Variable-Length Integers

MOQT requires a variable-length integer encoding with the following properties:

1. The encoded length can be determined from the first encoded byte.

2. The range of 1 byte values is as large as possible.

3. All 64 bit numbers can be encoded.

The variable-length integer encoding uses the number of leading 1 bits of the first byte to indicate the length of the encoding in bytes. The remaining bits after the first 0 and subsequent bytes, if any, represent the integer value, encoded in network byte order.

Integers are encoded in 1 to 9 bytes and can encode up to 64 bit unsigned integers. The following table summarizes the encoding properties.

Table 3: Summary of Integer Encodings

Leading Bits	Length (bytes)	Usable Bits	Range
0	1	7	0-127
10	2	14	0-16383
110	3	21	0-2097151
1110	4	28	0-268435455
11110	5	35	0-34359738367
111110	6	42	0-4398046511103
1111110	7	49	0-562949953421311
11111110	8	56	0-72057594037927935
11111111	9	64	0-18446744073709551615

The following table contains some example encodings:

Table 4: Example Integer Encodings

Byte Sequence	Decimal Value
0x25	37
0x8025	37
0xbbbd	15,293
0xed7f3e7d	226,442,877
0xfaa1a0e403d8	2,893,212,287,960
0xfc8998abc66bc0	151,288,809,941,952
0xfefa318fa8e3ca11	70,423,237,261,249,041
0xffffffffffffffffff	18,446,744,073,709,551,615

Variable-length integers do not need to be encoded using the minimum number of bytes; any encoding length that can represent the value is valid. Note that, as a result, the same numeric value can be represented by more than one byte sequence. For example, the value 0 can be encoded as 0x00, 0x8000, 0xc00000, or any longer form.

x (vi64):

Indicates that x holds an integer value using the variable-length encoding as described above.