///////////////////////////////////////////////////////////////////////////////
//
//  Microsoft Research Singularity
//
//  Copyright (c) Microsoft Corporation.  All rights reserved.
//
//  File:   BlockCache.sg
//
//  Todo: Look at giving up blocks when there is system memory pressure.
//        This needs a channel to the shared heap or another entity to
//        provide the necessary hints.
//

using Microsoft.Singularity.Channels;
using Microsoft.SingSharp;

using System.Threading;

namespace Microsoft.Singularity.Services.Fat.Fs
{
    /// <remarks> States associated with BlockCacheNodes. </remarks>
    internal enum NodeState : uint {
        Unused        = 0, // Node is unused.  It has a buffer, but it does
                           // not correspond to data on the disk.
                           // Transitions := <Clean|Dirty>

        Clean         = 1, // Node has data and it's clean.
                           // Transitions := <CleanBorrowed|Unused|Dirty>

        CleanBorrowed = 2, // There is clean data associated with node,
                           // but it is temporarily in use elsewhere in
                           // fs code.
                           // Transitions := <Clean|Dirty>

        Dirty         = 3, // Node has data and it's dirty.  If a user
                           // requests the node's data we have to return
                           // a copy as the data is enqueued to be written
                           // written to disk.
                           // Transitions := <WritePending|DirtyBorrowed>

        DirtyBorrowed = 4, // There is dirty data associated with node,
                           // but it is temporarily in use elsewhere in
                           // fs code.
                           // Transitions := <Dirty>

        WritePending  = 5, // The disk has the data associated with the
                           // node.  The disk will return the data when
                           // the write is complete.  The node state will
                           // transition to Clean when done.
                           // Transitions := <Clean>

        ReadPending   = 6, // The node is tagged with sector id, but data
                           // from disk has yet to be transferred.
                           // Transitions := <Clean>
    }

    internal struct BlockCacheNode
    {
        NodeState        nodeState;
        ulong            sectorId;
        int              lruTicks;
        VContainer<byte> blockData; // VContainer has mutex for acquire/release
                                    // but we don't use it and would prefer
                                    // not to have it.
        bool             acquired;  // For sanity check

        internal BlockCacheNode(NodeState                  ns,
                                ulong                      sectorId,
                                [Claims] byte[]! in ExHeap blockData,
                                int                        lruTicks)
        {
            this.nodeState = ns;
            this.sectorId  = sectorId;
            this.blockData = new VContainer<byte> (blockData);
            this.lruTicks  = lruTicks;
            this.acquired  = false;
        }

        private static NodeState[,] Transitions = {
            // From State                  To State
            { NodeState.Unused,           NodeState.ReadPending     },
            { NodeState.Unused,           NodeState.Dirty           },
            { NodeState.ReadPending,      NodeState.Clean           },
            { NodeState.ReadPending,      NodeState.CleanBorrowed   },
            { NodeState.Unused,           NodeState.Dirty           },
            { NodeState.Clean,            NodeState.CleanBorrowed   },
            { NodeState.Clean,            NodeState.Dirty           },
            { NodeState.Clean,            NodeState.ReadPending     },
            { NodeState.CleanBorrowed,    NodeState.Clean           },
            { NodeState.CleanBorrowed,    NodeState.Dirty           },
            { NodeState.Dirty,            NodeState.Dirty           },
            { NodeState.Dirty,            NodeState.DirtyBorrowed   },
            { NodeState.Dirty,            NodeState.WritePending    },
            { NodeState.DirtyBorrowed,    NodeState.Dirty           },
            { NodeState.WritePending,     NodeState.Clean           }
        };

