using System.Collections.Generic;
using UnityEngine;

namespace Pathfinding {
	using Pathfinding.Jobs;
	using Pathfinding.Util;
	using Unity.Jobs;
	using Unity.Profiling;
	using UnityEngine.Assertions;
	using UnityEngine.Profiling;

	/// <summary>
	/// Promise representing a graph update.
	///
	/// This is used internally by the system to represent a graph update.
	/// Generally you shouldn't need to care about it, unless you are implementing your own graph type.
	/// </summary>
	public interface IGraphUpdatePromise {
		/// <summary>
		/// Returns the progress of the update.
		///
		/// This should be a value between 0 and 1.
		/// </summary>
		float Progress => 0.0f;

		/// <summary>
		/// Coroutine to prepare an update asynchronously.
		///
		/// If a JobHandle is returned, it will be awaited before the coroutine is ticked again, and before the <see cref="Apply"/> method is called.
		///
		/// After this coroutine has finished, the <see cref="Apply"/> method will be called.
		///
		/// Note: No changes must be made to the graph in this method. Those should only be done in the <see cref="Apply"/> method.
		///
		/// May return null if no async work is required.
		/// </summary>
		IEnumerator<JobHandle> Prepare() => null;

		/// <summary>
		/// Applies the update in a single atomic update.
		///
		/// It is done as a single atomic update (from the main thread's perspective) to ensure
		/// that even if one does an async scan or update, the graph will always be in a valid state.
		/// This guarantees that things like GetNearest will still work during an async scan.
		///
		/// Warning: Must only be called after the <see cref="Prepare"/> method has finished.
		/// </summary>
		// TODO: Pass in a JobHandle and allow returning a JobHandle?
		void Apply(IGraphUpdateContext context);
	}

	/// <summary>
	/// Helper functions for graph updates.
	///
	/// A context is passed to graphs when they are updated, and to work items when they are executed.
	/// The <see cref="IWorkItemContext"/> interface inherits from this interface.
	/// </summary>
	public interface IGraphUpdateContext {
		/// <summary>
		/// Mark a particular region of the world as having been changed.
		///
		/// This should be used whenever graphs are changed.
		///
		/// This is used to recalculate off-mesh links that touch these bounds, and it will also ensure <see cref="GraphModifier"/> events are callled.
		///
		/// The bounding box should cover the surface of all nodes that have been updated.
		/// It is fine to use a larger bounding box than necessary (even an infinite one), though this may be slower, since more off-mesh links need to be recalculated.
		/// You can even use an infinitely large bounding box if you don't want to bother calculating a more accurate one.
		/// You can also call this multiple times to dirty multiple bounding boxes.
		/// </summary>
		void DirtyBounds(Bounds bounds);
	}

	class GraphUpdateProcessor {
		/// <summary>Holds graphs that can be updated</summary>
		readonly AstarPath astar;

		/// <summary>Used for IsAnyGraphUpdateInProgress</summary>
		bool anyGraphUpdateInProgress;

		/// <summary>
		/// Queue containing all waiting graph update queries. Add to this queue by using \link AddToQueue \endlink.
		/// See: AddToQueue
		/// </summary>
		readonly Queue<GraphUpdateObject> graphUpdateQueue = new Queue<GraphUpdateObject>();
		readonly List<(IGraphUpdatePromise, IEnumerator<JobHandle>)> pendingPromises = new List<(IGraphUpdatePromise, IEnumerator<JobHandle>)>();
		readonly List<GraphUpdateObject> pendingGraphUpdates = new List<GraphUpdateObject>();

		/// <summary>Returns if any graph updates are waiting to be applied</summary>
		public bool IsAnyGraphUpdateQueued { get { return graphUpdateQueue.Count > 0; } }

		/// <summary>Returns if any graph updates are in progress</summary>
		public bool IsAnyGraphUpdateInProgress { get { return anyGraphUpdateInProgress; } }

		public GraphUpdateProcessor (AstarPath astar) {
			this.astar = astar;
		}

		/// <summary>Work item which can be used to apply all queued updates</summary>
		public AstarWorkItem GetWorkItem () {
			return new AstarWorkItem(QueueGraphUpdatesInternal, ProcessGraphUpdates);
		}

		/// <summary>
		/// Update all graphs using the GraphUpdateObject.
		/// This can be used to, e.g make all nodes in an area unwalkable, or set them to a higher penalty.
		/// The graphs will be updated as soon as possible (with respect to AstarPath.batchGraphUpdates)
		///
		/// See: FlushGraphUpdates
		/// </summary>
		public void AddToQueue (GraphUpdateObject ob) {
			// Put the GUO in the queue
			graphUpdateQueue.Enqueue(ob);
		}

		/// <summary>
		/// Discards all queued graph updates.
		///
		/// Graph updates that are already in progress will not be discarded.
		/// </summary>
		public void DiscardQueued () {
			while (graphUpdateQueue.Count > 0) {
				graphUpdateQueue.Dequeue().internalStage = GraphUpdateObject.STAGE_ABORTED;
			}
		}

