The Ion 1.1 Draft Specification

FlexUInt

A variable-length unsigned integer.

The bytes of a FlexUInt are written in little-endian byte order. This means that the first bytes will contain the FlexUInt's least significant bits.

The least significant bits in the FlexUInt indicate the number of bytes that were used to encode the integer. If a FlexUInt is N bytes long, its N-1 least significant bits will be 0; a terminal 1 bit will be in the next most significant position.

All bits that are more significant than the terminal 1 represent the magnitude of the FlexUInt.

FlexUInt encoding of 14

              ┌──── Lowest bit is 1 (end), indicating
              │     this is the only byte.
0 0 0 1 1 1 0 1
└─────┬─────┘
unsigned int 14

FlexUInt 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 unsigned  of the unsigned
integer          integer

FlexUInt encoding of 21,043

            ┌───── There are 2 zeros in the least significant bits, so this
            │      integer is three bytes wide.
          ┌─┴─┐
1 0 0 1 1 1 0 0  1 0 0 1 0 0 0 1  0 0 0 0 0 0 1 0
└───┬───┘        └──────┬──────┘  └──────┬──────┘
lowest 6 bits    next 8 bits of   highest 8 bits
of the unsigned  the unsigned     of the unsigned
integer          integer          integer