import { expect, test, vi } from 'vitest';
import { ExponentialBackoff } from '../source/exponential-backoff.ts';

/**
 * Tests that the static {@link ExponentialBackoff.run} helper creates a throwaway instance
 * with library defaults (including the default 1000ms base delay) when no options are passed.
 */
const testExponentialBackoffRunUsesDefaultOptions = async (): Promise<void> => {
    // Fake timers let us advance time without waiting real seconds between retries.
    vi.useFakeTimers();
    // Pin Math.random so jitter does not randomize the delay we are about to measure.
    vi.spyOn(Math, 'random').mockReturnValue(0.5);

    try {
        // The wrapped function fails on its first invocation and succeeds on the second.
        // That forces ExponentialBackoff.run down the retry path using default options.
        const fn = vi.fn().mockRejectedValueOnce(new Error('retry me'))
.mockResolvedValueOnce('static-result');

        // Call the static helper with no onError and no options — defaults apply entirely.
        const promise = ExponentialBackoff.run(fn);

        // Yield one microtask so the first (immediate) attempt completes and schedules the retry timer.
        await Promise.resolve();
        expect(fn).toHaveBeenCalledTimes(1);

        // Default baseDelay is 1000ms; advancing less would not trigger the retry yet.
        await vi.advanceTimersByTimeAsync(1_000);

        // The retry should have succeeded and returned the resolved value from the mock.
        await expect(promise).resolves.toBe('static-result');
        expect(fn).toHaveBeenCalledTimes(2);
    } finally {
        vi.useRealTimers();
        vi.restoreAllMocks();
    }
};

/**
 * Tests that {@link ExponentialBackoff.run} accepts a partial options object and merges it
 * with defaults, still retrying when only some fields are overridden.
 */
const testExponentialBackoffRunWithPartialOptions = async (): Promise<void> => {
    // Same fail-then-succeed pattern; we only care that partial options still enable a retry.
    const fn = vi.fn().mockRejectedValueOnce(new Error('retry me'))
.mockResolvedValueOnce('done');

    // baseDelay/jitter of 0 skip real waiting; maxAttempts: 3 gives headroom for one retry.
    // onError is explicitly undefined to verify the default no-op handler is used.
    const result = await ExponentialBackoff.run(fn, undefined, {
        baseDelay: 0,
        jitter: 0,
        maxAttempts: 3,
    });

    expect(result).toBe('done');
    expect(fn).toHaveBeenCalledTimes(2);
};

/**
 * Tests that calling {@link ExponentialBackoff.run} on a constructed instance applies
 * the instance's stored options when no per-run options are supplied.
 */
const testExponentialBackoffInstanceRunUsesDefaultOnError = async (): Promise<void> => {
    const fn = vi.fn().mockRejectedValueOnce(new Error('retry me'))
.mockResolvedValueOnce('instance-result');

    // Options live on the instance; run(fn) should read them instead of static defaults.
    const backoff = new ExponentialBackoff({ baseDelay: 0, jitter: 0, maxAttempts: 3 });

    const result = await backoff.run(fn);

    expect(result).toBe('instance-result');
    expect(fn).toHaveBeenCalledTimes(2);
};

/**
 * Tests the happy path: the wrapped function succeeds immediately and no retry machinery runs.
 */
const testExponentialBackoffSucceedsOnFirstAttempt = async (): Promise<void> => {
    // Always resolves — never enters the catch/retry branch.
    const fn = vi.fn(async () => 'success');
    const onError = vi.fn();

    const result = await ExponentialBackoff.run(fn, onError, {
        baseDelay: 0,
        jitter: 0,
    });

    expect(result).toBe('success');
    expect(fn).toHaveBeenCalledOnce();
    expect(onError).not.toHaveBeenCalled();
};

/**
 * Tests that retries continue across multiple failures until the function eventually resolves.
 */
const testExponentialBackoffRetriesUntilSuccess = async (): Promise<void> => {
    // Three invocations: two rejections then a success on the third call.
    const fn = vi.fn().mockRejectedValueOnce(new Error('attempt 1'))
.mockRejectedValueOnce(new Error('attempt 2'))
.mockResolvedValueOnce('success');

    // maxAttempts: 5 is high enough that we stop because fn succeeded, not because we hit the cap.
    const result = await ExponentialBackoff.run(fn, () => {}, {
        baseDelay: 0,
        jitter: 0,
        maxAttempts: 5,
    });

    expect(result).toBe('success');
    expect(fn).toHaveBeenCalledTimes(3);
};

/**
 * Tests that the onError callback is invoked once for every failed attempt, including the last one
 * before the final rejection is thrown to the caller.
 */
const testExponentialBackoffCallsOnErrorForEachFailure = async (): Promise<void> => {
    const error = new Error('temporary failure');

