#algorithm# memory copy

mymemcopy.cpp

#include <stdio.h>

// A homemade memcpy function.
// 1. Handle the memory overlap
// 2. Copy 4 bytes a time first as int, then copy the left ones as char (at most 3)
void* mymemcpy(void* dest, const void* src, size_t count)
{
	if(NULL == dest || NULL == src || dest == src)
		return dest;

	size_t num_int = count / sizeof(int);
	size_t num_byte = count % sizeof(int);

	int*  piDest = (int*)dest;
	int*  piSrc = (int*)src;

	char* pcDest = (char*)dest + num_int * 4;
	char* pcSrc = (char*)src + num_int * 4;

	// pSrc->pDest overlap
	if(pcDest > pcSrc && size_t(pcDest-pcDest) < count)
	{
		// this for must come first
		for(size_t i = num_byte; i >= 1; i--)
		{
			pcDest[i-1] = pcSrc[i-1];
		}

		for(size_t i = num_int; i >= 1; i--)
		{
			piDest[i-1] = piSrc[i-1];
		}

	}
	// no overlap or pDest->pSrc overlap
	else
	{
		for(size_t i = 0; i < num_int; i++)
		{
			piDest[i] = piSrc[i];
		}

		for(size_t i = 0; i < num_byte; i++)
		{
			pcDest[i] = pcSrc[i];
		}
	}
	return (void*)dest;
}

int main()
{
	// non overlap
	char name[100] = "I am Henry";
	char newname[100];
	char* mynewname1 = (char*)mymemcpy(newname, name, 11);
	printf(mynewname1);

	printf("\n");

	// overlap
	char* p = name + 3;
	char* mynewname2 = (char*)mymemcpy(p, name, 11);
	printf(mynewname2);

}