Return probability (Javascript)

Question

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.

Answer 1

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()

Answer 2

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);
}