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encodingsignedcbor

Rationale for CBOR negative integers

I am confused as to why CBOR chooses to encode negative integers as unsigned binary numbers with the value defined as -1 minus the unsigned value, instead of e.g. regular two's complement representation. Is there an obvious advantage that I'm missing, apart from increased negative range (which, IMO, is of questionable value weighed against increased complexity)?

Solution

  • Advantages:

    1. There's only one allowed encoding type for each integer value, so all encoders will emit consistent output. If the encoders use the shortest encoding for each value as recommended by the spec, they'll emit identical output.
    2. Picking the shortest numeric field is easier for non-negative numbers than for signed negative numbers, and CBOR aims for tiny IOT devices to readily transmit data.
    3. It fits twice as many values into each integer encoding field width, thus making the data more compact. (It'd be yet more compact if the integer encodings didn't overlap, but that'd be notably more complicated.)
    4. It can handle twice as large a negative value before needing the bignum extension.