import { ExponentialBackoffMaxRetriesHitError } from './errors.ts';

/**
 * Exponential backoff is a technique used to retry a function after a delay.
 *
 * The delay increases exponentially with each attempt, up to a maximum delay.
 *
 * The jitter is a random amount of time added to the delay to prevent thundering herd problems.
 *
 * The growth rate is the factor by which the delay increases with each attempt.
 */
export class ExponentialBackoff {
    /**
     * Create a new ExponentialBackoff instance
     *
     * @param config - The configuration for the exponential backoff
     * @returns The ExponentialBackoff instance
     */
    static from(config?: Partial<ExponentialBackoffOptions>): ExponentialBackoff {
        const backoff = new ExponentialBackoff(config);

        return backoff;
    }

    /**
     * Run the function with exponential backoff
     *
     * @param fn - The function to run
     * @param onError - The callback to call when an error occurs
     * @param options - The configuration for the exponential backoff
     *
     * @throws The last error if the function fails and we have hit the max attempts
     *
     * @returns The result of the function
     */
    static run<T>(taskFn: () => Promise<T>, onError = (_error: Error): void => {}, options?: Partial<ExponentialBackoffOptions>): Promise<T> {
        const backoff = ExponentialBackoff.from(options);

        return backoff.run(taskFn, onError);
    }

    private readonly options: ExponentialBackoffOptions;

    /**
     * Creates a new exponential-backoff instance.
     *
     * Unspecified options use the defaults listed below.
     *
     * @param options - Exponential-backoff configuration overrides.
     * @param options.maxDelay - Maximum delay between retries. Default: `10_000` ms.
     * @param options.maxAttempts - Maximum number of attempts; `0` retries indefinitely. Default: `10`.
     * @param options.baseDelay - Delay used as the basis for the first retry. Default: `1_000` ms.
     * @param options.growthRate - Multiplier applied to the delay after each attempt. Default: `2`.
     * @param options.jitter - Random proportional variation applied to each delay. Default: `0.1`.
     */
    constructor(options: Partial<ExponentialBackoffOptions> = {}) {
        this.options = {
            maxDelay: 10_000,
            maxAttempts: 10,
            baseDelay: 1_000,
            growthRate: 2,
            jitter: 0.1,
            ...options,
        };
    }

    /**
     * Run the function with exponential backoff
     *
     * If the function fails but we have not hit the max attempts, the error will be passed to the onError callback
     * and the function will be retried with an exponential delay
     *
     * If the function fails and we have hit the max attempts, an ExponentialBackoffMaxRetriesHitError will be thrown with all the errors that were thrown by the task function
     *
     * @param fn - The function to run
     * @param onError - The callback to call when an error occurs
     *
     * @throws An ExponentialBackoffMaxRetriesHitError with all the errors that were thrown by the task function
     *
     * @returns The result of the function
     */
    async run<T>(taskFn: () => Promise<T>, onError = (_error: Error): void => {}): Promise<T> {
        // Initialize an empty array to store the errors
        const errors: Error[] = [];

        let attempt = 0;

        // If the max attempts is 0, we should continue indefinitely.
        const unlimitedAttempts = this.options.maxAttempts === 0;

        while (attempt < this.options.maxAttempts || unlimitedAttempts) {
            try {
                return await taskFn();
            } catch (error) {
                // Store the error in case we fail every attempt
                const errorInstance = error instanceof Error ? error : new Error(`${error}`);
                errors.push(errorInstance);
                onError(errorInstance);

                // Wait before going to the next attempt
                const delay = ExponentialBackoff.calculateDelay(this.options, attempt);
                await new Promise((resolve) => setTimeout(resolve, delay));
            }

            attempt++;
        }

        // We completed the loop without ever succeeding. Throw an ExponentialBackoffMaxRetriesHitError with all the errors we got
        throw new ExponentialBackoffMaxRetriesHitError(errors);
    }

    /**
     * Calculate the delay before we should attempt to retry
     *
     * NOTE: The maximum delay is (maxDelay * (1 + jitter))
     *
     * @param attempt
     * @returns The time in milliseconds before another attempt should be made
     */
    public static calculateDelay(options: ExponentialBackoffOptions, attempt: number): number {
        // Get the power of the growth rate
        const power = options.growthRate ** attempt;

        // Get the delay before jitter or limit
        const rawDelay = options.baseDelay * power;

        // Cap the delay to the maximum. Do this before the jitter so jitter does not become larger than delay
        const cappedDelay = Math.min(rawDelay, options.maxDelay);

        // Get the jitter direction. This will be between -1 and 1
        const jitterDirection = 2 * Math.random() - 1;

        // Calculate the jitter
        const jitter = jitterDirection * options.jitter * cappedDelay;

        // Add the jitter to the delay
        return cappedDelay + jitter;
    }
}

export type ExponentialBackoffOptions = {

    /**
     * The maximum delay between attempts in milliseconds
     */
    maxDelay: number;

    /**
     * The maximum number of attempts. Passing 0 will result in infinite attempts.
     */
    maxAttempts: number;

    /**
     * The base delay between attempts in milliseconds
     */
    baseDelay: number;

    /**
     * The growth rate of the delay
     */
    growthRate: number;

    /**
     * The jitter of the delay as a percentage of growthRate
     */
    jitter: number;
};