///////////////////////////////////////////////////////////////////////////////
//
//  Microsoft Research Singularity
//
//  Copyright (c) Microsoft Corporation.  All rights reserved.
//
//  File:   BlockIndex.sg
//
//  Note:
//          The BlockIndex is used to provide fast block location within
//  files and directories.  Its structure is fairly similar to the direct
//  and indirect blocks of a traditional inode.
//
//  The maximum size of a FAT file is 4GB.  The code support 3 levels
//  of indirection.  The direct block fan out is 32 and the
//  indirect block fan out is 64.  On the third indirection we can
//  address all of a 4GB file with 512 byte clusters:
//
//  Addressable region = 32 * 64^3 * 512 == 4GB
//
//  In practice, we should expect to see 4k cluster sizes and upwards.
//

using System;
using System.Diagnostics;

namespace Microsoft.Singularity.Services.Fat.Fs
{
    internal sealed class BlockIndex
    {
        ///////////////////////////////////////////////////////////////////////
        // Internal class definitions

        internal abstract class BaseBlock
        {
        }

        internal sealed class DirectBlock : BaseBlock
        {
            internal const int Shift    = 6;
            internal const int MaxItems = 1 << Shift;

            private int []! items;

            internal DirectBlock()
            {
                items = new int [MaxItems];
                base();
            }

            public int this[int index]
            {
                get { return items[index]; }
                set { items[index] = value; }
            }
        }

        internal sealed class IndirectBlock : BaseBlock
        {
            internal const int Shift    = 7;
            internal const int MaxItems = 1 << Shift;

            private BaseBlock [] items;

            internal IndirectBlock()
            {
                items = new BaseBlock [MaxItems];
            }

            public BaseBlock this[int index]
            {
                get { return items[index]; }
                set { Debug.Assert(items[index] == null); items[index] = value; }
            }
        }

        ///////////////////////////////////////////////////////////////////////
        // Constants

        private const int MaxIndirects = 3;

        ///////////////////////////////////////////////////////////////////////
        // Members

        int           count;
        readonly int  maxCount;
        DirectBlock   direct;
        IndirectBlock [] indirects;

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

        internal BlockIndex()
        {
            this.indirects = new IndirectBlock[MaxIndirects];
            this.direct = new DirectBlock();

            maxCount = DirectBlock.MaxItems;
            int delta = IndirectBlock.MaxItems * DirectBlock.MaxItems;
            for (int i = 0; i < MaxIndirects; i++) {
                maxCount += delta;
                delta *= IndirectBlock.MaxItems;
            }
        }

        static private int DirectOffset(int value)
        {
            return value & ((1 << DirectBlock.Shift) - 1);
        }

        static private int IndirectOffset(int maxDepth, int depth, int value)
        {
            value >>= DirectBlock.Shift;
            value >>= (IndirectBlock.Shift * (maxDepth - depth));
            return value & ((1 << IndirectBlock.Shift) - 1);
        }

        /// <summary>
        /// Accessor providing number of blocks indexed.
        /// </summary>
        internal int Count { get { return count; } }

        /// <summary>
        /// Lookup the block id of the nth block in the index.
        ///
        /// <returns> true if block exists in index,
        /// false otherwise. </returns>
        /// </summary>
        internal bool Lookup(int nthBlock, out int blockId)
        {
            if (count <= nthBlock) {
                blockId = Int32.MinValue;
                return false;
            }

            // Check if block is in direct blocks
            if (nthBlock < DirectBlock.MaxItems) {
                blockId = direct[nthBlock];
                return true;
            }
            nthBlock -= DirectBlock.MaxItems;

            // Must be an indirect block.  Find which indirect block to
            // start from.
            int threshold  = IndirectBlock.MaxItems * DirectBlock.MaxItems;
            int depth      = 0;

            while (nthBlock >= threshold)
            {
                nthBlock  -= threshold;
                threshold *= IndirectBlock.MaxItems;
                depth++;
            }

            // Walk from root indirect block down to block containing
            // appropriate direct block.
            IndirectBlock! ib = (!) indirects[depth];
            for (int i = 0; i < depth; i++) {
                ib = (IndirectBlock!) ib[IndirectOffset(depth, i, nthBlock)];
            }

