singrdk/base/Services/Fat/Fs/DHashTable.sg

///////////////////////////////////////////////////////////////////////////////
//
//  Microsoft Research Singularity
//
//  Copyright (c) Microsoft Corporation.  All rights reserved.
//
//  File:   DHashtable.sg
//
//  Note:
//         A hashtable customized for FAT directory hashing.  Table entries
//  are composed of <nameKey, (entry offset, entry length)>.
//
//         Earlier versions of this file implemented a chain based
//  Hashtable in place.  The code was updated to use open address hashing
//  as it has lower memory overhead and faster lookup when the load is
//  is low.
//

namespace Microsoft.Singularity.Services.Fat.Fs
{
    using System;
    using System.Diagnostics;

    /// <remarks> Specialized hashtable for storing filename
    /// hashes for directories. </remarks>
    internal sealed class DHashtable
    {
        public const int MaxKeyValue = OpenHash.MaxKeyValue;
        public const int MaxKeyBits  = OpenHash.MaxKeyBits;

        OpenHash table;

        internal DHashtable()
        {
            this.table = new OpenHash();
        }

        /// <value>Property <c>Count</c> represents the number of key-value
        /// pairs stored in the hash table. </value>
        internal int Count
        {
            get { return table.Count; }
        }

        /// <value>Property <c>Capacity</c> represents the current capacity
        /// of the hash table. </value>
        /// <para>The capacity of the hash table changes dynamically.</para>
        internal int Capacity
        {
            get { return table.Capacity; }
        }

        private static int ComposeValue(ushort x, ushort y)
        {
            return (((int)x) << 16) | ((int)y);
        }

        private static void DecomposeValue(int v, out ushort x, out ushort y)
        {
            x = (ushort)(v >> 16);
            y = (ushort)(v);
        }

        /// <summary>
        /// Insert name-key, entry offset, length triplet.
        /// </summary>
        internal bool Insert(int    nameKey,
                             ushort entryOffset,
                             ushort entryLength)
            requires nameKey <= MaxKeyValue;
        {
            return table.Insert(nameKey,
                                ComposeValue(entryOffset, entryLength));
        }

        /// <summary> Remove key-value pair. </summary>
        /// <returns> true on success, false if key-value pair
        /// is not present. </returns>
        internal bool Remove(int    nameKey,
                             ushort entryOffset,
                             ushort entryLength)
            requires nameKey <= MaxKeyValue;
        {
            return table.Remove(nameKey,
                                ComposeValue(entryOffset, entryLength));
        }

        /// <summary>
        /// Prepare to begin a search.  This methods clears out
        /// the state associated with any previous searches.
        /// </summary>
        internal void ResetSearch()
        {
            table.BeginSearch();
        }

        /// <summary> Performs value lookup after GetFirstValue
        /// has retrieved first value for key.  This methods
        /// allows callers to iterate over hash items with the
        /// same key.  </summary>
        /// <exception cref="InvalidOperationException"> The
        /// hash table was modified or the key does not match
        /// the key in the last GetFirstValue call. </exception>
        /// <seealso cref="GetFirstValue"/>
        internal bool Search(int        nameKey,
                             out ushort entryOffset,
                             out ushort entryLength)
            requires nameKey <= MaxKeyValue;
        {
            int value;
            bool success = table.Search(nameKey, out value);
            DecomposeValue(value, out entryOffset, out entryLength);
            return success;
        }
    }
}