//------------------------------------------------------------------------------
// <copyright file="CacheUsage.cs" company="Microsoft">
//     Copyright (c) Microsoft Corporation.  All rights reserved.
// </copyright>                                                                
//------------------------------------------------------------------------------

// 
// This is the implementation of an LRU2 list for the cache, which is used
// to flush the least frequently used items when memory pressure is low.
// 
// LRU2 is similar to LRU (Least Recently Used), but it keeps track of the 
// second to last use of an item, thereby measuring the average interval
// between references to an item. This has the advantage over LRU of keeping
// items that are frequently used, but haven't been used just at the time that
// we need to flush.
//
// Please note that when we insert a new item in the list, we treat the addition
// as a reference by itself.  So the 1st reference goes to the head, 
// and the 2nd reference goes to the head of the 2nd reference list.
//
// ...................................................
// For example:
// Action       List (Head...Tail)
// ------       ------------------
// Insert A     A1 A2
// Insert B     B1 A1 B2 A2
// Insert C     C1 B1 A1 C2 B2 A2
// Ref B        B1 C1 B2 A1 C2 A2
// Ref A        A1 B1 C1 B2 A2 C2
// Ref B        B1 A1 B2 C1 A2 C2
// Ref C        C1 B1 A1 B2 C2 A2
// Ref A        A1 C1 B1 A2 B2 C2
//
// When flushing, we will start scanning from the tail, and flush all 2nd references we find.
// For example, if we want to flush 2 items, we will find C2 and B2,
// and thus we will flush item B and C.
// ...................................................
// 
// An item in the LRU2 list is represented by a UsageEntry struct. This struct
// contains the reference to the cache entry, and the linked list entries for
// both the most recent first and second references to the entry. To distinguish
// between a link to an item's first reference and its second reference, we use
// a positive index for the first reference, and a negative index for the second
// reference.
//
// ...................................................
// For example:
// Logically I have this list
// Head                 Tail
// A1 B1 C2 C1 B2 A2
//
// Physically, A, B and C are stored in an array, and their indexes are 1, 2 and 3.
//
// So their ref1 and ref2 values will be:
//
// Entry(index) Ref1.Prev   Ref1.Next   Ref2.Prev   Ref2.Next
// -----        ---------   ---------   ---------   ---------
// (Head == 1 (point to A1))
// A(1)         0           2 (B1)      -2 (B2)     0
// B(2)         1 (A1)      -3 (C2)     3 (C1)      -1 (A2)
// C(3)         -3 (C2)     -2 (B2)     2 (B1)      3 (C1)
// ...................................................
// 
// To efficiently store potentially millions of cache items in the LRU2, 
// we adopt a paging model. Each UsagePage contains an array of 127 UsageEntry
// structs (whose size is slightly less than one x86 memory page). A reference
// to a UsageEntry is thus defined by 2 index, the index of the page, and the 
// index in the page's array of UsageEntry's.
//     
// When the number of free entries rises above a threshold, we reduce the number
// of pages in use.
// 
// In order to efficiently items in an array of arrays, we must use the C++ macro
// preprocessor. The jitter will not inline small functions, and we cannot represent
// access to a UsageEntry as a property, since the propery requires an index parameter.
// 

namespace System.Web.Caching {
    using System.Runtime.InteropServices;
    using System.Text;
    using System.Threading;
    using System.Web;
    using System.Web.Util;
    using System.Collections;
    using System;

    // UsageEntryRef defines a reference to a UsageEntry in the UsageBucket data structure.
    // An entry is identified by its index into the UsageBucket._pages array, and its
    // index into the UsagePage._entries array. 
    //
    // Bits 0-7 of the reference are for the index into the UsagePage._entries array.
    // Bits 8-31 of the reference are for the index into the UsageBucket._pages array.
    //
    // A reference to a UsageEntry may be to either its first or second reference in 
    // the UsageBucket._lastRef list. The second reference will have a negative index.
    //
    struct UsageEntryRef {
        // The invalid reference is 0.
        static internal readonly UsageEntryRef INVALID = new UsageEntryRef(0, 0);

        const uint   ENTRY_MASK  = 0x000000ffu;
        const uint   PAGE_MASK   = 0xffffff00u;
        const int    PAGE_SHIFT  = 8;

        uint _ref;

        internal UsageEntryRef(int pageIndex, int entryIndex) {
            Debug.Assert((pageIndex & 0x00ffffff) == pageIndex, "(pageIndex & 0x00ffffff) == pageIndex");
            Debug.Assert((Math.Abs(entryIndex) & ENTRY_MASK) == (Math.Abs(entryIndex)), "(Math.Abs(entryIndex) & ENTRY_MASK) == Math.Abs(entryIndex)");
            Debug.Assert(entryIndex != 0 || pageIndex == 0, "entryIndex != 0 || pageIndex == 0");

            // Please note that because the range of valid entryIndex is -127 to 127, so 
            // 1 byte is big enough to hold the value.

            _ref = ( (((uint)pageIndex) << PAGE_SHIFT) | (((uint)(entryIndex)) & ENTRY_MASK) );
        }

        public override bool Equals(object value) {
            if (value is UsageEntryRef) {
                return _ref == ((UsageEntryRef)value)._ref;
            }

            return false;
        }

#if NOT_USED
        public static bool Equals(UsageEntryRef r1, UsageEntryRef r2) {
            return r1._ref == r2._ref;
        }
#endif
        public static bool operator ==(UsageEntryRef r1, UsageEntryRef r2) {
            return r1._ref == r2._ref;
        }

        public static bool operator !=(UsageEntryRef r1, UsageEntryRef r2) {
            return r1._ref != r2._ref;
        }
        
        public override int GetHashCode() {
            return (int) _ref;
        }
    
#if DBG
        public override string ToString() {
            if (IsRef1) {
                return PageIndex + ":" + Ref1Index;
            }
            else if (IsRef2) {
                return PageIndex + ":" + -Ref2Index;
            }
            else {
                return "0";
            }
        }
#endif

        // The index into the UsageBucket._pages array.
        internal int PageIndex {
            get {
                int result = (int) (_ref >> PAGE_SHIFT);
                return result;
            }
        }

        
        // The index for UsageEntry._ref1 in the UsagePage._entries array.
        internal int Ref1Index {
            get {
                int result = (int) (sbyte) (_ref & ENTRY_MASK);
                Debug.Assert(result > 0, "result > 0");
                return result;
            }
        }

        // The index for UsageEntry._ref2 in the UsagePage._entries array.
        internal int Ref2Index {
            get {
                int result = (int) (sbyte) (_ref & ENTRY_MASK);
                Debug.Assert(result < 0, "result < 0");
                return -result;
            }
        }

        // Is the reference for the first reference?
        internal bool IsRef1 {
            get {
                return ((int) (sbyte) (_ref & ENTRY_MASK)) > 0;
            }
        }

        // Is the reference for the second reference?
        internal bool IsRef2 {
            get {
                return ((int) (sbyte) (_ref & ENTRY_MASK)) < 0;
            }
        }

        // Is the reference invalid?
        internal bool IsInvalid {
            get {
                return _ref == 0;
            }
        }
    }

    // A link to an item in the last references list.
    struct UsageEntryLink {
        internal UsageEntryRef _next;
        internal UsageEntryRef _prev;
    }

    // A cache entry in the LRU2 list. It contains the pointer to the 
    // cache entry, the date it was added to the list, and the links
    // to the first and second references to the item in the list.
    [StructLayout(LayoutKind.Explicit)]
    struct UsageEntry {
        // The entry's first reference in the last reference list.
        // _ref1._next is also used for the following purposes when
        // the entry is not on the last reference list:
        //  * As a link in the free entry list for a page.
        //  * As a link in the list of items to flush in FlushUnderUsedItems.
        //
        [FieldOffset(0)]
        internal UsageEntryLink _ref1;

