/* $Id$
 *
 * Copyright 2007-2008 Cisco Systems Inc.
 *
 * Licensed 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 Etch.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;
	}
}