// $Id$
// 
// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements. See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership. The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License. You may obtain a copy of the License at
// 
//   http://www.apache.org/licenses/LICENSE-2.0
// 
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied. See the License for the
// specific language governing permissions and limitations
// under the License.
// 
using System;
using System.Runtime.InteropServices;

namespace Org.Apache.Etch.Bindings.Csharp.Util
{
	public abstract class HPTimer
	{
		[DllImport("kernel32.dll")]
		private static extern bool QueryPerformanceCounter(out long x);

		[DllImport("kernel32.dll")]
		private static extern bool QueryPerformanceFrequency(out long x);

		static HPTimer()
		{
			long freq = 0;

			if (!QueryPerformanceFrequency(out freq))
				throw new Exception( "QueryPerformanceFrequency not supported" );
			
			// freq is ticks / second.
			// we wanna compute seconds from ticks.
			// the easy computation is seconds = ticks / freq
			// to Get ns, we compute ns = ticks * 1,000,000,000 / freq
			// the problem is ticks * 1,000,000,000 will likely overflow.
			// what we want to do is precompute nsPerTick = 1,000,000,000 / freq
			// and then compute ns = ticks * nsPerTick.

			//Console.WriteLine( "HPTimer: freq = "+freq );
			nsPerTick = ((double) NS_PER_SECOND)/freq;
			//Console.WriteLine( "HPTimer: nsPerTick = "+nsPerTick );
		}

        public const long NS_PER_MICROSECOND = 1000;
        public const long NS_PER_MILLISECOND = 1000 * NS_PER_MICROSECOND;
        public const double NS_PER_SECOND = 1000.0 * NS_PER_MILLISECOND;

		/// <summary>
		/// Returns the current high precision timer value in nanos.
		/// </summary>
		/// <returns></returns>
		public static long Now()
		{
			long x = 0;
			
			if (!QueryPerformanceCounter(out x))
				throw new Exception( "QueryPerformanceCounter not supported" );

			return (long) (x * nsPerTick);
		}

		/// <summary>
		/// Returns the difference in nanos between the current timer value and
		/// a timer value previously returned by Now.
		/// </summary>
		/// <param name="y"></param>
		/// <returns></returns>
		public static long NsSince( long y )
		{
			return Now() - y;
		}

		/// <summary>
		/// Returns the difference in millis between the current timer value and
		/// a timer value previously returned by Now.
		/// </summary>
		/// <param name="y"></param>
		/// <returns></returns>
		public static long MillisSince( long y )
		{
            return NsSince(y) / NS_PER_MILLISECOND;
		}

		/// <summary>
		/// Returns the difference in seconds between the current timer value and
		/// a timer value previously returned by Now.
		/// </summary>
		/// <param name="y"></param>
		/// <returns></returns>
		public static double SecondsSince( long y )
		{
            return NsSince(y) / NS_PER_SECOND;
		}

		/// <summary>
		/// Returns the number of high precision clock ticks per nano.
		/// </summary>
		/// <returns></returns>
		public static double NsPerTick()
		{
			return nsPerTick;
		}

		private static double nsPerTick;
	}
}