		/// <summary>Schedules graph updates internally</summary>
		void QueueGraphUpdatesInternal (IWorkItemContext context) {
			Assert.IsTrue(pendingGraphUpdates.Count == 0);
			while (graphUpdateQueue.Count > 0) {
				var ob = graphUpdateQueue.Dequeue();
				pendingGraphUpdates.Add(ob);
				if (ob.internalStage != GraphUpdateObject.STAGE_PENDING) {
					Debug.LogError("Expected remaining graph update to be pending");
					continue;
				}
			}

			foreach (IUpdatableGraph g in astar.data.GetUpdateableGraphs()) {
				NavGraph gr = g as NavGraph;
				var updates = ListPool<GraphUpdateObject>.Claim();
				for (int i = 0; i < pendingGraphUpdates.Count; i++) {
					GraphUpdateObject ob = pendingGraphUpdates[i];
					if (ob.nnConstraint == null || ob.nnConstraint.SuitableGraph((int)gr.graphIndex, gr)) {
						updates.Add(ob);
					}
				}
				if (updates.Count > 0) {
					var promise = g.ScheduleGraphUpdates(updates);
					if (promise != null) {
						var it = promise.Prepare();
						pendingPromises.Add((promise, it));
					} else {
						ListPool<GraphUpdateObject>.Release(ref updates);
					}
				} else {
					ListPool<GraphUpdateObject>.Release(ref updates);
				}
			}

			context.PreUpdate();
			anyGraphUpdateInProgress = true;
		}

		static readonly ProfilerMarker MarkerSleep = new ProfilerMarker(ProfilerCategory.Loading, "Sleep");
		static readonly ProfilerMarker MarkerCalculate = new ProfilerMarker("Calculating Graph Update");
		static readonly ProfilerMarker MarkerApply = new ProfilerMarker("Applying Graph Update");

		/// <summary>
		/// Updates graphs.
		/// Will do some graph updates, possibly signal another thread to do them.
		/// Will only process graph updates added by QueueGraphUpdatesInternal
		///
		/// Returns: True if all graph updates have been done and pathfinding (or other tasks) may resume.
		/// False if there are still graph updates being processed or waiting in the queue.
		/// </summary>
		/// <param name="context">Helper methods for the work items.</param>
		/// <param name="force">If true, all graph updates will be processed before this function returns. The return value
		/// will be True.</param>
		bool ProcessGraphUpdates (IWorkItemContext context, bool force) {
			Assert.IsTrue(anyGraphUpdateInProgress);

			if (pendingPromises.Count > 0) {
				if (!ProcessGraphUpdatePromises(pendingPromises, context, force)) {
					return false;
				}

				pendingPromises.Clear();
			}

			anyGraphUpdateInProgress = false;
			for (int i = 0; i < pendingGraphUpdates.Count; i++) {
				pendingGraphUpdates[i].internalStage = GraphUpdateObject.STAGE_APPLIED;
			}
			pendingGraphUpdates.Clear();
			return true;
		}

		public static bool ProcessGraphUpdatePromises (List<(IGraphUpdatePromise, IEnumerator<JobHandle>)> promises, IGraphUpdateContext context, bool force = false) {
			TimeSlice timeSlice = TimeSlice.MillisFromNow(2f);
			TimeSlice idleTimeSlice = default;

			while (true) {
				int firstNonFinished = -1;
				bool anyMainThreadProgress = false;
				for (int i = 0; i < promises.Count; i++) {
					var(promise, it) = promises[i];
					if (it == null) continue;
					if (force) {
						it.Current.Complete();
					} else {
						if (it.Current.IsCompleted) {
							it.Current.Complete();
						} else {
							if (firstNonFinished == -1) firstNonFinished = i;
							continue;
						}
					}

					anyMainThreadProgress = true;
					MarkerCalculate.Begin();
					try {
						if (it.MoveNext()) {
							if (firstNonFinished == -1) firstNonFinished = i;
						} else promises[i] = (promise, null);
					} catch (System.Exception e) {
						Debug.LogError(new System.Exception("Error while updating graphs.", e));
						promises[i] = (null, null);
					}
					MarkerCalculate.End();
				}

				if (firstNonFinished == -1) {
					break;
				} else if (!force) {
					if (timeSlice.expired) {
						return false;
					} else if (anyMainThreadProgress) {
						// Reset the idle time slice if we got something done on the main thread.
						// This allows us to wait on more very short jobs.
						idleTimeSlice = TimeSlice.MillisFromNow(0.1f);
					} else if (idleTimeSlice.expired) {
						return false;
					} else {
						// Allow waiting for a short amount of time to allow very short running
						// jobs in graph updates to complete without having to wait until the next frame.
						// While waiting we release our thread's time slice to make sure other threads get priority.
						if (!anyMainThreadProgress) {
							MarkerSleep.Begin();
							System.Threading.Thread.Yield();
							MarkerSleep.End();
						}
					}
				}
			}

			for (int i = 0; i < promises.Count; i++) {
				var(promise, it) = promises[i];
				Assert.IsNull(it);
				if (promise != null) {
					MarkerApply.Begin();
					try {
						promise.Apply(context);
					} catch (System.Exception e) {
						Debug.LogError(new System.Exception("Error while updating graphs.", e));
					}
					MarkerApply.End();
				}
			}

			return true;
		}
	}
}