        // _entries[0]._ref1._next is used to hold the head of the free entries
        // list. We use the space in _ref1._prev to hold the number
        // of free entries in the list.
        [FieldOffset(4)]
        internal int            _cFree;

        // The entry's second reference in the last reference list.
        [FieldOffset(8)]
        internal UsageEntryLink _ref2;

        // The date the entry was added to the list.
        // If the date is 0 (DateTime.MinValue), the entry is free.
        [FieldOffset(16)]
        internal DateTime       _utcDate; 
                                          
        // The cache entry.
        [FieldOffset(24)]
        internal CacheEntry     _cacheEntry;
    }

    // A page to hold the array of UsageEntry. 
    struct UsagePage {
        internal UsageEntry[]   _entries;       // array of UsagEntry.
        internal int            _pageNext;      // next page on the free page or free entry list
        internal int            _pagePrev;      // prev page on the free page or free entry list
    }

    // A list of UsagePages.
    struct UsagePageList {
        internal int            _head;          // head of list
        internal int            _tail;          // tail of list
    }

    sealed class UsageBucket {
        // We cannot use array index 0 for entries, because we need an array index to 
        // be different than its negation, and -0 = 0.
        // We use _entries[0] to hold the head of the free list, and the size of the free list.
        internal const int NUM_ENTRIES = 127;
        const int LENGTH_ENTRIES = 128;

        const int           MIN_PAGES_INCREMENT = 10;   
        const int           MAX_PAGES_INCREMENT = 340;   // (size of a page on x86) / (12 bytes per UsagePage)
        const double        MIN_LOAD_FACTOR = 0.5;  // minimum ratio of used to total entries before we will reduce

        CacheUsage          _cacheUsage;            // parent usage object
        byte                _bucket;                // priority of this bucket

        UsagePage[]         _pages;                 // list of pages
        int                 _cEntriesInUse;         // count of UsageEntry's in use
        int                 _cPagesInUse;           // count of UsagePage's in use
        int                 _cEntriesInFlush;       // count of UsageEntry's in process of being flushed
        int                 _minEntriesInUse;       // minimum number of entries in use before we reduce

        UsagePageList       _freePageList;          // list of free pages (_entries == null)
        UsagePageList       _freeEntryList;         // list of pages with free entries (entry.FreeCountCount > 0)

        UsageEntryRef       _lastRefHead;           // head of list of last refs
        UsageEntryRef       _lastRefTail;           // tail of list of last refs
        UsageEntryRef       _addRef2Head;           // head of ref2 list

        bool                _blockReduce;           // block calls to Reduce() while in FlushUnderUsedItems


        internal UsageBucket(CacheUsage cacheUsage, byte bucket) {
            _cacheUsage = cacheUsage;
            _bucket = bucket;
            InitZeroPages();
        }

        void InitZeroPages() {
            Debug.Assert(_cPagesInUse == 0, "_cPagesInUse == 0");
            Debug.Assert(_cEntriesInUse == 0, "_cEntriesInUse == 0");
            Debug.Assert(_cEntriesInFlush == 0, "_cEntriesInFlush == 0");
            Debug.Assert(_lastRefHead.IsInvalid, "_lastRefHead.IsInvalid");
            Debug.Assert(_lastRefTail.IsInvalid, "_lastRefTail.IsInvalid");
            Debug.Assert(_addRef2Head.IsInvalid, "_addRef2Head.IsInvalid");

            _pages = null;
            _minEntriesInUse = -1;
            _freePageList._head = -1;
            _freePageList._tail = -1;
            _freeEntryList._head = -1;
            _freeEntryList._tail = -1;
        }


// Use macros so that the code is inlined in the function
#define EntriesI(i)             (_pages[(i)]._entries)

#define EntriesR(entryRef)      (_pages[(entryRef.PageIndex)]._entries)

#define PagePrev(i)             (_pages[(i)]._pagePrev)

#define PageNext(i)             (_pages[(i)]._pageNext)

#define FreeEntryHead(entries)  ((entries)[0]._ref1._next)

#define FreeEntryCount(entries) ((entries)[0]._cFree)

#define CreateRef1(entryRef)    (new UsageEntryRef((entryRef).PageIndex, (entryRef).Ref2Index))

#define CreateRef2(entryRef)    (new UsageEntryRef((entryRef).PageIndex, -(entryRef).Ref1Index))


#if DBG
        bool EntryIsFree(UsageEntryRef entryRef) {
            return EntriesR(entryRef)[entryRef.Ref1Index]._cacheEntry == null &&
                   EntriesR(entryRef)[entryRef.Ref1Index]._utcDate == DateTime.MinValue;
        }

        bool EntryIsUsed(UsageEntryRef entryRef) {
            if (entryRef.IsRef1) {
                return EntriesR(entryRef)[entryRef.Ref1Index]._cacheEntry != null &&
                       EntriesR(entryRef)[entryRef.Ref1Index]._utcDate != DateTime.MinValue;
            }
            else {
                return EntriesR(entryRef)[entryRef.Ref2Index]._cacheEntry != null &&
                       EntriesR(entryRef)[entryRef.Ref2Index]._utcDate != DateTime.MinValue;
            }
        }
#endif

        // Add a page to the head of a list.
        void AddToListHead(int pageIndex, ref UsagePageList list) {
            Debug.Assert((list._head == -1) == (list._tail == -1), "(list._head == -1) == (list._tail == -1)");

            PagePrev(pageIndex) = -1;
            PageNext(pageIndex) = list._head;
            if (list._head != -1) {
                Debug.Assert(PagePrev(list._head) == -1, "PagePrev(list._head) == -1");
                PagePrev(list._head) = pageIndex;
            }
            else {
                list._tail = pageIndex;
            }

            list._head = pageIndex;
        }

        // Add a page to the tail of a list.
        void AddToListTail(int pageIndex, ref UsagePageList list) {
            Debug.Assert((list._head == -1) == (list._tail == -1), "(list._head == -1) == (list._tail == -1)");

            PageNext(pageIndex) = -1;
            PagePrev(pageIndex) = list._tail;
            if (list._tail != -1) {
                Debug.Assert(PageNext(list._tail) == -1, "PageNext(list._tail) == -1");
                PageNext(list._tail) = pageIndex;
            }
            else {
                list._head = pageIndex;
            }

            list._tail = pageIndex;
        }

        // Remove a page from the head of a list.
        int RemoveFromListHead(ref UsagePageList list) {
            Debug.Assert(list._head != -1, "list._head != -1");

            int oldHead = list._head;
            RemoveFromList(oldHead, ref list);
            return oldHead;
        }

        // Remove a page from the list.
        void RemoveFromList(int pageIndex, ref UsagePageList list) {
            Debug.Assert((list._head == -1) == (list._tail == -1), "(list._head == -1) == (list._tail == -1)");

            if (PagePrev(pageIndex) != -1) {
                Debug.Assert(PageNext(PagePrev(pageIndex)) == pageIndex, "PageNext(PagePrev(pageIndex)) == pageIndex");
                PageNext(PagePrev(pageIndex)) = PageNext(pageIndex);
            }
            else {
                Debug.Assert(list._head == pageIndex, "list._head == pageIndex");
                list._head = PageNext(pageIndex);
            }

            if (PageNext(pageIndex) != -1) {
                Debug.Assert(PagePrev(PageNext(pageIndex)) == pageIndex, "PagePrev(PageNext(pageIndex)) == pageIndex");
                PagePrev(PageNext(pageIndex)) = PagePrev(pageIndex);
            }
            else {
                Debug.Assert(list._tail == pageIndex, "list._tail == pageIndex");
                list._tail = PagePrev(pageIndex);
            }

