using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;

using MfGames.Gallium;

using Humanizer;

using Serilog;

namespace MfGames.Nitride.Pipelines;

/// <summary>
/// A wrapper class to handle a pipeline along with various methods for
/// tracking the operation of the pipeline and thread coordination.
/// </summary>
/// <remarks>
/// This is intended to only be used within a PipelineManager and there
/// should be little reason to use or extend this class.
/// </remarks>
public class PipelineRunner
{
    /// <summary>
    /// The manual reset event used to coordinate thread operations.
    /// </summary>
    private readonly ManualResetEventSlim blockDependencies;

    /// <summary>
    /// A manual reset event to tell the thread when consumers are done.
    /// </summary>
    private readonly ManualResetEventSlim consumersDone;

    private readonly ILogger logger;

    private DateTime changed;

    private bool signaledDoneWithInputs;

    private DateTime started;

    /// <summary>
    /// Contains the number of consumers we're currently waiting to finish
    /// processing.
    /// </summary>
    private int waitingOnConsumers;

    public PipelineRunner(
        ILogger logger,
        IPipeline pipeline)
    {
        this.Pipeline = pipeline ?? throw new ArgumentNullException(nameof(pipeline));
        this.Incoming = new List<PipelineRunner>();
        this.Outgoing = new List<PipelineRunner>();
        this.Outputs = new List<Entity>();
        this.logger = logger.ForContext<PipelineRunner>();
        this.blockDependencies = new ManualResetEventSlim(false);
        this.consumersDone = new ManualResetEventSlim(false);
        this.started = DateTime.Now;
        this.changed = DateTime.Now;
    }

    /// <summary>
    /// Public factory method for creating a new pipeline manager entry.
    /// </summary>
    public delegate PipelineRunner Factory(IPipeline pipeline);

    public TimeSpan ElapsedFromInitialized => DateTime.Now - this.started;

    public TimeSpan ElapsedFromState => DateTime.Now - this.changed;

    /// <summary>
    /// A collection of incoming entries that produce data for the pipeline.
    /// </summary>
    public ICollection<PipelineRunner> Incoming { get; }

    /// <summary>
    /// Gets a value indicating whether the output of this entry is not
    /// used by any other pipeline.
    /// </summary>
    public bool IsFinal => this.Outgoing.Count == 0;

    /// <summary>
    /// Gets a value indicating whether this pipeline is done running.
    /// </summary>
    public bool IsFinished => this.State is PipelineRunnerState.Finalized or PipelineRunnerState.Errored;

    /// <summary>
    /// Gets a value indicating whether this entry is a starting one
    /// that consumes no data.
    /// </summary>
    public bool IsStarting => this.Incoming.Count == 0;

    /// <summary>
    /// The collection of outgoing entries that consume the results of
    /// the pipeline in this entry.
    /// </summary>
    public ICollection<PipelineRunner> Outgoing { get; }

    /// <summary>
    /// Contains the list of all the outputs from this pipeline. This is
    /// only ensured to be valid after the pipeline is in the `Providing`
    /// state.
    /// </summary>
    public List<Entity> Outputs { get; }

    /// <summary>
    /// The pipeline associated with the entry.
    /// </summary>
    public IPipeline Pipeline { get; }

    /// <summary>
    /// Gets the current state of the pipeline.
    /// </summary>
    public PipelineRunnerState State { get; private set; }

    /// <summary>
    /// A method that tells the pipeline that one of the dependencies has
    /// completed consuming the input.
    /// </summary>
    public void ConsumerDoneWithOutputs()
    {
        int current = Interlocked.Decrement(ref this.waitingOnConsumers);

        this.logger.Verbose("{Pipeline:l}: Consumer signalled, waiting for {Count:n0}", this.Pipeline, current);

        if (current == 0)
        {
            this.consumersDone.Set();
        }
    }

    /// <summary>
    /// Initializes the runner after all external properties have been
    /// set and configured.
    /// </summary>
    public void Initialize()
    {
        this.ChangeState(PipelineRunnerState.Initialized);
    }

    /// <summary>
    /// Executes the pipeline, including waiting for any or all
    /// dependencies.
    /// </summary>
    public async Task RunAsync()
    {
        try
        {
            // Make sure we have a valid state.
            switch (this.State)
            {
                case PipelineRunnerState.Initialized:
                case PipelineRunnerState.Finalized:
                    break;

                default:
                    this.logger.Error(
                        "{Pipeline:l}: Pipeline cannot be started in a {State}"
                        + " state (not Initialized or Finalized)",
                        this.Pipeline,
                        this.State);

                    break;
            }

            // Prepare ourselves for running. We have a start/stop state because
            // this may be non-zero time.
            this.started = DateTime.Now;
            this.changed = DateTime.Now;
            this.ChangeState(PipelineRunnerState.Preparing);
            this.signaledDoneWithInputs = false;
            this.ChangeState(PipelineRunnerState.Prepared);

            // Go through the incoming and wait for each of the manual resets
            // on the dependency pipelines.
            if (this.WaitForDependencies())
            {
                this.SignalDoneWithInputs();

                return;
            }

            // Grab the outputs from the incoming. They will be populated
            // because we have waited for the reset events.
            this.ChangeState(PipelineRunnerState.Started);
            List<Entity> input = this.GatherDependencyOutputs();

