bicubic interpolation algorithm using Blake L. Carlson's code

Question

Could someone please explain to me how this code manages to interpolate images...

// bicubic interpolation by Blake L. Carlson <blake-carlson(at)uiowa(dot)edu
float f_x, f_y, a, b, rr, gg, bb, r1, r2;
int   i_x, i_y, xx, yy;
for(long y=0; y<newy; y++){
    //info.nProgress = (long)(100*y/newy);
    //if (info.nEscape) break;
    f_y = (float) y * yScale;
    i_y = (int) floor(f_y);
    a   = f_y - (float)floor(f_y);
    for(long x=0; x<newx; x++){
        f_x = (float) x * xScale;
        i_x = (int) floor(f_x);
        b   = f_x - (float)floor(f_x);

        rr = gg = bb = 0.0F;
        for(int m=-1; m<3; m++) {
            r1 = b3spline((float) m - a);
            for(int n=-1; n<3; n++) {
                r2 = b3spline(-1.0F*((float)n - b)); 
                xx = i_x+n+2;
                yy = i_y+m+2;
                if (xx<0) xx=0;
                if (yy<0) yy=0;
                if (xx>=source.GetWidth()) xx=source.GetWidth()-1;
                if (yy>=source.GetHeight()) yy=source.GetHeight()-1;

                srcPos = (yy * source.GetWidth() + xx) * 3;

                rr += src[srcPos] * r1 * r2;
                gg += src[srcPos+1] * r1 * r2;
                bb += src[srcPos+2] * r1 * r2;
            }
        }
        target.SetRGB(x, y, rr, gg, bb);
    }
}

source = target;

return source;

}

float wxImageResampler::b3spline(float x) { float a, b, c, d;

if((x + 2.0f) <= 0.0f) a = 0.0f; else a = (float)pow((x + 2.0f), 3.0f);
if((x + 1.0f) <= 0.0f) b = 0.0f; else b = (float)pow((x + 1.0f), 3.0f);
if(x <= 0) c = 0.0f; else c = (float)pow(x, 3.0f);  
if((x - 1.0f) <= 0.0f) d = 0.0f; else d = (float)pow((x - 1.0f), 3.0f);

return (0.16666666666666666667f * (a - (4.0f * b) + (6.0f * c) - (4.0f * d)));

}

im really confused with all the float data types.