gini/index.js

Calculate the Gini coefficient of a data set.

"use strict";

module.exports.ordered = function(data) {
  if (!Array.isArray(data)) {
    throw new Error("Data set is not an array.");
  }

  if (data.length <= 0) {
    throw new Error("Data set is an empty array.");
  }

  var sum1 = 0;
  var sum2 = 0;
  var previousValue;

  for (var i = 0; i < data.length; i++) {
    var value = Number(data[i]);

    if (isNaN(value)) {
      throw new Error("Data set contains non-numbers.");
    }

    if (value < 0) {
      throw new Error("Data set contains negative numbers.");
    }

    if (i > 0 && value < previousValue) {
      throw new Error("Data set is not ordered ascendingly.");
    }

    sum1 += ((2 * (i + 1)) - data.length - 1) * value;
    sum2 += value;
    previousValue = value;
  }

  if (sum2 == 0) {
    return 0; // If data set contains only zeroes, the equation further below won't work.
  }

  return sum1 / (Math.pow(data.length, 2) * (sum2 / data.length));
};

module.exports.unordered = function(data) {
  if (!Array.isArray(data)) {
    throw new Error("Data set is not an array.");
  }

  if (data.length <= 0) {
    throw new Error("Data set is an empty array.");
  }

  data = data.map(function(value) {
    value = Number(value);

    if (isNaN(value)) {
      throw new Error("Data set contains non-numbers.");
    }

    if (value < 0) {
      throw new Error("Data set contains negative numbers.");
    }

    return value;
  });

  var sum1 = 0;
  var sum2 = 0;

  for (var i = 0; i < data.length; i++) {
    for (var j = 0; j < data.length; j++) {
      if (i != j) {
        sum1 += Math.abs(data[i] - data[j]);
      }
    }

    sum2 += data[i];
  }

  if (sum2 == 0) {
    return 0; // If data set contains only zeroes, the equation further below won't work.
  }

  return sum1 / (2 * Math.pow(data.length, 2) * (sum2 / data.length));
};