#nullable enable
using System;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace Swan {
  /// <summary>
  /// Provides methods to help create external processes, and efficiently capture the
  /// standard error and standard output streams.
  /// </summary>
  public static class ProcessRunner {
    /// <summary>
    /// Defines a delegate to handle binary data reception from the standard 
    /// output or standard error streams from a process.
    /// </summary>
    /// <param name="processData">The process data.</param>
    /// <param name="process">The process.</param>
    // [Obsolete("NEED", false)]
    public delegate void ProcessDataReceivedCallback(Byte[] processData, Process process);

    /// <summary>
    /// Runs the process asynchronously and if the exit code is 0,
    /// returns all of the standard output text. If the exit code is something other than 0
    /// it returns the contents of standard error.
    /// This method is meant to be used for programs that output a relatively small amount of text.
    /// </summary>
    /// <param name="filename">The filename.</param>
    /// <param name="arguments">The arguments.</param>
    /// <param name="workingDirectory">The working directory.</param>
    /// <param name="cancellationToken">The cancellation token.</param>
    /// <returns>The type of the result produced by this Task.</returns>
    /// <example>
    /// The following code explains how to run an external process using the 
    /// <see cref="GetProcessOutputAsync(String, String, CancellationToken)"/> method.
    /// <code>
    /// class Example
    /// {
    ///     using System.Threading.Tasks;
    ///     using Swan;
    ///     
    ///     static async Task Main()
    ///     {
    ///         // execute a process and save its output
    ///          var data = await ProcessRunner.
    ///          GetProcessOutputAsync("dotnet", "--help");
    ///     
    ///         // print the output
    ///         data.WriteLine();
    ///     }
    /// }
    /// </code>
    /// </example>
    // [Obsolete("NEED", false)]
    public static async Task<String> GetProcessOutputAsync(String filename, String arguments = "", String? workingDirectory = null, CancellationToken cancellationToken = default) {
      ProcessResult result = await GetProcessResultAsync(filename, arguments, workingDirectory, cancellationToken: cancellationToken).ConfigureAwait(false);
      return result.ExitCode == 0 ? result.StandardOutput : result.StandardError;
    }

    /// <summary>
    /// Executes a process asynchronously and returns the text of the standard output and standard error streams
    /// along with the exit code. This method is meant to be used for programs that output a relatively small
    /// amount of text.
    /// </summary>
    /// <param name="filename">The filename.</param>
    /// <param name="arguments">The arguments.</param>
    /// <param name="cancellationToken">The cancellation token.</param>
    /// <returns>
    /// Text of the standard output and standard error streams along with the exit code as a <see cref="ProcessResult" /> instance.
    /// </returns>
    /// <exception cref="ArgumentNullException">filename.</exception>
    // [Obsolete("NEED", false)]
    public static Task<ProcessResult> GetProcessResultAsync(String filename, String arguments = "", CancellationToken cancellationToken = default) => GetProcessResultAsync(filename, arguments, null, Definitions.CurrentAnsiEncoding, cancellationToken);

    /// <summary>
    /// Executes a process asynchronously and returns the text of the standard output and standard error streams
    /// along with the exit code. This method is meant to be used for programs that output a relatively small
    /// amount of text.
    /// </summary>
    /// <param name="filename">The filename.</param>
    /// <param name="arguments">The arguments.</param>
    /// <param name="workingDirectory">The working directory.</param>
    /// <param name="encoding">The encoding.</param>
    /// <param name="cancellationToken">The cancellation token.</param>
    /// <returns>
    /// Text of the standard output and standard error streams along with the exit code as a <see cref="ProcessResult" /> instance.
    /// </returns>
    /// <exception cref="ArgumentNullException">filename.</exception>
    /// <example>
    /// The following code describes how to run an external process using the <see cref="GetProcessResultAsync(String, String, String, Encoding, CancellationToken)" /> method.
    /// <code>
    /// class Example
    /// {
    ///     using System.Threading.Tasks;
    ///     using Swan;
    /// 
    ///     static async Task Main()
    ///     {
    ///         // Execute a process asynchronously
    ///         var data = await ProcessRunner.GetProcessResultAsync("dotnet", "--help");
    /// 
    ///         // print out the exit code
    ///         $"{data.ExitCode}".WriteLine();
    ///
    ///         // print out the output
    ///         data.StandardOutput.WriteLine();
    ///         // and the error if exists
    ///         data.StandardError.Error();
    ///     }
    /// }
    /// </code></example>
    // [Obsolete("NEED", false)]
    public static async Task<ProcessResult> GetProcessResultAsync(String filename, String arguments, String? workingDirectory, Encoding? encoding = null, CancellationToken cancellationToken = default) {
      if(filename == null) {
        throw new ArgumentNullException(nameof(filename));
      }

      if(encoding == null) {
        encoding = Definitions.CurrentAnsiEncoding;
      }

