Unique Value for Two Strings

Question

This seems like a fairly simple problem, but the solution is eluding me:

If I have the following two strings:

string str1 = "Ping"; string str2 = "Pong";

I'm needing an algorithm to create a unique value for the two strings, no matter what order they are in. For example:

UniqueValue(str1, str2) and UniqueValue(str2, str1) should return the exact same value.

EDIT: UniqueValue returns a string. I understand it won't be "truly" unique, but a hash-like value will work.

Thanks, Tyler

Could you be more specific about the return type of UniqueValue? — lsuarez
– lsuarez, Commented Jun 22, 2011 at 2:06
How unique is unique? Could never be replicated by any other combination? Ping and Pong result in the same unique value until the end of time? What's the resulting type? You have left a lot out. — Anthony Pegram
– Anthony Pegram, Commented Jun 22, 2011 at 2:09
See Justin's answer. It can be truly, truly unique, and you don't have to settle for a hash if you have O(N) storage. — user684934
– user684934, Commented Jun 22, 2011 at 2:27

Justin Simon · Accepted Answer · 2011-06-22 02:10:50Z

7

Sort the strings.

Convert / to // and ; to /; in each of the strings.

Concatenate the strings, putting a ; between each.

answered Jun 22, 2011 at 2:10

Justin Simon

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2 Comments

Justin Simon Over a year ago

This guarantees that the created value is unique, and that the combinations "ab" "cdef" and "abc" "def" produce different results, even if they contan the separator character (i.e., ;). The downside is that it will be as long as the strings themselves, but that's necessary if you want the hash to be one-to-one.

user684934 Over a year ago

Excellent! Beat me to the punch.

agent-j · Accepted Answer · 2011-06-22 02:13:15Z

3

You can Xor the hashcodes.

static string UniqueValue (string str1, string str2) { return (str1.GetHashCode () ^ str2.GetHashCode()).ToString (); }

Edit: Or, you can generate this string:

if (str2 > str1) return str1+str2; return str2+str1;

edited Jun 22, 2011 at 2:13

answered Jun 22, 2011 at 2:07

agent-j

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4 Comments

agent-j Over a year ago

Alternatively, you could concatenate the strings and sort them.

Chris Laplante Over a year ago

I think xoring is better. Sorting would introduce unneeded overhead.

Justin Simon Over a year ago

Your edit produces the same result for "ab" + "cdef" and "abc" + "def". You need a unique separator, and to escape that separator where it may exist in the composing strings.

agent-j Over a year ago

As I understood the question, the result had to be the same for given input, not unique accross the domain.

Jonathan Wood · Accepted Answer · 2011-06-22 02:06:26Z

I would use some sort of hash, but I don't see any algorithm that would produce a 100% guaranteed unique string that could never be duplicated by any other two strings.

Chris Laplante · Accepted Answer · 2011-06-22 02:08:26Z

I would hash them with SHA1: http://msdn.microsoft.com/en-us/library/system.security.cryptography.sha1.aspx

To determine in which orientation they should be in when concatenated, use String.CompareTo: http://msdn.microsoft.com/en-us/library/system.string.compareto.aspx.

Andrew · Accepted Answer · 2011-06-22 02:10:09Z

public int UniqueValue(params string[] strings) { return String.Join("~", strings.AsEnumerable<string>() .OrderBy<string, string>(s => s) .ToArray<string>()) .GetHashCode(); }

This, of course, is provided that you don't need complete uniqueness and that a hash code is good enough.

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