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reimplement CBigNum's compact encoding of difficulty targets

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Use shifts instead of going through the MPI representation of BIGNUMs.
Be careful to keep the meaning of 0x00800000 as sign bit.
This commit is contained in:
Christian von Roques 2012-09-15 19:09:17 +02:00
parent 6f0cecfc47
commit 48a10a3780
1 changed files with 54 additions and 14 deletions
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68
src/bignum.h
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@ -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;
}