using System;
using System.Collections.Generic;
using System.Linq;

using Microsoft.Msagl.Core.Geometry;
using Microsoft.Msagl.Routing.Visibility;

namespace Microsoft.Msagl.Routing.Rectilinear {
    internal class MsmtRectilinearPath {
        private readonly double bendPenaltyAsAPercentageOfDistance = SsstRectilinearPath.DefaultBendPenaltyAsAPercentageOfDistance;

        // Waypoints define path stages: source -> wp1, wp1 -> wp2 ... wpn -> target.
        class WaypointEntry {
            internal readonly VisibilityVertex[] waypointVector = new VisibilityVertex[1];
            internal readonly VertexEntry[] entryVector = new VertexEntry[4];

            internal WaypointEntry(VisibilityVertex waypoint) {
                this.waypointVector[0] = waypoint;
            }
        }

        // Temporary for accumulating target entries.
        private readonly VertexEntry[] currentPassTargetEntries = new VertexEntry[4];

        public MsmtRectilinearPath(double bendPenalty) {
            this.bendPenaltyAsAPercentageOfDistance = bendPenalty;
        }

        /// <summary>
        /// Get the lowest-cost path from one of one or more sources to one of one or more targets, without waypoints.
        /// </summary>
        /// <param name="sources">One or more source vertices</param>
        /// <param name="targets">One or more target vertices</param>
        /// <returns>A single enumeration of path points.</returns>
        internal IEnumerable<Point> GetPath(IEnumerable<VisibilityVertex> sources, IEnumerable<VisibilityVertex> targets) {
            return SsstRectilinearPath.RestorePath(GetPathStage(null, sources, null, targets));
        }

        /// <summary>
        /// Get the lowest-cost path from one of one or more sources to one of one or more targets, with waypoints.
        /// </summary>
        /// <param name="sources">One or more source vertices</param>
        /// <param name="waypoints">One or more waypoint vertices</param>
        /// <param name="targets">One or more target vertices</param>
        /// <returns>An enumeration of each stage's enumeration of path points</returns>
        internal IEnumerable<IEnumerable<Point>> GetPath(IEnumerable<VisibilityVertex> sources, IEnumerable<VisibilityVertex> waypoints,
                                            IEnumerable<VisibilityVertex> targets) {
            var waypointEntries = new List<WaypointEntry>();

            var waypointEnum = waypoints.GetEnumerator();
            waypointEnum.MoveNext();
            var sourceWaypointEntry = new WaypointEntry(waypointEnum.Current);
            waypointEntries.Add(sourceWaypointEntry);

            VertexEntry lastEntry;
            if (!GetFirstPathStage(sourceWaypointEntry, sources)
                    || !GetWaypointPathStages(waypointEnum, waypointEntries, ref sourceWaypointEntry)
                    || !GetLastPathStage(sourceWaypointEntry, targets, out lastEntry)) {
                return null;
            }
            return RestorePathStages(waypointEntries, lastEntry);
        }

        private bool GetFirstPathStage(WaypointEntry sourceWaypointEntry, IEnumerable<VisibilityVertex> sources) {
            // Get the first stage: no prior entries to the sources; this target waypoint becomes the first waypoint stage's source.
            return null != this.GetPathStage(null, sources, sourceWaypointEntry.entryVector, sourceWaypointEntry.waypointVector);
        }

        private bool GetWaypointPathStages(IEnumerator<VisibilityVertex> waypointEnum, List<WaypointEntry> waypointEntries, ref WaypointEntry sourceWaypointEntry) {
            // Get the path stages between each waypoint: send the prior entries to source and get the entries to target.
            while (waypointEnum.MoveNext()) {
                var targetWaypointEntry = new WaypointEntry(waypointEnum.Current);
                waypointEntries.Add(targetWaypointEntry);

                if (null == this.GetPathStage(sourceWaypointEntry.entryVector, sourceWaypointEntry.waypointVector,
                        targetWaypointEntry.entryVector, targetWaypointEntry.waypointVector)) {
                    return false;
                }
                sourceWaypointEntry = targetWaypointEntry;
            }
            return true;
        }