            PagePrev(pageIndex) = -1;
            PageNext(pageIndex) = -1;
        }

        // Move a page to the head of the list
        void MoveToListHead(int pageIndex, ref UsagePageList list) {
            Debug.Assert(list._head != -1, "list._head != -1");
            Debug.Assert(list._tail != -1, "list._tail != -1");

            // already at head?
            if (list._head == pageIndex)
                return;

            // remove from list
            RemoveFromList(pageIndex, ref list);

            // add to head
            AddToListHead(pageIndex, ref list);
        }

        // Move to the tail of the list
        void MoveToListTail(int pageIndex, ref UsagePageList list) {
            Debug.Assert(list._head != -1, "list._head != -1");
            Debug.Assert(list._tail != -1, "list._tail != -1");

            // already at tail?
            if (list._tail == pageIndex)
                return;

            // remove from list
            RemoveFromList(pageIndex, ref list);

            // add to head
            AddToListTail(pageIndex, ref list);
        }

        // Update _minEntriesInUse when _cPagesInUse changes.
        // When _cEntriesInUse falls below _minEntriesInUse,
        // a call to Reduce() will consolidate entries onto fewer pages.
        // If _minEntries == -1, then a call to Reduce() will never reduce the number of pages.
        void UpdateMinEntries() {
            if (_cPagesInUse <= 1) {
                _minEntriesInUse = -1;
            }
            else {
                int capacity = _cPagesInUse * NUM_ENTRIES;
                Debug.Assert(capacity > 0, "capacity > 0");
                Debug.Assert(MIN_LOAD_FACTOR < 1.0, "MIN_LOAD_FACTOR < 1.0");

                _minEntriesInUse = (int) (capacity * MIN_LOAD_FACTOR);

                // Don't allow a reduce if there are not enough free entries to
                // remove a page.
                // Note: _minEntriesInUse - 1 == max # of entries still in use when Reduce() happens
                //       (_cPagesInUse - 1) * NUM_ENTRIES == capacity after one page is freed
                if ((_minEntriesInUse - 1) > ((_cPagesInUse - 1) * NUM_ENTRIES)) {
                    _minEntriesInUse = -1;
                }
            }

#if DBG
            if (Debug.IsTagPresent("CacheUsageNoReduce") && Debug.IsTagEnabled("CacheUsageNoReduce")) {
                _minEntriesInUse = -1;
            }
#endif

        }

        // Remove a UsagePage that is in use, and put in on the list of free pages.
        void RemovePage(int pageIndex) {
            Debug.Assert(FreeEntryCount(EntriesI(pageIndex)) == NUM_ENTRIES, "FreeEntryCount(EntriesI(pageIndex)) == NUM_ENTRIES");

            // release the page from the free entries list
            RemoveFromList(pageIndex, ref _freeEntryList);

            // Add the page to the free pages list
            AddToListHead(pageIndex, ref _freePageList);

            // remove reference to page
            Debug.Assert(EntriesI(pageIndex) != null, "EntriesI(pageIndex) != null");
            EntriesI(pageIndex) = null;

            // decrement count of pages and update _cMinEntriesInUse
            _cPagesInUse--;
            if (_cPagesInUse == 0) {
                InitZeroPages();
            }
            else {
                UpdateMinEntries();
            }
        }

#if DBG
        UsageEntryRef GetLastRefNext(UsageEntryRef entryRef) {
            if (entryRef.IsRef1) {
                return EntriesR(entryRef)[entryRef.Ref1Index]._ref1._next;
            }
            else if (entryRef.IsRef2) {
                return EntriesR(entryRef)[entryRef.Ref2Index]._ref2._next;
            }
            else {
                return _lastRefHead;
            }
        }

        UsageEntryRef GetLastRefPrev(UsageEntryRef entryRef) {
            if (entryRef.IsRef1) {
                return EntriesR(entryRef)[entryRef.Ref1Index]._ref1._prev;
            }
            else if (entryRef.IsRef2) {
                return EntriesR(entryRef)[entryRef.Ref2Index]._ref2._prev;
            }
            else {
                return _lastRefTail;
            }
        }
#endif

        // Set the _next reference in the last reference list.
        // We must do this with a macro to force inlining.
#define SetLastRefNext(entryRef, next) {                                                \
            if ((entryRef).IsRef1) {                                                    \
                EntriesR((entryRef))[(entryRef).Ref1Index]._ref1._next = (next);        \
            }                                                                           \
            else if ((entryRef).IsRef2) {                                               \
                EntriesR((entryRef))[(entryRef).Ref2Index]._ref2._next = (next);        \
            }                                                                           \
            else {                                                                      \
                _lastRefHead = (next);                                                  \
            }                                                                           \
        }                                                                               \

        // Set the _prev reference in the last reference list.
        // We must do this with a macro to force inlining.
#define SetLastRefPrev(entryRef, prev) {                                                \
            if ((entryRef).IsRef1) {                                                    \
                EntriesR((entryRef))[(entryRef).Ref1Index]._ref1._prev = (prev);        \
            }                                                                           \
            else if ((entryRef).IsRef2) {                                               \
                EntriesR((entryRef))[(entryRef).Ref2Index]._ref2._prev = (prev);        \
            }                                                                           \
            else {                                                                      \
                _lastRefTail = (prev);                                                  \
            }                                                                           \
        }                                                                               \

        // Get a free UsageEntry.
        UsageEntryRef GetFreeUsageEntry() {
            // get the page of the free entry
            Debug.Assert(_freeEntryList._head >= 0, "_freeEntryList._head >= 0");
            int pageIndex = _freeEntryList._head;

            // get a free entry from _entries
            UsageEntry[] entries = EntriesI(pageIndex);
            int entryIndex = FreeEntryHead(entries).Ref1Index;

            // fixup free list and count
            FreeEntryHead(entries) = entries[entryIndex]._ref1._next;
            FreeEntryCount(entries)--;
            if (FreeEntryCount(entries) == 0) {
                // remove page from list of free pages
                Debug.Assert(FreeEntryHead(entries).IsInvalid, "FreeEntryHead(entries).IsInvalid");
                RemoveFromList(pageIndex, ref _freeEntryList);
            }

#if DBG
            Debug.Assert(EntryIsFree(new UsageEntryRef(pageIndex, entryIndex)), "EntryIsFree(new UsageEntryRef(pageIndex, entryIndex))");
            if (!FreeEntryHead(entries).IsInvalid) {
                Debug.Assert(FreeEntryHead(entries).Ref1Index != entryIndex, "FreeEntryHead(entries).Ref1Index != entryIndex");
                Debug.Assert(EntryIsFree(new UsageEntryRef(pageIndex, FreeEntryHead(entries).Ref1Index)), "EntryIsFree(new UsageEntryRef(pageIndex, FreeEntryHead(entries).Ref1Index))");
            }
#endif

            return new UsageEntryRef(pageIndex, entryIndex);
        }

        // Add a UsageEntry to the free entry list.
        void AddUsageEntryToFreeList(UsageEntryRef entryRef) {
            Debug.Assert(entryRef.IsRef1, "entryRef.IsRef1");

            UsageEntry[] entries = EntriesR(entryRef);
            int entryIndex = entryRef.Ref1Index;

            Debug.Assert(entries[entryIndex]._cacheEntry == null, "entries[entryIndex]._cacheEntry == null");
            entries[entryIndex]._utcDate = DateTime.MinValue;
            entries[entryIndex]._ref1._prev = UsageEntryRef.INVALID;
            entries[entryIndex]._ref2._next = UsageEntryRef.INVALID;
            entries[entryIndex]._ref2._prev = UsageEntryRef.INVALID;

            entries[entryIndex]._ref1._next = FreeEntryHead(entries);
            FreeEntryHead(entries) = entryRef;

