Random weighted 'tick' function

Question

This is a very simple function, but it gets run on a roughly 10ms interval, so I'm looking for ways to improve it. The first obvious thing is that it has to run the comparisons nearly every time as it starts with the lowest probabilities first. Is there a way I can reverse that without breaking it, or a way I can generally improve the performance of this function to achieve the same end result?

function find_coin() {
    var roll = Math.random() * 100;

    if (roll < 0.01)
        currency['platinum']++;
    else if (roll < 0.1)
        currency['gold']++;
    else if (roll < 1)
        currency['silver']++;
    else if (roll < 10)
        currency['bronze']++;
};

Answer 1

Reversing the order of the tests is probably your best option here. In your current code:

function find_coin() {
    var roll = Math.random() * 100;

    if (roll < 0.01)
        currency['platinum']++;
    else if (roll < 0.1)
        currency['gold']++;
    else if (roll < 1)
        currency['silver']++;
    else if (roll < 10)
        currency['bronze']++;
};

90% of your random values will be >= 10, additionally, 9 % will be between 1 and 10 %, so, 99% of your tests will have to check.... 4 roll conditions (< 0.01, < 0.1, < 1, and < 10). So, 99% of all your rolls will check 100% of your conditions.

If you reverse the logic, and do:

function find_coin() {
    var roll = Math.random() * 100;

    if (roll >= 10) {
        return
    }

    if (roll >= 1) { 
        currency['bronze']++;
    } else if (roll >= 0.1) {
        currency['silver']++;
    } else if (roll >= 0.01) {
        currency['gold']++;
    } else {
        currency['platinum']++;
    }

};

With the above code, 90% of your rolls will do just one test, 9% will do 2 tests, 0.9 will do 3 tests, and so on.

Note, also, that I have used braces even for the 1-liner blocks in the conditions. This is a good practice and it avoids bugs when maintaining your code, later.

Answer 2

Depending on your requirements and your performance needs you can pre-populate a lookup table of random numbers and implement flip_coin() as a table lookup:

function flip_coin() {
  return ++i >= lookupTable.length ? lookupTable[i=0] : lookupTable[i];
}