// 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.Buffers.Binary;
using System.Collections.Immutable;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.IO;
using System.Linq;
using System.Text;
using Microsoft.Cci;
using Roslyn.Utilities;
 
namespace Microsoft.CodeAnalysis.CSharp;
 
[Experimental(RoslynExperiments.Interceptors, UrlFormat = RoslynExperiments.Interceptors_Url)]
public abstract class InterceptableLocation
{
    private protected InterceptableLocation() { }
 
    /// <summary>
    /// The version of the location encoding. Used as an argument to 'InterceptsLocationAttribute'.
    /// </summary>
    public abstract int Version { get; }
 
    /// <summary>
    /// Opaque data which references a call when used as an argument to 'InterceptsLocationAttribute'.
    /// The value does not require escaping, i.e. it is valid in a string literal when wrapped in " (double-quote) characters.
    /// </summary>
    public abstract string Data { get; }
 
    /// <summary>
    /// Gets a human-readable representation of the location, suitable for including in comments in generated code.
    /// </summary>
    public abstract string GetDisplayLocation();
 
    public abstract override bool Equals(object? obj);
    public abstract override int GetHashCode();
}
 
#pragma warning disable RSEXPERIMENTAL002 // internal usage of experimental API
/// <summary>
/// Version 1 of the InterceptableLocation encoding.
/// </summary>
internal sealed class InterceptableLocation1 : InterceptableLocation
{
    internal const int ContentHashLength = 16;
    private static readonly UTF8Encoding s_encoding = new UTF8Encoding(encoderShouldEmitUTF8Identifier: false, throwOnInvalidBytes: true);
 
    private readonly ImmutableArray<byte> _checksum;
    private readonly string _path;
    private readonly int _position;
    private readonly int _lineNumberOneIndexed;
    private readonly int _characterNumberOneIndexed;
    private string? _lazyData;
 
    internal InterceptableLocation1(ImmutableArray<byte> checksum, string path, int position, int lineNumberOneIndexed, int characterNumberOneIndexed)
    {
        Debug.Assert(checksum.Length == ContentHashLength);
        Debug.Assert(path is not null);
        Debug.Assert(position >= 0);
        Debug.Assert(lineNumberOneIndexed > 0);
        Debug.Assert(characterNumberOneIndexed > 0);
 
        _checksum = checksum;
        _path = path;
        _position = position;
        _lineNumberOneIndexed = lineNumberOneIndexed;
        _characterNumberOneIndexed = characterNumberOneIndexed;
    }
 
    public override string GetDisplayLocation()
    {
        // e.g. `C:\project\src\Program.cs(12,34)`
        return $"{_path}({_lineNumberOneIndexed},{_characterNumberOneIndexed})";
    }
 
    public override string ToString() => GetDisplayLocation();
 
    public override int Version => 1;
    public override string Data
    {
        get
        {
            if (_lazyData is null)
                _lazyData = makeData();
 
            return _lazyData;
 
            string makeData()
            {
                var builder = PooledBlobBuilder.GetInstance();
                builder.WriteBytes(_checksum, start: 0, 16);
                builder.WriteInt32(_position);
 
                var displayFileName = Path.GetFileName(_path);
                builder.WriteUTF8(displayFileName);
 
                var bytes = builder.ToArray();
                builder.Free();
                return Convert.ToBase64String(bytes);
            }
        }
    }
 
    internal static (ReadOnlyMemory<byte> checksum, int position, string displayFileName)? Decode(string? data)
    {
        if (data is null)
        {
            return null;
        }
 
        byte[] bytes;
        try
        {
            bytes = Convert.FromBase64String(data);
        }
        catch (FormatException)
        {
            return null;
        }
 
        // format:
        // - 16 bytes of target file content hash (xxHash128)
        // - int32 position (little endian)
        // - utf-8 display filename
        const int hashIndex = 0;
        const int hashSize = 16;
        const int positionIndex = hashIndex + hashSize;
        const int positionSize = sizeof(int);
        const int displayNameIndex = positionIndex + positionSize;
        const int minLength = displayNameIndex;
 
        if (bytes.Length < minLength)
        {
            return null;
        }
 
        var hash = bytes.AsMemory(start: hashIndex, length: hashSize);
        var position = BinaryPrimitives.ReadInt32LittleEndian(bytes.AsSpan(start: positionIndex));
 
        string displayFileName;
        try
        {
            displayFileName = s_encoding.GetString(bytes, index: displayNameIndex, count: bytes.Length - displayNameIndex);
        }
        catch (ArgumentException)
        {
            return null;
        }
 
        return (hash, position, displayFileName);
    }
 
    // Note: the goal of implementing equality here is so that incremental state tables etc. can detect and use it.
    // This encoding which uses the checksum of the referenced file may not be stable across incremental runs in practice, but it seems correct in principle to implement equality here anyway.
    public override bool Equals(object? obj)
    {
        if ((object)this == obj)
            return true;
 
        return obj is InterceptableLocation1 other
            && _checksum.SequenceEqual(other._checksum)
            && _path == other._path
            && _position == other._position
            && _lineNumberOneIndexed == other._lineNumberOneIndexed
            && _characterNumberOneIndexed == other._characterNumberOneIndexed;
    }
 
    public override int GetHashCode()
    {
        // Use only the _checksum and _position in the hash as these are the most distinctive fields of the location.
        // i.e. if these are equal across instances, then other fields are likely to be equal as well.
        return Hash.Combine(
            BinaryPrimitives.ReadInt32LittleEndian(_checksum.AsSpan()),
            _position);
    }
}