//******************************************************************************************************
//  ReaderWriterSpinLock.cs - Gbtc
//
//  Copyright © 2012, Grid Protection Alliance.  All Rights Reserved.
//
//  Licensed to the Grid Protection Alliance (GPA) under one or more contributor license agreements. See
//  the NOTICE file distributed with this work for additional information regarding copyright ownership.
//  The GPA licenses this file to you under the Eclipse Public License -v 1.0 (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.opensource.org/licenses/eclipse-1.0.php
//
//  Unless agreed to in writing, the subject software distributed under the License is distributed on an
//  "AS-IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. Refer to the
//  License for the specific language governing permissions and limitations.
//
//  Code Modification History:
//  ----------------------------------------------------------------------------------------------------
//  03/21/2011 - Ritchie
//       Generated original version of source code based on Joe Duffy's Weblog articles:
//           "A single-word reader/writer spin lock", January 29, 2009
//              http://www.bluebytesoftware.com/blog/2009/01/30/ASinglewordReaderwriterSpinLock.aspx
//           "A more scalable reader/writer lock, and a bit less harsh consideration of the idea", February 20, 2009
//              http://www.bluebytesoftware.com/blog/2009/02/21/AMoreScalableReaderwriterLockAndABitLessHarshConsiderationOfTheIdea.aspx
//  12/14/2012 - Starlynn Danyelle Gilliam
//       Modified Header.
//
//******************************************************************************************************

using System;
using System.Runtime.InteropServices;
using System.Threading;

// We use plenty of interlocked operations on volatile fields below.  Safe.
#pragma warning disable 0420

namespace GSF.Threading
{
    /// <summary>
    /// Represents a fast, lightweight reader/writer lock that uses spinning to perform locking with no thread affinity. No recursive
    /// acquires or upgradable locks are allowed (i.e., all entered locks must be exited before entering another lock).
    /// </summary>
    /// <remarks>
    /// <para>
    /// <see cref="ReaderWriterSpinLock"/> uses a single word of memory and only spins when contention arises so no events are necessary.
    /// </para>
    /// <para>
    /// This reader/writer lock uses <see cref="SpinWait"/> to spin the CPU instead of engaging event based locking. As a result it
    /// should only be used in cases where lock times are expected to be very small, reads are very frequent and writes are rare.
    /// If hold times for write locks can be lengthy, it will be better to use <see cref="ReaderWriterLockSlim"/> instead to avoid
    /// unnecessary CPU utilization due to spinning incurred by waiting reads.
    /// </para>
    /// </remarks>
    public class ReaderWriterSpinLock
    {
        #region [ Members ]

        // Nested Types
        [StructLayout(LayoutKind.Sequential, Size = 128)]
        struct ReadEntry
        {
            internal volatile int m_taken;
        }

        // Fields
        private volatile int m_writer;
        private volatile ReadEntry[] m_readers = new ReadEntry[Environment.ProcessorCount * 16];

        #endregion

        #region [ Properties ]

        private int ReadLockIndex
        {
            get
            {
                return Thread.CurrentThread.ManagedThreadId % m_readers.Length;
            }
        }

        #endregion

        #region [ Methods ]

        /// <summary>
        /// Enters the lock in write mode.
        /// </summary>
        /// <remarks>
        /// Upon successful aquistion of a write lock, use the <c>finally</c> block of a <c>try/finally</c> statement to call <see cref="ExitWriteLock"/>.
        /// One <see cref="ExitWriteLock"/> should be called for each <see cref="EnterWriteLock"/>.
        /// </remarks>
        public void EnterWriteLock()
        {
            SpinWait sw = new SpinWait();

            while (true)
            {
                if (m_writer == 0 && Interlocked.Exchange(ref m_writer, 1) == 0)
                {
                    // We now hold the write lock, and prevent new readers -
                    // but we must ensure no readers exist before proceeding.
                    for (int i = 0; i < m_readers.Length; i++)
                    {
                        while (m_readers[i].m_taken != 0)
                            sw.SpinOnce();
                    }

                    break;
                }

                // We failed to take the write lock; wait a bit and retry.
                sw.SpinOnce();
            }
        }

        /// <summary>
        /// Exits write mode.
        /// </summary>
        public void ExitWriteLock()
        {
            // No need for a CAS.
            m_writer = 0;
        }

        /// <summary>
        /// Enters the lock in read mode.
        /// </summary>
        /// <remarks>
        /// Upon successful aquistion of a read lock, use the <c>finally</c> block of a <c>try/finally</c> statement to call <see cref="ExitReadLock"/>.
        /// One <see cref="ExitReadLock"/> should be called for each <see cref="EnterReadLock"/>.
        /// </remarks>
        public void EnterReadLock()
        {
            SpinWait sw = new SpinWait();
            int tid = ReadLockIndex;

            // Wait until there are no writers.
            while (true)
            {
                while (m_writer == 1)
                    sw.SpinOnce();

                // Try to take the read lock.
                Interlocked.Increment(ref m_readers[tid].m_taken);

                if (m_writer == 0)
                {
                    // Success, no writer, proceed.
                    break;
                }

                // Back off, to let the writer go through.
                Interlocked.Decrement(ref m_readers[tid].m_taken);
            }
        }

        /// <summary>
        /// Exits read mode.
        /// </summary>
        /// <exception cref="InvalidOperationException">Cannot exit read lock when there are no readers.</exception>
        public void ExitReadLock()
        {
            // Just note that the current reader has left the lock.
            Interlocked.Decrement(ref m_readers[ReadLockIndex].m_taken);
        }

        #endregion
    }
}