XCOR - Compute z = x cross-correlated with y

xcor	compute z= x cross-correlated with y

Function Prototype:
void xcor (int lx, int ifx, float *x, int ly, int ify, float *y ,
	int lz, int ifz, float *z);

Input:
lx		length of x array
ifx		sample index of first x
x		array[lx] to be cross-correlated with y
ly		length of y array
ify		sample index of first y
y		array[ly] with which x is to be cross-correlated
lz		length of z array
ifz		sample index of first z

Output:
z		array[lz] containing x cross-correlated with y

Notes:
See notes for convolution function conv().

The operation "x cross correlated with y"  is defined to be:

           ifx+lx-1
    z[i] =   sum    x[j]*y[i+j]  ;  i = ifz,...,ifz+lz-1
            j=ifx

This function performs cross-correlation by
(1) reversing the samples in the x array while copying
    them to a temporary array, and
(2) calling function conv() with ifx set to 1-ifx-lx.
Assuming that the overhead of reversing the samples in x is negligible,
this method enables cross-correlation to be performed as efficiently as
convolution, while reducing the amount of code that must be optimized
and maintained.

Author:  Dave Hale, Colorado School of Mines, 11/23/91