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private static String calculateBodyLength(String message) {
    // Find the position of the BodyLength placeholder
    int bodyLengthIndex = message.indexOf("9=#");
    if (bodyLengthIndex == -1) {
        throw new RuntimeException("BodyLength placeholder not found");
    }
    
    // Find the end of the BodyLength field (after the SOH)
    int bodyStart = message.indexOf(SOH, bodyLengthIndex) + 1;
    if (bodyStart == 0) {
        throw new RuntimeException("Invalid message format");
    }
    
    // Calculate length from bodyStart to end of message (before checksum)
    String body = message.substring(bodyStart);
    int length = body.getBytes(StandardCharsets.US_ASCII).length;
    
    return message.replaceFirst("9=#", "9=" + length);
}

private static String addCheckSum(String message) {
    // Calculate checksum for entire message except the checksum field
    int checksum = 0;
    byte[] messageBytes = message.getBytes(StandardCharsets.US_ASCII);
    for (byte b : messageBytes) {
        checksum += b & 0xFF;
    }
    checksum %= 256;
    
    // Append checksum field
    return message + "10=" + String.format("%03d", checksum) + SOH;
}