It is actually doing a specific batch linear interpolation job for a 3D array.

nakeinterp3.c

/* nakeinterp3.c, use specifically for calIsenFlux            */
/* interpolate a 3d array A(i,j,k) only in k direction        */
/* interface as PT3dInterp = nakeinterp3(numx,numy,z,PT3d,zi) */
/* numx, numy are the first two dimensions                    */
/* z is the coordinate, zi is the interoplated coordinate     */
/* PT3d is the function to be interoplated, only done so in z */
/* z and zi should be column vector                           */

#include "mex.h"
#include "matrix.h"

void mexFunction(int nlhs, mxArray *plhs[],int nrhs, const mxArray *prhs[])
{
  double *PT3d, *PT3dInterp;
  double *z, *zi;
  mwSize numx, numy, numz, numzi;
  mwSize dims[3] = {0};
  
  mwSize  nx, m, k, i1, i9, imid, baseindex, baseArea;
  mwSize  loopX, loopY;
  double zik;
  
  numx = (int) * mxGetPr(prhs[0]);
  numy = (int) * mxGetPr(prhs[1]);
  z    =   mxGetPr(prhs[2]);
  PT3d =   mxGetPr(prhs[3]);
  zi   =   mxGetPr(prhs[4]);
  
  numz =  (int) mxGetM(prhs[2]);
  numzi=  (int) mxGetM(prhs[4]);
  
  dims[0] = numx;
  dims[1] = numy;
  dims[2] = numzi;
  plhs[0] = mxCreateNumericArray(3, dims, mxDOUBLE_CLASS, mxREAL);
  PT3dInterp = mxGetPr(plhs[0]);
  baseArea = numx*numy;
  
  for (k=numzi; k--;)
  {
    zik = zi[k];
    i1  = 0;
    i9  = numz-1;
    while (i9>i1+1)
    {
      imid = (i1+i9+1)/2;
      if (z[imid] < zik) i1 = imid;
      else i9 = imid;
    }
    if (i1 == i9) {
      for (loopX=0;loopX<numx;loopX++)
      {
	for (loopY=0;loopY<numy;loopY++)
	{
	  baseindex = loopX+numx*loopY;
        PT3dInterp[baseindex+baseArea*k] =
              PT3d[baseindex+baseArea*i1];
	}
      }
    }
    else
    {
     for (loopX=0;loopX<numx;loopX++)
      {
	for (loopY=0;loopY<numy;loopY++)
	{
	  baseindex = loopX+numx*loopY;
      PT3dInterp[baseindex+baseArea*k] =
             PT3d[baseindex+baseArea*i1] +
        (PT3d[baseindex+baseArea*i9]-PT3d[baseindex+baseArea*i1]) *
        (zik-z[i1]) / (z[i9]-z[i1]);
	}
      }
    }
  }
  
}