            // Get entry in direct block and pray for forgiveness.
            DirectBlock! db =
                (DirectBlock)(!)ib[IndirectOffset(depth, depth, nthBlock)];
            blockId = db[DirectOffset(nthBlock)];

            return true;
        }

        private bool ScanDirectBlockForId(DirectBlock! db,
                                          int          blockLength,
                                          int          blockId,
                                          out int      dbIndex)
        {
            for (dbIndex = 0; dbIndex < blockLength; dbIndex++) {
                if (db[dbIndex] == blockId) {
                    return true;
                }
            }
            dbIndex = -1;
            return false;
        }

        private bool ScanIndirectBlockForId(int             depth,
                                            IndirectBlock!  idb,
                                            int             done,
                                            int             stepSize,
                                            int             blockId,
                                            out DirectBlock db,
                                            out int         dbIndex,
                                            out int         absoluteIndex)
        {
            if (depth == 0) {
                int i = 0;
                while (i < IndirectBlock.MaxItems && done < this.count) {
                    db = (!)((DirectBlock)idb[i]);
                    if (ScanDirectBlockForId(db,
                                             Math.Min(this.count - done,
                                                      DirectBlock.MaxItems),
                                             blockId, out dbIndex)) {
                        absoluteIndex = done + dbIndex;
                        return true;
                    }
                    done += stepSize;
                    i++;
                }
            }
            else {
                int i = 0;
                while (i < IndirectBlock.MaxItems && done < this.count) {
                    IndirectBlock! child = (IndirectBlock!) idb[i];
                    int childStepSize    = stepSize / IndirectBlock.MaxItems;
                    if (ScanIndirectBlockForId(depth - 1, child,
                                               done, childStepSize, blockId,
                                               out db,
                                               out dbIndex,
                                               out absoluteIndex)) {
                        return true;
                    }
                    done += stepSize;
                    i++;
                }
            }

            db            = null;
            dbIndex       = 0;
            absoluteIndex = 0;

            return false;
        }

        private bool FindId(int blockId,
                            out DirectBlock db,
                            out int         dbIndex,
                            out int         absoluteIndex)
        {
            db            = null;
            dbIndex       = -1;
            absoluteIndex = -1;

            // NB need to watch end of index bounds
            if (ScanDirectBlockForId(this.direct,
                                     Math.Min(count, DirectBlock.MaxItems),
                                     blockId, out dbIndex)) {
                db = this.direct;
                absoluteIndex = dbIndex;
                return true;
            }

            if (this.count <= DirectBlock.MaxItems) {
                return false;
            }

            int done  = DirectBlock.MaxItems;
            int step  = DirectBlock.MaxItems;
            int depth = 0;
            while (done < this.count) {
                IndirectBlock! idb = (!)((IndirectBlock)this.indirects[depth]);
                if (ScanIndirectBlockForId(
                        depth, idb, done, step, blockId,
                        out db, out dbIndex, out absoluteIndex)
                    ) {
                    return true;
                }
                depth++;
                step *= IndirectBlock.MaxItems;
                done += step;
            }

            return false;
        }

        /// <summary>
        /// Replace bad block id with new block id.
        /// </summary>
        ///
        /// <para> This method performs a linear search through the
        /// known block id's and replaces the target id if found.
        /// </para>
        internal bool Replace(int oldBlockId, int newBlockId)
        {
            DirectBlock db;
            int         dbIndex;
            int         absIndex;
            if (FindId(oldBlockId, out db, out dbIndex, out absIndex)) {
                assert db != null;
                assert dbIndex >= 0 && dbIndex < DirectBlock.MaxItems;
                assert db[dbIndex] == oldBlockId;
                db[dbIndex] = newBlockId;
                return true;
            }
            return false;
        }

        /// <summary>
        /// Truncate at specified block id.  Remove blockId and all block
        /// id's later in index.
        /// </summary>
        ///
        /// <para> This method performs a linear search through
        /// the known block id's truncates the collection at the
        /// specified point if found.
        /// </para>
        internal bool TruncateAtBlockId(int lastBlockId)
        {
            DirectBlock db;
            int         dbIndex;
            int         absIndex;
            if (FindId(lastBlockId, out db, out dbIndex, out absIndex)) {
                assert absIndex >= 0 && absIndex < count;
                assert db != null;
                assert db[dbIndex] == lastBlockId;
                this.count = absIndex;
                return true;
            }
            return false;
        }

        internal bool TruncateToLength(int clusterLength)
            requires clusterLength >= 0 && clusterLength <= this.Count;
        {
            this.count = clusterLength;
            return true;
        }

        /// <summary>
        /// Append blockId at end of index.
        ///
        /// The caller should check that file being added does not push
        /// the file size over FAT's maximum supported size (4GB).
        /// </summary>
        internal bool Append(int blockId)
        {
            if (this.count == this.maxCount) {
                return false;
            }

            int nthBlock = this.count;
            this.count++;

            // Check if we can place this in direct block
            if (nthBlock < DirectBlock.MaxItems) {
                if (this.direct == null) this.direct = new DirectBlock();
                this.direct[nthBlock] = blockId;
                return true;
            }

            // Must be placed in an indirect block.  Find which
            // indirect block to start search from.
            nthBlock -= DirectBlock.MaxItems;
            int threshold = IndirectBlock.MaxItems * DirectBlock.MaxItems;
            int depth = 0;
            while (nthBlock >= threshold)
            {
                nthBlock  -= threshold;
                threshold *= IndirectBlock.MaxItems;
                depth++;
            }

            if (indirects[depth] == null) {
                indirects[depth] = new IndirectBlock();
            }

