// Copyright 2017-2018 Alexander Luzgarev

using System;
using System.Globalization;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices;

namespace MatFileHandler
{
    /// <summary>
    /// Header of a .mat file.
    /// </summary>
    internal class Header
    {
        private Header(string text, long subsystemDataOffset, int version)
        {
            Text = text;
            SubsystemDataOffset = subsystemDataOffset;
            Version = version;
        }

        /// <summary>
        /// Gets the header text.
        /// </summary>
        public string Text { get; }

        /// <summary>
        /// Gets subsystem data offset.
        /// </summary>
        public long SubsystemDataOffset { get; }

        /// <summary>
        /// Gets file version.
        /// </summary>
        public int Version { get; }

        /// <summary>
        /// Creates a new header for a .mat file.
        /// </summary>
        /// <returns>A header.</returns>
        public static Header CreateNewHeader()
        {
            var subsystemDataOffset = Enumerable.Repeat((byte)0, 8).ToArray();
            var dateTime = DateTime.Now.ToString(CultureInfo.InvariantCulture);
            var length = 116 - "MATLAB 5.0 MAT-file, Platform: , Created on: ".Length - dateTime.Length;
            var platform = GetOperatingSystem();
            var padding = string.Empty;
            if (platform.Length < length)
            {
                padding = padding.PadRight(length - platform.Length);
            }
            else
            {
                platform = platform.Remove(length);
            }
            var text = $"MATLAB 5.0 MAT-file, Platform: {platform}, Created on: {dateTime}{padding}";
            return new Header(text, 0, 256);
        }

        /// <summary>
        /// Read header from file.
        /// </summary>
        /// <param name="reader">Binary reader.</param>
        /// <returns>The header read.</returns>
        public static Header Read(BinaryReader reader)
        {
            var textBytes = reader.ReadBytes(116);
            var text = System.Text.Encoding.UTF8.GetString(textBytes);
            var subsystemDataOffsetBytes = reader.ReadBytes(8);
            var subsystemDataOffset = BitConverter.ToInt64(subsystemDataOffsetBytes, 0);
            var version = reader.ReadInt16();
            var endian = reader.ReadInt16();
            var isLittleEndian = endian == 19785;
            if (!isLittleEndian)
            {
                throw new NotSupportedException("Big-endian files are not supported.");
            }
            return new Header(text, subsystemDataOffset, version);
        }

        private static string GetOperatingSystem()
        {
#if NET461
            return "Windows";
#else
            if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
            {
                return "Windows";
            }
            if (RuntimeInformation.IsOSPlatform(OSPlatform.OSX))
            {
                return "macOS";
            }
            if (RuntimeInformation.IsOSPlatform(OSPlatform.Linux))
            {
                return "Linux";
            }
            return "Unknown";
#endif
        }
    }
}