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POA particular type of hash on a String concatenation
 

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    <p>I need a specialised hash function h(X,Y) in Java with the following properties.</p> <ol> <li>X and Y are strings.</li> <li>h(X,Y) = h(Y,X).</li> <li>X and Y are arbitrary length strings and there is no length limit on the result of h(X,Y) either.</li> <li>h(X,Y) and h(Y,X) should not collide with h(A,B) = h(B,A) if X is not equal to A and Y is not equal to B.</li> <li>h() does not need to be a secure hash function unless it is necessary to meet the aforementioned requirements.</li> <li>Fairly high-performant but this is an open-ended criterion.</li> </ol> <p>In my mind, I see requirements 2 and 4 slightly contradictory but perhaps I am worrying too much.</p> <p>At the moment, what I am doing in Java is the following:</p> <pre><code>public static BigInteger hashStringConcatenation(String str1, String str2) { BigInteger bA = BigInteger.ZERO; BigInteger bB = BigInteger.ZERO; for(int i=0; i&lt;str1.length(); i++) { bA = bA.add(BigInteger.valueOf(127L).pow(i+1).multiply(BigInteger.valueOf(str1.codePointAt(i)))); } for(int i=0; i&lt;str2.length(); i++) { bB = bB.add(BigInteger.valueOf(127L).pow(i+1).multiply(BigInteger.valueOf(str2.codePointAt(i)))); } return bA.multiply(bB); } </code></pre> <p>I think this is hideous but that's why I am looking for nicer solutions. Thanks.</p> <p>Forgot to mention that on a 2.53GHz dual core Macbook Pro with 8GB RAM and Java 1.6 on OS X 10.7, the hash function takes about 270 micro-seconds for two 8 (ASCII) character Strings. I suspect this would be higher with the increase in the String size, or if Unicode characters are used.</p>
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    1. COI have given it a good thought... And @Anirban I believe you've a pretty neat implementation. Good Work.
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    2. CO@BharatSinha, thanks but I am working with hideously large precision numbers, which is a worry regarding performance. Plus, the return statement code read `return bA.add(bB)` instead. My main concern is, however, what is the probability that my solution breaks requirement 4.
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    3. COWell As far as I can see the design of the method; it looks perfect... Since you are adding the powers of 127 so I don't see a chance of sum coming the same...
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