/****************************************************************************
 * Func: interp                                                             *
 *                                                                          *
 * Desc: This function inputs a set of knots and outputs an array of values *
 *       interpolated (linearly) between the knots                          *
 *                                                                          *
 * Params: knot_x- array of knot x values                                   *
 *         knot_y- array of knot y values                                   *
 *         spline_x- array of input x values of interpolated points         *
 *         spline_y- array of interpolated values                           *
 *         spline_length- number of values to interpolate                   *
 ****************************************************************************/

void interp(float *knot_x, float *knot_y,
	    float *spline_x, float *spline_y, int spline_length)
    {
    int i, index;           /* loop variable and array index */
    float left_x, right_x;  /* pointer to left and right of current interval */
    float left_y, right_y;  /* pointer to left and right value */
    float frac_x;           /* fraction x is within current knot interval */

    /* intialize variables */
    index =0;
    left_x=knot_x[index];
    left_y=knot_y[index];
    right_x=knot_x[index+1];
    right_y=knot_y[index+1];

    for(i=0; i<spline_length; i++)
	{
	if(spline_x[i] > knot_x[index])
	    {
	    left_x=knot_x[index];
	    left_y=knot_y[index];
	    index++;
	    right_x=knot_x[index];
	    right_y=knot_y[index];
	    }
	frac_x = (spline_x[i] - left_x) / (right_x - left_x);
	spline_y[i] = left_y + frac_x * (right_y - left_y);
	}
    }