Question

What are some simple ways to hash a 32-bit integer (e.g. IP address, e.g. Unix time_t, etc.) down to a 16-bit integer?

E.g. hash_32b_to_16b(0x12345678) might return 0xABCD.

Let’s start with this as a horrible but functional example solution:

function hash_32b_to_16b(val32b) {
    return val32b % 0xffff;
}

Question is specifically about JavaScript, but feel free to add any language-neutral solutions, preferably without using library functions.

The context for this question is generating unique IDs (e.g. a 64-bit ID might be composed of several 16-bit hashes of various 32-bit values). Avoiding collisions is important.

Simple = good. Wacky+obfuscated = amusing.

Answer 1

This depends on the nature of the integers.
If they can contain some bit-masks, or can differ by powers of two, then simple XORs will have high probability of collisions.
You can try something like (i>>16) ^ ((i&0xffff) * p) with p being a prime number.

Security-hashes like MD5 are all good, but they are obviously an overkill here. Anything more complex than CRC16 is overkill.