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reimplement CBigNum's compact encoding of difficulty targets

Use shifts instead of going through the MPI representation of BIGNUMs.
Be careful to keep the meaning of 0x00800000 as sign bit.
0.8
Christian von Roques 12 years ago
parent
commit
48a10a3780
  1. 68
      src/bignum.h

68
src/bignum.h

@ -262,28 +262,68 @@ public: @@ -262,28 +262,68 @@ public:
return vch;
}
// 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 = (-1^sign) * mantissa * 256^(exponent-3)
//
// 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)
// (0x05c0de00) would be -0x40de000000
//
// 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.
//
// This implementation directly uses shifts instead of going
// through an intermediate MPI representation.
CBigNum& SetCompact(unsigned int nCompact)
{
unsigned int nSize = nCompact >> 24;
std::vector<unsigned char> vch(4 + nSize);
vch[3] = nSize;
if (nSize >= 1) vch[4] = (nCompact >> 16) & 0xff;
if (nSize >= 2) vch[5] = (nCompact >> 8) & 0xff;
if (nSize >= 3) vch[6] = (nCompact >> 0) & 0xff;
BN_mpi2bn(&vch[0], vch.size(), this);
bool fNegative =(nCompact & 0x00800000) != 0;
unsigned int nWord = nCompact & 0x007fffff;
if (nSize <= 3)
{
nWord >>= 8*(3-nSize);
BN_set_word(this, nWord);
}
else
{
BN_set_word(this, nWord);
BN_lshift(this, this, 8*(nSize-3));
}
BN_set_negative(this, fNegative);
return *this;
}
unsigned int GetCompact() const
{
unsigned int nSize = BN_bn2mpi(this, NULL);
std::vector<unsigned char> vch(nSize);
nSize -= 4;
BN_bn2mpi(this, &vch[0]);
unsigned int nCompact = nSize << 24;
if (nSize >= 1) nCompact |= (vch[4] << 16);
if (nSize >= 2) nCompact |= (vch[5] << 8);
if (nSize >= 3) nCompact |= (vch[6] << 0);
unsigned int nSize = BN_num_bytes(this);
unsigned int nCompact = 0;
if (nSize <= 3)
nCompact = BN_get_word(this) << 8*(3-nSize);
else
{
CBigNum bn;
BN_rshift(&bn, this, 8*(nSize-3));
nCompact = BN_get_word(&bn);
}
// The 0x00800000 bit denotes the sign.
// Thus, if it is already set, divide the mantissa by 256 and increase the exponent.
if (nCompact & 0x00800000)
{
nCompact >>= 8;
nSize++;
}
nCompact |= nSize << 24;
nCompact |= (BN_is_negative(this) ? 0x00800000 : 0);
return nCompact;
}

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