    // Always rejects with the same error — we will exhaust all attempts.
    const fn = vi.fn().mockRejectedValue(error);
    const onError = vi.fn();

    // maxAttempts: 3 means three tries total, all of which will fail.
    await expect(ExponentialBackoff.run(fn, onError, {
            baseDelay: 0,
            jitter: 0,
            maxAttempts: 3,
        })).rejects.toThrow('temporary failure');

    expect(onError).toHaveBeenCalledTimes(3);
    expect(onError).toHaveBeenCalledWith(error);
};

/**
 * Tests that when all attempts are exhausted the caller receives the error from the final attempt,
 * not an earlier one.
 */
const testExponentialBackoffThrowsLastErrorWhenExhausted = async (): Promise<void> => {
    const firstError = new Error('first');
    const lastError = new Error('last');

    // Two distinct errors so we can prove the last one surfaces.
    const fn = vi.fn().mockRejectedValueOnce(firstError)
.mockRejectedValueOnce(lastError);

    await expect(ExponentialBackoff.run(fn, () => {}, {
            baseDelay: 0,
            jitter: 0,
            maxAttempts: 2,
        })).rejects.toThrow('last');

    expect(fn).toHaveBeenCalledTimes(2);
};

/**
 * Tests that rejections which are not Error instances are coerced to Error before onError runs,
 * so callers always observe a consistent error type in the callback.
 */
const testExponentialBackoffWrapsNonErrorThrows = async (): Promise<void> => {
    // Reject with a plain string — not an Error subclass.
    const fn = vi.fn().mockRejectedValue('not-an-error');
    const onError = vi.fn();

    // Single attempt — we fail fast and inspect what onError received.
    await expect(ExponentialBackoff.run(fn, onError, {
            baseDelay: 0,
            jitter: 0,
            maxAttempts: 1,
        })).rejects.toThrow('not-an-error');

    expect(onError).toHaveBeenCalledOnce();
    expect(onError.mock.calls[0][0]).toBeInstanceOf(Error);
    expect(onError.mock.calls[0][0].message).toBe('not-an-error');
};

/**
 * Tests the {@link ExponentialBackoff.from} factory and subsequent instance {@link ExponentialBackoff.run}
 * as an alternative to the static helper.
 */
const testExponentialBackoffFromAndInstanceRun = async (): Promise<void> => {
    const fn = vi.fn(async () => 42);

    // from() is a convenience constructor; run() on the result should behave like the static path.
    const backoff = ExponentialBackoff.from({
        baseDelay: 0,
        jitter: 0,
    });

    const result = await backoff.run(fn);

    expect(result).toBe(42);
    expect(fn).toHaveBeenCalledOnce();
};

/**
 * Tests that maxAttempts: 0 disables the attempt cap so retries continue until the function succeeds.
 */
const testExponentialBackoffRetriesIndefinitelyWhenMaxAttemptsIsZero = async (): Promise<void> => {
    // Four invocations: three failures then success — would exceed a cap of 3 if one existed.
    const fn = vi
        .fn()
        .mockRejectedValueOnce(new Error('attempt 1'))
        .mockRejectedValueOnce(new Error('attempt 2'))
        .mockRejectedValueOnce(new Error('attempt 3'))
        .mockResolvedValueOnce('eventually');

    const result = await ExponentialBackoff.run(fn, () => {}, {
        baseDelay: 0,
        jitter: 0,
        maxAttempts: 0,
    });

    expect(result).toBe('eventually');
    expect(fn).toHaveBeenCalledTimes(4);
};

/**
 * Tests the delay formula: each retry waits baseDelay * growthRate^attemptIndex milliseconds
 * (with jitter disabled so the math is exact).
 */
const testExponentialBackoffIncreasesDelayExponentially = async (): Promise<void> => {
    vi.useFakeTimers();
    vi.spyOn(Math, 'random').mockReturnValue(0.5);

    try {
        const fn = vi
            .fn()
            .mockRejectedValueOnce(new Error('attempt 1'))
            .mockRejectedValueOnce(new Error('attempt 2'))
            .mockResolvedValueOnce('success');

        const promise = ExponentialBackoff.run(fn, () => {}, {
            baseDelay: 100,
            growthRate: 2,
            jitter: 0,
            maxDelay: 10_000,
            maxAttempts: 5,
        });

        // Attempt 0 fires synchronously on the first microtask tick.
        await Promise.resolve();
        expect(fn).toHaveBeenCalledTimes(1);

        // After attempt 0 fails, delay = 100 * 2^0 = 100ms before attempt 1.
        await vi.advanceTimersByTimeAsync(100);
        expect(fn).toHaveBeenCalledTimes(2);

