// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using System.Collections.Generic;
using System.Collections.Immutable;
using System.Diagnostics.CodeAnalysis;
using System.Reflection.Internal;
 
namespace System.Reflection.Metadata
{
    public readonly struct BlobContentId : IEquatable<BlobContentId>
    {
        private const int Size = BlobUtilities.SizeOfGuid + sizeof(uint);
 
        private readonly Guid _guid;
        private readonly uint _stamp;
 
        public Guid Guid => _guid;
        public uint Stamp => _stamp;
 
        public BlobContentId(Guid guid, uint stamp)
        {
            _guid = guid;
            _stamp = stamp;
        }
 
        public BlobContentId(ImmutableArray<byte> id)
        {
            if (id.IsDefault)
            {
                Throw.ArgumentNull(nameof(id));
            }
 
            Initialize(id.AsSpan(), out _guid, out _stamp);
        }
 
        public BlobContentId(byte[] id)
        {
            if (id is null)
            {
                Throw.ArgumentNull(nameof(id));
            }
 
            Initialize(id, out _guid, out _stamp);
        }
 
        private static unsafe void Initialize(ReadOnlySpan<byte> id, out Guid guid, out uint stamp)
        {
            if (id.Length != Size)
            {
                throw new ArgumentException(SR.Format(SR.UnexpectedArrayLength, Size), nameof(id));
            }
 
            fixed (byte* ptr = &id[0])
            {
                var reader = new BlobReader(ptr, id.Length);
                guid = reader.ReadGuid();
                stamp = reader.ReadUInt32();
            }
        }
 
        public bool IsDefault => Guid == default(Guid) && Stamp == 0;
 
        public static BlobContentId FromHash(ImmutableArray<byte> hashCode)
        {
            if (hashCode.IsDefault)
            {
                Throw.ArgumentNull(nameof(hashCode));
            }
 
            return FromHash(hashCode.AsSpan());
        }
 
        public static BlobContentId FromHash(byte[] hashCode)
        {
            if (hashCode is null)
            {
                Throw.ArgumentNull(nameof(hashCode));
            }
 
            return FromHash(hashCode.AsSpan());
        }
 
        private static BlobContentId FromHash(ReadOnlySpan<byte> hashCode)
        {
            const int minHashSize = 20;
 
            if (hashCode.Length < minHashSize)
            {
                throw new ArgumentException(SR.Format(SR.HashTooShort, minHashSize), nameof(hashCode));
            }
 
            // extract guid components from input data
            uint a = ((uint)hashCode[3] << 24 | (uint)hashCode[2] << 16 | (uint)hashCode[1] << 8 | hashCode[0]);
            ushort b = (ushort)(hashCode[5] << 8 | hashCode[4]);
            ushort c = (ushort)(hashCode[7] << 8 | hashCode[6]);
            byte d = hashCode[8];
            byte e = hashCode[9];
            byte f = hashCode[10];
            byte g = hashCode[11];
            byte h = hashCode[12];
            byte i = hashCode[13];
            byte j = hashCode[14];
            byte k = hashCode[15];
 
            // modify the guid data so it decodes to the form of a "random" guid ala rfc4122
            c = (ushort)((c & 0x0fff) | (4 << 12));
            d = (byte)((d & 0x3f) | (2 << 6));
            Guid guid = new Guid((int)a, (short)b, (short)c, d, e, f, g, h, i, j, k);
 
            // compute a random-looking stamp from the remaining bits, but with the upper bit set
            uint stamp = 0x80000000u | ((uint)hashCode[19] << 24 | (uint)hashCode[18] << 16 | (uint)hashCode[17] << 8 | hashCode[16]);
 
            return new BlobContentId(guid, stamp);
        }
 
        public static Func<IEnumerable<Blob>, BlobContentId> GetTimeBasedProvider()
        {
            // In the PE File Header this is a "Time/Date Stamp" whose description is "Time and date
            // the file was created in seconds since January 1st 1970 00:00:00 or 0"
            // However, when we want to make it deterministic we fill it in (later) with bits from the hash of the full PE file.
            uint timestamp = (uint)(DateTime.UtcNow - new DateTime(1970, 1, 1)).TotalSeconds;
            return content => new BlobContentId(Guid.NewGuid(), timestamp);
        }
 
        public bool Equals(BlobContentId other) => Guid == other.Guid && Stamp == other.Stamp;
        public override bool Equals([NotNullWhen(true)] object? obj) => obj is BlobContentId bcid && Equals(bcid);
        public override int GetHashCode() => Hash.Combine(Stamp, Guid.GetHashCode());
        public static bool operator ==(BlobContentId left, BlobContentId right) => left.Equals(right);
        public static bool operator !=(BlobContentId left, BlobContentId right) => !left.Equals(right);
    }
}