        private bool GetLastPathStage(WaypointEntry sourceWaypointEntry, IEnumerable<VisibilityVertex> targets, out VertexEntry lastEntry) {
            // Final stage: no need to get separate entries to the targets.  And because we send null for the
            // target entry vector, this will return the final vertexEntry of the path.
            lastEntry = this.GetPathStage(sourceWaypointEntry.entryVector, sourceWaypointEntry.waypointVector, null, targets);
            return lastEntry != null;
        }

        private static IEnumerable<IEnumerable<Point>> RestorePathStages(List<WaypointEntry> waypointEntries, VertexEntry lastEntry) {
            // Back up to restore the path stages back to each waypoint.
            var paths = new List<IEnumerable<Point>>();
            waypointEntries.Reverse();
            foreach (var waypointEntry in waypointEntries) {
                paths.Add(SsstRectilinearPath.RestorePath(ref lastEntry, waypointEntry.waypointVector[0]));
            }

            // Add the first stage.
            paths.Add(SsstRectilinearPath.RestorePath(lastEntry));
            paths.Reverse();
            return paths;
        }

        /// <summary>
        /// Route a single stage of a possibly multi-stage (due to waypoints) path.
        /// </summary>
        /// <param name="sourceVertexEntries">The VertexEntry array that was in the source vertex if it was the target of a prior stage.</param>
        /// <param name="sources">The enumeration of source vertices; must be only one if sourceVertexEntries is non-null.</param>
        /// <param name="targets">The enumeration of target vertex entries; must be only one if targetVertexEntries is non-null.</param>
        /// <param name="targetVertexEntries">The VertexEntry array that is in the target at the end of the stage.</param>
        private VertexEntry GetPathStage(VertexEntry[] sourceVertexEntries, IEnumerable<VisibilityVertex> sources,
                                                VertexEntry[] targetVertexEntries, IEnumerable<VisibilityVertex> targets) {
            var ssstCalculator = new SsstRectilinearPath();
            VertexEntry bestEntry = null;

            // This contains the best (lowest) path cost after normalizing origins to the center of the sources
            // and targets.  This is used to avoid selecting a vertex pair whose path has more bends than another pair of
            // vertices, but the bend penalty didn't total enough to offset the additional length between the "better" pair.
            // This also plays the role of an upper bound on the path length; if a path cost is greater than adjustedMinCost 
            // then we stop exploring it, which saves considerable time after low-cost paths have been found.
            double bestCost = double.MaxValue / ScanSegment.OverlappedWeight;
            double bestPathCostRatio = double.PositiveInfinity;

            // Calculate the bend penalty multiplier.  This is a percentage of the distance between the source and target,
            // so that we have the same relative importance if we have objects of about size 20 that are about 100 apart
            // as for objects of about size 200 that are about 1000 apart.
            Point sourceCenter = GetBarycenterOfUniquePortLocations(sources);
            Point targetCenter = GetBarycenterOfUniquePortLocations(targets);
            var distance = SsstRectilinearPath.ManhattanDistance(sourceCenter, targetCenter);
            ssstCalculator.BendsImportance = Math.Max(0.001, distance * (this.bendPenaltyAsAPercentageOfDistance * 0.01));

            // We'll normalize by adding (a proportion of) the distance (only; not bends) from the current endpoints to
            // their centers. This is similar to routeToCenter, but routing multiple paths like this means we'll always
            // get at least a tie for the best vertex pair, whereas routeToCenter can introduce extraneous bends
            // if the sources/targets are not collinear with the center (such as an E-R diagram).
            // interiorLengthAdjustment is a way to decrease the cost adjustment slightly to allow a bend if it saves moving
            // a certain proportion of the distance parallel to the object before turning to it.
            var interiorLengthAdjustment = ssstCalculator.LengthImportance;