        [ Microsoft.Contracts.Pure ]
        static bool TransitionValid(NodeState oldState, NodeState newState)
        {
            for (int i = 0; i < Transitions.Length; i += Transitions.Rank) {
                if (Transitions[i/2, 0] == oldState &&
                    Transitions[i/2, 1] == newState) {
                    return true;
                }
            }
            return false;
        }

        internal byte[]! in ExHeap Acquire()
        {
            byte []! in ExHeap bytes = blockData.Acquire();
            assert (State == NodeState.Unused ||
                    State == NodeState.Clean ||
                    State == NodeState.Dirty);
            this.acquired  = true;
            return bytes;
        }

        internal void AcquiredTransition(NodeState newNodeState, int lruTicks)
        {
            assert TransitionValid(nodeState, newNodeState);
            this.nodeState = newNodeState;
            this.lruTicks  = lruTicks;
        }

        internal void AcquiredSetSectorId(ulong newSectorId)
        {
            assert acquired == true;
            sectorId = newSectorId;
        }

        internal void Release([Claims] byte[]! in ExHeap bytes,
                              NodeState                  newNodeState,
                              int                        lruTicks)
        {
            assert TransitionValid(nodeState, newNodeState);

            assert (newNodeState == NodeState.Unused ||
                    newNodeState == NodeState.Clean ||
                    newNodeState == NodeState.Dirty);

            this.nodeState = newNodeState;
            this.lruTicks  = lruTicks;
            this.acquired  = false;
            this.blockData.Release(bytes);
        }

        internal NodeState State
        {
            get { return nodeState; }
        }

        internal ulong SectorId
        {
            get { return sectorId; }
        }

        internal int LruTicks
        {
            get { return lruTicks; }
        }

        internal bool Acquired
        {
            get { return acquired; }
        }
    }

    internal pointerfree struct BlockCacheConfiguration
    {
        ulong startSectorId;        // SectorId of first block in cache range
        uint  bytesPerSector;
        uint  sectorsPerBlock;
        uint  totalBlocks;          // Number of blocks in cache range
        uint  maxCacheKB;           // Maximum block data cached

        internal BlockCacheConfiguration(ulong startSectorId,
                                         uint  bytesPerSector,
                                         uint  sectorsPerBlock,
                                         uint  totalBlocks,
                                         uint  maxCacheKB)
            requires (bytesPerSector != 0 &&
                      (bytesPerSector & (bytesPerSector - 1)) == 0)
                /* otherwise ArgumentException */;
            requires (sectorsPerBlock != 0 &&
                      (sectorsPerBlock & (sectorsPerBlock - 1)) == 0)
                /* otherwise ArgumentException */;
            requires (totalBlocks != 0)
                /* otherwise ArgumentException */;
        {
            this.startSectorId   = startSectorId;
            this.bytesPerSector  = bytesPerSector;
            this.sectorsPerBlock = sectorsPerBlock;
            this.totalBlocks     = totalBlocks;
            this.maxCacheKB      = maxCacheKB;
        }

       internal ulong StartSectorId     { get { return startSectorId; } }
       internal uint  BytesPerSector    { get { return sectorsPerBlock; } }
       internal uint  SectorsPerBlock   { get { return sectorsPerBlock; } }
       internal uint  BytesPerBlock
       {
           get { return sectorsPerBlock * bytesPerSector; }
       }
       internal uint  TotalBlocks       { get { return totalBlocks; } }
       internal uint  MaxCacheKB        { get { return maxCacheKB; } }
    }

    /// <summary>
    /// The BlockCache is an n-way associative cache of a set of blocks on
    /// a disk.  All of the blocks are assumed to have the same size.
    /// </summary>
    internal sealed class BlockCache : IBlockWriterUser
    {
        ///////////////////////////////////////////////////////////////////////
        // Fields

        private BlockCacheConfiguration config;

        private uint                cacheStride;            // bucketCount
        private uint                cacheAssociativity;     // bucketDepth
        private BlockCacheNode [][] cache;
        private int []              cacheTicks;             // lru clock for
                                                            // each cache line
        private Disk!               disk;
        private BlockWriter!        blockWriter;

        ///////////////////////////////////////////////////////////////////////
        // Methods

        [ Microsoft.Contracts.NotDelayed ]
        internal BlockCache(BlockCacheConfiguration configuration,
                            Disk!                       disk,
                            BlockWriter!                blockWriter)

        {
            this.config = configuration;
            this.disk   = disk;
            this.blockWriter = blockWriter;

            base();

            SizeCache(ref config, out cacheStride, out cacheAssociativity);

            ulong usedKB = ((ulong)cacheStride) * cacheAssociativity * config.BytesPerBlock / 1024;

