// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System;
using System.Diagnostics;

namespace Microsoft.Extensions.Internal
{
    internal readonly struct ValueStopwatch
    {
        private static readonly double s_timestampToTicks = TimeSpan.TicksPerSecond / (double)Stopwatch.Frequency;

        private readonly long _startTimestamp;

        public bool IsActive => _startTimestamp != 0;

        private ValueStopwatch(long startTimestamp)
        {
            _startTimestamp = startTimestamp;
        }

        public static ValueStopwatch StartNew() => new ValueStopwatch(Stopwatch.GetTimestamp());

        public TimeSpan GetElapsedTime()
        {
            // Start timestamp can't be zero in an initialized ValueStopwatch. It would have to be literally the first thing executed when the machine boots to be 0.
            // So it being 0 is a clear indication of default(ValueStopwatch)
            if (!IsActive)
            {
                throw new InvalidOperationException("An uninitialized, or 'default', ValueStopwatch cannot be used to get elapsed time.");
            }

            long end = Stopwatch.GetTimestamp();
            long timestampDelta = end - _startTimestamp;
            long ticks = (long)(s_timestampToTicks * timestampDelta);
            return new TimeSpan(ticks);
        }
    }
}

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System;
using System.Threading.Tasks;
using Xunit;

namespace Microsoft.Extensions.Internal.Test
{
    public class ValueStopwatchTest
    {
        [Fact]
        public void IsActiveIsFalseForDefaultValueStopwatch()
        {
            Assert.False(default(ValueStopwatch).IsActive);
        }

        [Fact]
        public void IsActiveIsTrueWhenValueStopwatchStartedWithStartNew()
        {
            Assert.True(ValueStopwatch.StartNew().IsActive);
        }

        [Fact]
        public void GetElapsedTimeThrowsIfValueStopwatchIsDefaultValue()
        {
            var stopwatch = default(ValueStopwatch);
            Assert.Throws<InvalidOperationException>(() => stopwatch.GetElapsedTime());
        }

        [Fact]
        public async Task GetElapsedTimeReturnsTimeElapsedSinceStart()
        {
            var stopwatch = ValueStopwatch.StartNew();
            await Task.Delay(200);
            Assert.True(stopwatch.GetElapsedTime().TotalMilliseconds > 0);
        }
    }
}