MKV file cluster size which is encoded?

Question

Element IDs (also called EBML IDs), beginning with the ID itself, followed by the Data Size.

Data size, in octets, is also coded with an UTF-8 like system.

How can I decode the data size in decimal

Answer 1

You can find the below simple sample code in https://github.com/wangf1978/DumpTS/blob/master/Matroska.h

static uint64_t UnpackUnsignedIntVal(CBitstream&bs, uint8_t max_octs = 8, bool unPackVal=true, uint8_t* pcbValLen=nullptr)
{
    uint8_t nLeadingZeros = 0;
    uint64_t u64Val = bs.GetByte();
    for (uint8_t i = 0; i < max_octs; i++)
        if ((u64Val&(1ULL << (7 - i))) == 0)
            nLeadingZeros++;
        else
            break;

    if (nLeadingZeros >= max_octs)  // Unexpected
        return UINT64_MAX;

    if (unPackVal)
        u64Val &= ~(1 << (7 - nLeadingZeros));

    for (uint8_t i = 0; i<nLeadingZeros; i++)
        u64Val = (((uint64_t)u64Val) << 8) | (uint8_t)bs.GetBits(8);

    if (pcbValLen != nullptr)
        *pcbValLen = nLeadingZeros + 1;

    return u64Val;
}

virtual int Unpack(CBitstream& bs)
{
    // Read the element ID
    uint64_t u64Val = UnpackUnsignedIntVal(bs, 4, false);
    if (u64Val == UINT64_MAX)
        return -1;

    ID = (uint32_t)u64Val;

    if ((u64Val = UnpackUnsignedIntVal(bs)) == UINT64_MAX)
        return -1;

    Size = u64Val;

    //printf("ID: 0X%X, Size: %lld(0X%llX)\n", ID, Size, Size);

    return 0;
}