            DebugStub.Print(
                "BlockCache suggested size = {0} K, actual size = {1} K ({2} x {3} x {4})\n",
                            __arglist(
                                config.MaxCacheKB,
                                usedKB,
                                cacheStride,
                                cacheAssociativity,
                                config.BytesPerBlock));

            cache      = new BlockCacheNode [cacheStride][];
            cacheTicks = new int[cacheStride];

            ulong allocated = 0;

            for (uint i = 0; i < cacheStride; i++) {
                BlockCacheNode [] cacheLine = new BlockCacheNode[cacheAssociativity];
                cache[i]      = cacheLine;
                cacheTicks[i] = 0;
                for (uint j = 0; j < cacheAssociativity; j++) {
                    cacheLine[j] = new BlockCacheNode(
                        NodeState.Unused,
                        0 /* no sector id */,
                        new [ExHeap] byte [config.BytesPerBlock],
                        cacheTicks[i]);
                    allocated += config.BytesPerBlock;
                }
            }
            DebugStub.Print("Cache allocated {0}\n",
                            __arglist(allocated));
        }

        internal ulong StartSectorId
        {
            get { return config.StartSectorId; }
        }

        internal uint TotalBlocks
        {
            get { return config.TotalBlocks; }
        }

        internal uint BytesPerBlock
        {
            get { return config.BytesPerBlock; }
        }

        private uint GetCacheLine(uint blockId)
            requires blockId < TotalBlocks /* otherwise ArgumentException */;
        {
            return blockId % cacheStride;
        }

        private ulong BlockToSectorId(uint blockId)
            requires blockId < TotalBlocks /* otherwise ArgumentException */;
        {
            return config.StartSectorId + blockId * (ulong)config.SectorsPerBlock;
        }

        private uint SectorToBlockId(ulong sectorId)
            requires (long)sectorId >= (long)StartSectorId;
        {
            return ((uint)(sectorId - config.StartSectorId) / config.SectorsPerBlock);
        }

        enum CandidateResult : uint
        {
            FoundExact       = 1,           // Found exact block
            FoundVictim      = 2,           // Found candidate for replacement
            NodeBlocked      = 0x80000001,  // Found block but in use
            CacheLineBlocked = 0x80000002   // No blocks found or available
        }

        /// <remarks> Search for identified node or a victim for
        /// replacement if not present.
        ///
        /// <para>Looks for cache node matching sector id or
        /// most suitable node to use for sector id.  If it is
        /// not in the cache, a suitable candidate for
        /// replacement may be returned.</para>
        ///
        /// <para>If the node is in the cache, but in use, then
        /// NodeBlocked is returned.</para>
        ///
        /// <para>If the node is not present and there are no
        /// available nodes in cache line for replacement, then
        /// CacheLineBlocked is returned.</para>
        /// </remarks>
        private static CandidateResult
        GetCandidateNode(BlockCacheNode[]! cacheLine,
                         int               clockTicks,
                         ulong             sectorId,
                         out int           nodeIndex)
        {
            // requires Monitor.Entered(cacheLine) == true
            int freeIndex   = -1;
            int lruIndex    = -1;
            uint lastLruAge = 0;

            for (int i = 0; i < cacheLine.Length; i++) {
                if (cacheLine[i].State == NodeState.Unused) {
                    freeIndex = i;
                }
                else if (cacheLine[i].SectorId == sectorId) {
                    if (cacheLine[i].State == NodeState.Clean ||
                        cacheLine[i].State == NodeState.Dirty) {
                        nodeIndex = i;
#if RECORD_BLOCK_CACHE_STATISTICS
                        DebugStub.AddToPerfCounter(0, 1);   // Block found
#endif
                        return CandidateResult.FoundExact;
                    }
                    else {
                        nodeIndex = i;
                        // Node exists in cache but is in use
#if RECORD_BLOCK_CACHE_STATISTICS
                        DebugStub.AddToPerfCounter(1, 1);   // Block in use, block.
#endif
                        return CandidateResult.NodeBlocked;
                    }
                }
                else if (cacheLine[i].State == NodeState.Clean) {
                    // Candidate for eviction from cache.  Compute age
                    // and note if eldest clean node to date.
                    if (lruIndex < -0) {
                        lruIndex = i;
                        lastLruAge = (uint) unchecked(clockTicks - cacheLine[i].LruTicks);
                    }
                    else {
                        uint lruAge = (uint) unchecked(clockTicks - cacheLine[i].LruTicks);
                        if (lruAge > lastLruAge) {
                            lruIndex   = i;
                            lastLruAge = lruAge;
                        }
                    }
                }
            }

