A simple quicksort

First pass at a simple (i.e., non-optimal) quicksort function implemented in coffeescript.

Notably, does not sort in-place and is less than optimal in runtime because of its choice of pivot element (viz., the first element).

---

A very naive version of quicksort might look as follows:

qsort = ([p, rest...]) ->
  return [] unless x?
  lt = (x for x in rest when x <= p)
  gt = (x for x in rest when x > p)
  qsort(lt)
    .concat(x)
    .concat(qsort(gt))

This is nice and compact, but we’re iterating over the input array (minus its first element p) twice. So, in lieu of the double list comprehensions, we’re going to reduce the array to a partition object (part) with less-than (lt) and greater-than (gt) properties.

---

We first create a function for creating a partition iterator. We’ll use this to reduce the input to a partition object. This contains the items less than or equal to p (viz. part.lt) and those greater than p (part.gt). The resulting partition object will have the following form:

part =
  lt: [ input items <= p ]
  gt: [ input items > p  ]

The following returns an iterator that takes a partition object part for its initial value / previous value / accumulator. The returned partition should be partitioned at p.

partitionAt = (p) ->
  (part, x) ->
    if x > p then part.gt.push(x) else part.lt.push(x)
    part

Initial partition object:

init = ->
    lt: []
    gt: []

Our simple quicksort routine:

qsort = ([x, xs...]) ->
  return [] unless x?
  part = xs.reduce(partitionAt(x), init())
  qsort(part.lt)
    .concat(x)
    .concat(qsort(part.gt))

Testing:

{ok, deepEqual} = require 'assert'
eq = deepEqual

input    = [3, 2, 1, 3, 4, 6, 5]
expected = [1, 2, 3, 3, 4, 5, 6]

eq qsort(input), expected