///////////////////////////////////////////////////////////////////////////////
//
//  Microsoft Research Singularity
//
//  Copyright (c) Microsoft Corporation.  All rights reserved.
//
//  File:   FsInfo32.sg
//
//  Note:
//
//  Based on:
//  "Microsoft Extensible Firmware Initiative FAT32 File System Specification",
//  Version 1.03, December 6, 2000, Microsoft Corporation.
//

using System.Runtime.InteropServices;

using Microsoft.Singularity.Io;

namespace Microsoft.Singularity.Services.Fat.Fs
{
    /// <remarks>
    /// FAT32 FsInfo sector structure.  The FsInfo structure
    /// occupies a whole sector and contains information on the
    /// number of free blocks and the location of the first free
    /// block.
    /// </remarks>
    [StructLayout(LayoutKind.Explicit, Pack = 1, Size = 512)]
    internal pointerfree struct FsInfo32
    {
        ///////////////////////////////////////////////////////////////////////
        // Constants

        internal const int  Length = 512;

        internal const uint ExpectedLeadingSignature   = 0x41615252;
        internal const uint ExpectedStructureSignature = 0x61417272;
        internal const uint ExpectedTrailingSignature  = 0xaa550000;

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

        [FieldOffset(000)] private uint leadingSignature;      // 0
        // Reserved1                                           // 4..483
        [FieldOffset(484)] private uint structureSignature;    // 484..487
        [FieldOffset(488)] private uint freeCount;             // 488..491
        [FieldOffset(492)] private uint nextFree;              // 492..495
        // Reserved2                                           // 496..507
        [FieldOffset(508)] private uint trailingSignature;     // 508..511

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

        internal void Initialize(uint theFreeCount, uint theNextFree)
        {
            LeadingSignature   = ExpectedLeadingSignature;
            StructureSignature = ExpectedStructureSignature;
            TrailingSignature  = ExpectedTrailingSignature;
            FreeCount          = theFreeCount;
            NextFree           = theNextFree;
        }

        private uint LeadingSignature
        {
            get { return ByteOrder.LittleEndianToHost(leadingSignature); }
            set { leadingSignature = ByteOrder.HostToLittleEndian(value); }
        }

        private uint StructureSignature
        {
            get { return ByteOrder.LittleEndianToHost(structureSignature); }
            set { structureSignature = ByteOrder.HostToLittleEndian(value); }
        }

        internal uint FreeCount
        {
            get { return ByteOrder.LittleEndianToHost(freeCount); }
            set { freeCount = ByteOrder.HostToLittleEndian(value); }
        }

        internal uint NextFree
        {
            get { return ByteOrder.LittleEndianToHost(nextFree); }
            set { nextFree = ByteOrder.HostToLittleEndian(value); }
        }

        private uint TrailingSignature
        {
            get { return ByteOrder.LittleEndianToHost(trailingSignature); }
            set { trailingSignature = ByteOrder.HostToLittleEndian(value); }
        }

        [Microsoft.Contracts.Pure]
        internal bool ValidSignatures()
        {
            return (this.LeadingSignature   == ExpectedLeadingSignature     &&
                    this.StructureSignature == ExpectedStructureSignature   &&
                    this.TrailingSignature  == ExpectedTrailingSignature);
        }
    }
}