SpaceNinjaServer/src/services/rngService.ts

import { TRarity } from "warframe-public-export-plus";

export interface IRngResult {
    type: string;
    itemCount: number;
    probability: number;
}

export const getRandomElement = <T>(arr: readonly T[]): T | undefined => {
    return arr[Math.floor(Math.random() * arr.length)];
};

// Returns a random integer between min (inclusive) and max (inclusive).
// https://stackoverflow.com/a/1527820
export const getRandomInt = (min: number, max: number): number => {
    min = Math.ceil(min);
    max = Math.floor(max);
    return Math.floor(Math.random() * (max - min + 1)) + min;
};

export const generateRewardSeed = (): bigint => {
    const hiDword = getRandomInt(0, 0x7fffffff);
    const loDword = getRandomInt(0, 0xffffffff);
    let seed = (BigInt(hiDword) << 32n) | BigInt(loDword);
    if (Math.random() < 0.5) {
        seed *= -1n;
        seed -= 1n;
    }
    return seed;
};

export const getRewardAtPercentage = <T extends { probability: number }>(
    pool: T[],
    percentage: number
): T | undefined => {
    if (pool.length == 0) return;

    const totalChance = pool.reduce((accum, item) => accum + item.probability, 0);
    const randomValue = percentage * totalChance;

    let cumulativeChance = 0;
    for (const item of pool) {
        cumulativeChance += item.probability;
        if (randomValue <= cumulativeChance) {
            return item;
        }
    }
    return pool[pool.length - 1];
};

export const getRandomReward = <T extends { probability: number }>(pool: T[]): T | undefined => {
    return getRewardAtPercentage(pool, Math.random());
};

export const getRandomWeightedReward = <T extends { rarity: TRarity }>(
    pool: T[],
    weights: Record<TRarity, number>
): (T & { probability: number }) | undefined => {
    const resultPool: (T & { probability: number })[] = [];
    const rarityCounts: Record<TRarity, number> = { COMMON: 0, UNCOMMON: 0, RARE: 0, LEGENDARY: 0 };
    for (const entry of pool) {
        ++rarityCounts[entry.rarity];
    }
    for (const entry of pool) {
        resultPool.push({
            ...entry,
            probability: weights[entry.rarity] / rarityCounts[entry.rarity]
        });
    }
    return getRandomReward(resultPool);
};

export const getRandomWeightedRewardUc = <T extends { Rarity: TRarity }>(
    pool: T[],
    weights: Record<TRarity, number>
): (T & { probability: number }) | undefined => {
    const resultPool: (T & { probability: number })[] = [];
    const rarityCounts: Record<TRarity, number> = { COMMON: 0, UNCOMMON: 0, RARE: 0, LEGENDARY: 0 };
    for (const entry of pool) {
        ++rarityCounts[entry.Rarity];
    }
    for (const entry of pool) {
        resultPool.push({
            ...entry,
            probability: weights[entry.Rarity] / rarityCounts[entry.Rarity]
        });
    }
    return getRandomReward(resultPool);
};

// ChatGPT generated this. It seems to have a good enough distribution.
export const mixSeeds = (seed1: number, seed2: number): number => {
    let seed = seed1 ^ seed2;
    seed ^= seed >>> 21;
    seed ^= seed << 35;
    seed ^= seed >>> 4;
    return seed >>> 0;
};

// Seeded RNG with identical results to the game client. Based on work by Donald Knuth.
export class SRng {
    state: bigint;

    constructor(seed: bigint | number) {
        this.state = BigInt(seed);
    }

    randomInt(min: number, max: number): number {
        const diff = max - min;
        if (diff != 0) {
            this.state = (0x5851f42d4c957f2dn * this.state + 0x14057b7ef767814fn) & 0xffffffffffffffffn;
            min += (Number(this.state >> 32n) & 0x3fffffff) % (diff + 1);
        }
        return min;
    }

    randomElement<T>(arr: readonly T[]): T | undefined {
        return arr[this.randomInt(0, arr.length - 1)];
    }

    randomElementPop<T>(arr: T[]): T | undefined {
        if (arr.length != 0) {
            const index = this.randomInt(0, arr.length - 1);
            const elm = arr[index];
            arr.splice(index, 1);
            return elm;
        }
        return undefined;
    }

    randomFloat(): number {
        this.state = (0x5851f42d4c957f2dn * this.state + 0x14057b7ef767814fn) & 0xffffffffffffffffn;
        return (Number(this.state >> 38n) & 0xffffff) * 0.000000059604645;
    }

    randomReward<T extends { probability: number }>(pool: T[]): T | undefined {
        return getRewardAtPercentage(pool, this.randomFloat());
    }

    churnSeed(its: number): void {
        while (its--) {
            this.state = (0x5851f42d4c957f2dn * this.state + 0x14057b7ef767814fn) & 0xffffffffffffffffn;
        }
    }

    shuffleArray<T>(arr: T[]): void {
        for (let lastIdx = arr.length - 1; lastIdx >= 1; --lastIdx) {
            const swapIdx = this.randomInt(0, lastIdx);
            const tmp = arr[swapIdx];
            arr[swapIdx] = arr[lastIdx];
            arr[lastIdx] = tmp;
        }
    }

    shuffledArray<T>(inarr: readonly T[]): T[] {
        const arr = [...inarr];
        this.shuffleArray(arr);
        return arr;
    }
}

export const sequentiallyUniqueRandomElement = <T>(
    deck: readonly T[],
    idx: number,
    lookbehind: number,
    seed: number = 0
): T | undefined => {
    // This algorithm may modify a shuffle up to index `lookbehind + 1`. It assumes that the last `lookbehind` cards are not adjusted.
    if (lookbehind + 1 >= deck.length - lookbehind) {
        throw new Error(
            `this algorithm cannot guarantee ${lookbehind} unique cards in a row with a deck of size ${deck.length}`
        );
    }

    const iteration = Math.trunc(idx / deck.length);
    const card = idx % deck.length;
    const currentShuffle = new SRng(mixSeeds(new SRng(iteration).randomInt(0, 100_000), seed)).shuffledArray(deck);
    if (card < currentShuffle.length - lookbehind) {
        // We are indexing before the end of the deck, so adjustments may be needed to achieve uniqueness.
        const window: T[] = [];
        {
            const previousShuffle = new SRng(
                mixSeeds(new SRng(iteration - 1).randomInt(0, 100_000), seed)
            ).shuffledArray(deck);
            for (let i = previousShuffle.length - lookbehind; i != previousShuffle.length; ++i) {
                window.push(previousShuffle[i]);
            }
        }
        // From this point on, `window.length == lookbehind` should hold.
        for (let i = 0; i != lookbehind; ++i) {
            if (window.indexOf(currentShuffle[i]) != -1) {
                for (let j = i; ; ++j) {
                    // `j < currentShuffle.length - lookbehind` should hold.
                    if (window.indexOf(currentShuffle[j]) == -1) {
                        const tmp = currentShuffle[j];
                        currentShuffle[j] = currentShuffle[i];
                        currentShuffle[i] = tmp;
                        break;
                    }
                }
            }
            window.splice(0, 1);
            window.push(currentShuffle[i]);
        }
    }
    return currentShuffle[card];
};