            if (freeIndex >= 0) {
                nodeIndex = freeIndex;
#if RECORD_BLOCK_CACHE_STATISTICS
                DebugStub.AddToPerfCounter(2, 1);   // Will fetch.
#endif
                return CandidateResult.FoundVictim;
            }
            else if (lruIndex >= 0) {
                nodeIndex = lruIndex;
#if RECORD_BLOCK_CACHE_STATISTICS
                DebugStub.AddToPerfCounter(3, 1);   // Will fetch.
#endif
                return CandidateResult.FoundVictim;
            }
            nodeIndex = -1;
#if RECORD_BLOCK_CACHE_STATISTICS
            DebugStub.AddToPerfCounter(3, 1);       // Wait to evict cache.
#endif
            return CandidateResult.CacheLineBlocked;
        }

        /// <remarks>
        /// Claims an unused or clean block in the cache and tags it
        /// with the requested block id.  It is assumed the caller wants
        /// to write block as new without reading it first. The block
        /// is optionally initialized with zeros and always marked as dirty.
        /// The caller must pair this method with EndQuickBlockOperation.
        /// Typical usage would be creating a new directory or first block
        /// for a file.
        /// </remarks>
        internal byte[]! in ExHeap
        CreateBlockAndBeginQuickOperation(uint blockId, bool zeroBuffer)
        {
            uint line = GetCacheLine(blockId);
            BlockCacheNode[]! cacheLine = (!) cache[line];
            ulong sectorId = BlockToSectorId(blockId);

            bool enqueue = false;

            // Take cache line lock
            Monitor.Enter(cacheLine);

          retry:
            int nodeIndex;
            CandidateResult cr =
                GetCandidateNode(cacheLine, cacheTicks[line], sectorId,
                                 out nodeIndex);
            if (cr == CandidateResult.NodeBlocked) {
                Monitor.Wait(cacheLine);
                goto retry;
            }
            else if (cr == CandidateResult.CacheLineBlocked) {
                // Wake up thread that flushes blocks to disk and wait
                // for notification that block state has changed.
                blockWriter.WakeUp();
                Monitor.Wait(cacheLine);
                goto retry;
            }

            byte []! in ExHeap bytes = cacheLine[nodeIndex].Acquire();

            if (cr == CandidateResult.FoundExact) {
                // Block exists in cache
                assert cacheLine[nodeIndex].SectorId == sectorId;

                assert (cacheLine[nodeIndex].State == NodeState.Clean ||
                        cacheLine[nodeIndex].State == NodeState.Dirty);

                if (cacheLine[nodeIndex].State == NodeState.Clean) {
                    cacheLine[nodeIndex].AcquiredTransition(
                        NodeState.Dirty, cacheTicks[line]++);
                    enqueue = true;
                }

                if (zeroBuffer) {
                    Bitter.Zero(bytes, 0, bytes.Length);
                }

                cacheLine[nodeIndex].AcquiredTransition(
                    NodeState.DirtyBorrowed, cacheTicks[line]++);

                Monitor.Exit(cacheLine);
            }
            else {
                // Block was not in cache, relabel block and mark as dirty.

                assert cr == CandidateResult.FoundVictim;

                assert (cacheLine[nodeIndex].State == NodeState.Clean ||
                        cacheLine[nodeIndex].State == NodeState.Unused);

                cacheLine[nodeIndex].AcquiredSetSectorId(sectorId);

                cacheLine[nodeIndex].AcquiredTransition(
                    NodeState.Dirty, cacheTicks[line]++);

                if (zeroBuffer) {
                    Bitter.Zero(bytes, 0, bytes.Length);
                }

                cacheLine[nodeIndex].AcquiredTransition(
                    NodeState.DirtyBorrowed, cacheTicks[line]++);

                enqueue = true;

