using System;
using System.Collections.Generic;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using MinecraftClient.Mapping;
using MinecraftClient.Pathing.Core;
using MinecraftClient.Pathing.Execution.Telemetry;
using MinecraftClient.Pathing.Goals;
using MinecraftClient.Physics;

namespace MinecraftClient.Pathing.Execution
{
    /// <summary>
    /// Top-level navigation controller. Holds a PathExecutor, monitors its progress,
    /// and triggers replanning on failure or deviation.
    ///
    /// Replans and look-ahead plans run on a background Task (Baritone equivalent:
    /// findPathInNewThread). The main tick only reads task state: either applies a
    /// completed plan or installs the next one. A look-ahead plan is speculatively
    /// started as the current executor nears the end of its segment list so that the
    /// next executor is ready to splice in without a user-visible pause.
    /// </summary>
    public sealed class PathSegmentManager
    {
        private const int MaxReplans = 5;
        private const int ReplanTimeoutMs = 3000;
        private const int LookaheadTriggerSegmentsRemaining = 2;

        private readonly Action<string>? _debugLog;
        private readonly Action<string>? _infoLog;
        private readonly IPathExecutionObserver? _observer;

        private PathExecutor? _executor;
        private PathExecutor? _nextExecutor;
        private IGoal? _goal;
        private int _replanCount;
        private bool _isInitialPlan;

        private Task<PathResult>? _pendingReplan;
        private CancellationTokenSource? _pendingReplanCts;
        private Task<PathResult>? _pendingLookahead;
        private CancellationTokenSource? _pendingLookaheadCts;
        private (int x, int y, int z)? _pendingLookaheadAnchor;

        /// <summary>
        /// Set to true to emit Info-level diagnostic traces (full path node dump,
        /// failing-segment context, recent position tail) on every plan/replan
        /// event. Users enable this via <c>/pathdiag on</c> when reporting pathing
        /// bugs so the default log level stays quiet.
        /// </summary>
        public static bool DiagnosticsEnabled { get; set; }

        private const int DiagnosticsTailSize = 200;
        private const int SlowSegmentDumpTickThreshold = 25;
        private readonly Queue<string> _diagnosticsTail = new(DiagnosticsTailSize + 1);
        private PathResult? _lastPlan;
        private int _lastObservedSegmentIndex = -1;
        private int _ticksSinceSegmentStart;

        // Diagnostic emission runs on the thread pool to keep the 20 TPS tick
        // unblocked. Each batch is appended to a single chained Task so that
        // line ordering is preserved across batches, even when the same tick
        // produces both a slow-segment dump and the next seg-> header. Without
        // the chain, multiple Task.Run calls could interleave and mangle the
        // log output.
        //
        // Without this offload, a 200-line failure dump issued synchronously
        // through ConsoleIO.WriteLogLine -> file logger took 200-500 ms on
        // the main tick. The stalled tick stops position packets so the
        // server view freezes, then snaps forward when the tick resumes -
        // exactly the "freeze, jump, freeze" the user reported when /pathdiag
        // was on.
        private Task _diagFlushTail = Task.CompletedTask;
        private readonly object _diagFlushLock = new();

        public bool IsNavigating =>
            (_executor is not null && !_executor.IsComplete)
            || _nextExecutor is not null
            || _pendingReplan is not null;

        public int ReplanCount => _replanCount;
        public IGoal? Goal => _goal;

        public PathSegmentManager(Action<string>? debugLog = null, Action<string>? infoLog = null, IPathExecutionObserver? observer = null)
        {
            _debugLog = debugLog;
            _infoLog = infoLog;
            _observer = observer;
        }

        public void StartNavigation(IGoal goal, PathResult result)
        {
            CancelPendingTasks();
            _nextExecutor = null;
            _goal = goal;
            _replanCount = 0;
            _isInitialPlan = false;
            if (result.Status == PathStatus.Failed || result.Path.Count < 2)
            {
                _infoLog?.Invoke("[PathMgr] Navigation rejected -- no path found.");
                _executor = null;
                _goal = null;
                return;
            }

            var segments = PathSegmentBuilder.FromPath(result.Path);
            _executor = new PathExecutor(segments, _debugLog, _observer);
            _lastObservedSegmentIndex = -1;
            _ticksSinceSegmentStart = 0;
            _infoLog?.Invoke($"[PathMgr] Navigation started: {segments.Count} segments");
        }