            // Walk from root indirect block down to block containing
            // appropriate direct block.
            IndirectBlock ib = indirects[depth];
            for (int i = 0; i < depth; i++) {
                int offset = IndirectOffset(depth, i, nthBlock);
                if (((!)ib)[offset] == null) {
                    IndirectBlock tmp = new IndirectBlock();
                    ib[offset] = tmp;
                    ib = tmp;
                }
                else {
                    ib = (IndirectBlock)ib[offset];
                }
            }

            // Allocate direct block if it does not exist
            int lastOffset = IndirectOffset(depth, depth, nthBlock);
            DirectBlock db;
            if (((!)ib)[lastOffset] == null) {
                db = new DirectBlock();
                ib[lastOffset] = db;
            }
            else {
                db = (DirectBlock)ib[lastOffset];
            }

            // Set entry in direct block and run.
            ((!)db)[DirectOffset(nthBlock)] = blockId;

            return true;
        }

        /// <summary>
        /// Append series of blockId's at end of index.
        /// </summary>
        /// <returns> true on success, false if capacity would be exceeded.
        /// </returns>
        internal bool Append(int startBlockId, int length)
            requires length >= 1;
        {
            if (this.count + length >= this.maxCount) {
                return false;
            }

            int done = 0;
            while (done != length) {
                if (this.Append(startBlockId + done) == false) {
                    // This shouldn't happen.
                    goto undo;
                }
                done++;
            }
            return true;

          undo:
            this.count -= done;
            return false;
        }

#if TEST
        private static void TestLookup()
        {
            Console.WriteLine("Testing Lookup.");

            const int MaxLookupItems = 4 * 1024 * 1024;
            const int TickPeriod = 1000000;

            BlockIndex bi = new BlockIndex();

            for (int i = 0; i < bi.maxCount; i++) {
                bi.Append(i);
                int j;
                bool r = bi.Lookup(i, out j);
                Debug.Assert(i == j);
                Debug.Assert(r == true);
                Debug.Assert(bi.Lookup(i + 1, out j) == false);
                if ((i % TickPeriod) == 0) Console.Write(".");
            }

            Debug.Assert(bi.Append(0) == false);

            for (int i = 0; i < MaxLookupItems; i++) {
                int u = bi.Count - i - 1;
                int j;
                bool r = bi.Lookup(u, out j);
                Debug.Assert(r == true);
                Debug.Assert(j == u);
                if ((i % TickPeriod) == 0) Console.Write("o");
            }

            Debug.Assert(bi.Append(0) == false);

            for (int i = 0; i < MaxLookupItems; i++) {
                int u = (7 * i) % MaxLookupItems;
                int j;
                bool r = bi.Lookup(u, out j);
                Debug.Assert(r == true);
                Debug.Assert(j == u);
                if ((i % TickPeriod) == 0) Console.Write("O");
            }
            Console.Write("\n");
        }

        static private void TestTruncate()
        {
            Console.WriteLine("Testing Truncate.");

            const int MaxTruncateItems = 4 * 1024 * 1024;

            BlockIndex bi = new BlockIndex();
            for (int i = 0; i < MaxTruncateItems; i++) {
                bi.Append(i);
            }

            Debug.Assert(!bi.TruncateAtBlockId(MaxTruncateItems));

            int done  = 0;
            int where = bi.Count - 1;
            do {
                Debug.Assert(where < bi.Count);
                bool r = bi.TruncateAtBlockId(where);
                Debug.Assert(r == true);
                Debug.Assert(bi.Count == where);
                done++;
                where = Math.Max((bi.Count * 97 / 100) - 1, 0);
            } while (bi.Count > 0);

            Debug.Assert(!bi.TruncateAtBlockId(MaxTruncateItems));
        }

        static private void TestReplace()
        {
            Console.WriteLine("Testing Replace.");

            const int MaxReplaceItems = 4 * 1024 * 1024;

            BlockIndex bi = new BlockIndex();
            for (int i = 0; i < MaxReplaceItems; i++) {
                bi.Append(i);
            }

            Debug.Assert(!bi.Replace(MaxReplaceItems, MaxReplaceItems));

            int done = 0;
            int where = MaxReplaceItems - 1;
            while (where >= 0) {
                int newWhere = MaxReplaceItems + where;
                bi.Replace(where, newWhere);

                int checkWhere = 0;
                bi.Lookup(where, out checkWhere);
                Debug.Assert(checkWhere == newWhere);

                where = (where * 98 / 100) - 1;
                done++;
            }

            Debug.Assert(!bi.Replace(MaxReplaceItems, MaxReplaceItems));
        }

        static public void Main()
        {
            TestTruncate();
            TestReplace();
            TestLookup();
        }
#endif // TEST
    }
}