            _cEntriesInUse--;
            int pageIndex = entryRef.PageIndex;
            FreeEntryCount(entries)++;
            if (FreeEntryCount(entries) == 1) {
                // add page to head of list of free pages
                AddToListHead(pageIndex, ref _freeEntryList);
            }
            else if (FreeEntryCount(entries) == NUM_ENTRIES) {
                RemovePage(pageIndex);
            }
        }

        // Expand the capacity of the UsageBucket to hold more CacheEntry's.
        // We will need to allocate a new page, and perhaps expand the _pages array.
        // Note that we never collapse the _pages array.
        void Expand() {
            Debug.Assert(_cPagesInUse * NUM_ENTRIES == _cEntriesInUse, "_cPagesInUse * NUM_ENTRIES == _cEntriesInUse");
            Debug.Assert(_freeEntryList._head == -1, "_freeEntryList._head == -1");
            Debug.Assert(_freeEntryList._tail == -1, "_freeEntryList._tail == -1");

            // exapnd _pages if there are no more
            if (_freePageList._head == -1) {
                // alloc new pages array
                int oldLength;
                if (_pages == null) {
                    oldLength = 0;
                }
                else {
                    oldLength = _pages.Length;
                }

                Debug.Assert(_cPagesInUse == oldLength, "_cPagesInUse == oldLength");
                Debug.Assert(_cEntriesInUse == oldLength * NUM_ENTRIES, "_cEntriesInUse == oldLength * ExpiresEntryRef.NUM_ENTRIES");

                int newLength = oldLength * 2;
                newLength = Math.Max(oldLength + MIN_PAGES_INCREMENT, newLength);
                newLength = Math.Min(newLength, oldLength + MAX_PAGES_INCREMENT);
                Debug.Assert(newLength > oldLength, "newLength > oldLength");

                UsagePage[] newPages = new UsagePage[newLength];

                // copy original pages
                for (int i = 0; i < oldLength; i++) {
                    newPages[i] = _pages[i];
                }

                // setup free list of new pages
                for (int i = oldLength; i < newPages.Length; i++) {
                    newPages[i]._pagePrev = i - 1;
                    newPages[i]._pageNext = i + 1;
                }

                newPages[oldLength]._pagePrev = -1;
                newPages[newPages.Length - 1]._pageNext = -1;

                // use new pages array
                _freePageList._head = oldLength;
                _freePageList._tail = newPages.Length - 1;

                _pages = newPages;
            }

            // move from free page list to free entries list
            int pageIndex = RemoveFromListHead(ref _freePageList);
            AddToListHead(pageIndex, ref _freeEntryList);

            // create the entries
            UsageEntry[] entries = new UsageEntry[LENGTH_ENTRIES];
            FreeEntryCount(entries) = NUM_ENTRIES;
            
            // init free list
            for (int i = 0; i < entries.Length - 1; i++) {
                entries[i]._ref1._next = new UsageEntryRef(pageIndex, i + 1);
            }
            entries[entries.Length - 1]._ref1._next = UsageEntryRef.INVALID;

            EntriesI(pageIndex) = entries;

            // increment count of pages and update _minEntriesInUse
            _cPagesInUse++;
            UpdateMinEntries();
        }

        // Consolidate UsageEntry's onto fewer pages when there are too many
        // free entries.
        void Reduce() {
            // Test if we need to consolidate.
            if (_cEntriesInUse >= _minEntriesInUse || _blockReduce)
                return;

            Debug.Assert(_freeEntryList._head != -1, "_freeEntryList._head != -1");
            Debug.Assert(_freeEntryList._tail != -1, "_freeEntryList._tail != -1");
            Debug.Assert(_freeEntryList._head != _freeEntryList._tail, "_freeEntryList._head != _freeEntryList._tail");

            // Rearrange free page list to put pages with more free entries at the tail
            int meanFree = (int) (NUM_ENTRIES - (NUM_ENTRIES * MIN_LOAD_FACTOR));
            int pageIndexLast = _freeEntryList._tail;
            int pageIndexCurrent = _freeEntryList._head;
            int pageIndexNext;
            UsageEntry[] entries;

            for (;;) {
                pageIndexNext = PageNext(pageIndexCurrent);

                // move pages with greater than mean number
                // of free items to tail, move the others to head
                if (FreeEntryCount(EntriesI(pageIndexCurrent)) > meanFree) {
                    MoveToListTail(pageIndexCurrent, ref _freeEntryList);
                }
                else {
                    MoveToListHead(pageIndexCurrent, ref _freeEntryList);
                }

                // check if entire list has been examined
                if (pageIndexCurrent == pageIndexLast)
                    break;

                // iterate
                pageIndexCurrent = pageIndexNext;
            }

            // Move entries from the free pages at the tail to the 
            // free pages at the front, and release the free pages at the tail.
            for (;;) {
                // See if there is room left to move entries used by the page.
                if (_freeEntryList._tail == -1)
                    break;

                entries = EntriesI(_freeEntryList._tail);
                Debug.Assert(FreeEntryCount(entries) > 0, "FreeEntryCount(entries) > 0");
                int availableFreeEntries = (_cPagesInUse * NUM_ENTRIES) - FreeEntryCount(entries) - _cEntriesInUse;
                if (availableFreeEntries < (NUM_ENTRIES - FreeEntryCount(entries)))
                    break;

                // Move each entry from the page at the tail to a page at the head.
                for (int i = 1; i < entries.Length; i++) {
                    // skip the free entries
                    if (entries[i]._cacheEntry == null)
                        continue;

                    // get a free UsageEntry from the head of the list.
                    Debug.Assert(_freeEntryList._head != _freeEntryList._tail, "_freeEntryList._head != _freeEntryList._tail");
                    UsageEntryRef newRef1 = GetFreeUsageEntry();
                    UsageEntryRef newRef2 = CreateRef2(newRef1);
                    Debug.Assert(newRef1.PageIndex != _freeEntryList._tail, "newRef1.PageIndex != _freeEntryList._tail");

                    UsageEntryRef oldRef1 = new UsageEntryRef(_freeEntryList._tail, i);
                    UsageEntryRef oldRef2 = CreateRef2(oldRef1);

                    // update the CacheEntry
                    CacheEntry cacheEntry = entries[i]._cacheEntry;
                    Debug.Assert(cacheEntry.UsageEntryRef == oldRef1, "cacheEntry.UsageEntryRef == oldRef1");
                    cacheEntry.UsageEntryRef = newRef1;

                    // copy old entry to new entry
                    UsageEntry[] newEntries = EntriesR(newRef1);
                    newEntries[newRef1.Ref1Index] = entries[i];

                    // Update free entry count for debugging. We don't bother
                    // to fix up the free entry list for this page as we are
                    // going to release the page.
                    FreeEntryCount(entries)++;

                    // Update the last ref list. We need to be careful when
                    // references to the entry refer to the same entry.

