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using System;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.Extensions.Time.Testing;
using NUnit.Framework;

namespace Opc.Ua.Core.Tests.Stack.Client
{
    /// <summary>
    /// Exercises monotonic reconnect deadlines, cancellation, and completion races with a fake clock.
    /// </summary>
    [TestFixture]
    [Parallelizable]
    public sealed class ReconnectDeadlineTests
    {
        /// <summary>
        /// The deadline stays live immediately before its bound and expires at the exact bound.
        /// </summary>
        [Test]
        public async Task DeadlineExpiresAtTheExactBoundAsync()
        {
            var time = new FakeTimeProvider();
            await using var deadline = new ReconnectDeadline(time, CancellationToken.None);
            deadline.Tighten(new RetryBudget(TimeSpan.FromSeconds(1), time));

            time.Advance(TimeSpan.FromSeconds(1) - TimeSpan.FromTicks(1));
            Assert.That(deadline.Expired, Is.False);
            Assert.That(deadline.Token.IsCancellationRequested, Is.False);
            time.Advance(TimeSpan.FromTicks(1));

            Assert.That(deadline.Expired, Is.True);
            Assert.That(deadline.TryComplete(), Is.False);
            await WaitForCancellationAsync(deadline.Token).ConfigureAwait(false);
        }

        /// <summary>
        /// Successful recovery cannot be expired by a later timer tick or tighter budget.
        /// </summary>
        [Test]
        public async Task SuccessfulCompletionRetiresTheDeadlineAsync()
        {
            var time = new FakeTimeProvider();
            await using var deadline = new ReconnectDeadline(time, CancellationToken.None);
            deadline.Tighten(new RetryBudget(TimeSpan.FromSeconds(1), time));
            time.Advance(TimeSpan.FromSeconds(1) - TimeSpan.FromTicks(1));

            Assert.That(deadline.TryComplete(), Is.True);
            deadline.Tighten(new RetryBudget(TimeSpan.Zero, time));
            time.Advance(TimeSpan.FromHours(1));

            Assert.That(deadline.Expired, Is.False);
            Assert.That(deadline.Token.IsCancellationRequested, Is.False);
        }

        /// <summary>
        /// A later budget can shorten, but never extend, the original recovery window.
        /// </summary>
        [Test]
        public async Task LaterJoinersCanOnlyShortenTheExistingWindowAsync()
        {
            var time = new FakeTimeProvider();
            await using var deadline = new ReconnectDeadline(time, CancellationToken.None);
            deadline.Tighten(new RetryBudget(TimeSpan.FromSeconds(10), time));
            time.Advance(TimeSpan.FromSeconds(2));
            deadline.Tighten(new RetryBudget(TimeSpan.FromSeconds(3), time));
            deadline.Tighten(new RetryBudget(TimeSpan.FromMinutes(1), time));

            Assert.That(deadline.Duration, Is.EqualTo(TimeSpan.FromSeconds(5)));
            time.Advance(TimeSpan.FromSeconds(3));
            Assert.That(deadline.Expired, Is.True);
            Assert.That(deadline.Elapsed, Is.EqualTo(TimeSpan.FromSeconds(5)));
        }

        /// <summary>
        /// Attaching a partly consumed budget preserves its original expiry time.
        /// </summary>
        [Test]
        public async Task PreviouslyConsumedCallerBudgetKeepsItsOriginalDeadlineAsync()
        {
            var time = new FakeTimeProvider();
            var budget = new RetryBudget(TimeSpan.FromSeconds(5), time);
            Assert.That(budget.TryConsume(out _), Is.True);
            time.Advance(TimeSpan.FromSeconds(2));
            await using var deadline = new ReconnectDeadline(time, CancellationToken.None);

            deadline.Tighten(budget);
            Assert.That(deadline.Duration, Is.EqualTo(TimeSpan.FromSeconds(3)));
            time.Advance(TimeSpan.FromSeconds(3));
            Assert.That(deadline.Expired, Is.True);
        }

        /// <summary>
        /// Resetting a caller's budget does not renew a deadline that has already sampled it.
        /// </summary>
        [Test]
        public async Task ResettingACallerBudgetCannotExtendAnAttachedDeadlineAsync()
        {
            var time = new FakeTimeProvider();
            var budget = new RetryBudget(TimeSpan.FromSeconds(5), time);
            await using var deadline = new ReconnectDeadline(time, CancellationToken.None);
            deadline.Tighten(budget);
            time.Advance(TimeSpan.FromSeconds(2));
            budget.Reset();
            deadline.Tighten(budget);
            time.Advance(TimeSpan.FromSeconds(3));

            Assert.That(deadline.Duration, Is.EqualTo(TimeSpan.FromSeconds(5)));
            Assert.That(deadline.Expired, Is.True);
        }

