The "compact" format is a representation of a whole number N using an unsigned 32bit number similar to a floating point format. The most significant 8 bits are the unsigned exponent of base 256. This exponent can be thought of as "number of bytes of N". The lower 23 bits are the mantissa. Bit number 24 (0x800000) represents the sign of N. N = (‐1ˆsign) * mantissa * 256ˆ(exponent‐3)

Synopsis

Declared in <arith_uint256.h>

arith_uint256&
SetCompact(
    uint32_t nCompact,
    bool* pfNegative = nullptr,
    bool* pfOverflow = nullptr);

Description

Satoshi's original implementation used BN_bn2mpi() and BN_mpi2bn(). MPI uses the most significant bit of the first byte as sign. Thus 0x1234560000 is compact (0x05123456) and 0xc0de000000 is compact (0x0600c0de)

Bitcoin only uses this "compact" format for encoding difficulty targets, which are unsigned 256bit quantities. Thus, all the complexities of the sign bit and using base 256 are probably an implementation accident.

Decodes a value from the compact difficulty‐target format described above.

Return Value

A reference to this object holding the decoded value.

Parameters

Name

Description

nCompact

The 32‐bit compact encoding to decode.

pfNegative

If not null, set to true when the encoded sign bit is set.

pfOverflow

If not null, set to true when the encoded value overflows 256 bits.

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