Why 5 bit left shift in hashing function?

Question

A popular answer for generating a hashing function in JS is given in Simple (non-secure) hash function for JavaScript? and Generate a Hash from string in Javascript

One example of the code is:

String.prototype.hashCode = function() {
    var hash = 0;
    if (this.length == 0) {
        return hash;
    }
    for (var i = 0; i < this.length; i++) {
        var char = this.charCodeAt(i);
        hash = ((hash<<5)-hash)+char;
        hash = hash & hash; // Convert to 32bit integer
    }
    return hash;
}

One line that doesn't make sense to me is hash = ((hash<<5)-hash)+char;

Could someone please explain WHY this is done? I gather that we are doing a 5 bit left shift on the hash. Is there any reason why it is 5 bits and not 4 or 6? Also why do we then minus the hash and add the char?

Answer 1

(hash << 5) is (hash * 32), so ((hash << 5) - hash) is (hash * 31). And the reason for multiplication by 31 is described in the answers to question Why does Java's hashCode() in String use 31 as a multiplier?

So, if this is changed to (hash * 31), the result is the same. Possibly (hash << 5) - hash is slightly faster, as shift / subtraction might be faster than multiplication. However, if that's really the case depends on many factors (whether JIT compilation is used, and optimizations in the JIT, and even the processor). So I assume the author of the code tested it, and found it to be faster for his case.