        /// <summary>
        /// Null and infinite budgets impose no deadline on the recovery cycle.
        /// </summary>
        [Test]
        public async Task UnlimitedBudgetLeavesRecoveryUnboundedAsync()
        {
            var time = new FakeTimeProvider();
            await using var deadline = new ReconnectDeadline(time, CancellationToken.None);
            deadline.Tighten(null);
            deadline.Tighten(new RetryBudget(Timeout.InfiniteTimeSpan, time));
            time.Advance(TimeSpan.FromDays(365));

            Assert.That(deadline.Duration, Is.EqualTo(TimeSpan.MaxValue));
            Assert.That(deadline.Expired, Is.False);
            Assert.That(deadline.TryComplete(), Is.True);
        }

        /// <summary>
        /// Durations beyond one timer's range rearm until the full recovery budget has elapsed.
        /// </summary>
        [Test]
        public async Task LongExplicitBudgetRearmsWithoutExceedingTheTimerRangeAsync()
        {
            var time = new FakeTimeProvider();
            await using var deadline = new ReconnectDeadline(time, CancellationToken.None);
            var duration = TimeSpan.FromDays(60);
            deadline.Tighten(new RetryBudget(duration, time));
            var timerMaximum = TimeSpan.FromMilliseconds(uint.MaxValue - 1);
            time.Advance(timerMaximum);

            Assert.That(deadline.Expired, Is.False);
            time.Advance(duration - timerMaximum);
            Assert.That(deadline.Expired, Is.True);
            Assert.That(deadline.Duration, Is.EqualTo(duration));
        }

        /// <summary>
        /// Explicit closing cancels recovery and retires the timer without reporting expiry.
        /// </summary>
        [Test]
        public async Task ClosingRecoveryCancelsWithoutReportingExpiryAsync()
        {
            var time = new FakeTimeProvider();
            await using var deadline = new ReconnectDeadline(time, CancellationToken.None);
            deadline.Tighten(new RetryBudget(TimeSpan.FromSeconds(1), time));
            deadline.Cancel();
            Assert.That(deadline.TryComplete(), Is.False);
            time.Advance(TimeSpan.FromHours(1));

            await WaitForCancellationAsync(deadline.Token).ConfigureAwait(false);
            Assert.That(deadline.Expired, Is.False);
            Assert.That(deadline.TryComplete(), Is.False);
        }

        /// <summary>
        /// Manager shutdown cancels the linked token without producing a deadline-expired outcome.
        /// </summary>
        [Test]
        public async Task ShutdownIsNotReportedAsDeadlineExpiryAsync()
        {
            var time = new FakeTimeProvider();
            using var shutdown = new CancellationTokenSource();
            await using var deadline = new ReconnectDeadline(time, shutdown.Token);
            deadline.Tighten(new RetryBudget(TimeSpan.FromSeconds(1), time));
            await shutdown.CancelAsync().ConfigureAwait(false);

            Assert.That(deadline.Token.IsCancellationRequested, Is.True);
            Assert.That(deadline.Expired, Is.False);
            Assert.That(deadline.TryComplete(), Is.False);
        }

        /// <summary>
        /// Simultaneous completion and expiry produce exactly one successful or expired outcome.
        /// </summary>
        [Test]
        [Repeat(20)]
        public async Task CompletionRacingWithExpiryHasOneVerdictAsync()
        {
            var time = new FakeTimeProvider();
            await using var deadline = new ReconnectDeadline(time, CancellationToken.None);
            deadline.Tighten(new RetryBudget(TimeSpan.FromSeconds(1), time));
            var start = new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
            Task<bool> completion = Task.Run(async () =>
            {
                await start.Task.ConfigureAwait(false);
                return deadline.TryComplete();
            });
            Task expiration = Task.Run(async () =>
            {
                await start.Task.ConfigureAwait(false);
                time.Advance(TimeSpan.FromSeconds(1));
            });
            start.TrySetResult(true);
            await Task.WhenAll(completion, expiration).WaitAsync(TimeSpan.FromSeconds(5)).ConfigureAwait(false);

            Assert.That(await completion.ConfigureAwait(false), Is.EqualTo(!deadline.Expired));
            if (deadline.Expired)
            {
                await WaitForCancellationAsync(deadline.Token).ConfigureAwait(false);
            }
            Assert.That(deadline.Token.IsCancellationRequested, Is.EqualTo(deadline.Expired));
        }

        private static async Task WaitForCancellationAsync(CancellationToken token)
        {
            var cancelled = new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
            using CancellationTokenRegistration registration = token.Register(() => cancelled.TrySetResult(true));
            await cancelled.Task.WaitAsync(TimeSpan.FromSeconds(5), CancellationToken.None).ConfigureAwait(false);
            Assert.That(token.IsCancellationRequested, Is.True);
        }
    }
}