Problem Statement

The   Knuth shuffle   (a.k.a. the Fisher-Yates shuffle)   is an algorithm for randomly shuffling the elements of an array.

Implement the Knuth shuffle for an integer array (or, if possible, an array of any type).

Given an array items with indices ranging from 0 to last, the algorithm can be defined as follows (pseudo-code):

(These are listed here just for your convenience; no need to demonstrate them on the page.)

Let's start with the solution:

knuth_shuffle = (a) ->
  n = a.length
  while n > 1
    r = Math.floor(n * Math.random())
    n -= 1
    [a[n], a[r]] = [a[r], a[n]]
  a

counts =
  "1,2,3": 0
  "1,3,2": 0
  "2,1,3": 0
  "2,3,1": 0
  "3,1,2": 0
  "3,2,1": 0

for i in [1..100000]
  counts[knuth_shuffle([ 1, 2, 3 ]).join(",")] += 1

for key, val of counts
  console.log "#{key}: #{val}"