        /// <summary>
        /// Kicks off the initial A* search on a background task and returns
        /// immediately. The main tick loop drains the task via
        /// <see cref="DrainPendingReplan"/>, installing the executor when the
        /// plan completes. Use this from interactive entry points (e.g.
        /// <c>/goto</c>) so the 20 TPS tick loop never blocks on a long A*
        /// search -- a complex climb can take the full planning budget
        /// (several seconds) and freezing the tick causes the player to
        /// desync, stop sending keep-alives, and miss chunk updates.
        /// </summary>
        public void StartNavigationAsync(IGoal goal, Location startPos, World world, long timeoutMs)
        {
            ArgumentNullException.ThrowIfNull(goal);
            ArgumentNullException.ThrowIfNull(world);

            CancelPendingTasks();
            _executor = null;
            _nextExecutor = null;
            _goal = goal;
            _replanCount = 0;
            _isInitialPlan = true;

            int sx = (int)Math.Floor(startPos.X);
            int sy = (int)Math.Floor(startPos.Y);
            int sz = (int)Math.Floor(startPos.Z);

            var ctx = new CalculationContext(world, allowParkour: true, allowParkourAscend: true);
            if (!ctx.CanWalkThrough(sx, sy, sz) && ctx.CanWalkThrough(sx, sy + 1, sz))
                sy++;

            _debugLog?.Invoke($"[PathMgr] Initial plan kicked off from ({sx},{sy},{sz}) to {goal}");

            _pendingReplanCts = new CancellationTokenSource();
            CancellationToken token = _pendingReplanCts.Token;
            Action<string>? debugLog = _debugLog;
            long budget = timeoutMs;
            _pendingReplan = Task.Run(() =>
            {
                var finder = new AStarPathFinder { DebugLog = debugLog };
                return finder.Calculate(ctx, sx, sy, sz, goal, token, budget);
            }, token);
        }

        public void Tick(Location pos, PlayerPhysics physics, MovementInput input, World world)
        {
            DrainPendingReplan(pos, world);
            DrainPendingLookahead();

            if (_executor is null)
            {
                // Navigation is still alive if we are waiting on a background plan to
                // come back. The next tick will promote it into _executor.
                if (_pendingReplan is not null || _nextExecutor is not null)
                {
                    TryPromoteNextExecutor();
                    input.Reset();
                    return;
                }

                return;
            }

            if (DiagnosticsEnabled)
                RecordDiagnosticsSample(pos, physics);

            var state = _executor.Tick(pos, physics, input, world);

            switch (state)
            {
                case PathExecutorState.Complete:
                    HandleExecutorComplete(pos, world, input);
                    break;

                case PathExecutorState.Failed:
                    if (DiagnosticsEnabled)
                        EmitSegmentFailureDiagnostics(pos);
                    _infoLog?.Invoke("[PathMgr] Segment failed, replanning...");
                    // The prepared next path assumes we finished the current segment
                    // cleanly, so drop it when we fail.
                    DiscardLookahead();
                    _nextExecutor = null;
                    StartReplanAsync(pos, world);
                    break;

                case PathExecutorState.InProgress:
                    MaybeStartLookahead(world);
                    break;
            }
        }

        private void RecordDiagnosticsSample(Location pos, PlayerPhysics physics)
        {
            if (_executor is null)
                return;
            int segIdx = _executor.CurrentIndex;
            PathSegment? seg = _executor.CurrentSegment;
            string segStr = seg is null
                ? "none"
                : $"seg{segIdx}/{_executor.TotalSegments} {seg.MoveType} ({seg.Start.X:F1},{seg.Start.Y:F1},{seg.Start.Z:F1})->({seg.End.X:F1},{seg.End.Y:F1},{seg.End.Z:F1})";