            // Run the pipeline. This may not be resolved until we gather
            // the output below.
            await this.RunPipeline(input);

            // At this point, we are completely done with our inputs, so signal
            // to them in case they have to clean up any of their structures.
            this.SignalDoneWithInputs();

            // If we have outgoing runners, provide them data until they are
            // done.
            this.SendToDependants();

            // Finalize ourselves.
            this.ChangeState(PipelineRunnerState.Finalized);
        }
        catch (Exception exception)
        {
            // Report the exception.
            this.logger.Error(exception, "{Pipeline:l}: There was an exception running pipeline", this.Pipeline);

            // Change our state and then release any pipeline waiting for us
            // so they can pick up the error and fail themselves.
            this.ChangeState(PipelineRunnerState.Errored);
            this.blockDependencies.Set();
            this.SignalDoneWithInputs();
        }
    }

    /// <summary>
    /// A method to block the call until this runner is done processing and
    /// is ready to provide output.
    /// </summary>
    public void WaitUntilProviding()
    {
        this.blockDependencies.Wait();
    }

    /// <summary>
    /// Changes the state of the pipeline into a new state.
    /// </summary>
    /// <param name="newState">The state to change the pipeline into.</param>
    private void ChangeState(PipelineRunnerState newState)
    {
        this.logger.Verbose(
            "{Pipeline:l}: Switching from state {Old} to {New} (elapsed {Elapsed}, duration {Duration})",
            this.Pipeline,
            this.State,
            newState,
            this.ElapsedFromInitialized,
            this.ElapsedFromState);

        this.changed = DateTime.Now;
        this.State = newState;
    }

    private List<Entity> GatherDependencyOutputs()
    {
        if (this.Incoming.Count <= 0)
        {
            return new List<Entity>();
        }

        // Report that we are gathering our outputs.
        this.logger.Verbose(
            "{Pipeline:l}: Gathering outputs from {Count:n0} dependencies",
            this.Pipeline,
            this.Incoming.Count);

        var input = this.Incoming.SelectMany(x => x.Outputs)
            .ToList();

        this.logger.Debug(
            "{Pipeline:l}: Got {Count:l} from dependencies",
            this.Pipeline,
            "entity".ToQuantity(input.Count, "N0"));

        return input;
    }

    private async Task RunPipeline(List<Entity> input)
    {
        // Get the sequence of data, but this doesn't drain the enumeration.
        List<Entity> output = await this.Pipeline
            .RunAsync(input)
            .ToListAsync();

        // Gather all the output.
        this.logger.Debug("{Pipeline:l}: Gathering output", this.Pipeline);
        this.Outputs.Clear();
        this.Outputs.AddRange(output);
    }

    private void SendToDependants()
    {
        if (this.Outgoing.Count <= 0)
        {
            return;
        }

        // Make sure our internal wait for the consumers it set.
        this.logger.Verbose("{Pipeline:l}: Setting up internal thread controls", this.Pipeline);
        this.waitingOnConsumers = this.Outgoing.Count;
        this.consumersDone.Reset();

        // Report how many files we're sending out and then use manual
        // reset and the semaphore to control the threads.
        this.logger.Debug(
            "{Pipeline:l}: Output {Count:l} from pipeline",
            this.Pipeline,
            "entity".ToQuantity(this.Outputs.Count, "N0"));

        // Release our manual reset to allow operations to continue.
        this.ChangeState(PipelineRunnerState.Providing);
        this.logger.Verbose("{Pipeline:l}: Release manual reset for consumers", this.Pipeline);
        this.blockDependencies.Set();

        // Wait until all consumers have finished processing.
        this.consumersDone.Wait();
    }

    private void SignalDoneWithInputs()
    {
        if (this.Incoming.Count <= 0 || this.signaledDoneWithInputs)
        {
            return;
        }

        this.signaledDoneWithInputs = true;

        this.logger.Debug("{Pipeline:l}: Signaling {Count:n0} dependencies done", this.Pipeline, this.Incoming.Count);

        foreach (PipelineRunner? dependency in this.Incoming)
        {
            dependency.ConsumerDoneWithOutputs();
        }
    }

    private bool WaitForDependencies()
    {
        if (this.Incoming.Count <= 0)
        {
            return false;
        }

        // Wait for the dependencies to run first.
        this.ChangeState(PipelineRunnerState.Waiting);

        this.logger.Verbose(
            "{Pipeline:l}: Waiting for {Count:l} to complete",
            this.Pipeline,
            "dependency".ToQuantity(this.Incoming.Count));

        foreach (PipelineRunner? dependency in this.Incoming)
        {
            dependency.WaitUntilProviding();
        }

        // Check for any error state in the dependency, if we have one,
        // then we need to stop ourselves.
        bool hasError = this.Incoming.Any(x => x.State == PipelineRunnerState.Errored);

        if (!hasError)
        {
            return false;
        }

        this.logger.Error("{Pipeline:l}: There was an exception in an dependency", this.Pipeline);
        this.ChangeState(PipelineRunnerState.Errored);
        this.blockDependencies.Set();

        return true;
    }
}