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    <p>I recommend using <strong>92821</strong>. Here's why.</p> <p>To give a meaningful answer to this you have to know something about the possible values of <code>i</code> and <code>j</code>. The only thing I can think of in general is, that in many cases small values will be more common than large values. (The odds of 15 appearing as a value in your program are much better than, say, 438281923.) So it seems a good idea to make the smallest hashcode collision as large as possible by choosing an appropriate prime. For 31 this rather bad - already for <code>i=-1</code> and <code>j=31</code> you have the same hash value as for <code>i=0</code> and <code>j=0</code>.</p> <p>Since this is interesting, I've written a little program that searched the whole int range for the best prime in this sense. That is, for each prime I searched for the minimum value of <code>Math.abs(i) + Math.abs(j)</code> over all values of <code>i,j</code> that have the same hashcode as <code>0,0</code>, and then took the prime where this minimum value is as large as possible. </p> <p><em>Drumroll</em>: the best prime in this sense is 486187739 (with the smallest collision being <code>i=-25486, j=67194</code>). Nearly as good and much easier to remember is 92821 with the smallest collision being <code>i=-46272 and j=46016</code>.</p> <p>If you give "small" another meaning and want to be the minimum of <code>Math.sqrt(i*i+j*j)</code> for the collision as large as possible, the results are a little different: the best would be 1322837333 with <code>i=-6815 and j=70091</code>, but my favourite 92821 (smallest collision <code>-46272,46016</code>) is again almost as good as the best value.</p> <p>I do acknowledge that it is quite debatable whether these calculation make much sense in practice. But I do think that taking 92821 as prime makes much more sense than 31, unless you have good reasons not to.</p>
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    1. POWhat is a sensible prime for hashcode calculation?
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    1. USHans-Peter Störr
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    1. COYou're looking for a magic number for a perfect hash, or a nearly perfect one at any rate. I'd be more interested in seeing a solution for arbitrary inputs up to the hash size (eg, 4 2-byte values in an 8 byte hashcode), than this particular case of simple transposition.
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    2. CO8 byte hashcode? At least in Java this is 4 bytes. Anyway: you could just continue the scheme that is used in eclipse hashCode generation: result = prime * result + i; result = prime * result + j; and so forth. For this 92821 is probably a good choice as prime - at least much better than the eclipse default 31.
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