// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using Microsoft.JSInterop;
 
namespace Microsoft.AspNetCore.Components;
 
/// <Summary>
/// A stream that pulls each chunk on demand using JavaScript interop. This implementation is used for
/// WebAssembly and WebView applications.
/// </Summary>
internal sealed class PullFromJSDataStream : Stream
{
    private readonly IJSRuntime _runtime;
    private readonly IJSStreamReference _jsStreamReference;
    private readonly long _totalLength;
    private readonly CancellationToken _streamCancellationToken;
    private long _offset;
 
    public static PullFromJSDataStream CreateJSDataStream(
        IJSRuntime runtime,
        IJSStreamReference jsStreamReference,
        long totalLength,
        CancellationToken cancellationToken = default)
    {
        var jsDataStream = new PullFromJSDataStream(runtime, jsStreamReference, totalLength, cancellationToken);
        return jsDataStream;
    }
 
    private PullFromJSDataStream(
        IJSRuntime runtime,
        IJSStreamReference jsStreamReference,
        long totalLength,
        CancellationToken cancellationToken)
    {
        _runtime = runtime;
        _jsStreamReference = jsStreamReference;
        _totalLength = totalLength;
        _streamCancellationToken = cancellationToken;
        _offset = 0;
    }
 
    public override bool CanRead => true;
 
    public override bool CanSeek => false;
 
    public override bool CanWrite => false;
 
    public override long Length => _totalLength;
 
    public override long Position
    {
        get => _offset;
        set => throw new NotSupportedException();
    }
 
    public override void Flush()
    {
        // No-op
    }
 
    public override Task FlushAsync(CancellationToken cancellationToken) => Task.CompletedTask;
 
    public override int Read(byte[] buffer, int offset, int count)
        => throw new NotSupportedException("Synchronous reads are not supported.");
 
    public override long Seek(long offset, SeekOrigin origin)
        => throw new NotSupportedException();
 
    public override void SetLength(long value)
        => throw new NotSupportedException();
 
    public override void Write(byte[] buffer, int offset, int count)
        => throw new NotSupportedException();
 
    public override async Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
        => await ReadAsync(buffer.AsMemory(offset, count), cancellationToken);
 
    public override async ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken cancellationToken = default)
    {
        var bytesRead = await RequestDataFromJSAsync(buffer.Length);
        ThrowIfCancellationRequested(cancellationToken);
        bytesRead.CopyTo(buffer);
 
        return bytesRead.Length;
    }
 
    private void ThrowIfCancellationRequested(CancellationToken cancellationToken)
    {
        if (cancellationToken.IsCancellationRequested ||
            _streamCancellationToken.IsCancellationRequested)
        {
            throw new TaskCanceledException();
        }
    }
 
    private async ValueTask<byte[]> RequestDataFromJSAsync(int numBytesToRead)
    {
        numBytesToRead = (int)Math.Min(numBytesToRead, _totalLength - _offset);
        var bytesRead = await _runtime.InvokeAsync<byte[]>("Blazor._internal.getJSDataStreamChunk", _jsStreamReference, _offset, numBytesToRead);
        if (bytesRead.Length != numBytesToRead)
        {
            throw new EndOfStreamException("Failed to read the requested number of bytes from the stream.");
        }
 
        _offset += bytesRead.Length;
        if (_offset == _totalLength)
        {
            Dispose(true);
        }
        return bytesRead;
    }
}