# Return probability (Javascript)

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Problem

I’ve been working on a personal project. There’s this game called TFT which is an autochess game. Each match is played by 8 players and there’s a shared pool of champions. Each champion has their cost and how many of each there are, below is the table:

12 Champs that cost 1 gold, 39 of each champ.

12 Champs that cost 2 gold, 26 of each champ.

12 Champs that cost 3 gold, 18 of each champ.

9 Champs that cost 4 gold, 13 of each champ.

6 Champs that cost 5 gold, 10 of each champ.

So, I’ve been doing some functions to get the probability of getting x champ in a 5 champion deck, something I forgot to say is that in the game, you have your player level which somewhat influences in the pool, below is the multiplicator array, each row is a player level starting from Level 2 (which is the zeroth row) to Level 9, and each column is the champions cost (cost 1 to 5).

``````// pool.class.js
class Pool {
constructor(champPool) {
this.champPool = champPool; //this is an array of Champions(name,pool,cost)
this.chancePerTier = [
[1, 0, 0, 0, 0], //Level 2
[0.7, 0.25, 0.05, 0, 0], //Level 3
[0.5, 0.35, 0.15, 0, 0], //Level 4
[0.35, 0.35, 0.25, 0.05, 0], //Level 5
[0.25, 0.35, 0.3, 0.1, 0], //Level 6
[0.2, 0.3, 0.33, 0.15, 0.02], //Level 7
[0.15, 0.2, 0.35, 0.24, 0.06], //Level 8
[0.1, 0.15, 0.3, 0.3, 0.15] //Level 9
];
}

getChampProbability(champName, playerLevel) {
let poolSize = this.getPoolBasedOnLevel(playerLevel);
let poolIndex = this.champPool.findIndex(function(champ) {
return champ.getName() === champName;
});
if (poolIndex === -1) return 0;
let champPool = this.champPool[poolIndex].getPool();
let champCost = this.champPool[poolIndex].getCost();

return (
(champPool / poolSize) *
5 *
100 *
this.chancePerTier[playerLevel - 2][champCost - 1]
).toFixed(2);
}

getPoolBasedOnLevel(summonerLevel) {
let pool;
if (summonerLevel == 2) {
pool = this.champPool.reduce(function(prevChamp, currentChamp) {
if (currentChamp.getCost() == 1) {
return prevChamp + currentChamp.getPool();
} else {
return prevChamp;
}
}, 0);
} else if (summonerLevel == 3 || summonerLevel == 4) {
pool = this.champPool.reduce(function(prevChamp, currentChamp) {
if (currentChamp.getCost() >= 1 && currentChamp.getCost() <= 3) {
return prevChamp + currentChamp.getPool();
} else {
return prevChamp;
}
}, 0);
} else if (summonerLevel == 5 || summonerLevel == 6) {
pool = this.champPool.reduce(function(prevChamp, currentChamp) {
if (currentChamp.getCost() >= 1 && currentChamp.getCost() <= 4) {
return prevChamp + currentChamp.getPool();
} else {
return prevChamp;
}
}, 0);
} else {
pool = this.getWholePool();
}
return pool;
}

getWholePool() {
let allPool = this.champPool.reduce(function(prevChamp, currentChamp) {
return prevChamp + currentChamp.getPool();
}, 0);
return allPool;
}
}
module.exports = Pool;
``````
``````// champion.class.js
class Champion {
constructor(name, pool, cost, origin) {
this.name = name; // String
this.pool = pool; // Integer
this.cost = cost; // Integer
this.origin = origin; // Origin
}

getName() {
return this.name;
}

setPool(newPool) {
this.pool = newPool;
}

getPool() {
return this.pool;
}

getCost() {
return this.cost;
}

getOrigin() {
return this.origin;
}
}

module.exports = Champion;
``````

The code is working, but I think there’s a good-looking approach to this ugly mess.

Solution

As was already mentioned in the comments, a `switch` statement could be used to clean up the sets of `if` statements. Another option would be to abstract the similar callback functions passed to `.reduce()`– perhaps using partially applied functions to accept parameters – e.g. the minimum values in conditions like `currentChamp.getCost() <= 3`.

It seems like `let` is used for most variables. It is wise to default to using `const` to avoid accidental re-assignment. When you decide you do need to reassign a value then use `let`.

The method `Pool::getChampProbability()` could use `this.champPool.find()` instead of `this.champPool.findIndex()`, since the `index` is only used to dereference an array element. This would mean `0` should be returned if `find()` returned `undefined` (instead of `if (poolIndex === -1) return 0;`).

The method `Pool::getWholePool()` has a single use variable- i.e. `allPool`. That variable can be eliminated. Use of a linter would help find things like this.

The `else` keywords can be eliminated in many spots following a `return` in a conditional e.g. many of the else’s in `Pool::getPoolBasedOnLevel()`

`getPoolBasedOnLevel()` is the oddity here. There are three arrow functions doing similar jobs. Using one function with a lookup should clean things up. Effectively, each of them is doing this:

``````const sum = list=>list.reduce((a,b)=>a+b,0)

function poolTotal(champ_pool,min_cost,max_cost) {
const champs_for_level = champ_pool.filter(champ=>
champ.getCost()>=min_cost &&
champ.getCost()<=max_cost
);
const pools = champs_for_level.map(
champ=>champ.getPool()
)
return sum(pools);
}
``````

That function also implicitly determines the min and maxchampion cost for each level. It would benefit readability to state this is explcitly.

``````function costsForLevel(level) {
if (level===2) return {min:1,max:1}
if (level===3 || level===4) return {min:1,max:3}
if (level===5 || level===6) return {min:1,max:4}
return {min:0,max:Number.MAX_VALUE}
}
``````

With both of these functions defined, then `getPoolBasedOnLevel()` can be written as so

``````getPoolBasedOnLevel(summonerLevel) {
const {min,max} = costForLevel(summonerLevel);
return poolTotal(this.champPool,min,max);
}
``````