// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using System;
using System.Collections.Generic;
using System.Collections.Immutable;
using System.IO.Pipelines;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Runtime.Serialization;
using Roslyn.Utilities;

namespace Microsoft.CodeAnalysis;

/// <summary>
/// Checksum of data can be used later to see whether two data are same or not
/// without actually comparing data itself
/// </summary>
[DataContract, StructLayout(LayoutKind.Explicit, Size = HashSize)]
internal readonly partial record struct Checksum(
    [field: FieldOffset(0)][property: DataMember(Order = 0)] long Data1,
    [field: FieldOffset(8)][property: DataMember(Order = 1)] long Data2) : IComparable<Checksum>
{
    /// <summary>
    /// The intended size of the <see cref="Checksum"/> structure. 
    /// </summary>
    public const int HashSize = 16;

    /// <summary>
    /// Represents a default/null/invalid Checksum, equivalent to <c>default(Checksum)</c>.  This values contains
    /// all zeros which is considered infinitesimally unlikely to ever happen from hashing data (including when
    /// hashing null/empty/zero data inputs).
    /// </summary>
    public static readonly Checksum Null = default;

    /// <summary>
    /// Create Checksum from given byte array. if byte array is bigger than <see cref="HashSize"/>, it will be
    /// truncated to the size.
    /// </summary>
    public static Checksum From(byte[] checksum)
        => From(checksum.AsSpan());

    /// <summary>
    /// Create Checksum from given byte array. if byte array is bigger than <see cref="HashSize"/>, it will be
    /// truncated to the size.
    /// </summary>
    public static Checksum From(ImmutableArray<byte> checksum)
        => From(checksum.AsSpan());

    public static Checksum From(ReadOnlySpan<byte> checksum)
    {
        if (checksum.Length < HashSize)
            throw new ArgumentException($"checksum must be equal or bigger than the hash size: {HashSize}", nameof(checksum));

        Contract.ThrowIfFalse(MemoryMarshal.TryRead(checksum, out Checksum result));
        return result;
    }

    public string ToBase64String()
    {
#if NET
        Span<byte> bytes = stackalloc byte[HashSize];
        this.WriteTo(bytes);
        return Convert.ToBase64String(bytes);
#else
        var bytes = new byte[HashSize];
        this.WriteTo(bytes.AsSpan());
        return Convert.ToBase64String(bytes);
#endif
    }

    public static Checksum FromBase64String(string value)
        => From(Convert.FromBase64String(value));

    public override string ToString()
        => ToBase64String();

    public void WriteTo(ObjectWriter writer)
    {
        writer.WriteInt64(Data1);
        writer.WriteInt64(Data2);
    }

    public void WriteTo(Span<byte> span)
    {
        Contract.ThrowIfTrue(span.Length < HashSize);
        Unsafe.WriteUnaligned(ref MemoryMarshal.GetReference(span), this);
    }

    public void WriteTo(PipeWriter pipeWriter)
    {
        var span = pipeWriter.GetSpan(HashSize);
        this.WriteTo(span);
        pipeWriter.Advance(HashSize);
    }

    public static Checksum ReadFrom(ObjectReader reader)
        => new(reader.ReadInt64(), reader.ReadInt64());

    public static Func<Checksum, string> GetChecksumLogInfo { get; }
        = checksum => checksum.ToString();

    public static Func<IEnumerable<Checksum>, string> GetChecksumsLogInfo { get; }
        = checksums => string.Join("|", checksums.Select(c => c.ToString()));

    // Explicitly implement this method as default jit for records on netfx doesn't properly devirtualize the
    // standard calls to EqualityComparer<long>.Default.Equals
    public bool Equals(Checksum other)
        => this.Data1 == other.Data1 && this.Data2 == other.Data2;

    // Directly override to any overhead that records add when hashing things like the EqualityContract
    public override int GetHashCode()
    {
        // The checksum is already a hash. Just read a 4-byte value to get a well-distributed hash code.
        return (int)Data1;
    }

    public int CompareTo(Checksum other)
    {
        var result = Data1.CompareTo(other.Data1);
        return result != 0 ? result : Data2.CompareTo(other.Data2);
    }
}

internal static class ChecksumExtensions
{
    public static void AddIfNotNullChecksum(this HashSet<Checksum> checksums, Checksum checksum)
    {
        if (checksum != Checksum.Null)
            checksums.Add(checksum);
    }
}