// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

namespace System
{
    public readonly partial struct DateTime
    {
        internal static readonly bool s_systemSupportsLeapSeconds = SystemSupportsLeapSeconds();

        public static unsafe DateTime UtcNow
        {
            get
            {
                if (s_systemSupportsLeapSeconds)
                {
                    FullSystemTime time;
                    GetSystemTimeWithLeapSecondsHandling(&time);
                    return CreateDateTimeFromSystemTime(in time);
                }

                return new DateTime(((ulong)(GetSystemTimeAsFileTime() + FileTimeOffset)) | KindUtc);
            }
        }

        internal static unsafe bool IsValidTimeWithLeapSeconds(int year, int month, int day, int hour, int minute, int second, DateTimeKind kind)
        {
            DateTime dt = new DateTime(year, month, day);
            FullSystemTime time = new FullSystemTime(year, month, dt.DayOfWeek, day, hour, minute, second);

            switch (kind)
            {
                case DateTimeKind.Local: return ValidateSystemTime(&time.systemTime, localTime: true);
                case DateTimeKind.Utc:   return ValidateSystemTime(&time.systemTime, localTime: false);
                default:
                    return ValidateSystemTime(&time.systemTime, localTime: true) || ValidateSystemTime(&time.systemTime, localTime: false);
            }
        }

        private static unsafe DateTime FromFileTimeLeapSecondsAware(long fileTime)
        {
            FullSystemTime time;
            if (FileTimeToSystemTime(fileTime, &time))
            {
                return CreateDateTimeFromSystemTime(in time);
            }

            throw new ArgumentOutOfRangeException(nameof(fileTime), SR.ArgumentOutOfRange_DateTimeBadTicks);
        }

        private static unsafe long ToFileTimeLeapSecondsAware(long ticks)
        {
            FullSystemTime time = new FullSystemTime(ticks);
            long fileTime;

            if (SystemTimeToFileTime(&time.systemTime, &fileTime))
            {
                return fileTime + ticks % TicksPerMillisecond;
            }

            throw new ArgumentOutOfRangeException(null, SR.ArgumentOutOfRange_FileTimeInvalid);
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static DateTime CreateDateTimeFromSystemTime(in FullSystemTime time)
        {
            long ticks  = DateToTicks(time.systemTime.Year, time.systemTime.Month, time.systemTime.Day);
            ticks += TimeToTicks(time.systemTime.Hour, time.systemTime.Minute, time.systemTime.Second);
            ticks += time.systemTime.Milliseconds * TicksPerMillisecond;
            ticks += time.hundredNanoSecond;
            return new DateTime( ((UInt64)(ticks)) | KindUtc);
        }

        // FullSystemTime struct is the SYSTEMTIME struct with extra hundredNanoSecond field to store more precise time.
        [StructLayout(LayoutKind.Sequential)]
        private struct FullSystemTime
        {
            internal Interop.Kernel32.SYSTEMTIME systemTime;
            internal long hundredNanoSecond;

            internal FullSystemTime(int year, int month, DayOfWeek dayOfWeek, int day, int hour, int minute, int second)
            {
                systemTime.Year = (ushort) year;
                systemTime.Month = (ushort) month;
                systemTime.DayOfWeek = (ushort) dayOfWeek;
                systemTime.Day = (ushort) day;
                systemTime.Hour = (ushort) hour;
                systemTime.Minute = (ushort) minute;
                systemTime.Second = (ushort) second;
                systemTime.Milliseconds = 0;
                hundredNanoSecond = 0;
            }

            internal FullSystemTime(long ticks)
            {
                DateTime dt = new DateTime(ticks);

                int year, month, day;
                dt.GetDatePart(out year, out month, out day);

                systemTime.Year = (ushort) year;
                systemTime.Month = (ushort) month;
                systemTime.DayOfWeek = (ushort) dt.DayOfWeek;
                systemTime.Day = (ushort) day;
                systemTime.Hour = (ushort) dt.Hour;
                systemTime.Minute = (ushort) dt.Minute;
                systemTime.Second = (ushort) dt.Second;
                systemTime.Milliseconds = (ushort) dt.Millisecond;
                hundredNanoSecond = 0;
            }
        };

