unfair_shuffle.rb

# in-place shuffle
def wrong_shuffle arr
  arr.size.times {
    i, j = rand(arr.size), rand(arr.size)
    arr[i], arr[j] = arr[j], arr[i]
  }
  arr
end

# in-place shuffle
def wrong_shuffle2 arr
  (1...arr.size).to_a.reverse.each{|i| 
    j = rand(arr.size)
    arr[i], arr[j] = arr[j], arr[i]
  }
  arr
end

def benchmark(arr,samples,target,&shuffle)
  (1..samples).map{
    shuffled = yield arr.clone
    shuffled.find_index target
  }.group_by{|idx| idx}.map{|idx,l| [idx,l.count]}.sort_by(&:first).each{|a,b|puts [a,'.'*(b*100/samples)].join(': ')}
end

# benchmark((?a..?f).to_a, 10000, ?a, &method(:wrong_shuffle))
# 0: .......................
# 1: ...............
# 2: ...............
# 3: ...............
# 4: ...............
# 5: ...............
# => [[0, 23961], [1, 15265], [2, 15048], [3, 15236], [4, 15169], [5, 15321]]

# benchmark((?a..?f).to_a, 10000, ?a, &method(:wrong_shuffle2))
# 0: ........................................
# 1: ................
# 2: .............
# 3: ...........
# 4: ..........
# 5: ........
# => [[0, 4012], [1, 1639], [2, 1340], [3, 1149], [4, 1024], [5, 836]]

# fair shuffling algorithm
# benchmark((?a..?f).to_a, 100000, ?a) {|arr|
#   (1...arr.size).to_a.reverse.each{|i| j = rand(i+1); arr[i], arr[j] = arr[j], arr[i]}
#   arr
# }
# 0: ................
# 1: ................
# 2: ................
# 3: ................
# 4: ................
# 5: ................
# => [[0, 16676], [1, 16577], [2, 16640], [3, 16850], [4, 16732], [5, 16525]]