import type { NumberArray } from 'cheminfo-types';
import { isAnyArray } from 'is-any-array';

import { xMean } from './xMean.ts';

export interface XVarianceOptions {
  /**
   * Unbiased option
   * @default true
   */
  unbiased?: boolean;

  /**
   * Precalculated mean of the data. If undefined it will be recalculated internally
   * @default mean calculated
   */
  mean?: number;
}

/**
 * Finds the variance of the data
 * @param values - the values of the array
 * @param options - options
 * @returns variance
 */
export function xVariance(values: NumberArray, options: XVarianceOptions = {}) {
  if (!isAnyArray(values)) {
    throw new TypeError('input must be an array');
  }

  const { unbiased = true, mean = xMean(values) } = options;
  let sqrError = 0;

  for (const value of values) {
    const x = value - mean;
    sqrError += x * x;
  }

  if (unbiased) {
    return sqrError / (values.length - 1);
  } else {
    return sqrError / values.length;
  }
}