using System.Runtime.InteropServices;

namespace BurnOutSharp.Models.LinearExecutable
{
    /// <summary>
    /// The entry table contains object and offset information that is used to resolve
    /// fixup references to the entry points within this module. Not all entry points
    /// in the entry table will be exported, some entry points will only be used
    /// within the module. An ordinal number is used to index into the entry table.
    /// The entry table entries are numbered starting from one. 
    /// 
    /// The list of entries are compressed into 'bundles', where possible. The entries
    /// within each bundle are all the same size. A bundle starts with a count field
    /// which indicates the number of entries in the bundle. The count is followed by
    /// a type field which identifies the bundle format. This provides both a means
    /// for saving space as well as a mechanism for extending the bundle types.
    /// </summary>
    /// <see href="https://faydoc.tripod.com/formats/exe-LE.htm"/>
    /// <see href="http://www.edm2.com/index.php/LX_-_Linear_eXecutable_Module_Format_Description"/>
    [StructLayout(LayoutKind.Explicit)]
    public class EntryTableEntry
    {
        /// <summary>
        /// Number of entries.
        /// </summary>
        /// <remarks>
        /// This is the number of entries in this bundle.
        /// 
        /// A zero value for the number of entries identifies the end of the
        /// entry table. There is no further bundle information when the number
        /// of entries is zero. In other words the entry table is terminated by
        /// a single zero byte.
        /// 
        /// For <see cref="BundleType.UnusedEntry"/>, this is the number of unused
        ///     entries to skip.
        /// For <see cref="BundleType.SixteenBitEntry"/>, this is the number of 16-bit
        ///     entries in this bundle. The flags and offset value are repeated this
        ///     number of times.
        /// For <see cref="BundleType.TwoEightySixCallGateEntry"/>, this is the number
        ///     of 286 call gate entries in this bundle. The flags, callgate, and offset
        ///     value are repeated this number of times.
        /// For <see cref="BundleType.ThirtyTwoBitEntry"/>, this is the number
        ///     of 32-bit entries in this bundle. The flags and offset value are repeated
        ///     this number of times.
        /// For <see cref="BundleType.ForwarderEntry"/>, this field is reserved for future use.
        /// </remarks>
        [FieldOffset(0)] public byte Entries;

        /// <summary>
        /// This defines the bundle type which determines the contents of the BUNDLE INFO.
        /// </summary>
        [FieldOffset(1)] public BundleType BundleType;

        #region 16-bit Entry

        /// <summary>
        /// Object number.
        /// </summary>
        /// <remarks>
        /// This is the object number for the entries in this bundle.
        /// </remarks>
        [FieldOffset(2)] public ushort SixteenBitObjectNumber;

        /// <summary>
        /// Entry flags.
        /// </summary>
        /// <remarks>
        /// These are the flags for this entry point.
        /// </remarks>
        [FieldOffset(4)] public EntryFlags SixteenBitEntryFlags;

        /// <summary>
        /// Offset in object.
        /// </summary>
        /// <remarks>
        /// This is the offset in the object for the entry point defined at this ordinal number.
        /// </remarks>
        [FieldOffset(5)] public ushort SixteenBitOffset;

        #endregion

        #region 286 Call Gate Entry

        /// <summary>
        /// Object number.
        /// </summary>
        /// <remarks>
        /// This is the object number for the entries in this bundle.
        /// </remarks>
        [FieldOffset(2)] public ushort TwoEightySixObjectNumber;

        /// <summary>
        /// Entry flags.
        /// </summary>
        /// <remarks>
        /// These are the flags for this entry point.
        /// </remarks>
        [FieldOffset(4)] public EntryFlags TwoEightySixEntryFlags;

        /// <summary>
        /// Offset in object.
        /// </summary>
        /// <remarks>
        /// This is the offset in the object for the entry point defined at this ordinal number.
        /// </remarks>
        [FieldOffset(5)] public ushort TwoEightySixOffset;

        /// <summary>
        /// Callgate selector.
        /// </summary>
        /// <remarks>
        /// The callgate selector is a reserved field used by the loader to store a call
        /// gate selector value for references to ring 2 entry points. When a ring 3
        /// reference to a ring 2 entry point is made, the callgate selector with a zero
        /// offset is place in the relocation fixup address. The segment number and offset
        /// in segment is placed in the LDT callgate.
        /// </remarks>
        [FieldOffset(7)] public ushort TwoEightySixCallgate;

