◆ memdecr_BE()

bool Chusov::Memory::memdecr_BE	(	void *	pDest,
		std::size_t	cbDest,
		const void *	pSrc,
		std::size_t	cbSrc
	)

noexcept

Decrements an arbitrary precision integer specified by a vector of bytes in the Big-Endian byte order doing it in place or writing the result into a separate buffer with zero-extension or contraction to fit the buffer and returns the resulting borrow bit.

Parameters

[out]	pDest	A pointer to the destination buffer which receives the result of the decrement modulo `b^cbDest`, where `b = 2^CHAR_BIT` is a number of values that can be represented using one byte. That is, the capacity `cbDest` of the buffer can be different from the size of the input value: if the capacity is greater than `cbSrc`, the input value is zero-extended to `cbDest` bytes before the decrement; otherwise, if the capacity is less than the size of the input integer, the necessary number of high-order bytes of the result of the decrement are discarded.
	cbDest	Capacity of the output buffer pointed to by `pDest` as well as the size of the resulting integer.
[in]	pSrc	A pointer to the input integer to decrement. Both pointers, `pSrc` and `pDest`, may refer to the same buffer in memory, in which case the operation is performed in place. Other than that, the buffers may not overlap.
	cbSrc	A size of the input integer in bytes.

Returns: true if a borrow from the digit in the position cbDest has taken place, i.e. the input value is zero. Otherwise, the return value is false.

The function considers the input vector to specify the integer in the Big-Endian byte order. That is, given that \(V_X = \{x_0, \dots, x_n\}\) is the input vector pointed to by pSrc, such that \(x_0\) is the first byte in memory of the vector, \(x_n\) is the last byte and n = cbSrc - 1, the input value for the operation is \(X = \sum_{i=0}^{n}x_{n - i}\cdot b^i\).

The operation is performed modulo b^cbDest. That is, before the decrement takes place the input value \(X\) is zero-extended or contracted to fit the buffer of cbDest bytes. The input value is therefore transformed to the value \(X' = \sum_{i=0}^{m}x'_i\cdot b^i\), where m = cbDest - 1, \(x'_i = x_{n - i}\) if \(0 \le i \le n\), and \(x'_i = 0\) otherwise. The value \(X'\) is then decremented and written into the destination buffer pDest, again, in the Big-Endian byte order (after the decrement the \(m\)-th byte of the result is written to pDest[0]).

To decrement Little-Endian integers use memdecr_LE_inplace.

See also: memdecr_BE;
memdecr_LE_inplace;
memdecr_LE_copy;
memincr_LE;
memadd_LE;
memsub_LE;
memmul_LE;
memdiv_LE.