        // After attempt 1 fails, delay = 100 * 2^1 = 200ms before attempt 2.
        await vi.advanceTimersByTimeAsync(200);
        expect(fn).toHaveBeenCalledTimes(3);

        await expect(promise).resolves.toBe('success');
    } finally {
        vi.useRealTimers();
        vi.restoreAllMocks();
    }
};

/**
 * Tests that computed delay never exceeds maxDelay even when exponential growth would go higher.
 */
const testExponentialBackoffCapsDelayAtMaxDelay = async (): Promise<void> => {
    vi.useFakeTimers();
    vi.spyOn(Math, 'random').mockReturnValue(0.5);

    try {
        const fn = vi
            .fn()
            .mockRejectedValueOnce(new Error('attempt 1'))
            .mockRejectedValueOnce(new Error('attempt 2'))
            .mockResolvedValueOnce('success');

        const promise = ExponentialBackoff.run(fn, () => {}, {
            baseDelay: 1_000,
            growthRate: 4,
            jitter: 0,
            maxDelay: 2_000,
            maxAttempts: 5,
        });

        await Promise.resolve();
        expect(fn).toHaveBeenCalledTimes(1);

        // attempt 0: 1000 * 4^0 = 1000ms, below the 2000ms cap.
        await vi.advanceTimersByTimeAsync(1_000);
        expect(fn).toHaveBeenCalledTimes(2);

        // attempt 1: uncapped would be 4000ms but maxDelay clamps to 2000ms.
        await vi.advanceTimersByTimeAsync(2_000);
        expect(fn).toHaveBeenCalledTimes(3);

        await expect(promise).resolves.toBe('success');
    } finally {
        vi.useRealTimers();
        vi.restoreAllMocks();
    }
};

/**
 * Tests that jitter scales the capped delay up or down by jitter * cappedDelay based on Math.random.
 */
const testExponentialBackoffAppliesJitter = async (): Promise<void> => {
    vi.useFakeTimers();
    // random = 1 → jitterDirection = 2*1 - 1 = +1 → full positive 10% jitter on the delay.
    vi.spyOn(Math, 'random').mockReturnValue(1);

    try {
        const fn = vi.fn().mockRejectedValueOnce(new Error('attempt 1'))
.mockResolvedValueOnce('success');

        const promise = ExponentialBackoff.run(fn, () => {}, {
            baseDelay: 1_000,
            growthRate: 1,
            jitter: 0.1,
            maxDelay: 10_000,
            maxAttempts: 3,
        });

        await Promise.resolve();
        expect(fn).toHaveBeenCalledTimes(1);

        // cappedDelay = 1000; with +10% jitter the retry fires after 1100ms, not 1000ms.
        await vi.advanceTimersByTimeAsync(1_100);
        expect(fn).toHaveBeenCalledTimes(2);

        await expect(promise).resolves.toBe('success');
    } finally {
        vi.useRealTimers();
        vi.restoreAllMocks();
    }
};

const runTests = async (): Promise<void> => {
    test('ExponentialBackoff.run: delegates to a new instance using default options', testExponentialBackoffRunUsesDefaultOptions);
    test('ExponentialBackoff.run: retries and succeeds with partial options', testExponentialBackoffRunWithPartialOptions);
    test('ExponentialBackoff.run: uses the instance default onError when omitted', testExponentialBackoffInstanceRunUsesDefaultOnError);
    test('ExponentialBackoff: returns the result on first success', testExponentialBackoffSucceedsOnFirstAttempt);
    test('ExponentialBackoff: retries until the function succeeds', testExponentialBackoffRetriesUntilSuccess);
    test('ExponentialBackoff: calls onError for each failed attempt', testExponentialBackoffCallsOnErrorForEachFailure);
    test('ExponentialBackoff: throws the last error when max attempts are exhausted', testExponentialBackoffThrowsLastErrorWhenExhausted);
    test('ExponentialBackoff: wraps non-Error throws before calling onError', testExponentialBackoffWrapsNonErrorThrows);
    test('ExponentialBackoff: works via from and instance run', testExponentialBackoffFromAndInstanceRun);
    test('ExponentialBackoff: retries indefinitely when maxAttempts is 0', testExponentialBackoffRetriesIndefinitelyWhenMaxAttemptsIsZero);
    test('ExponentialBackoff: increases delay exponentially between attempts', testExponentialBackoffIncreasesDelayExponentially);
    test('ExponentialBackoff: caps delay at maxDelay', testExponentialBackoffCapsDelayAtMaxDelay);
    test('ExponentialBackoff: applies jitter around the capped delay', testExponentialBackoffAppliesJitter);
};

await runTests();