import defined from "./defined.js";
import DeveloperError from "./DeveloperError.js";

/**
 * An {@link InterpolationAlgorithm} for performing linear interpolation.
 *
 * @namespace LinearApproximation
 */
const LinearApproximation = {
  type: "Linear",
};

/**
 * Given the desired degree, returns the number of data points required for interpolation.
 * Since linear interpolation can only generate a first degree polynomial, this function
 * always returns 2.
 * @param {number} degree The desired degree of interpolation.
 * @returns {number} This function always returns 2.
 *
 */
LinearApproximation.getRequiredDataPoints = function (degree) {
  return 2;
};

/**
 * Interpolates values using linear approximation.
 *
 * @param {number} x The independent variable for which the dependent variables will be interpolated.
 * @param {number[]} xTable The array of independent variables to use to interpolate.  The values
 * in this array must be in increasing order and the same value must not occur twice in the array.
 * @param {number[]} yTable The array of dependent variables to use to interpolate.  For a set of three
 * dependent values (p,q,w) at time 1 and time 2 this should be as follows: {p1, q1, w1, p2, q2, w2}.
 * @param {number} yStride The number of dependent variable values in yTable corresponding to
 * each independent variable value in xTable.
 * @param {number[]} [result] An existing array into which to store the result.
 * @returns {number[]} The array of interpolated values, or the result parameter if one was provided.
 */
LinearApproximation.interpolateOrderZero = function (
  x,
  xTable,
  yTable,
  yStride,
  result
) {
  //>>includeStart('debug', pragmas.debug);
  if (xTable.length !== 2) {
    throw new DeveloperError(
      "The xTable provided to the linear interpolator must have exactly two elements."
    );
  } else if (yStride <= 0) {
    throw new DeveloperError(
      "There must be at least 1 dependent variable for each independent variable."
    );
  }
  //>>includeEnd('debug');

  if (!defined(result)) {
    result = new Array(yStride);
  }

  let i;
  let y0;
  let y1;
  const x0 = xTable[0];
  const x1 = xTable[1];

  //>>includeStart('debug', pragmas.debug);
  if (x0 === x1) {
    throw new DeveloperError(
      "Divide by zero error: xTable[0] and xTable[1] are equal"
    );
  }
  //>>includeEnd('debug');

  for (i = 0; i < yStride; i++) {
    y0 = yTable[i];
    y1 = yTable[i + yStride];
    result[i] = ((y1 - y0) * x + x1 * y0 - x0 * y1) / (x1 - x0);
  }

  return result;
};
export default LinearApproximation;