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//////////////////////
// Helper
//////////////////////

// Convert a hex string to a byte array
function hexToBytes(hex) {
    for (var bytes = [], c = 0; c < hex.length; c += 2)
        bytes.push(parseInt(hex.substr(c, 2), 16));
    return bytes;
}

// Convert a byte array to a hex string
function bytesToHex(bytes) {
    for (var hex = [], i = 0; i < bytes.length; i++) {
        var current = bytes[i] < 0 ? bytes[i] + 256 : bytes[i];
        hex.push((current >>> 4).toString(16));
        hex.push((current & 0xF).toString(16));
    }
    return hex.join("");
}

// MFieldToString is Calculated from MField (15 bit) and represented as 3 ASCII chars
// First bit is ignored and used to define the uniqueness of the Meter Id
// AAA = 0x0421 = 0 00001 00001 00001
// ZZZ = 0x6b5a = 0 11010 11010 11010
function MFieldToString(m) {
    var letters = ""
    var char3 = (m&0x1F)+64
    var char2 = ((m&0x3E0) >> 5)+64
    var char1 = ((m&0x7C00) >> 10)+64
    letters += String.fromCharCode(char1)
    letters += String.fromCharCode(char2)
    letters += String.fromCharCode(char3)
    return letters
}


// ParseWmbus returns a struct with the wMbus telegram header information plus the raw telegram
// telegram is the whole telegram as byte slice (each element of the slice contains one byte value as number)
//
// Data Link Layer:
// ----------------
//   0  |   1  |  2   |   3  |  4   |   5  |   6  |   7  |   8  |   9    |  10  |  11  | -> Application Layer
// 0x2E | 0x44 | 0xB0 | 0x5C | 0x12 | 0x13 | 0x00 | 0x00 | 0x02 | 0x1B   | 0x8A |  ... |
// Len  |  C   |  M   |  M   |   ID |  ID  |  ID  |  ID  | Ver  | Device |  CRC |  CRC |
function ParseWmbus(telegram) {
    var id = bytesToHex(telegram.slice(4, 8).reverse());
    var mField = MFieldToString((telegram[2] << 0) + (telegram[3] << 8));
    var version = telegram[8];
    var deviceType = telegram[9];

    return {
        "Id": id,
        "MField": mField,
        "Version": version,
        "Device": deviceType,
        "Telegram": bytesToHex(telegram)
    }
}