            // VertexEntries for the current pass of the current stage, if multistage.
            var tempTargetEntries = (targetVertexEntries != null) ? this.currentPassTargetEntries : null;

            // Process closest pairs first, so we can skip longer ones (jump out of SsstRectilinear sooner, often immediately).
            // This means that we'll be consistent on tiebreaking for equal scores with differing bend counts (the shorter
            // path will win).  In overlapped graphs the shortest path may have more higher-weight edges. 
            foreach (var pair in
                    from VisibilityVertexRectilinear source in sources
                    from VisibilityVertexRectilinear target in targets
                    orderby SsstRectilinearPath.ManhattanDistance(source.Point, target.Point)
                    select new { sourceV = source, targetV = target }) {
                var source = pair.sourceV;
                var target = pair.targetV;
                if (PointComparer.Equal(source.Point, target.Point)) {
                    continue;
                }
                var sourceCostAdjustment = SsstRectilinearPath.ManhattanDistance(source.Point, sourceCenter) * interiorLengthAdjustment;
                var targetCostAdjustment = SsstRectilinearPath.ManhattanDistance(target.Point, targetCenter) * interiorLengthAdjustment;

                var adjustedBestCost = bestCost;
                if (targetVertexEntries != null) {
                    Array.Clear(tempTargetEntries, 0, tempTargetEntries.Length);
                    adjustedBestCost = ssstCalculator.MultistageAdjustedCostBound(bestCost);
                }
                VertexEntry lastEntry = ssstCalculator.GetPathWithCost(sourceVertexEntries, source, sourceCostAdjustment,
                                                                        tempTargetEntries, target, targetCostAdjustment, 
                                                                        adjustedBestCost);
                if (tempTargetEntries != null) {
                    UpdateTargetEntriesForEachDirection(targetVertexEntries, tempTargetEntries, ref bestCost, ref bestEntry);
                    continue;
                }

                // This is the final (or only) stage. Break ties by picking the lowest ratio of cost to ManhattanDistance between the endpoints.
                if (lastEntry == null) {
                    continue;
                }
                var costRatio = lastEntry.Cost / SsstRectilinearPath.ManhattanDistance(source.Point, target.Point);
                if ((lastEntry.Cost < bestCost) || ApproximateComparer.Close(lastEntry.Cost, bestCost) && (costRatio < bestPathCostRatio)) {
                    bestCost = lastEntry.Cost;
                    bestEntry = lastEntry;
                    bestPathCostRatio = lastEntry.Cost / SsstRectilinearPath.ManhattanDistance(source.Point, target.Point);
                }
            }
            return bestEntry;
        }

        private static void UpdateTargetEntriesForEachDirection(VertexEntry[] targetVertexEntries, VertexEntry[] tempTargetEntries,
                            ref double bestCost, ref VertexEntry bestEntry) {
            for (int ii = 0; ii < tempTargetEntries.Length; ++ii) {
                var tempEntry = tempTargetEntries[ii];
                if (tempEntry == null) {
                    continue;
                }
                if ((targetVertexEntries[ii] == null) || (tempEntry.Cost < targetVertexEntries[ii].Cost)) {
                    targetVertexEntries[ii] = tempEntry;
                    if (tempEntry.Cost < bestCost) {
                        // This does not have the ratio tiebreaker because the individual stage path is only used as a success indicator.
                        bestCost = tempEntry.Cost;
                        bestEntry = tempEntry;
                    }
                }
            }
            return;
        }

        private static Point GetBarycenterOfUniquePortLocations(IEnumerable<VisibilityVertex> vertices) {
            var center = new Point();
            VisibilityVertex prevVertex = null;
            int count = 0;
            foreach (var vertex in vertices.OrderBy(s => s.Point)) {
                if ((prevVertex != null) && ApproximateComparer.CloseIntersections(vertex.Point, prevVertex.Point)) {
                    continue;
                }
                prevVertex = vertex;
                ++count;
                center += vertex.Point;
            }
            return center / count;
        }
    }
}