            // Emit an Info-level transition event whenever the executor steps to a
            // new segment so the caller can reconstruct the full execution timeline
            // without relying on the bounded tail buffer. Resets on plan install.
            if (segIdx != _lastObservedSegmentIndex)
            {
                if (_lastObservedSegmentIndex >= 0
                    && _ticksSinceSegmentStart >= SlowSegmentDumpTickThreshold)
                {
                    int toDump = Math.Min(_diagnosticsTail.Count, _ticksSinceSegmentStart);
                    int skipCount = _diagnosticsTail.Count - toDump;
                    var batch = new List<string>(toDump + 1)
                    {
                        $"[PathDiag] Slow segment {_lastObservedSegmentIndex}/{_executor.TotalSegments} took {_ticksSinceSegmentStart} ticks, dumping last {toDump} ticks:"
                    };
                    int i = 0;
                    foreach (string line in _diagnosticsTail)
                    {
                        if (i++ < skipCount)
                            continue;
                        batch.Add($"[PathDiag]   t-{toDump - (i - skipCount)}: {line}");
                    }
                    DispatchDiagnosticsBatch(batch);
                }

                _lastObservedSegmentIndex = segIdx;
                _ticksSinceSegmentStart = 0;
                DispatchDiagnosticsBatch(new[]
                {
                    $"[PathDiag] seg->{segIdx}/{_executor.TotalSegments} pos=({pos.X:F2},{pos.Y:F2},{pos.Z:F2}) yaw={physics.Yaw:F1} vy={physics.DeltaMovement.Y:F3} og={physics.OnGround} "
                    + (seg is null ? "none" : $"{seg.MoveType} ({seg.Start.X:F1},{seg.Start.Y:F1},{seg.Start.Z:F1})->({seg.End.X:F1},{seg.End.Y:F1},{seg.End.Z:F1}) exit={seg.ExitTransition}")
                });
            }
            else
            {
                _ticksSinceSegmentStart++;
            }

            _diagnosticsTail.Enqueue(
                $"pos=({pos.X:F2},{pos.Y:F2},{pos.Z:F2}) yaw={physics.Yaw:F1} vy={physics.DeltaMovement.Y:F3} vx={physics.DeltaMovement.X:F3} vz={physics.DeltaMovement.Z:F3} og={physics.OnGround} {segStr}");
            while (_diagnosticsTail.Count > DiagnosticsTailSize)
                _diagnosticsTail.Dequeue();
        }

        private void EmitSegmentFailureDiagnostics(Location pos)
        {
            if (_executor is null)
                return;
            PathSegment? seg = _executor.CurrentSegment;
            int segIdx = _executor.CurrentIndex;
            var batch = new List<string>(_diagnosticsTail.Count + 2)
            {
                $"[PathDiag] Failure context: pos=({pos.X:F2},{pos.Y:F2},{pos.Z:F2}) failingSeg={segIdx}/{_executor.TotalSegments} "
                + (seg is null ? "seg=<none>" : $"seg={seg.MoveType} ({seg.Start.X:F1},{seg.Start.Y:F1},{seg.Start.Z:F1})->({seg.End.X:F1},{seg.End.Y:F1},{seg.End.Z:F1}) exit={seg.ExitTransition}")
            };
            if (_diagnosticsTail.Count > 0)
            {
                batch.Add($"[PathDiag] Recent tick trace (last {_diagnosticsTail.Count}):");
                int i = 0;
                int total = _diagnosticsTail.Count;
                foreach (string line in _diagnosticsTail)
                    batch.Add($"[PathDiag]   t-{total - i++ - 1}: {line}");
            }
            DispatchDiagnosticsBatch(batch);
        }

