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1/**2 * Obliterator Combinations Function3 * ==================================4 *5 * Iterator returning combinations of the given array.6 */7var Iterator = require('./iterator.js');8 9/**10 * Helper mapping indices to items.11 */12function indicesToItems(target, items, indices, r) {13  for (var i = 0; i < r; i++)14    target[i] = items[indices[i]];15}16 17/**18 * Combinations.19 *20 * @param  {array}    array - Target array.21 * @param  {number}   r     - Size of the subsequences.22 * @return {Iterator}23 */24module.exports = function combinations(array, r) {25  if (!Array.isArray(array))26    throw new Error('obliterator/combinations: first argument should be an array.');27 28  var n = array.length;29 30  if (typeof r !== 'number')31    throw new Error('obliterator/combinations: second argument should be omitted or a number.');32 33  if (r > n)34    throw new Error('obliterator/combinations: the size of the subsequences should not exceed the length of the array.');35 36  if (r === n)37    return Iterator.of(array.slice());38 39  var indices = new Array(r),40      subsequence = new Array(r),41      first = true,42      i;43 44  for (i = 0; i < r; i++)45    indices[i] = i;46 47  return new Iterator(function next() {48    if (first) {49      first = false;50 51      indicesToItems(subsequence, array, indices, r);52      return {value: subsequence};53    }54 55    if (indices[r - 1]++ < n - 1) {56      indicesToItems(subsequence, array, indices, r);57      return {value: subsequence};58    }59 60    i = r - 2;61 62    while (i >= 0 && indices[i] >= (n - (r - i)))63      --i;64 65    if (i < 0)66      return {done: true};67 68    indices[i]++;69 70    while (++i < r)71      indices[i] = indices[i - 1] + 1;72 73    indicesToItems(subsequence, array, indices, r);74    return {value: subsequence};75  });76};77