Bitcoin Cash Specification
Compact Variable Length Integer
A variable-width data format for unsigned integers allowing a more compact representation for smaller values. This is sometimes referred to as “compact size” or “var int”.
Format
Numbers are encoded with the first rule that applies of the following:
- If the number < 253 (0xFD), store it in 1 byte, left-padded with zeros.
- If the number fits in 16 bits (but is greater than 252), store it in 3 bytes: a 1-byte value 253 (0xFD) followed by the 2 byte little-endian number.
Byte Index | C-Style Calculation |
---|---|
0 | 0xFD |
1 | value & 255 |
2 | value » 8 |
- If the number fits in 32 bits (but not 8 or 16), store it in 5 bytes: a 1-byte value 254 (0xFE) followed by the 4 byte little-endian number
Byte Index | C-Style Calculation |
---|---|
0 | 0xFE |
1 | value & 255 |
2 | (value » 8) & 255 |
3 | (value » 16) & 255 |
4 | (value » 24) & 255 |
- If the number fits in 64 bits (but not 8, 16, or 32), store it in 9 bytes: a 1-byte value 255 (0xFF) followed by the 8 byte little-endian number
Byte Index | C-Style Calculation |
---|---|
0 | 0xFF |
1 | value & 255 |
2 | (value » 8) & 255 |
3 | (value » 16) & 255 |
4 | (value » 24) & 255 |
5 | (value » 32) & 255 |
6 | (value » 40) & 255 |
7 | (value » 48) & 255 |
8 | (value » 56) & 255 |