                Monitor.Exit(cacheLine);
            }

            if (enqueue) {
                blockWriter.Enqueue(this, sectorId, nodeIndex);
            }
            return bytes;
        }

        /// <remarks>
        /// Fetches and locks a data block in the cache.  Only
        /// used by non-blocking internal FS operations.  This
        /// method may block if a fetch from from the disk is
        /// required to get the data or the data block is
        /// already locked.
        /// </remarks>
        internal byte[]! in ExHeap
        BeginQuickBlockOperation(uint blockId)
        {
            uint line = GetCacheLine(blockId);
            BlockCacheNode[]! cacheLine = (!) cache[line];
            ulong sectorId = BlockToSectorId(blockId);

            // Take cache line lock
            Monitor.Enter(cacheLine);

          retry:
            int nodeIndex;
            CandidateResult cr =
                GetCandidateNode(cacheLine, cacheTicks[line], sectorId,
                                 out nodeIndex);
            if (cr == CandidateResult.NodeBlocked) {
                Monitor.Wait(cacheLine);
                goto retry;
            }
            else if (cr == CandidateResult.CacheLineBlocked) {
                // Wake up thread that flushes blocks to disk and wait
                // for notification that block state has changed.
                blockWriter.WakeUp();
                Monitor.Wait(cacheLine);
                goto retry;
            }

            // Get buffer
            byte []! in ExHeap bytes = cacheLine[nodeIndex].Acquire();

            if (cr == CandidateResult.FoundExact) {
                // Block exists in cache
                assert cacheLine[nodeIndex].SectorId == sectorId;

                assert (cacheLine[nodeIndex].State == NodeState.Clean ||
                        cacheLine[nodeIndex].State == NodeState.Dirty);

                if (cacheLine[nodeIndex].State == NodeState.Clean) {
                    cacheLine[nodeIndex].AcquiredTransition(
                        NodeState.CleanBorrowed, cacheTicks[line]++);
                }
                else if (cacheLine[nodeIndex].State == NodeState.Dirty) {
                    cacheLine[nodeIndex].AcquiredTransition(
                        NodeState.DirtyBorrowed, cacheTicks[line]++);
                }

                // Drop cache line lock
                Monitor.Exit(cacheLine);

                return bytes;
            }
            else {
                assert cr == CandidateResult.FoundVictim;

                assert (cacheLine[nodeIndex].State == NodeState.Clean ||
                        cacheLine[nodeIndex].State == NodeState.Unused);

                // Block needs to be fetched from disk.
                // Change sector id on node about to be replaced
                cacheLine[nodeIndex].AcquiredSetSectorId(sectorId);

                // Mark State
                cacheLine[nodeIndex].AcquiredTransition(
                    NodeState.ReadPending, cacheTicks[line]++);

                // Drop lock for duration of read
                Monitor.Exit(cacheLine);

                byte[] in ExHeap inData = disk.Read(sectorId, bytes);
                assert inData != null;

                Monitor.Enter(cacheLine);
                cacheLine[nodeIndex].AcquiredTransition(
                    NodeState.CleanBorrowed, cacheTicks[line]++);
                Monitor.Exit(cacheLine);

                return inData;
            }
        }

        /// <remarks>
        /// Returns block to cache and unlocks it.  Must be paired with
        /// BeginQuickBlockOperation.
        /// </remarks>
        internal void
        EndQuickBlockOperation(uint                       blockId,
                               [Claims] byte[]! in ExHeap blockData,
                               bool                       dirtiedBuffer)
        {
            uint line = GetCacheLine(blockId);
            BlockCacheNode[]! cacheLine = (!) cache[line];

            // Lock cache line
            Monitor.Enter(cacheLine);

            // Get node
            int nodeIndex;
            CandidateResult cr = GetCandidateNode(cacheLine,
                                                  cacheTicks[line],
                                                  BlockToSectorId(blockId),
                                                  out nodeIndex);

