The Ion 1.1 Draft Specification

FlexInt

A variable-length signed integer.

From an encoding perspective, FlexInts are structurally similar to a FlexUInt. Both encode their bytes using little-endian byte order, and both use the count of least-significant zero bits to indicate how many bytes were used to encode the integer. They differ in the interpretation of their bits; while a FlexUInt's bits are unsigned, a FlexInt's bits are encoded using two's complement notation.

TIP: An implementation could choose to read a FlexInt by instead reading a FlexUInt and then reinterpreting its bits as two's complement.

FlexInt encoding of 14

              ┌──── Lowest bit is 1 (end), indicating
              │     this is the only byte.
0 0 0 1 1 1 0 1
└─────┬─────┘
 2's comp. 14

FlexInt encoding of -14

              ┌──── Lowest bit is 1 (end), indicating
              │     this is the only byte.
1 1 1 0 0 1 0 1
└─────┬─────┘
 2's comp. -14

FlexInt encoding of 729

             ┌──── There's 1 zero in the least significant bits, so this
             │     integer is two bytes wide.
            ┌┴┐
0 1 1 0 0 1 1 0  0 0 0 0 1 0 1 1
└────┬────┘      └──────┬──────┘
lowest 6 bits    highest 8 bits
of the 2's       of the 2's
comp. integer    comp. integer

FlexInt encoding of -729

             ┌──── There's 1 zero in the least significant bits, so this
             │     integer is two bytes wide.
            ┌┴┐
1 0 0 1 1 1 1 0  1 1 1 1 0 1 0 0
└────┬────┘      └──────┬──────┘
lowest 6 bits    highest 8 bits
of the 2's       of the 2's
comp. integer    comp. integer