        private void EmitPathDumpDiagnostics(string label, PathResult result, int startIdx = 0)
        {
            if (!DiagnosticsEnabled)
                return;
            int count = result.Path.Count;
            var batch = new List<string>(count + 1)
            {
                $"[PathDiag] {label}: {count} waypoints, status={result.Status}, nodes={result.NodesExplored}, time={result.ElapsedMs}ms"
            };
            for (int i = 0; i < count; i++)
            {
                var node = result.Path[i];
                string move = i == 0 ? "Start" : node.MoveUsed.ToString();
                batch.Add($"[PathDiag]   [{startIdx + i:D2}] {move,-22} ({node.X},{node.Y},{node.Z})");
            }
            DispatchDiagnosticsBatch(batch);
        }

        /// <summary>
        /// Schedule a diagnostics line batch for emission on a background task,
        /// chained behind any prior batch so output order is preserved. The
        /// caller's snapshot is captured by reference; the input list MUST not
        /// be mutated after dispatch.
        /// </summary>
        private void DispatchDiagnosticsBatch(IReadOnlyList<string> lines)
        {
            Action<string>? infoLog = _infoLog;
            if (infoLog is null || lines.Count == 0)
                return;

            lock (_diagFlushLock)
            {
                _diagFlushTail = _diagFlushTail.ContinueWith(_ =>
                {
                    for (int i = 0; i < lines.Count; i++)
                    {
                        try
                        {
                            infoLog(lines[i]);
                        }
                        catch
                        {
                            // Swallow logger faults so a downstream sink failure
                            // never tears down the chain (which would silently
                            // drop every subsequent diagnostics batch).
                        }
                    }
                }, TaskScheduler.Default);
            }
        }

        public void Cancel()
        {
            if (_executor is not null || _pendingReplan is not null || _nextExecutor is not null)
            {
                _infoLog?.Invoke("[PathMgr] Navigation cancelled.");
            }

            CancelPendingTasks();
            _executor = null;
            _nextExecutor = null;
            _goal = null;
        }

        private void HandleExecutorComplete(Location pos, World world, MovementInput input)
        {
            if (_goal is not null)
            {
                int px = (int)Math.Floor(pos.X);
                int py = (int)Math.Floor(pos.Y);
                int pz = (int)Math.Floor(pos.Z);
                if (!_goal.IsInGoal(px, py, pz))
                {
                    if (TryPromoteNextExecutor())
                    {
                        _debugLog?.Invoke("[PathMgr] Spliced to prepared next segment chain.");
                        return;
                    }

                    _infoLog?.Invoke("[PathMgr] Planned route ended before reaching goal, replanning...");
                    StartReplanAsync(pos, world);
                    input.Reset();
                    return;
                }
            }

            _observer?.OnNavigationCompleted(_executor!.TotalTicks);
            _infoLog?.Invoke("[PathMgr] Navigation complete!");
            CancelPendingTasks();
            _executor = null;
            _nextExecutor = null;
            _goal = null;
        }

        private bool TryPromoteNextExecutor()
        {
            if (_nextExecutor is null)
                return false;

            _executor = _nextExecutor;
            _nextExecutor = null;
            return true;
        }

        private void StartReplanAsync(Location pos, World world)
        {
            _replanCount++;
            _observer?.OnReplanStarted(_replanCount, pos);
            if (_replanCount > MaxReplans)
            {
                _observer?.OnReplanFailed(_replanCount, pos);
                _infoLog?.Invoke($"[PathMgr] Giving up after {MaxReplans} replans.");
                CancelPendingTasks();
                _executor = null;
                _nextExecutor = null;
                _goal = null;
                return;
            }

            if (_goal is null)
            {
                _executor = null;
                return;
            }