#if !CORECLR
        internal static readonly bool s_systemSupportsPreciseSystemTime = SystemSupportsPreciseSystemTime();

        private static unsafe bool SystemSupportsPreciseSystemTime()
        {
            if (Environment.IsWindows8OrAbove)
            {
                // GetSystemTimePreciseAsFileTime exists and we'd like to use it.  However, on
                // misconfigured systems, it's possible for the "precise" time to be inaccurate:
                //     https://github.com/dotnet/coreclr/issues/14187
                // If it's inaccurate, though, we expect it to be wildly inaccurate, so as a
                // workaround/heuristic, we get both the "normal" and "precise" times, and as
                // long as they're close, we use the precise one. This workaround can be removed
                // when we better understand what's causing the drift and the issue is no longer
                // a problem or can be better worked around on all targeted OSes.

                long systemTimeResult;
                Interop.Kernel32.GetSystemTimeAsFileTime(&systemTimeResult);

                long preciseSystemTimeResult;
                Interop.Kernel32.GetSystemTimePreciseAsFileTime(&preciseSystemTimeResult);

                return Math.Abs(preciseSystemTimeResult - systemTimeResult) <= 100 * TicksPerMillisecond;
            }

            return false;
        }

        private static unsafe bool ValidateSystemTime(Interop.Kernel32.SYSTEMTIME* time, bool localTime)
        {
            if (localTime)
            {
                Interop.Kernel32.SYSTEMTIME st;
                return Interop.Kernel32.TzSpecificLocalTimeToSystemTime(IntPtr.Zero, time, &st) != Interop.BOOL.FALSE;
            }
            else
            {
                long timestamp;
                return Interop.Kernel32.SystemTimeToFileTime(time, &timestamp) != Interop.BOOL.FALSE;
            }
        }

        private static unsafe bool FileTimeToSystemTime(long fileTime, FullSystemTime* time)
        {
            if (Interop.Kernel32.FileTimeToSystemTime(&fileTime, &time->systemTime) != Interop.BOOL.FALSE)
            {
                // to keep the time precision
                time->hundredNanoSecond = fileTime % TicksPerMillisecond;
                if (time->systemTime.Second > 59)
                {
                    // we have a leap second, force it to last second in the minute as DateTime doesn't account for leap seconds in its calculation.
                    // we use the maxvalue from the milliseconds and the 100-nano seconds to avoid reporting two out of order 59 seconds
                    time->systemTime.Second = 59;
                    time->systemTime.Milliseconds = 999;
                    time->hundredNanoSecond = 9999;
                }
                return true;
            }
            return false;
        }

        private static unsafe void GetSystemTimeWithLeapSecondsHandling(FullSystemTime* time)
        {
            if (!FileTimeToSystemTime(GetSystemTimeAsFileTime(), time))
            {
                Interop.Kernel32.GetSystemTime(&time->systemTime);
                time->hundredNanoSecond = 0;
                if (time->systemTime.Second > 59)
                {
                    // we have a leap second, force it to last second in the minute as DateTime doesn't account for leap seconds in its calculation.
                    // we use the maxvalue from the milliseconds and the 100-nano seconds to avoid reporting two out of order 59 seconds
                    time->systemTime.Second = 59;
                    time->systemTime.Milliseconds = 999;
                    time->hundredNanoSecond = 9999;
                }
            }
        }

        private static unsafe bool SystemTimeToFileTime(Interop.Kernel32.SYSTEMTIME* time, long* fileTime)
        {
            return Interop.Kernel32.SystemTimeToFileTime(time, fileTime) != Interop.BOOL.FALSE;
        }

        private static unsafe long GetSystemTimeAsFileTime()
        {
            long timestamp;

            if (s_systemSupportsPreciseSystemTime)
            {
                Interop.Kernel32.GetSystemTimePreciseAsFileTime(&timestamp);
            }
            else
            {
                Interop.Kernel32.GetSystemTimeAsFileTime(&timestamp);
            }

            return timestamp;
        }
#endif
    }
}