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@ -262,28 +262,68 @@ public: |
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return vch; |
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return vch; |
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} |
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} |
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// The "compact" format is a representation of a whole
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// number N using an unsigned 32bit number similar to a
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// floating point format.
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// The most significant 8 bits are the unsigned exponent of base 256.
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// This exponent can be thought of as "number of bytes of N".
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// The lower 23 bits are the mantissa.
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// Bit number 24 (0x800000) represents the sign of N.
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// N = (-1^sign) * mantissa * 256^(exponent-3)
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//
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// Satoshi's original implementation used BN_bn2mpi() and BN_mpi2bn().
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// MPI uses the most significant bit of the first byte as sign.
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// Thus 0x1234560000 is compact (0x05123456)
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// and 0xc0de000000 is compact (0x0600c0de)
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// (0x05c0de00) would be -0x40de000000
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//
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// Bitcoin only uses this "compact" format for encoding difficulty
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// targets, which are unsigned 256bit quantities. Thus, all the
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// complexities of the sign bit and using base 256 are probably an
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// implementation accident.
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//
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// This implementation directly uses shifts instead of going
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// through an intermediate MPI representation.
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CBigNum& SetCompact(unsigned int nCompact) |
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CBigNum& SetCompact(unsigned int nCompact) |
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{ |
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{ |
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unsigned int nSize = nCompact >> 24; |
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unsigned int nSize = nCompact >> 24; |
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std::vector<unsigned char> vch(4 + nSize); |
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bool fNegative =(nCompact & 0x00800000) != 0; |
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vch[3] = nSize; |
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unsigned int nWord = nCompact & 0x007fffff; |
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if (nSize >= 1) vch[4] = (nCompact >> 16) & 0xff; |
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if (nSize <= 3) |
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if (nSize >= 2) vch[5] = (nCompact >> 8) & 0xff; |
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{ |
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if (nSize >= 3) vch[6] = (nCompact >> 0) & 0xff; |
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nWord >>= 8*(3-nSize); |
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BN_mpi2bn(&vch[0], vch.size(), this); |
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BN_set_word(this, nWord); |
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} |
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else |
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{ |
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BN_set_word(this, nWord); |
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BN_lshift(this, this, 8*(nSize-3)); |
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} |
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BN_set_negative(this, fNegative); |
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return *this; |
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return *this; |
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} |
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} |
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unsigned int GetCompact() const |
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unsigned int GetCompact() const |
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{ |
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{ |
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unsigned int nSize = BN_bn2mpi(this, NULL); |
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unsigned int nSize = BN_num_bytes(this); |
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std::vector<unsigned char> vch(nSize); |
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unsigned int nCompact = 0; |
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nSize -= 4; |
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if (nSize <= 3) |
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BN_bn2mpi(this, &vch[0]); |
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nCompact = BN_get_word(this) << 8*(3-nSize); |
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unsigned int nCompact = nSize << 24; |
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else |
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if (nSize >= 1) nCompact |= (vch[4] << 16); |
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{ |
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if (nSize >= 2) nCompact |= (vch[5] << 8); |
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CBigNum bn; |
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if (nSize >= 3) nCompact |= (vch[6] << 0); |
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BN_rshift(&bn, this, 8*(nSize-3)); |
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nCompact = BN_get_word(&bn); |
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} |
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// The 0x00800000 bit denotes the sign.
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// Thus, if it is already set, divide the mantissa by 256 and increase the exponent.
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if (nCompact & 0x00800000) |
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{ |
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nCompact >>= 8; |
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nSize++; |
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} |
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nCompact |= nSize << 24; |
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nCompact |= (BN_is_negative(this) ? 0x00800000 : 0); |
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return nCompact; |
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return nCompact; |
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} |
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} |
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