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 Post subject: Strategies for calculating hash codes from composite keys?
PostPosted: Fri Jul 23, 2010 6:21 pm 
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Joined: Thu May 20, 2010 12:31 pm
Posts: 28
Are there any general collision-free best practices to generate hash codes for any (atomic) type composite primary keys?

I thought about it for a few hours and came to the conclusion, that a string concatenated by *all* primary key columns would be the only reliable way to do so. Calling Java's hashCode method on that concatenated string should yield a unique integer. (it would in fact somehow mimic what a database index does *eyerolling*)

Code:
    @Override
    public int hashCode()
    {
        String sSurrogate =   String.format("%011d", this.gameId)         // 11 chars
                            + String.format("%01d" , this.isHome ? 1 : 0) //  1 char
                            + String.format("%011d", this.playerId)       // 11 chars
                            + String.format("%011d", this.rosterId)       // 11 chars
                            + String.format("%06d" , this.period);        //  6 chars
       
        //System.out.println("surrogate = '" + sSurrogate + "'");
       
        return sSurrogate.hashCode();
    }


However, I don't believe my solution is optimal. There must be algorithms out there that are better. The above probably runs rather slow, but it's still faster than accessing a DB generator. What about standard hashing functions like MD5, SHA1, CRC, and Adler?

Karsten

PS: This is for overriding composite key ID classes...


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