SteamTools/System.Common.CoreLib/IO/FileFormats/FileFormat.MagicNumber.cs

using System.Collections.Generic;
using System.Linq;

namespace System.IO.FileFormats
{
    partial class FileFormat
    {
        /// <summary>
        /// 文件头标识(幻数)
        /// </summary>
        public static class MagicNumber
        {
            /// <summary>
            /// 对 MagicNumber 进行比较，以确定两个序列是否相等，如果某一个序列为 <see cref="" langword="null"/> 则返回 <see langword="false"/>
            /// </summary>
            /// <typeparam name="T"></typeparam>
            /// <param name="first"></param>
            /// <param name="second"></param>
            /// <param name="equatable"></param>
            /// <returns></returns>
            static bool SequenceEqual<T>(
                IReadOnlyList<byte> first,
                IReadOnlyCollection<T> second,
                Func<byte, T, bool> equatable)
            {
                if (first == null || second == null) return false;
                if (first.Count < second.Count) return false;
                return !second.Where((t, i) => !equatable(first[i], t)).Any();
            }

            static bool Equals(byte left, byte right) => left.Equals(right);

            static bool Equals(byte left, byte? right) => !right.HasValue || left.Equals(right);

            public static bool SequenceEqual(IReadOnlyList<byte> first, IReadOnlyCollection<byte?> second) => SequenceEqual(first, second, Equals);

            public static bool SequenceEqual(IReadOnlyList<byte> first, IReadOnlyCollection<byte> second) => SequenceEqual(first, second, Equals);

            public static bool SequenceEqual(IReadOnlyList<byte> first, IEnumerable<IReadOnlyCollection<byte>> seconds)
                => seconds.Any(second => SequenceEqual(first, second));

            public static bool SequenceEqual(IReadOnlyList<byte> first, IEnumerable<IReadOnlyCollection<byte?>> seconds)
                => seconds.Any(second => SequenceEqual(first, second));

            public static byte[] ReadHeaderBuffer(Stream stream, int length, bool resetPosition = true)
            {
                var currentPosition = stream.Position;
                if (currentPosition != 0) stream.Position = 0;
                byte[] result;
                if (stream.Length < length)
                {
                    var buffer = new byte[stream.Length];
                    stream.Read(buffer, 0, buffer.Length);
                    result = buffer;
                }
                else
                {
                    var buffer = new byte[length];
                    stream.Read(buffer, 0, buffer.Length);
                    result = buffer;
                }
                if (resetPosition) stream.Position = currentPosition;
                return result;
            }

            public static int GetLength(object magicNumber)
            {
                if (magicNumber is IReadOnlyCollection<byte> byteArray)
                {
                    return byteArray.Count;
                }
                else if (magicNumber is IReadOnlyCollection<byte?> byteArrayNullable)
                {
                    return byteArrayNullable.Count;
                }
                else if (magicNumber is IEnumerable<IReadOnlyCollection<byte>> byteArrayArray)
                {
                    return byteArrayArray.Max(x => x.Count);
                }
                else if (magicNumber is IEnumerable<IReadOnlyCollection<byte?>> byteArrayArrayNullable)
                {
                    return byteArrayArrayNullable.Max(x => x.Count);
                }
                else
                {
                    throw new ArgumentOutOfRangeException(nameof(magicNumber), "type mismatch.");
                }
            }

            public static bool Match(object magicNumber, IReadOnlyList<byte>? buffer, Stream? stream)
            {
                bool result;
                if (magicNumber is IReadOnlyCollection<byte> byteArray)
                {
                    if (buffer == null)
                    {
                        if (stream == null) goto null_ex;
                        buffer = ReadHeaderBuffer(stream, byteArray.Count);
                    }
                    result = SequenceEqual(buffer, byteArray);
                }
                else if (magicNumber is IReadOnlyCollection<byte?> byteArrayNullable)
                {
                    if (buffer == null)
                    {
                        if (stream == null) goto null_ex;
                        buffer = ReadHeaderBuffer(stream, byteArrayNullable.Count);
                    }
                    result = SequenceEqual(buffer, byteArrayNullable);
                }
                else if (magicNumber is IEnumerable<IReadOnlyCollection<byte>> byteArrayArray)
                {
                    if (buffer == null)
                    {
                        if (stream == null) goto null_ex;
                        buffer = ReadHeaderBuffer(stream, byteArrayArray.Max(x => x.Count));
                    }
                    result = SequenceEqual(buffer, byteArrayArray);
                }
                else if (magicNumber is IEnumerable<IReadOnlyCollection<byte?>> byteArrayArrayNullable)
                {
                    if (buffer == null)
                    {
                        if (stream == null) goto null_ex;
                        buffer = ReadHeaderBuffer(stream, byteArrayArrayNullable.Max(x => x.Count));
                    }
                    result = SequenceEqual(buffer, byteArrayArrayNullable);
                }
                else
                {
                    result = false;
                }
                return result;
            null_ex: throw new ArgumentNullException(nameof(stream), "stream or buffer one must be non null.");
            }
        }
    }
}