected JoinIR (Phase 258 P0 dynamic needle):
//   - needle_len = substr.length()  // Dynamic calculation
//   - bound = len - needle_len
//   - exit_cond = (i > bound)       // Not found case
//   - window = s.substring(i, i + needle_len)  // Dynamic window
//   - if window == substr { return i }  // Found case
//   - i = i + 1  // Step

static box StringUtils {
    index_of_string(s, substr) {
        local i
        i = 0
        loop(i <= s.length() - substr.length()) {
            if s.substring(i, i + substr.length()) == substr {
                return i
            }
            i = i + 1
        }
        return -1
    }
}

static box Main {
    main() {
        local result
        result = StringUtils.index_of_string("hello world", "world")
        return result  // Expected: 6 (index of "world" in "hello world")
    }
}