byte copy

[John de la Garza <john@xxxxxxxxx>]

Can anone see why these two pieces of code would behave differently?
They are used in a mergesort merge

the top one ends up with c (cp points to an offset in c)
1 2 3 1

the bottom one (not commented out)
results in 1 2 3 4 (correct)

        /*
        if (alen == 0)
                memcpy(cp, bp, blen);
        else if (blen == 0){
                memcpy(cp, ap, alen);
        }
        */
        if (alen == 0)
                while (blen--)
                        *cp++ = *bp++;
        else
                while (alen--)
                        *cp++ = *ap++;


if you want to read it with more context see below
---------------------------------------------------

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

void pr_array(int *a, int len)
{
	int i;
	for (i = 0; i < len; i++)
		printf("%d ", a[i]);
	printf("\n");
}
void merge(int *a, int alen, int *b, int blen, int *c)
{
	int *ap = a;
	int *bp = b;
	int *cp = c;

	while (alen && blen) {
		if (*ap <= *bp) {
			*cp++ = *ap++;
			alen--;
		} else{
			*cp++ = *bp++;
			blen--;
		}
	}
	if (alen == 0)
		memcpy(cp, bp, blen);
	else if (blen == 0){
		memcpy(cp, ap, alen);
	}
	/*
	if (alen == 0)
		while (blen--)
			*cp++ = *bp++;
	else
		while (alen--)
			*cp++ = *ap++;
	*/
}

void msort(int *arr, int len)
{
	int mid = len / 2;
	int llen = mid;
	int rlen = len - mid;
	int *l = malloc(llen);
	int *r = malloc(rlen);
	int i;

	if (len <= 1)
		return;	
	for (i = 0; i < llen; i++)
		l[i] = arr[i];
	for (i = mid; i < mid + rlen; i++)
		r[i-mid] = arr[i];
	msort(l, llen);
	msort(r, rlen);
	merge(l, llen, r, rlen, arr);
	free(l);
	free(r);
}
int main()
{
	int arr[] = {4,3, 2, 1};
	int len = sizeof(arr) /sizeof(int);
	int x[] = {1,3,5,7};
	int y[] = {2,4,6,8};

	msort(arr, len);
	pr_array(arr, len);
	return(0);
}

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