Produce MD5 or SHA1 hash code to long (64 bits)

You can truncate the hash and use just the first 64 bits. The hash will be somewhat less strong, but the first 64 bits are still extremely likely to be unique.

For most uses of a hash this is both a common and perfectly acceptable practice.

You can also store the complete hash in two 64-bit integers.

I'm using this (Java):

public class SimpleLongHash {
    final MessageDigest md;
    //
    public SimpleLongHash() throws NoSuchAlgorithmException {
        md = MessageDigest.getInstance("MD5");
    }
    //
    public long hash(final String str) {
        return hash(str.getBytes());
    }
    public long hash(final byte[] buf) {
        md.reset();
        final byte[] digest = md.digest(buf);
        return (getLong(digest, 0) ^ getLong(digest, 8));
    }
    //
    private static final long getLong(final byte[] array, final int offset) {
        long value = 0;
        for (int i = 0; i < 8; i++) {
            value = ((value << 8) | (array[offset+i] & 0xFF));
        }
        return value;
    }
}