using System;
using System.IO;
using System.Runtime.InteropServices;
using System.Text;

namespace Packaging.Targets.IO
{
    /// <summary>
    /// The cpio archive format collects any number of files, directories, and
    /// other file system objects(symbolic links, device nodes, etc.) into a
    /// single stream of bytes.
    /// </summary>
    /// <remarks>
    /// Each file system object in a cpio archive comprises a header record with
    /// basic numeric metadata followed by the full pathname of the entry and the
    /// file data.The header record stores a series of integer values that generally
    /// follow the fields in struct stat.  (See stat(2) for details.)  The
    /// variants differ primarily in how they store those integers(binary,
    /// octal, or hexadecimal).  The header is followed by the pathname of the
    /// entry(the length of the pathname is stored in the header) and any file
    /// data.The end of the archive is indicated by a special record with the
    /// pathname <c>TRAILER!!!</c>.
    /// </remarks>
    /// <seealso href="https://people.freebsd.org/~kientzle/libarchive/man/cpio.5.txt"/>
    public class CpioFile : ArchiveFile, IDisposable
    {
        /// <summary>
        /// The entry which is currently available.
        /// </summary>
        private CpioHeader entryHeader;

        /// <summary>
        /// The position at which the data for the current entry starts.
        /// </summary>
        private long entryDataOffset;

        /// <summary>
        /// The length of the data for the current entry.
        /// </summary>
        private long entryDataLength;

        /// <summary>
        /// Initializes a new instance of the <see cref="CpioFile"/> class.
        /// </summary>
        /// <param name="stream">
        /// A <see cref="Stream"/> which represents the CPIO data.
        /// </param>
        /// <param name="leaveOpen">
        /// <see langword="true"/> to leave the underlying <paramref name="stream"/> open when this <see cref="CpioFile"/>
        /// is disposed of; otherwise, <see langword="false"/>.
        /// </param>
        public CpioFile(Stream stream, bool leaveOpen)
            : base(stream, leaveOpen)
        {
        }

        /// <summary>
        /// Gets the header of the current entry.
        /// </summary>
        public CpioHeader EntryHeader
        {
            get { return this.entryHeader; }
        }

        /// <summary>
        /// Gets a value indicating whether the current entry is a directory.
        /// </summary>
        public bool IsDirectory
        {
            get { return this.entryDataLength == 0; }
        }

        /// <summary>
        /// Adds an entry to the <see cref="CpioFile"/>
        /// </summary>
        /// <param name="header">
        /// A <see cref="CpioHeader"/> with the item metaata. The <see cref="CpioHeader.Signature"/>,
        /// <see cref="CpioHeader.NameSize"/> and <see cref="CpioHeader.Filesize"/> values are overwritten.
        /// </param>
        /// <param name="name">
        /// The file name of the entry.
        /// </param>
        /// <param name="data">
        /// A <see cref="Stream"/> which contains the file data.
        /// </param>
        public void Write(CpioHeader header, string name, Stream data)
        {
            if (name == null)
            {
                throw new ArgumentNullException(nameof(name));
            }

            if (data == null)
            {
                throw new ArgumentNullException(nameof(data));
            }

            byte[] nameBytes = Encoding.UTF8.GetBytes(name);

            // We make sure the magic and size fields have the correct values. All other fields
            // are the responsibility of the caller.
            header.Signature = "070701";
            header.NameSize = (uint)(nameBytes.Length + 1);
            header.FileSize = (uint)data.Length;

            this.Stream.WriteStruct(header);
            this.Stream.Write(nameBytes, 0, nameBytes.Length);
            this.Stream.WriteByte(0); // Trailing 0

            // The pathname is followed by NUL bytes so that the total size of the fixed
            // header plus pathname is a multiple of four.
            var headerSize = Marshal.SizeOf<CpioHeader>() + (int)header.NameSize;
            var paddingSize = PaddingSize(4, headerSize);

            for (int i = 0; i < paddingSize; i++)
            {
                this.Stream.WriteByte(0);
            }

            data.Position = 0;
            data.CopyTo(this.Stream);

            // The file data is padded to a multiple of four bytes.
            paddingSize = PaddingSize(4, (int)data.Length);

            for (int i = 0; i < paddingSize; i++)
            {
                this.Stream.WriteByte(0);
            }
        }

        /// <summary>
        /// Writes the trailer entry.
        /// </summary>
        public void WriteTrailer()
        {
            this.Write(CpioHeader.Empty, "TRAILER!!!", new MemoryStream(Array.Empty<byte>()));
        }

        /// <summary>
        /// Reads the next entry in the <see cref="CpioFile"/>.
        /// </summary>
        /// <returns>
        /// <see langword="true"/> if more data is available; otherwise, <see langword="false"/>.
        /// </returns>
        public override bool Read()
        {
            if (this.EntryStream != null)
            {
                this.EntryStream.Dispose();
            }

            this.Align(4);

            this.entryHeader = this.Stream.ReadStruct<CpioHeader>();
            this.FileHeader = this.entryHeader;

            if (this.entryHeader.Signature != "070701")
            {
                throw new InvalidDataException("The magic for the file entry is invalid");
            }

            byte[] nameBytes = new byte[this.entryHeader.NameSize];
            this.Stream.Read(nameBytes, 0, nameBytes.Length);

            this.FileName = Encoding.UTF8.GetString(nameBytes, 0, (int)this.entryHeader.NameSize - 1);

            // The pathname is followed by NUL bytes so that the total size of the fixed
            // header plus pathname is a multiple of four.
            var headerSize = Marshal.SizeOf<CpioHeader>() + nameBytes.Length;
            var paddingSize = PaddingSize(4, headerSize);

            if (nameBytes.Length < paddingSize)
            {
                nameBytes = new byte[paddingSize];
            }

            this.Stream.Read(nameBytes, 0, paddingSize);

            this.entryDataOffset = this.Stream.Position;
            this.entryDataLength = this.entryHeader.FileSize;
            this.EntryStream = new SubStream(this.Stream, this.entryDataOffset, this.entryDataLength, leaveParentOpen: true);

            return this.FileName != "TRAILER!!!";
        }
    }
}