        #endregion

        #region 32-bit Entry

        /// <summary>
        /// Object number.
        /// </summary>
        /// <remarks>
        /// This is the object number for the entries in this bundle.
        /// </remarks>
        [FieldOffset(2)] public ushort ThirtyTwoBitObjectNumber;

        /// <summary>
        /// Entry flags.
        /// </summary>
        /// <remarks>
        /// These are the flags for this entry point.
        /// </remarks>
        [FieldOffset(4)] public EntryFlags ThirtyTwoBitEntryFlags;

        /// <summary>
        /// Offset in object.
        /// </summary>
        /// <remarks>
        /// This is the offset in the object for the entry point defined at this ordinal number.
        /// </remarks>
        [FieldOffset(5)] public uint ThirtyTwoBitOffset;

        #endregion

        #region Forwarder Entry

        /// <summary>
        /// 0
        /// </summary>
        /// <remarks>
        /// This field is reserved for future use.
        /// </remarks>
        [FieldOffset(2)] public ushort ForwarderReserved;

        /// <summary>
        /// Forwarder flags.
        /// </summary>
        /// <remarks>
        /// These are the flags for this entry point.
        /// </remarks>
        [FieldOffset(4)] public ForwarderFlags ForwarderFlags;

        /// <summary>
        /// Module Ordinal Number
        /// </summary>
        /// <remarks>
        /// This is the index into the Import Module Name Table for this forwarder.
        /// </remarks>
        [FieldOffset(5)] public ushort ForwarderModuleOrdinalNumber;

        /// <summary>
        /// Procedure Name Offset
        /// </summary>
        /// <remarks>
        /// If the FLAGS field indicates import by ordinal, then this field is the
        /// ordinal number into the Entry Table of the target module, otherwise this
        /// field is the offset into the Procedure Names Table of the target module.
        /// 
        /// A Forwarder entry (type = 4) is an entry point whose value is an imported
        /// reference. When a load time fixup occurs whose target is a forwarder, the
        /// loader obtains the address imported by the forwarder and uses that imported
        /// address to resolve the fixup.
        /// 
        /// A forwarder may refer to an entry point in another module which is itself a
        /// forwarder, so there can be a chain of forwarders. The loader will traverse
        /// the chain until it finds a non-forwarded entry point which terminates the
        /// chain, and use this to resolve the original fixup. Circular chains are
        /// detected by the loader and result in a load time error. A maximum of 1024
        /// forwarders is allowed in a chain; more than this results in a load time error.
        /// 
        /// Forwarders are useful for merging and recombining API calls into different
        /// sets of libraries, while maintaining compatibility with applications. For
        /// example, if one wanted to combine MONCALLS, MOUCALLS, and VIOCALLS into a
        /// single libraries, one could provide entry points for the three libraries
        /// that are forwarders pointing to the common implementation.
        /// </remarks>
        [FieldOffset(7)] public uint ProcedureNameOffset;

        /// <summary>
        /// Import Ordinal Number
        /// </summary>
        /// <remarks>
        /// If the FLAGS field indicates import by ordinal, then this field is the
        /// ordinal number into the Entry Table of the target module, otherwise this
        /// field is the offset into the Procedure Names Table of the target module.
        /// 
        /// A Forwarder entry (type = 4) is an entry point whose value is an imported
        /// reference. When a load time fixup occurs whose target is a forwarder, the
        /// loader obtains the address imported by the forwarder and uses that imported
        /// address to resolve the fixup.
        /// 
        /// A forwarder may refer to an entry point in another module which is itself a
        /// forwarder, so there can be a chain of forwarders. The loader will traverse
        /// the chain until it finds a non-forwarded entry point which terminates the
        /// chain, and use this to resolve the original fixup. Circular chains are
        /// detected by the loader and result in a load time error. A maximum of 1024
        /// forwarders is allowed in a chain; more than this results in a load time error.
        /// 
        /// Forwarders are useful for merging and recombining API calls into different
        /// sets of libraries, while maintaining compatibility with applications. For
        /// example, if one wanted to combine MONCALLS, MOUCALLS, and VIOCALLS into a
        /// single libraries, one could provide entry points for the three libraries
        /// that are forwarders pointing to the common implementation.
        /// </remarks>
        [FieldOffset(7)] public uint ImportOrdinalNumber;

        #endregion
    }
}