                    // ref1
                    UsageEntryRef prev = newEntries[newRef1.Ref1Index]._ref1._prev;
                    Debug.Assert(prev != oldRef2, "prev != oldRef2");

                    UsageEntryRef next = newEntries[newRef1.Ref1Index]._ref1._next;
                    if (next == oldRef2) {
                        next = newRef2;
                    }

#if DBG
                    Debug.Assert(GetLastRefNext(prev) == oldRef1, "GetLastRefNext(prev) == oldRef1");
                    Debug.Assert(GetLastRefPrev(next) == oldRef1, "GetLastRefPrev(next) == oldRef1");
#endif

                    SetLastRefNext(prev, newRef1);
                    SetLastRefPrev(next, newRef1);

                    // ref2
                    prev = newEntries[newRef1.Ref1Index]._ref2._prev; 
                    if (prev == oldRef1) {
                        prev = newRef1;
                    }

                    next = newEntries[newRef1.Ref1Index]._ref2._next; 
                    Debug.Assert(next != oldRef1, "next != oldRef1");

#if DBG
                    Debug.Assert(GetLastRefNext(prev) == oldRef2, "GetLastRefNext(prev) == oldRef2");
                    Debug.Assert(GetLastRefPrev(next) == oldRef2, "GetLastRefPrev(next) == oldRef2");
#endif

                    SetLastRefNext(prev, newRef2);
                    SetLastRefPrev(next, newRef2);
                    
                    // _addRef2Head
                    if (_addRef2Head == oldRef2) {
                        _addRef2Head = newRef2;
                    }
                }

                // now the page is free - release its memory
                RemovePage(_freeEntryList._tail);

                Debug.Validate("CacheValidateUsage", this);
            }
        }

        // Add a new UsageEntry for a CacheEntry.
        internal void AddCacheEntry(CacheEntry cacheEntry) {
            lock (this) {
                // Expand if there are no free UsageEntry's available.
                if (_freeEntryList._head == -1) {
                    Expand();
                }

                // get the free entry
                UsageEntryRef freeRef1 = GetFreeUsageEntry();
                UsageEntryRef freeRef2 = CreateRef2(freeRef1);
                Debug.Assert(cacheEntry.UsageEntryRef.IsInvalid, "cacheEntry.UsageEntryRef.IsInvalid");
                cacheEntry.UsageEntryRef = freeRef1;

                // initialize index
                UsageEntry[] entries = EntriesR(freeRef1);
                int entryIndex = freeRef1.Ref1Index;
                entries[entryIndex]._cacheEntry = cacheEntry;
                entries[entryIndex]._utcDate = DateTime.UtcNow;

                // add ref1 to head of entire list, ref2 to head of new ref2 list
#if DBG
                Debug.Assert(!_addRef2Head.IsRef1, "!_addRef2Head.IsRef1");
                Debug.Assert(!_lastRefTail.IsRef1, "!_lastRefTail.IsRef1");
                Debug.Assert(!_lastRefHead.IsRef2, "!_lastRefHead.IsRef2");
                Debug.Assert(_lastRefTail.IsInvalid == _lastRefHead.IsInvalid, "_lastRefTail.IsInvalid == _lastRefHead.IsInvalid");
                Debug.Assert(!_lastRefTail.IsInvalid || _addRef2Head.IsInvalid, "!_lastRefTail.IsInvalid || _addRef2Head.IsInvalid");
                Debug.Assert(GetLastRefNext(_lastRefTail).IsInvalid, "GetLastRefNext(_lastRefTail).IsInvalid");
                Debug.Assert(GetLastRefPrev(_lastRefHead).IsInvalid, "GetLastRefPrev(_lastRefHead).IsInvalid");
#endif

                entries[entryIndex]._ref1._prev = UsageEntryRef.INVALID;
                entries[entryIndex]._ref2._next = _addRef2Head;
                if (_lastRefHead.IsInvalid) {
                    entries[entryIndex]._ref1._next = freeRef2;
                    entries[entryIndex]._ref2._prev = freeRef1;
                    _lastRefTail = freeRef2;
                }
                else {
                    entries[entryIndex]._ref1._next = _lastRefHead;
                    SetLastRefPrev(_lastRefHead, freeRef1);

                    UsageEntryRef next, prev;
                    if (_addRef2Head.IsInvalid) {
                        prev = _lastRefTail;
                        next = UsageEntryRef.INVALID;
                    }
                    else {
                        prev = EntriesR(_addRef2Head)[_addRef2Head.Ref2Index]._ref2._prev;
                        next = _addRef2Head;
                    }

                    entries[entryIndex]._ref2._prev = prev;
                    SetLastRefNext(prev, freeRef2);
                    SetLastRefPrev(next, freeRef2);
                }

                _lastRefHead = freeRef1;
                _addRef2Head = freeRef2;

                _cEntriesInUse++;

                Debug.Trace("CacheUsageAdd", 
                            "Added item=" + cacheEntry.Key + 
                            ",_bucket=" + _bucket + 
                            ",ref=" + freeRef1);

                Debug.Validate("CacheValidateUsage", this);
                Debug.Dump("CacheUsageAdd", this);
            }
        }

        // Remove an entry from the last references list.
        void RemoveEntryFromLastRefList(UsageEntryRef entryRef) {
            Debug.Assert(entryRef.IsRef1, "entryRef.IsRef1");
            UsageEntry[] entries = EntriesR(entryRef);
            int entryIndex = entryRef.Ref1Index;

            // remove ref1 from list
            UsageEntryRef prev = entries[entryIndex]._ref1._prev;
            UsageEntryRef next = entries[entryIndex]._ref1._next;

#if DBG
            Debug.Assert(GetLastRefNext(prev) == entryRef, "GetLastRefNext(prev) == entryRef");
            Debug.Assert(GetLastRefPrev(next) == entryRef, "GetLastRefPrev(next) == entryRef");
#endif

            SetLastRefNext(prev, next);
            SetLastRefPrev(next, prev);

            // remove ref2 from list
            prev = entries[entryIndex]._ref2._prev;
            next = entries[entryIndex]._ref2._next;

            UsageEntryRef entryRef2 = CreateRef2(entryRef);

#if DBG
            Debug.Assert(GetLastRefNext(prev) == entryRef2, "GetLastRefNext(prev) == entryRef2");
            Debug.Assert(GetLastRefPrev(next) == entryRef2, "GetLastRefPrev(next) == entryRef2");
#endif

            SetLastRefNext(prev, next);
            SetLastRefPrev(next, prev);

            // fixup _addRef2Head
            if (_addRef2Head == entryRef2) {
                _addRef2Head = next;
            }
        }

        // Remove a CacheEntry from the UsageBucket.
        internal void RemoveCacheEntry(CacheEntry cacheEntry) {
            lock (this) {
                // The cache entry could have been removed from the cache while 
                // we are in the middle of FlushUnderUsedItems, after we have
                // released the lock and before we ourselves call Cache.Remove().
                // Guard against that here.
                UsageEntryRef entryRef = cacheEntry.UsageEntryRef;
                if (entryRef.IsInvalid)
                    return;

                UsageEntry[]    entries = EntriesR(entryRef);
                int             entryIndex = entryRef.Ref1Index;

#if DBG
                Debug.Assert(entryRef.IsRef1, "entryRef.IsRef1");
                Debug.Assert(EntryIsUsed(entryRef), "EntryIsUsed(entryRef)");
                Debug.Assert(cacheEntry == entries[entryIndex]._cacheEntry, "cacheEntry == entries[entryIndex]._cacheEntry");
#endif
    
                // update the cache entry
                cacheEntry.UsageEntryRef = UsageEntryRef.INVALID;
                entries[entryIndex]._cacheEntry = null;
    
                // remove from last ref list
                RemoveEntryFromLastRefList(entryRef);

                // add to free list
                AddUsageEntryToFreeList(entryRef);

                // remove pages if necessary
                Reduce();

                Debug.Trace("CacheUsageRemove", 
                            "Removed item=" + cacheEntry.Key + 
                            ",_bucket=" + _bucket + 
                            ",ref=" + entryRef);

                Debug.Validate("CacheValidateUsage", this);
                Debug.Dump("CacheUsageRemove", this);
            }
        }

        // Update the CacheEntry in the last references list.
        internal void UpdateCacheEntry(CacheEntry cacheEntry) {
            lock (this) {
                // The cache entry could have been retreived from the cache while 
                // we are in the middle of FlushUnderUsedItems, after we have
                // released the lock and before we ourselves call Cache.Remove().
                // Guard against that here.
                UsageEntryRef entryRef = cacheEntry.UsageEntryRef;
                if (entryRef.IsInvalid)
                    return;