            // BeginQuickBlockOperation should have blocked node
            assert cr == CandidateResult.NodeBlocked;

            assert nodeIndex >= 0;
            assert cacheLine[nodeIndex].Acquired == true;
            assert (cacheLine[nodeIndex].State == NodeState.CleanBorrowed ||
                    cacheLine[nodeIndex].State == NodeState.DirtyBorrowed);

            bool enqueueBlock = false;
            NodeState newState;

            // Return data to cache node and update state
            if (cacheLine[nodeIndex].State == NodeState.CleanBorrowed) {
                if (dirtiedBuffer) {
                    enqueueBlock = true;
                    newState = NodeState.Dirty;
                }
                else {
                    newState = NodeState.Clean;
                }
            }
            else {
                assert cacheLine[nodeIndex].State == NodeState.DirtyBorrowed;
                newState = NodeState.Dirty;
            }

            cacheLine[nodeIndex].Release(blockData, newState,
                                         cacheTicks[line]++);

            // Enqueue request *after* returning node to cache.
            // This is a must to maintain the correct state associated with
            // the node because writer may work synchronously or
            // asynchronously and the enqueue operation may trigger a write
            // which will update the state of the block both when it
            // starts and completes.
            if (enqueueBlock) {
                // NB cookie is index
                // in cache line of node.
                blockWriter.Enqueue(this,
                                    cacheLine[nodeIndex].SectorId,
                                    nodeIndex);
            }

            // Wakeup anyone waiting for this block or this cache-line
            // NB Threads may be sleeping because node has data transiently
            // unavailable (CleanBorrowed,DirtyBorrowed,ReadPending, or
            // WritePending), or because it's waiting for a victim to replace
            // and this node might now be eligible.
            Monitor.Pulse(cacheLine);

            Monitor.Exit(cacheLine);
        }

        internal void Read(uint              blockId,
                           int               blockOffset,
                           byte[]! in ExHeap dstBuffer,
                           int               dstOffset,
                           int               readBytes)
            requires blockId < TotalBlocks;
            requires blockOffset >= 0;
            requires dstOffset >= 0;
            requires readBytes >= 0;
            requires blockOffset + readBytes <= this.BytesPerBlock;
            requires dstOffset + readBytes <= dstBuffer.Length;
        {
            byte[]! in ExHeap blockData = BeginQuickBlockOperation(blockId);

            Bitter.Copy(dstBuffer, dstOffset, readBytes,
                        blockData, blockOffset);

            EndQuickBlockOperation(blockId, blockData, false);
        }

        internal void WriteEntireBlock(uint              blockId,
                                       byte[]! in ExHeap srcBuffer,
                                       int               srcOffset)
            requires blockId < TotalBlocks;
            requires srcOffset >= 0;
            requires srcOffset + BytesPerBlock <= srcBuffer.Length;
        {
            uint line = GetCacheLine(blockId);
            BlockCacheNode[]! cacheLine = (!) cache[line];
            ulong sectorId = BlockToSectorId(blockId);

            // Take cache line lock
            Monitor.Enter(cacheLine);

          retry:
            int nodeIndex;

            CandidateResult cr = GetCandidateNode(cacheLine,
                                                  cacheTicks[line],
                                                  sectorId,
                                                  out nodeIndex);
            if (cr == CandidateResult.NodeBlocked) {
                Monitor.Wait(cacheLine);
                goto retry;
            }
            else if (cr == CandidateResult.CacheLineBlocked) {
                // Wake up thread that flushes blocks to disk and wait
                // for notification that block state has changed.
                blockWriter.WakeUp();
                Monitor.Wait(cacheLine);
                goto retry;
            }
            assert (cr == CandidateResult.FoundExact ||
                    cr == CandidateResult.FoundVictim);
            assert ((cacheLine[nodeIndex].SectorId == sectorId &&
                     // Node has correct sector id, so is dirty/clean/unused
                     (cacheLine[nodeIndex].State == NodeState.Dirty ||
                      cacheLine[nodeIndex].State == NodeState.Clean ||
                      cacheLine[nodeIndex].State == NodeState.Unused)) ||
                    // Node is about to be reassigned, so is clean/unused
                    (cacheLine[nodeIndex].State == NodeState.Clean ||
                     cacheLine[nodeIndex].State == NodeState.Unused));

            byte []! in ExHeap blockData = cacheLine[nodeIndex].Acquire();