            // Once we decide to replan, the currently-installed executor is discarded;
            // the new plan will replace it. We keep the executor reference until the
            // plan returns so IsNavigating reflects that work is in flight.
            if (_pendingReplan is not null)
                return;

            int sx = (int)Math.Floor(pos.X);
            int sy = (int)Math.Floor(pos.Y);
            int sz = (int)Math.Floor(pos.Z);

            var ctx = new CalculationContext(world, allowParkour: true, allowParkourAscend: true);
            if (!ctx.CanWalkThrough(sx, sy, sz) && ctx.CanWalkThrough(sx, sy + 1, sz))
                sy++;

            IGoal goal = _goal;
            _debugLog?.Invoke($"[PathMgr] Replan #{_replanCount} kicked off from ({pos.X:F2},{pos.Y:F2},{pos.Z:F2})");

            _pendingReplanCts = new CancellationTokenSource();
            CancellationToken token = _pendingReplanCts.Token;
            _pendingReplan = Task.Run(() =>
            {
                var finder = new AStarPathFinder { DebugLog = _debugLog };
                return finder.Calculate(ctx, sx, sy, sz, goal, token, ReplanTimeoutMs);
            }, token);

            // Clear the current executor so IsNavigating stays true via the pending
            // task branch. This prevents the main client from believing navigation
            // ended between "plan completes" and "plan gets installed".
            _executor = null;
        }

        private void DrainPendingReplan(Location pos, World world)
        {
            if (_pendingReplan is null)
                return;

            // When the current executor has been cleared (we are waiting for a plan
            // to install), give the background task a short budget to finish. The
            // player is standing still anyway, so trading a few ms of pause for
            // installing the new plan immediately is a clear win over letting the
            // tick return with _executor == null. This also makes tests with a
            // tight Tick poll loop deterministic: Task.Run needs the caller to
            // yield at some point so the thread-pool worker can complete.
            if (!_pendingReplan.IsCompleted && _executor is null && _nextExecutor is null)
            {
                try
                {
                    _pendingReplan.Wait(20);
                }
                catch
                {
                    // Exceptions are inspected via Task.IsFaulted below.
                }
            }

            if (!_pendingReplan.IsCompleted)
                return;

            Task<PathResult> task = _pendingReplan;
            _pendingReplan = null;
            var cts = _pendingReplanCts;
            _pendingReplanCts = null;
            cts?.Dispose();

            bool isInitial = _isInitialPlan;
            _isInitialPlan = false;

            if (task.IsFaulted || task.IsCanceled)
            {
                _infoLog?.Invoke(isInitial
                    ? "[PathMgr] Initial plan failed or was cancelled."
                    : "[PathMgr] Replan task failed or was cancelled.");
                if (!isInitial)
                    _observer?.OnReplanFailed(_replanCount, pos);
                _goal = null;
                _executor = null;
                _nextExecutor = null;
                return;
            }

            PathResult result = task.Result;

            int sx = (int)Math.Floor(pos.X);
            int sy = (int)Math.Floor(pos.Y);
            int sz = (int)Math.Floor(pos.Z);
            bool alreadyInGoal = _goal is not null && (_goal.IsInGoal(sx, sy, sz)
                || (result.Path.Count == 1 && _goal.IsInGoal(result.Path[0].X, result.Path[0].Y, result.Path[0].Z)));

            if (alreadyInGoal)
            {
                _infoLog?.Invoke("[PathMgr] Navigation complete!");
                _executor = null;
                _nextExecutor = null;
                _goal = null;
                return;
            }

            if (result.Status == PathStatus.Failed || result.Path.Count < 2)
            {
                if (!isInitial)
                    _observer?.OnReplanFailed(_replanCount, pos);
                _infoLog?.Invoke(isInitial
                    ? $"[PathMgr] No path found (nodes={result.NodesExplored}, time={result.ElapsedMs}ms)."
                    : "[PathMgr] Replan failed -- no path found.");
                _executor = null;
                _nextExecutor = null;
                _goal = null;
                return;
            }