#if DBG
                Debug.Assert(entryRef.IsRef1, "entryRef.IsRef1");
                Debug.Assert(EntryIsUsed(entryRef), "EntryIsUsed(entryRef)");
                Debug.Assert(!_lastRefHead.IsInvalid, "!_lastRefHead.IsInvalid");
                Debug.Assert(!_lastRefTail.IsInvalid, "!_lastRefTail.IsInvalid");
#endif

                UsageEntry[] entries = EntriesR(entryRef);
                int entryIndex = entryRef.Ref1Index;
                UsageEntryRef entryRef2 = CreateRef2(entryRef);

                // remove ref2 from list
                UsageEntryRef prev = entries[entryIndex]._ref2._prev;
                UsageEntryRef next = entries[entryIndex]._ref2._next;

#if DBG
                Debug.Assert(GetLastRefNext(prev) == entryRef2, "GetLastRefNext(prev) == entryRef2");
                Debug.Assert(GetLastRefPrev(next) == entryRef2, "GetLastRefPrev(next) == entryRef2");
#endif

                SetLastRefNext(prev, next);
                SetLastRefPrev(next, prev);

                // fixup _addRef2Head
                if (_addRef2Head == entryRef2) {
                    _addRef2Head = next;
                }

                // move ref1 to ref2
                entries[entryIndex]._ref2 = entries[entryIndex]._ref1;
                prev = entries[entryIndex]._ref2._prev;
                next = entries[entryIndex]._ref2._next;

#if DBG
                Debug.Assert(GetLastRefNext(prev) == entryRef, "GetLastRefNext(prev) == entryRef");
                Debug.Assert(GetLastRefPrev(next) == entryRef, "GetLastRefPrev(next) == entryRef");
#endif

                SetLastRefNext(prev, entryRef2);
                SetLastRefPrev(next, entryRef2);

                // put ref1 at head of list
                entries[entryIndex]._ref1._prev = UsageEntryRef.INVALID;
                entries[entryIndex]._ref1._next = _lastRefHead;

#if DBG
                Debug.Assert(GetLastRefPrev(_lastRefHead).IsInvalid, "GetLastRefPrev(_lastRefHead).IsInvalid");
#endif

                SetLastRefPrev(_lastRefHead, entryRef);
                _lastRefHead = entryRef;

                Debug.Trace("CacheUsageUpdate", 
                            "Updated item=" + cacheEntry.Key + 
                            ",_bucket=" + _bucket + 
                            ",ref=" + entryRef);

                Debug.Validate("CacheValidateUsage", this);
                Debug.Dump("CacheUsageUpdate", this);
            }
        }

        // Flush under used items from the cache.
        // If force is false, then we will skip items that have not aged enough
        // to accumulate history.
        // publicEntriesFlushed keeps track of the number of public entries that are flushed
        internal int FlushUnderUsedItems(int maxFlush, bool force, ref int publicEntriesFlushed, ref int ocEntriesFlushed) {            
#if DBG
            if (Debug.IsTagPresent("CacheUsageNoFlush") && Debug.IsTagEnabled("CacheUsageNoFlush"))
                return 0;
#endif

            // Check if there is something to flush
            if (_cEntriesInUse == 0)
                return 0;

            Debug.Assert(maxFlush > 0, "maxFlush is not greater than 0, instead is " + maxFlush);
            Debug.Assert(_cEntriesInFlush == 0, "_cEntriesInFlush == 0");

            // We create a list of UsageEntry's that we wish to flush. These entries
            // are not considered free, so the page that holds them will not be removed.
            // inFlushHead will point to the head of that list, while we use the same
            // UsageEntry._ref1 to chain them together (after we remove them from
            // the LastRef list.)
            UsageEntryRef inFlushHead = UsageEntryRef.INVALID;

            UsageEntryRef prev, prevNext;
            DateTime utcDate;
            UsageEntry[] entries;
            int entryIndex;
            CacheEntry cacheEntry;
            int flushed = 0;

            try {
                // Block insertion into the Cache if we're under high memory pressure
                _cacheUsage.CacheSingle.BlockInsertIfNeeded();
                
                lock (this) {
                    Debug.Assert(_blockReduce == false, "_blockReduce == false");

                    // Recheck if there is something to flush.
                    if (_cEntriesInUse == 0)
                        return 0;

                    DateTime utcNow = DateTime.UtcNow;

                    // Walk the ref2 list backwards, as these are the items that have
                    // been used least.
                    for (prev = _lastRefTail; _cEntriesInFlush < maxFlush && !prev.IsInvalid; prev = prevNext) {
                        Debug.Assert(_cEntriesInUse > 0, "_cEntriesInUse > 0");

                        // Set prevNext before possibly freeing an item.
                        // Examine only at ref2 items so we don't enumerate an
                        // item twice.
                        prevNext = EntriesR(prev)[prev.Ref2Index]._ref2._prev;
                        while (prevNext.IsRef1) {
                            prevNext = EntriesR(prevNext)[prevNext.Ref1Index]._ref1._prev;
                        }

#if DBG
                        Debug.Assert(prev.IsRef2, "prev.IsRef2");
                        Debug.Assert(EntryIsUsed(prev), "EntryIsUsed(prev)");
                        Debug.Assert(EntryIsUsed(prev), "EntryIsUsed(prev)");
#endif

                        entries = EntriesR(prev);
                        entryIndex = prev.Ref2Index;

                        // Do not remove an item if it was recently added to the last references list,
                        // as it has not had enough time to accumulate usage history.
                        if (!force) {
                            utcDate = entries[entryIndex]._utcDate;
                            Debug.Assert(utcDate != DateTime.MinValue, "utcDate != DateTime.MinValue");

                            if (utcNow - utcDate <= CacheUsage.NEWADD_INTERVAL && utcNow >= utcDate)
                                continue;
                        }

                        UsageEntryRef prev1 = CreateRef1(prev);
                        cacheEntry = entries[entryIndex]._cacheEntry;
                        Debug.Assert(cacheEntry.UsageEntryRef == prev1, "cacheEntry.UsageEntryRef == prev1");
                        Debug.Trace("CacheUsageFlushUnderUsedItem", "Flushing underused items, item=" + cacheEntry.Key + ", bucket=" + _bucket);

                        // Remove reference from CacheEntry. We must do this before we
                        // release the lock, otherwise the item would be corrupted if
                        // UpdateCacheEntry or RemoveCacheEntry were called.
                        cacheEntry.UsageEntryRef = UsageEntryRef.INVALID;
                        
                        // Keep track of how many public entries were flushed
                        if (cacheEntry.IsPublic) {
                            publicEntriesFlushed ++;
                        }
                        else if (cacheEntry.IsOutputCache) {
                            ocEntriesFlushed++;
                        }
                        
                        // remove from lastref list
                        RemoveEntryFromLastRefList(prev1);

                        // add it to the inFlush list
                        entries[entryIndex]._ref1._next = inFlushHead;
                        inFlushHead = prev1;

                        flushed++;
                        _cEntriesInFlush++;
                    }

                    if (flushed == 0) {
                        Debug.Trace("CacheUsageFlushTotal", "Flush(" + maxFlush + "," + force + ") removed " + flushed + 
                                    " underused items; Time=" + Debug.FormatLocalDate(DateTime.Now));

                        return 0;
                    }
                    

                    // We are about to leave the lock.  However, we still have to use the 
                    // locally created "inFlush list" after that.  That's why we have to
                    // set _blockReduce to true to prevent Reduce() from moving the
                    // entries around.
                    _blockReduce = true;
                }
            }
            finally {
                // Don't hold any insertblock before we remove Cache items.  If not, the following
                // deadlock scenario may happen:
                // - 3rd party code hold lock A, call Cache.Insert, which wait for the Cache insertblock
                // - FlushUnderUsedItems holds the Cache insertBlock, call Cache.Remove, which call
                //   3rd party CacheItemRemovedCallback, which then try to get lock A
                
                _cacheUsage.CacheSingle.UnblockInsert();
            }

            // We need to release the lock because when we do remove below,
            // some CacheRemoveCallback user code might run that might
            // do a cache insert in another thread, and will cause
            // a CacheUsage insert/remove/update, we will block
            // that thread, which cause a deadlock if the user code is
            // waiting for that thread to finish its job.