            NodeState oldState = cacheLine[nodeIndex].State;

            cacheLine[nodeIndex].AcquiredSetSectorId(sectorId);
            Bitter.Copy(blockData, 0, (int)BytesPerBlock,
                        srcBuffer, (int)srcOffset);
            cacheLine[nodeIndex].Release(blockData, NodeState.Dirty,
                                         cacheTicks[line]++);

            if (cacheLine[nodeIndex].State != oldState)
            {
                // NB cookie is index
                // in cache line of node.
                blockWriter.Enqueue(this,
                                    cacheLine[nodeIndex].SectorId,
                                    nodeIndex);
            }

            // Drop cache line lock
            Monitor.Exit(cacheLine);
            return;
        }

        /// <remarks> Creates a zero-filled block in the cache. </remarks>
        internal void ZeroBlock(uint blockId)
        {
            uint line = GetCacheLine(blockId);
            BlockCacheNode[]! cacheLine = (!) cache[line];
            ulong sectorId = BlockToSectorId(blockId);

            // Take cache line lock
            Monitor.Enter(cacheLine);

          retry:
            int nodeIndex;
            CandidateResult cr = GetCandidateNode(cacheLine,
                                                  cacheTicks[line],
                                                  sectorId,
                                                  out nodeIndex);
            if (cr == CandidateResult.NodeBlocked) {
                Monitor.Wait(cacheLine);
                goto retry;
            }
            else if (cr == CandidateResult.CacheLineBlocked) {
                // Wake up thread that flushes blocks to disk and wait
                // for notification that block state has changed.
                blockWriter.WakeUp();
                Monitor.Wait(cacheLine);
                goto retry;
            }
            assert (cr == CandidateResult.FoundExact ||
                    cr == CandidateResult.FoundVictim);
            assert ((cacheLine[nodeIndex].SectorId == sectorId &&
                     // Node has correct sector id, so is dirty/clean/unused
                     (cacheLine[nodeIndex].State == NodeState.Dirty ||
                      cacheLine[nodeIndex].State == NodeState.Clean ||
                      cacheLine[nodeIndex].State == NodeState.Unused)) ||
                    // Node is about to be reassigned, so is clean/unused
                    (cacheLine[nodeIndex].State == NodeState.Clean ||
                     cacheLine[nodeIndex].State == NodeState.Unused));

            byte []! in ExHeap blockData = cacheLine[nodeIndex].Acquire();

            NodeState oldState = cacheLine[nodeIndex].State;

            cacheLine[nodeIndex].AcquiredSetSectorId(sectorId);
            Bitter.Zero(blockData, 0, (int)BytesPerBlock);
            cacheLine[nodeIndex].Release(blockData, NodeState.Dirty,
                                         cacheTicks[line]++);

            if (cacheLine[nodeIndex].State != oldState)
            {
                // NB cookie is index
                // in cache line of node.
                blockWriter.Enqueue(this,
                                    cacheLine[nodeIndex].SectorId,
                                    nodeIndex);
            }

            // Drop cache line lock
            Monitor.Exit(cacheLine);
            return;
        }

        internal void Write(uint              blockId,
                            int               blockOffset,
                            byte[]! in ExHeap srcBuffer,
                            int               srcOffset,
                            int               writeBytes)
            requires blockId < TotalBlocks;
            requires blockOffset >= 0;
            requires srcOffset >= 0;
            requires writeBytes >= 0;
            requires blockOffset + writeBytes <= this.BytesPerBlock;
            requires srcOffset + writeBytes <= srcBuffer.Length;
        {
            if (writeBytes < BytesPerBlock) {
                byte[]! in ExHeap blockData = BeginQuickBlockOperation(blockId);

                Bitter.Copy(blockData, blockOffset, writeBytes,
                            srcBuffer, srcOffset);

                EndQuickBlockOperation(blockId, blockData, true);
            }
            else {
                WriteEntireBlock(blockId, srcBuffer, srcOffset);
            }
        }

        byte[]! in ExHeap
        IBlockWriterUser.GetDataForWrite(ulong sectorId, int cookie)
        {
            uint line = GetCacheLine(SectorToBlockId(sectorId));
            BlockCacheNode[]! cacheLine = (!) cache[line];

