How to create a consistent hashstring of a ruby array?

Question

I wonder how it is possible to create a consistent hash of a ruby array full of strings. The requirements are that the hash is always the same if the array contains the same values, independent of their order.

>> a = ["a", "b", "c", "d"]
>> SomeModule.hash(a)
=> "2aae6c35c94fcfb415dbe95f408b9ce91ee846ed"
>>
>> b = ["d", "b", "c", "a"]
>> SomeModule.hash(b)
=> "2aae6c35c94fcfb415dbe95f408b9ce91ee846ed"
>>
>> SomeModule.hash(a) == SomeModule.hash(b)
=> true

Zlib or digest only do strings, but I had to always sort the array and join it to get that working.

So is there anything better?

score 5 · Answer 1 · answered Oct 19 '12 at 08:21

5

You can convert your array to Set and call to_set method (don't foreget to `require 'set')

a = ["a", "b", "c", "d"]
a.to_set.hash # => 425494174200536878

b = ["d", "b", "c", "a"]
b.to_set.hash # => 425494174200536878

answered Oct 19 '12 at 08:21

Vasiliy Ermolovich

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Aah, that is a perfect solution thanks. Didn't know about the Set in Ruby. Fits perfectly! – Thomas Fankhauser Oct 19 '12 at 08:25
3

Looks easy but you will lose duplicate array elements. That way `['a','b']` would be the same as `['a','b','a']`. – iltempo Oct 19 '12 at 08:26
1

Is the `to_set.hash` method reliable to use here and will not change with other ruby 1.9 versions? – Thomas Fankhauser Oct 19 '12 at 09:45
3

I just realized that the `.hash` method just represents a runtime hash and is not consistent during multiple runs. What would you suggest there? – Thomas Fankhauser Oct 19 '12 at 10:33
2

Just unchecked this as the right answer, because unfortunately it isn't really consistent during multiple runs of the program.. – Thomas Fankhauser Oct 22 '12 at 02:34
The hash is consistent in a single process only. – iltempo Oct 22 '12 at 06:34

score 3 · Accepted Answer · answered Oct 19 '12 at 08:19

3

You can just sort the array, concatenate all elements to a string and hash it.

def hash(array)
   Digest::SHA1.digest(array.join)
end

answered Oct 19 '12 at 08:19

iltempo

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I see you last sentence now. But I'm not sure if you would come around sorting if the order of the elements should not matter. – iltempo Oct 19 '12 at 08:28
1

If you convert your array to a set you are going to lose duplicate elements http://www.ruby-doc.org/stdlib-1.9.3/libdoc/set/rdoc/Set.html. Don't know if this is what you want. – iltempo Oct 19 '12 at 08:38
That's - in my case - even desired behavior, so that would be totally acceptable. – Thomas Fankhauser Oct 19 '12 at 09:47
Seems, as with sort this is the only really "consistent" way how to do this.. so thanks! – Thomas Fankhauser Oct 22 '12 at 02:35

score 0 · Answer 3 · answered Oct 19 '12 at 08:21

0

There is already a standard library called set that introduces the Set class. You can also easily implement it by yourself. Instead of an array like this:

["a", "b", "c", "d"]

keep it as a hash:

{"a" => true, "b" => true, "c" => true, "d" => true}

answered Oct 19 '12 at 08:21

sawa

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score 0 · Answer 4 · answered Jan 20 '19 at 14:37

It seems the stateless aspect of .hash is being overlooked. Running the following respects time sensitivity of this issue:

a = %w(alpha bravo charlie delta echo foxtrot gulf hotel india july kilo sheep)
c = %w(alpha bravo charlie delta echo foxtrot gulf hotel india july kilo sheep)

require 'ap'

ap a
ap c

def h2(value)
    Digest::SHA512.hexdigest value.inspect
end

aa = h2(a)
cc = h2(c)
ap aa
ap cc

ap "they are equal" if aa==cc
puts

t = Turtle.new("Tom", "blue")  # an object with two ivars, name and colour
ap h2(t)

t.name = "Teth"
ap h2(t)

t.name = "Tom"
ap h2(t)
puts

t.colour = "ruby red"
ap h2(t)

t.colour = "blue"
ap h2(t)

How to create a consistent hashstring of a ruby array?

4 Answers4