            Debug.Assert(!inFlushHead.IsInvalid, "!inFlushHead.IsInvalid");

            // Remove items on the inFlush list from the rest of the cache.
            CacheSingle cacheSingle = _cacheUsage.CacheSingle;
            UsageEntryRef current = inFlushHead;
            UsageEntryRef next;
            while (!current.IsInvalid) {
                entries = EntriesR(current);
                entryIndex = current.Ref1Index;

                next = entries[entryIndex]._ref1._next;
                
                // remove the entry
                cacheEntry = entries[entryIndex]._cacheEntry;
                entries[entryIndex]._cacheEntry = null;
                Debug.Assert(cacheEntry.UsageEntryRef.IsInvalid, "cacheEntry.UsageEntryRef.IsInvalid");
                cacheSingle.Remove(cacheEntry, CacheItemRemovedReason.Underused);

                //iterate
                current = next;
            }

            try {
                // Block insertion into the Cache if we're under high memory pressure
                _cacheUsage.CacheSingle.BlockInsertIfNeeded();
                
                lock (this) {
                    // add each UsageEntry to the free list
                    current = inFlushHead;
                    while (!current.IsInvalid) {
                        entries = EntriesR(current);
                        entryIndex = current.Ref1Index;

                        next = entries[entryIndex]._ref1._next;

                        _cEntriesInFlush--;
                        AddUsageEntryToFreeList(current);

                        //iterate
                        current = next;
                    }

                    // try to reduce
                    Debug.Assert(_cEntriesInFlush == 0, "_cEntriesInFlush == 0");
                    _blockReduce = false;
                    Reduce();

                    Debug.Trace("CacheUsageFlushTotal", "Flush(" + maxFlush + "," + force + ") removed " + flushed + 
                                " underused items; Time=" + Debug.FormatLocalDate(DateTime.Now));

                    Debug.Validate("CacheValidateUsage", this);
                    Debug.Dump("CacheUsageFlush", this);
                }
            }
            finally {
                _cacheUsage.CacheSingle.UnblockInsert();
            }

            return flushed;
        }

#if DBG
        internal void DebugValidate() {
            int cFree = 0;               
            int cEntriesInUse = 0;              
            int cPagesInUse = 0;
            int pagesLength;

            if (_pages == null) {
                pagesLength = 0;
            }
            else {
                pagesLength = _pages.Length;
            }

            Debug.CheckValid(-1 <= _freePageList._head && _freePageList._head <= pagesLength, "-1 <= _freePageList._head && _freePageList._head <= pagesLength");
            Debug.CheckValid(-1 <= _freeEntryList._head && _freeEntryList._head <= pagesLength, "-1 <= _freeEntryList._head && _freeEntryList._head <= pagesLength");
            Debug.CheckValid(-1 <= _freeEntryList._tail && _freeEntryList._tail <= pagesLength, "-1 <= _freeEntryList._tail && _freeEntryList._tail <= pagesLength");
            Debug.CheckValid((_freeEntryList._head == -1) == (_freeEntryList._tail == -1), "(_freeEntryList._head == -1) == (_freeEntryList._tail == -1)");
            Debug.CheckValid(_minEntriesInUse >= -1, "_minEntriesInUse >= -1");
            Debug.CheckValid(_lastRefHead.IsInvalid == _lastRefTail.IsInvalid, "_lastRefHead.IsInvalid == _lastRefTail.IsInvalid");
            Debug.CheckValid(!_lastRefTail.IsInvalid || _addRef2Head.IsInvalid, "!_lastRefTail.IsInvalid || _addRef2Head.IsInvalid");
            Debug.CheckValid(!_lastRefHead.IsRef2, "!_lastRefHead.IsRef2");
            Debug.CheckValid(!_lastRefTail.IsRef1, "!_lastRefTail.IsRef1");
            Debug.CheckValid(!_addRef2Head.IsRef1, "!_addRef2Head.IsRef1");
            Debug.CheckValid(_cEntriesInFlush >= 0, "_cEntriesInFlush >= 0");

            // check counts
            for (int i = 0; i < pagesLength; i++) {
                UsageEntry[] entries = _pages[i]._entries;
                if (entries != null) {
                    cPagesInUse++;
                    cFree = 0;

                    Debug.CheckValid(entries[0]._cacheEntry == null, "entries[0]._cacheEntry == null");
                    Debug.CheckValid(entries[0]._utcDate == DateTime.MinValue, "entries[0]._utcDate == DateTime.MinValue");

                    for (int j = 1; j < entries.Length; j++) {
                        if (EntryIsFree(new UsageEntryRef(i, j))) {
                            cFree++;
                        }
                        else {
                            cEntriesInUse++;
                        }
                    }

                    Debug.CheckValid(cFree == FreeEntryCount(entries), "cFree == FreeEntryCount(entries)");

                    // walk the free list
                    cFree = 0;
                    if (!FreeEntryHead(entries).IsInvalid) {
                        int j = FreeEntryHead(entries).Ref1Index;
                        for (;;) {
                            cFree++;
                            Debug.CheckValid(cFree <= FreeEntryCount(entries), "cFree <= FreeEntryCount(entries)");

                            if (entries[j]._ref1._next.IsInvalid)
                                break;

                            j = entries[j]._ref1._next.Ref1Index;
                        }
                    }
                    Debug.CheckValid(cFree == FreeEntryCount(entries), "cFree == FreeEntryCount(entries)");
                }
            }

            Debug.CheckValid(cPagesInUse == _cPagesInUse, "cPagesInUse == _cPagesInUse");
            Debug.CheckValid(cEntriesInUse == _cEntriesInUse, "cEntriesInUse == _cEntriesInUse");

            // walk the free slot list
            int cFreeSlots = 0;
            if (_freePageList._head != -1) {
                for (int i = _freePageList._head; i != -1; i = _pages[i]._pageNext) {
                    cFreeSlots++;
                    Debug.CheckValid(cFreeSlots <= pagesLength, "cFreeSlots <= pagesLength");
                    Debug.CheckValid(_pages[i]._entries == null, "_pages[i]._entries == null");
                    if (_freePageList._head != i) {
                        Debug.CheckValid(PageNext(PagePrev(i)) == i, "PageNext(PagePrev(i)) == i");
                    }

                    if (_freePageList._tail != i) {
                        Debug.CheckValid(PagePrev(PageNext(i)) == i, "PagePrev(PageNext(i)) == i");
                    }
                }
            }
            Debug.CheckValid(cFreeSlots == pagesLength - _cPagesInUse, "cFreeSlots == pagesLength - _cPagesInUse");

            // walk the free page list
            int cFreeEntries = 0;
            int cFreePages = 0;
            if (_freeEntryList._head != -1) {
                for (int i = _freeEntryList._head; i != -1; i = _pages[i]._pageNext) {
                    cFreePages++;
                    Debug.CheckValid(cFreePages <= pagesLength, "cFreePages < pagesLength");
                    UsageEntry[] entries = _pages[i]._entries;
                    Debug.CheckValid(entries != null, "entries != null");
                    cFreeEntries += FreeEntryCount(entries);
                    if (_freeEntryList._head != i) {
                        Debug.CheckValid(PageNext(PagePrev(i)) == i, "PageNext(PagePrev(i)) == i");
                    }

                    if (_freeEntryList._tail != i) {
                        Debug.CheckValid(PagePrev(PageNext(i)) == i, "PagePrev(PageNext(i)) == i");
                    }
                }
            }