      StringBuilder standardOutputBuilder = new StringBuilder();
      StringBuilder standardErrorBuilder = new StringBuilder();

      Int32 processReturn = await RunProcessAsync(filename, arguments, workingDirectory, (data, proc) => standardOutputBuilder.Append(encoding.GetString(data)), (data, proc) => standardErrorBuilder.Append(encoding.GetString(data)), encoding, true, cancellationToken).ConfigureAwait(false);

      return new ProcessResult(processReturn, standardOutputBuilder.ToString(), standardErrorBuilder.ToString());
    }

    /// <summary>
    /// Runs an external process asynchronously, providing callbacks to
    /// capture binary data from the standard error and standard output streams.
    /// The callbacks contain a reference to the process so you can respond to output or
    /// error streams by writing to the process' input stream.
    /// The exit code (return value) will be -1 for forceful termination of the process.
    /// </summary>
    /// <param name="filename">The filename.</param>
    /// <param name="arguments">The arguments.</param>
    /// <param name="workingDirectory">The working directory.</param>
    /// <param name="onOutputData">The on output data.</param>
    /// <param name="onErrorData">The on error data.</param>
    /// <param name="encoding">The encoding.</param>
    /// <param name="syncEvents">if set to <c>true</c> the next data callback will wait until the current one completes.</param>
    /// <param name="cancellationToken">The cancellation token.</param>
    /// <returns>
    /// Value type will be -1 for forceful termination of the process.
    /// </returns>
    // [Obsolete("NEED", false)]
    public static Task<Int32> RunProcessAsync(String filename, String arguments, String? workingDirectory, ProcessDataReceivedCallback onOutputData, ProcessDataReceivedCallback? onErrorData, Encoding encoding, Boolean syncEvents = true, CancellationToken cancellationToken = default) {
      if(filename == null) {
        throw new ArgumentNullException(nameof(filename));
      }

      return Task.Run(() => {
        // Setup the process and its corresponding start info
        Process process = new Process {
          EnableRaisingEvents = false,
          StartInfo = new ProcessStartInfo {
            Arguments = arguments,
            CreateNoWindow = true,
            FileName = filename,
            RedirectStandardError = true,
            StandardErrorEncoding = encoding,
            RedirectStandardOutput = true,
            StandardOutputEncoding = encoding,
            UseShellExecute = false,
          },
        };

        if(!String.IsNullOrWhiteSpace(workingDirectory)) {
          process.StartInfo.WorkingDirectory = workingDirectory;
        }

        // Launch the process and discard any buffered data for standard error and standard output
        _ = process.Start();
        process.StandardError.DiscardBufferedData();
        process.StandardOutput.DiscardBufferedData();

        // Launch the asynchronous stream reading tasks
        Task[] readTasks = new Task[2];
        readTasks[0] = CopyStreamAsync(process, process.StandardOutput.BaseStream, onOutputData, syncEvents, cancellationToken);
        readTasks[1] = CopyStreamAsync(process, process.StandardError.BaseStream, onErrorData, syncEvents, cancellationToken);

        try {
          // Wait for all tasks to complete
          Task.WaitAll(readTasks, cancellationToken);
        } catch(TaskCanceledException) {
          // ignore
        } finally {
          // Wait for the process to exit
          while(cancellationToken.IsCancellationRequested == false) {
            if(process.HasExited || process.WaitForExit(5)) {
              break;
            }
          }

          // Forcefully kill the process if it do not exit
          try {
            if(process.HasExited == false) {
              process.Kill();
            }
          } catch {
            // swallow
          }
        }

        try {
          // Retrieve and return the exit code.
          // -1 signals error
          return process.HasExited ? process.ExitCode : -1;
        } catch {
          return -1;
        }
      }, cancellationToken);
    }

