diff options
| -rw-r--r-- | src/bignum.h | 68 |
1 files changed, 54 insertions, 14 deletions
diff --git a/src/bignum.h b/src/bignum.h index 9fea3f70fb..a0e5f306b3 100644 --- a/src/bignum.h +++ b/src/bignum.h @@ -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; } |