            var segments = PathSegmentBuilder.FromPath(result.Path);
            _lastPlan = result;
            _diagnosticsTail.Clear();
            _lastObservedSegmentIndex = -1;
            _ticksSinceSegmentStart = 0;
            if (isInitial)
            {
                _executor = new PathExecutor(segments, _debugLog, _observer);
                _nextExecutor = null;
                string partial = result.Status == PathStatus.Partial ? " (partial)" : "";
                _infoLog?.Invoke($"[PathMgr] Navigation started: {segments.Count} segments, nodes={result.NodesExplored}, time={result.ElapsedMs}ms{partial}");
                EmitPathDumpDiagnostics("Initial plan", result);
            }
            else
            {
                _observer?.OnReplanSucceeded(_replanCount, segments);
                _executor = new PathExecutor(segments, _debugLog, _observer);
                _nextExecutor = null;
                _infoLog?.Invoke($"[PathMgr] Replanned: {segments.Count} segments (replan #{_replanCount})");
                EmitPathDumpDiagnostics($"Replan #{_replanCount} plan", result);
            }
        }

        private void MaybeStartLookahead(World world)
        {
            if (_pendingLookahead is not null || _nextExecutor is not null || _goal is null || _executor is null)
                return;

            int total = _executor.TotalSegments;
            int current = _executor.CurrentIndex;
            if (total - current > LookaheadTriggerSegmentsRemaining)
                return;

            // Anchor the lookahead at the final segment's end; that is where execution
            // will arrive if the current executor finishes without drift.
            PathSegment? lastSegment = _executor.LastSegment;
            if (lastSegment is null)
                return;

            int ax = (int)Math.Floor(lastSegment.End.X);
            int ay = (int)Math.Floor(lastSegment.End.Y);
            int az = (int)Math.Floor(lastSegment.End.Z);

            if (_goal.IsInGoal(ax, ay, az))
                return;

            // Avoid re-planning from the same anchor repeatedly.
            if (_pendingLookaheadAnchor is { } prev && prev == (ax, ay, az))
                return;

            var ctx = new CalculationContext(world, allowParkour: true, allowParkourAscend: true);
            IGoal goal = _goal;
            _debugLog?.Invoke($"[PathMgr] Lookahead plan kicked off from ({ax},{ay},{az})");

            _pendingLookaheadCts = new CancellationTokenSource();
            CancellationToken token = _pendingLookaheadCts.Token;
            _pendingLookaheadAnchor = (ax, ay, az);
            _pendingLookahead = Task.Run(() =>
            {
                var finder = new AStarPathFinder { DebugLog = _debugLog };
                return finder.Calculate(ctx, ax, ay, az, goal, token, ReplanTimeoutMs);
            }, token);
        }

        private void DrainPendingLookahead()
        {
            if (_pendingLookahead is null || !_pendingLookahead.IsCompleted)
                return;

            Task<PathResult> task = _pendingLookahead;
            _pendingLookahead = null;
            var cts = _pendingLookaheadCts;
            _pendingLookaheadCts = null;
            cts?.Dispose();

            if (task.IsFaulted || task.IsCanceled)
                return;

            PathResult result = task.Result;
            if (result.Status == PathStatus.Failed || result.Path.Count < 2)
                return;

            // The lookahead should continue from the anchor point. If the live
            // executor is still running, splice the prepared plan as _nextExecutor
            // so it can take over without a wait.
            var segments = PathSegmentBuilder.FromPath(result.Path);
            _nextExecutor = new PathExecutor(segments, _debugLog, _observer);
            _debugLog?.Invoke($"[PathMgr] Lookahead ready: {segments.Count} segments spliced into _nextExecutor");
        }

        private void DiscardLookahead()
        {
            _pendingLookaheadCts?.Cancel();
            _pendingLookaheadCts?.Dispose();
            _pendingLookaheadCts = null;
            _pendingLookahead = null;
            _pendingLookaheadAnchor = null;
        }

        private void CancelPendingTasks()
        {
            _pendingReplanCts?.Cancel();
            _pendingReplanCts?.Dispose();
            _pendingReplanCts = null;
            _pendingReplan = null;

            DiscardLookahead();
        }

    }
}