Playing around with combinations of generators.

generators.js

'use strict';

const items = [
  { id: 'a', group: 'x', count: 20 },
  { id: 'b', group: 'x', count: 10 },
  { id: 'c', group: 'x', count: 40 },
  { id: 'c', group: 'y', count: 40 },
  { id: 'c', group: 'y', count: 10 },
  { id: 'c', group: 'y', count: 30 },
  { id: 'c', group: 'z', count: 20 },
  { id: 'c', group: 'z', count: 30 }
];

function groupBy(key) {
  return items =>
    items.reduce(
      (grouped, item) => {
        const value = item[key];
        if (value in grouped) {
          grouped[value].push(item);
        } else {
          grouped[value] = [item];
        }
        return grouped;
      },
      {}
    );
}

function groupGenerator(key) {
  const groupFn = groupBy(key);

  return function*(items, i = 0) {
    const groups = groupFn(items);
    const keys = Object.keys(groups).sort();

    for (let k = 0; k < keys.length; k += 1) {
      const key = keys[(k + i) % keys.length];
      yield groups[key];
    }
  };
}

function sortBy(key) {
  return items => items.slice().sort((a, b) => {
      if (a[key] < b[key]) {
        return -1;
      }
      if (a[key] > b[key]) {
        return 1;
      }
      return 0;
    });
}

function sortGenerator(key) {
  const sortFn = sortBy(key);

  return function*(items) {
    const sorted = sortFn(items);

    for (let i of sorted) {
      yield i;
    }
  };
}

function groupedSortedGenerator(groupKey, sortKey) {
  const group = groupGenerator(groupKey);
  const sort = sortGenerator(sortKey);

  return function*(items) {
    for (let g of group(items)) {
      for (let i of sort(g)) {
        yield i;
      }
    }
  };
}

function interleavedGenerator(groupKey, sortKey) {
  const group = groupGenerator(groupKey);
  const sort = sortGenerator(sortKey);

  return function*(items) {
    const groups = Array.from(group(items));
    const iterators = groups.map(g => sort(g)[Symbol.iterator]());

    for (let i = 0; i < items.length; ) {
      const it = iterators[i % iterators.length];
      const next = it.next();
      if (!next.done) {
        i += 1;
        yield next.value;
      } else {
        continue;
      }
    }
  };
}

const groupedSorted = Array.from(groupedSortedGenerator('group', 'count')(items));
const interleaved = Array.from(interleavedGenerator('group', 'count')(items));

console.log('Grouped-sorted:');
console.log(groupedSorted);
console.log('Interleaved:');
console.log(interleaved);

// Grouped-sorted:
// [ { id: 'b', group: 'x', count: 10 },
//   { id: 'a', group: 'x', count: 20 },
//   { id: 'c', group: 'x', count: 40 },
//   { id: 'c', group: 'y', count: 10 },
//   { id: 'c', group: 'y', count: 30 },
//   { id: 'c', group: 'y', count: 40 },
//   { id: 'c', group: 'z', count: 20 },
//   { id: 'c', group: 'z', count: 30 } ]
// Interleaved:
// [ { id: 'b', group: 'x', count: 10 },
//   { id: 'c', group: 'y', count: 10 },
//   { id: 'c', group: 'z', count: 20 },
//   { id: 'a', group: 'x', count: 20 },
//   { id: 'c', group: 'y', count: 30 },
//   { id: 'c', group: 'z', count: 30 },
//   { id: 'c', group: 'x', count: 40 },
//   { id: 'c', group: 'y', count: 40 } ]