import { PickPartial } from '../types/shared.ts';
import { delay } from './delay.ts';

/** A Leaky Bucket.
 * Useful for rate limiting purposes.
 * This uses `performance.now()` instead of `Date.now()` for higher accuracy.
 *
 * NOTE: This bucket is lazy, means it only updates when a related method is called.
 */
export interface LeakyBucket {
    // ----------
    // PROPERTIES
    // ----------

    /** How many tokens this bucket can hold. */
    max: number;
    /** Amount of tokens gained per interval.
     * If bigger than `max` it will be pressed to `max`.
     */
    refillAmount: number;
    /** Interval at which the bucket gains tokens. */
    refillInterval: number;

    // ----------
    // METHODS
    // ----------

    /** Acquire tokens from the bucket.
     * Resolves when the tokens are acquired and available.
     * @param {boolean} [highPriority=false] Whether this acquire is should be done asap.
     */
    acquire(amount: number, highPriority?: boolean): Promise<void>;

    /** Returns the number of milliseconds until the next refill. */
    nextRefill(): number;

    /** Current tokens in the bucket. */
    tokens(): number;

    // ----------
    // INTERNAL STATES
    // ----------

    /** @private Internal track of when the last refill of tokens was.
     * DO NOT TOUCH THIS! Unless you know what you are doing ofc :P
     */
    lastRefill: number;

    /** @private Internal state of whether currently it is allowed to acquire tokens.
     * DO NOT TOUCH THIS! Unless you know what you are doing ofc :P
     */
    allowAcquire: boolean;

    /** @private Internal number of currently available tokens.
     * DO NOT TOUCH THIS! Unless you know what you are doing ofc :P
     */
    tokensState: number;

    /** @private Internal array of promises necessary to guarantee no race conditions.
     * DO NOT TOUCH THIS! Unless you know what you are doing ofc :P
     */
    waiting: ((_?: unknown) => void)[];
}

export function createLeakyBucket(
    { max, refillInterval, refillAmount, tokens, waiting, ...rest }:
        & Omit<
            PickPartial<
                LeakyBucket,
                'max' | 'refillInterval' | 'refillAmount'
            >,
            'tokens'
        >
        & {
            /** Current tokens in the bucket.
             * @default max
             */
            tokens?: number;
        },
): LeakyBucket {
    return {
        max,
        refillInterval,
        refillAmount: refillAmount > max ? max : refillAmount,
        lastRefill: performance.now(),
        allowAcquire: true,

        nextRefill: function () {
            return nextRefill(this);
        },

        tokens: function () {
            return updateTokens(this);
        },

        acquire: async function (amount, highPriority) {
            return await acquire(this, amount, highPriority);
        },

        tokensState: tokens ?? max,
        waiting: waiting ?? [],

        ...rest,
    };
}

/** Update the tokens of that bucket.
 * @returns {number} The amount of current available tokens.
 */
function updateTokens(bucket: LeakyBucket): number {
    const timePassed = performance.now() - bucket.lastRefill;
    const missedRefills = Math.floor(timePassed / bucket.refillInterval);

    // The refill shall not exceed the max amount of tokens.
    bucket.tokensState = Math.min(bucket.tokensState + (bucket.refillAmount * missedRefills), bucket.max);
    bucket.lastRefill += bucket.refillInterval * missedRefills;

    return bucket.tokensState;
}

function nextRefill(bucket: LeakyBucket): number {
    // Since this bucket is lazy update the tokens before calculating the next refill.
    updateTokens(bucket);

    return (performance.now() - bucket.lastRefill) + bucket.refillInterval;
}

async function acquire(bucket: LeakyBucket, amount: number, highPriority = false): Promise<void> {
    // To prevent the race condition of 2 acquires happening at once,
    // check whether its currently allowed to acquire.
    if (!bucket.allowAcquire) {
        // create, push, and wait until the current running acquiring is finished.
        await new Promise((resolve) => {
            if (highPriority) {
                bucket.waiting.unshift(resolve);
            } else {
                bucket.waiting.push(resolve);
            }
        });

        // Somehow another acquire has started,
        // so need to wait again.
        if (!bucket.allowAcquire) {
            return await acquire(bucket, amount);
        }
    }

    bucket.allowAcquire = false;
    // Since the bucket is lazy update the tokens now,
    // and also get the current amount of available tokens
    let currentTokens = updateTokens(bucket);

    // It's possible that more than available tokens have been acquired,
    // so calculate the amount of milliseconds to wait until this acquire is good to go.
    if (currentTokens < amount) {
        const tokensNeeded = amount - currentTokens;
        let refillsNeeded = Math.ceil(tokensNeeded / bucket.refillAmount);

        const waitTime = bucket.refillInterval * refillsNeeded;
        await delay(waitTime);

        // Update the tokens again to ensure nothing has been missed.
        updateTokens(bucket);
    }

    // In order to not subtract too much from the tokens,
    // calculate what is actually needed to subtract.
    const toSubtract = (amount % bucket.refillAmount) || amount;
    bucket.tokensState -= toSubtract;

    // Allow the next acquire to happen.
    bucket.allowAcquire = true;
    // If there is an acquire waiting, let it continue.
    bucket.waiting.shift()?.();
}