JavaScript: Weighted Random Generator

I'm attempting to create a function that will give me a "fish" at random, though a bias random depending on the weight value for the "fish".

var FISH = {
"level1": [
//["name",      xp, weight]
["Shrimp",    10, 95],
["Sardine",   20, 85],
["Herring",   30, 75],
["Anchovies", 40, 65],
["Mackerel",  40, 55]
]
};

function randomRange(min, max) {
return Math.random() * (max - min) + min;
}

function getRandomFish(level) {
var fishForLevel = FISH[level],
numberOfFish = fishForLevel.length,
chance = randomRange(0, 100);

if (numberOfFish > 1) {

var fish = fishForLevel[Math.floor(randomRange(0, numberOfFish))];

if (chance <= fish[2]) {
return fish;
} else {
return getRandomFish(level);
}

} else {
return fishForLevel[0];
}
}


Example of outcome: http://jsfiddle.net/qAvAs/

If this could be made more efficient, how would one do so?

I'd suggest you select a random number that's from 0 to the total weight of your fish. Then, each fish can have a piece of that random range according to it's weight which gives it the weighting you want. This ends up giving each fish a number of buckets that corresponds to it's weighting value and then you pick a random number from 0 to the total number of buckets and find out who's bucket the random number landed in.

It's the same idea as turning each weighting number into a percentage change and then picking a random number from 0 to 99.

This code should do that:

var FISH = {
"level1": [
//["name",      xp, weight]
["Shrimp",    10, 95],
["Sardine",   20, 85],
["Herring",   30, 75],
["Anchovies", 40, 65],
["Mackerel",  40, 55]
]
};

function getRandomFish(level) {
var fishForLevel = FISH[level];
var fishTotalWeight = 0, fishCumWeight = 0, i;
// sum up the weights
for (i = 0; i < fishForLevel.length; i++) {
fishTotalWeight += fishForLevel[i][2];
}
var random = Math.floor(Math.random() * fishTotalWeight);
// now find which bucket out random value is in

for (i = 0; i < fishForLevel.length; i++) {
fishCumWeight += fishForLevel[i][2];
if (random < fishCumWeight) {
return(fishForLevel[i]);
}
}
}

• I like this. Certainly seems more efficient then what I was using. :)
– Shaz
Aug 20 '11 at 17:52