See hash_clash.rb for a description

hash_clash.rb

# Produces a histogram illustrating key collisions.
# A round consists of repeatedly drawing a value from 0..255 
# until we first see a duplicate value. We count how many draws 
# were needed to hit that first duplicate.
# Repeat for 1 million rounds, then draw a histogram showing
# num draws vs num rounds with that num draws. 
#
# English escapes me - just read the code!
#
# Written to illustrate comments in this article 
# http://nerds-central.blogspot.co.uk/2013/10/ordered-vs-unorderedmap-in-c-some-facts.html

histo = Hash.new(0)

iters = 1_000_000

1.upto(iters) do 
  set = {}  # add to this till we get a clash
  i = 0
  loop do
  	i += 1
  	num = rand(256)
  	break if set[num]  # seen this num before => clash
  	set[num] = true
  end
  histo[i] += 1
end

puts "#values\t%age\tGraph"


histo.keys.sort.each do |k|
	print k, "\t",  "%.2f" % (100*histo[k]/iters.to_f), "\t", "-" * (histo[k]/1000), "\n"
end

output.txt

$ ruby hash_clash.rb
#values	%age	Graph
2	0.40	----
3	0.76	-------
4	1.14	-----------
5	1.51	---------------
6	1.89	------------------
7	2.22	----------------------
8	2.51	-------------------------
9	2.79	---------------------------
10	3.02	------------------------------
11	3.28	--------------------------------
12	3.44	----------------------------------
13	3.62	------------------------------------
14	3.70	------------------------------------
15	3.81	--------------------------------------
16	3.84	--------------------------------------
17	3.89	--------------------------------------
18	3.83	--------------------------------------
19	3.78	-------------------------------------
20	3.74	-------------------------------------
21	3.64	------------------------------------
22	3.54	-----------------------------------
23	3.43	----------------------------------
24	3.23	--------------------------------
25	3.07	------------------------------
26	2.95	-----------------------------
27	2.75	---------------------------
28	2.54	-------------------------
29	2.37	-----------------------
30	2.20	----------------------
31	1.99	-------------------
32	1.86	------------------
33	1.63	----------------
34	1.50	--------------
35	1.35	-------------
36	1.21	------------
37	1.07	----------
38	0.95	---------
39	0.81	--------
40	0.72	-------
41	0.64	------
42	0.54	-----
43	0.46	----
44	0.40	---
45	0.34	---
46	0.29	--
47	0.24	--
48	0.20	--
49	0.16	-
50	0.14	-
51	0.11	-
52	0.10	
53	0.07	
54	0.07	
55	0.05	
56	0.04	
57	0.03	
58	0.03	
59	0.02	
60	0.02	
61	0.01	
62	0.01	
63	0.01	
64	0.01	
65	0.00	
66	0.00	
67	0.00	
68	0.00	
69	0.00	
70	0.00	
71	0.00	
72	0.00	
73	0.00	
74	0.00	
76	0.00	
77	0.00	
80	0.00	
81	0.00