Make choice from array semi-randomly based on string

Question

I have this problem. I'd like to make random a choice from array [1,2,3,4] based on arbitrary 6 letter string in such way that this choice is always same if string is same.

So if i have string 'dogdog' function would return always '3' for example, but '4' for 'bigcat' etc.

I think the solution might be first hashing the string. How could one convert hash string into choice from array?

georg · Accepted Answer · 2016-11-29 16:22:30Z

1

You can calculate a hash from a string and take the array item at [hash % array.length]. An example with the DJB hashfunc (see http://www.cse.yorku.ca/~oz/hash.html for more):

function djbHash(s) {
    let hash = 5381;

    for (let c of s) {
        hash = hash * 33 + c.charCodeAt(0);
    }
    return hash;
}

function mapToValues(s, values) {
    return values[djbHash(s) % values.length];
}

console.log(mapToValues('dogdog', [1, 2, 3, 4]));
console.log(mapToValues('bigcat', [1, 2, 3, 4]));

answered Nov 29, 2016 at 16:22

georg

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Emil Vikström Over a year ago

This is a good solution because the hashing function have a good distribution. Better hashing functions give better results!

score 0 · Accepted Answer · 2016-11-29 16:08:50Z

0

A really simple hash function:

Change each letter of your word by a number (a is 1, b is 2, etc.). Let's call w the whole word changed in a number.
Calculate i = w mod 4. i will be a number between 0 and 3. Add 1 to it.
Congrats, you can now associate any word to a "random" number between 1 and 4. You can of course replace 4 by any other number to associate each world to a random number in any range.

edited Nov 29, 2016 at 16:08

answered Nov 29, 2016 at 16:00

user5455540

1 Comment

Emil Vikström Over a year ago

Your random number algorithm gives an exceptionally poor distribution.

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