    /// <summary>
    /// Runs an external process asynchronously, providing callbacks to
    /// capture binary data from the standard error and standard output streams.
    /// The callbacks contain a reference to the process so you can respond to output or
    /// error streams by writing to the process' input stream.
    /// The exit code (return value) will be -1 for forceful termination of the process.
    /// </summary>
    /// <param name="filename">The filename.</param>
    /// <param name="arguments">The arguments.</param>
    /// <param name="onOutputData">The on output data.</param>
    /// <param name="onErrorData">The on error data.</param>
    /// <param name="syncEvents">if set to <c>true</c> the next data callback will wait until the current one completes.</param>
    /// <param name="cancellationToken">The cancellation token.</param>
    /// <returns>Value type will be -1 for forceful termination of the process.</returns>
    /// <example>
    /// The following example illustrates how to run an external process using the 
    /// <see cref="RunProcessAsync(String, String, ProcessDataReceivedCallback, ProcessDataReceivedCallback, Boolean, CancellationToken)"/>
    /// method.
    /// <code>
    /// class Example
    /// {
    ///     using System.Diagnostics;
    ///     using System.Text;
    ///     using System.Threading.Tasks;
    ///     using Swan;
    ///     
    ///     static async Task Main()
    ///     {
    ///         // Execute a process asynchronously 
    ///         var data = await ProcessRunner
    ///         .RunProcessAsync("dotnet", "--help", Print, Print);
    ///     
    ///         // flush all messages
    ///         Terminal.Flush();
    ///     }
    ///     
    ///     // a callback to print both output or errors
    ///     static void Print(byte[] data, Process proc) =>
    ///         Encoding.GetEncoding(0).GetString(data).WriteLine();
    /// }
    /// </code>
    /// </example>
    // [Obsolete("NEED", false)]
    public static Task<Int32> RunProcessAsync(String filename, String arguments, ProcessDataReceivedCallback onOutputData, ProcessDataReceivedCallback? onErrorData, Boolean syncEvents = true, CancellationToken cancellationToken = default) => RunProcessAsync(filename, arguments, null, onOutputData, onErrorData, Definitions.CurrentAnsiEncoding, syncEvents, cancellationToken);

    /// <summary>
    /// Copies the stream asynchronously.
    /// </summary>
    /// <param name="process">The process.</param>
    /// <param name="baseStream">The source stream.</param>
    /// <param name="onDataCallback">The on data callback.</param>
    /// <param name="syncEvents">if set to <c>true</c> [synchronize events].</param>
    /// <param name="ct">The cancellation token.</param>
    /// <returns>Total copies stream.</returns>
    // [Obsolete("NEED", false)]
    private static Task<UInt64> CopyStreamAsync(Process process, Stream baseStream, ProcessDataReceivedCallback? onDataCallback, Boolean syncEvents, CancellationToken ct) => Task.Run(async () => {
      // define some state variables
      Byte[] swapBuffer = new Byte[2048]; // the buffer to copy data from one stream to the next
      UInt64 totalCount = 0; // the total amount of bytes read
      Boolean hasExited = false;

      while(ct.IsCancellationRequested == false) {
        try {
          // Check if process is no longer valid
          // if this condition holds, simply read the last bits of data available.
          Int32 readCount; // the bytes read in any given event
          if(process.HasExited || process.WaitForExit(1)) {
            while(true) {
              try {
                readCount = await baseStream.ReadAsync(swapBuffer, 0, swapBuffer.Length, ct);

                if(readCount > 0) {
                  totalCount += (UInt64)readCount;
                  onDataCallback?.Invoke(swapBuffer.Skip(0).Take(readCount).ToArray(), process);
                } else {
                  hasExited = true;
                  break;
                }
              } catch {
                hasExited = true;
                break;
              }
            }
          }

          if(hasExited) {
            break;
          }

          // Try reading from the stream. < 0 means no read occurred.
          readCount = await baseStream.ReadAsync(swapBuffer, 0, swapBuffer.Length, ct).ConfigureAwait(false);

          // When no read is done, we need to let is rest for a bit
          if(readCount <= 0) {
            await Task.Delay(1, ct).ConfigureAwait(false); // do not hog CPU cycles doing nothing.
            continue;
          }

          totalCount += (UInt64)readCount;
          if(onDataCallback == null) {
            continue;
          }

          // Create the buffer to pass to the callback
          Byte[] eventBuffer = swapBuffer.Skip(0).Take(readCount).ToArray();

          // Create the data processing callback invocation
          Task eventTask = Task.Run(() => onDataCallback.Invoke(eventBuffer, process), ct);

          // wait for the event to process before the next read occurs
          if(syncEvents) {
            eventTask.Wait(ct);
          }
        } catch {
          break;
        }
      }

      return totalCount;
    }, ct);
  }
}