            Debug.CheckValid(cFreeEntries == (_cPagesInUse * NUM_ENTRIES) - _cEntriesInUse, "cFreeEntries == (_cPagesInUse * NUM_ENTRIES) - _cEntriesInUse");

            // walk last ref list forwards
            int cTotalRefs = 2 * (_cEntriesInUse - _cEntriesInFlush);
            int cRefs = 0;
            UsageEntryRef last = UsageEntryRef.INVALID;
            UsageEntryRef next = _lastRefHead;
            while (!next.IsInvalid) {
                cRefs++;
                Debug.CheckValid(cRefs <= cTotalRefs, "cRefs <= cTotalRefs");
                Debug.CheckValid(EntryIsUsed(next), "EntryIsUsed(next)");
                Debug.CheckValid(GetLastRefPrev(next) == last, "GetLastRefPrev(next) == last");
                last = next;
                next = GetLastRefNext(next);
            }

            Debug.CheckValid(cRefs == cTotalRefs, "cRefs == cTotalRefs");

            // walk list backwards
            cRefs = 0;
            last = UsageEntryRef.INVALID;
            UsageEntryRef prev = _lastRefTail;
            while (!prev.IsInvalid) {
                cRefs++;
                Debug.CheckValid(cRefs <= cTotalRefs, "cRefs <= cTotalRefs");
                Debug.CheckValid(EntryIsUsed(prev), "EntryIsUsed(next)");
                Debug.CheckValid(GetLastRefNext(prev) == last, "GetLastRefPrev(next) == last");
                last = prev;
                prev = GetLastRefPrev(prev);
            }

            Debug.CheckValid(cRefs == cTotalRefs, "cRefs == cTotalRefs");
            
            // walk the add2ref list
            cRefs = 0;
            last = GetLastRefPrev(_addRef2Head);
            next = _addRef2Head;
            while (!next.IsInvalid) {
                cRefs++;
                Debug.CheckValid(cRefs <= (cTotalRefs / 2), "cRefs <= (cTotalRefs / 2)");
                Debug.CheckValid(EntryIsUsed(next), "EntryIsUsed(next)");
                Debug.CheckValid(GetLastRefPrev(next) == last, "GetLastRefPrev(next) == last");
                Debug.CheckValid(!next.IsRef1, "!next.IsRef1");
                last = next;
                next = GetLastRefNext(next);
            }

            Debug.CheckValid(cRefs <= (cTotalRefs / 2), "cRefs <= (cTotalRefs / 2)");
        }

        internal string DebugDescription(string indent) {
            StringBuilder   sb = new StringBuilder();
            string          i2 = indent + "    ";

            sb.Append(indent + 
                      "_bucket=" + _bucket + 
                      ",_cEntriesInUse=" + _cEntriesInUse + 
                      ",_cPagesInUse=" + _cPagesInUse + 
                      ",_pages is " + (_pages == null ? "null" : "non-null") + 
                      ",_minEntriesInUse=" + _minEntriesInUse + 
                      ",_freePageList._head=" + _freePageList._head + 
                      ",_freeEntryList._head=" + _freeEntryList._head + 
                      ",_freeEntryList._tail=" + _freeEntryList._tail +
                      "\n");

            sb.Append(indent + "Refs list, in order:\n");

            UsageEntryRef next = _lastRefHead;
            while (!next.IsInvalid) {
                if (next.IsRef1) {
                    sb.Append(i2 + next.PageIndex + ":" + next.Ref1Index + " (1): " + EntriesR(next)[next.Ref1Index]._cacheEntry.Key + "\n");
                }
                else {
                    sb.Append(i2 + next.PageIndex + ":" + next.Ref2Index + " (2): " + EntriesR(next)[next.Ref2Index]._cacheEntry.Key + "\n");
                }

                next = GetLastRefNext(next);
            }

            return sb.ToString();
        }
#endif
    }

    class CacheUsage {
        internal static readonly TimeSpan   NEWADD_INTERVAL = new TimeSpan(0, 0, 10);
        internal static readonly TimeSpan   CORRELATED_REQUEST_TIMEOUT = new TimeSpan(0, 0, 1);
        internal static readonly TimeSpan   MIN_LIFETIME_FOR_USAGE = NEWADD_INTERVAL;
        const byte                          NUMBUCKETS = (byte) (CacheItemPriority.High);
        const int                           MAX_REMOVE = 1024;  // one page of poiners to CacheEntry's

        readonly CacheSingle              _cacheSingle;
        internal readonly UsageBucket[]     _buckets;
        int                                 _inFlush;

        internal CacheUsage(CacheSingle cacheSingle) {
            Debug.Assert((int) CacheItemPriority.Low == 1, "(int) CacheItemPriority.Low == 1");

            _cacheSingle = cacheSingle;
            _buckets = new UsageBucket[NUMBUCKETS];
            for (byte b = 0; b < _buckets.Length; b++) {
                _buckets[b] = new UsageBucket(this, b);
            }
        }

        internal CacheSingle CacheSingle {
            get {
                return _cacheSingle;
            }
        }

        internal void Add(CacheEntry cacheEntry) {
            byte bucket = cacheEntry.UsageBucket;
            Debug.Assert(bucket != 0xff, "bucket != 0xff");
            _buckets[bucket].AddCacheEntry(cacheEntry);
        }

        internal void Remove(CacheEntry cacheEntry) {
            byte bucket = cacheEntry.UsageBucket;
            if (bucket != 0xff) {
                _buckets[bucket].RemoveCacheEntry(cacheEntry);
            }
        }

        internal void Update(CacheEntry cacheEntry) {
            byte bucket = cacheEntry.UsageBucket;
            if (bucket != 0xff) {
                _buckets[bucket].UpdateCacheEntry(cacheEntry);
            }
        }

        // publicEntriesFlushed keeps track of the number of public entries that are flushed
        internal int FlushUnderUsedItems(int toFlush, ref int publicEntriesFlushed, ref int ocEntriesFlushed) {
            int flushed = 0;

            if (Interlocked.Exchange(ref _inFlush, 1) == 0) {
                try { 
                    foreach (UsageBucket usageBucket in _buckets) {
                        int flushedOne = usageBucket.FlushUnderUsedItems(toFlush - flushed, 
                                                                         false,
                                                                         ref publicEntriesFlushed, 
                                                                         ref ocEntriesFlushed);
                        flushed += flushedOne;
                        if (flushed >= toFlush)
                            break;
                    }
        
                    if (flushed < toFlush) {
                        foreach (UsageBucket usageBucket in _buckets) {
                            int flushedOne = usageBucket.FlushUnderUsedItems(toFlush - flushed, 
                                                                             true, 
                                                                             ref publicEntriesFlushed, 
                                                                             ref ocEntriesFlushed);
                            flushed += flushedOne;
                            if (flushed >= toFlush)
                                break;
                        }
                    }
                }
                finally {
                    Interlocked.Exchange(ref _inFlush, 0);
                }
            }

            return flushed;
        }

#if DBG
        internal void DebugValidate() {
            foreach (UsageBucket usageBucket in _buckets) {
                usageBucket.DebugValidate();
            }
        }

        internal string DebugDescription(string indent) {
            StringBuilder   sb = new StringBuilder();
            string          i2 = indent + "    ";

            sb.Append(indent);
            sb.Append("Cache Usage\n");

            foreach (UsageBucket usageBucket in _buckets) {
                sb.Append(usageBucket.DebugDescription(i2));
            }

            return sb.ToString();
        }
#endif
    }
}