            Monitor.Enter(cacheLine);

            // Cookie is index in cache line of node.
            int nodeIndex = cookie;

            // Node should not have moved
            // and should be dirty.
            assert nodeIndex >= 0 && nodeIndex <= cacheLine.Length;
            assert cacheLine[nodeIndex].SectorId == sectorId;
            assert (cacheLine[nodeIndex].State == NodeState.Dirty ||
                    cacheLine[nodeIndex].State == NodeState.DirtyBorrowed);

            while (cacheLine[nodeIndex].State == NodeState.DirtyBorrowed) {
                Monitor.Wait(cacheLine);
            }

            byte[]! in ExHeap blockData = cacheLine[nodeIndex].Acquire();
            cacheLine[nodeIndex].AcquiredTransition(NodeState.WritePending,
                                                    cacheTicks[line]++);
            Monitor.Exit(cacheLine);
            return blockData;
        }

        void
        IBlockWriterUser.WriteComplete(ulong                      sectorId,
                                       [Claims] byte[]! in ExHeap blockData,
                                       int                        cookie)
        {
            uint line = GetCacheLine(SectorToBlockId(sectorId));
            BlockCacheNode[]! cacheLine = (!) cache[line];

            Monitor.Enter(cacheLine);

            // Cookie is index in cache line of node.
            int nodeIndex = cookie;

            // Node should not have moved
            // and should be pending write completion.
            assert nodeIndex >= 0 && nodeIndex <= cacheLine.Length;
            assert cacheLine[nodeIndex].SectorId == sectorId;
            assert cacheLine[nodeIndex].State == NodeState.WritePending;

            cacheLine[nodeIndex].Release(blockData,
                                         NodeState.Clean,
                                         cacheTicks[line]++);
            // Notify any waiters of clean node presence
            Monitor.Pulse(cacheLine);

            Monitor.Exit(cacheLine);
        }

        internal void ValidateAllClean()
        {
            assert this.cache != null;
            bool failed = false;
            for (int line = 0; line < this.cache.Length; line++) {
                BlockCacheNode []! bcns = (!)cache[line];
                for (int i = 0; i < bcns.Length; i++) {
                    if (bcns[i].State == NodeState.Unused &&
                        bcns[i].State == NodeState.Clean) {
                        DebugStub.Print("Failed on node[{0}][{1}] (sectorId {2}) -> {3}\n",
                                        __arglist(line, i,
                                                  bcns[i].SectorId,
                                                  bcns[i].State));
                        failed = true;
                    }
                }
            }
            assert !failed;
        }

        ///////////////////////////////////////////////////////////////////////
        // Static helper methods

        private static int Log2(uint value)
        {
            int l = 0;
            if ((value & 0xffff0000u) != 0) {
                l += 16;
                value >>= 16;
            }
            if ((value & 0xff00ff00u) != 0) {
                l += 8;
                value >>= 8;
            }
            if ((value & 0xf0f0f0f0) != 0) {
                l += 4;
                value >>= 4;
            }
            if ((value & 0xcccccccc) != 0) {
                l += 2;
                value >>= 2;
            }
            if ((value & 0xaaaaaaaa) != 0) {
                l += 1;
                value >>= 1;
            }
            return l;
        }

        /// <remarks>
        /// Computes cache dimensions from cache configuration.
        /// Output values are both powers of 2.
        /// </remarks>
        private static void SizeCache(ref BlockCacheConfiguration config,
                                      out uint stride,
                                      out uint associativity)
        {
            uint maxBlocks = config.MaxCacheKB * 1024 / config.BytesPerBlock;
            if (maxBlocks > config.TotalBlocks) {
                maxBlocks = config.TotalBlocks;
            }

            if (maxBlocks == 0) {
                stride        = 1;
                associativity = 1;
            }
            else {
                stride = 0;
                associativity = 64;
                while (stride == 0) {
                    associativity /= 2;
                    stride = maxBlocks / associativity;
                }
            }
        }
    }
}