using System;
using System.Collections.Generic;
using IjwFramework.Collections;
using System.Linq;
using OpenRa.FileFormats;
using OpenRa.Game.Graphics;

namespace OpenRa.Game
{
	class PathFinder
	{
		double[][,] passableCost = new double[4][,];
		Map map;

		public PathFinder(Map map, TileSet tileSet)
		{
			this.map = map;

			for (var umt = UnitMovementType.Foot; umt <= UnitMovementType.Float; umt++)
				passableCost[(int)umt] = new double[128, 128];
			for( int x = 0 ; x < 128 ; x++ )
				for( int y = 0 ; y < 128 ; y++ )
					for (var umt = UnitMovementType.Foot; umt <= UnitMovementType.Float; umt++ )
						passableCost[(int)umt][ x, y ] = ( map.IsInMap( x, y ) )
							? TerrainCosts.Cost( umt, tileSet.GetWalkability( map.MapTiles[ x, y ] ) )
							: double.PositiveInfinity;
		}

		public List<int2> FindUnitPath(int2 src, int2 dest, UnitMovementType umt)
		{
			return FindUnitPath(src, DefaultEstimator(dest), umt);
		}

		public List<int2> FindUnitPathToRange(int2 src, int2 dest, UnitMovementType umt, int range)
		{
			var tilesInRange = Game.FindTilesInCircle(dest, range)
				.Where(t => Game.IsCellBuildable(t, umt))
				.Select(t => t + map.Offset);

			var path = FindUnitPath(tilesInRange, DefaultEstimator(dest), umt);
			path.Reverse();
			return path;
		}

		List<int2> FindUnitPath( int2 unitLocation, Func<int2,double> estimator, UnitMovementType umt )
		{
			var startLocation = unitLocation + map.Offset;
			return FindUnitPath( new[] {startLocation}, estimator, umt );
		}

		List<int2> FindUnitPath(IEnumerable<int2> startLocations, Func<int2, double> estimator, UnitMovementType umt)
		{
			var cellInfo = new CellInfo[128, 128];
			var offset = map.Offset;

			for (int x = 0; x < 128; x++)
				for (int y = 0; y < 128; y++)
					cellInfo[x, y] = new CellInfo(double.PositiveInfinity, new int2(x, y), false);

			var queue = new PriorityQueue<PathDistance>();

			foreach (var sl in startLocations)
			{
				queue.Add(new PathDistance(estimator(sl - offset), sl));
				cellInfo[sl.X, sl.Y].MinCost = 0;
			}

			while( !queue.Empty )
			{
				PathDistance p = queue.Pop();
				int2 here = p.Location;
				cellInfo[ here.X, here.Y ].Seen = true;

				if( estimator( here - offset ) == 0.0 )
					return MakePath( cellInfo, here, offset );

				foreach( int2 d in Util.directions )
				{
					int2 newHere = here + d;

					if( cellInfo[ newHere.X, newHere.Y ].Seen )
						continue;
					if( passableCost[(int)umt][ newHere.X, newHere.Y ] == double.PositiveInfinity )
						continue;
					if (Game.BuildingInfluence.GetBuildingAt(newHere - offset) != null)
						continue;

					double cellCost = ( ( d.X * d.Y != 0 ) ? 1.414213563 : 1.0 ) * passableCost[(int)umt][ newHere.X, newHere.Y ];
					double newCost = cellInfo[ here.X, here.Y ].MinCost + cellCost;

					if( newCost >= cellInfo[ newHere.X, newHere.Y ].MinCost )
						continue;

					cellInfo[ newHere.X, newHere.Y ].Path = here;
					cellInfo[ newHere.X, newHere.Y ].MinCost = newCost;

					queue.Add( new PathDistance( newCost + estimator( newHere - offset ), newHere ) );
				}
			}

			// no path exists
			return new List<int2>();
		}

		List<int2> MakePath( CellInfo[ , ] cellInfo, int2 destination, int2 offset )
		{
			List<int2> ret = new List<int2>();
			int2 pathNode = destination;

			while( cellInfo[ pathNode.X, pathNode.Y ].Path != pathNode )
			{
				ret.Add( pathNode - offset );
				pathNode = cellInfo[ pathNode.X, pathNode.Y ].Path;
			}

			return ret;
		}

		static Func<int2, double> DefaultEstimator(int2 destination)
		{
			return here =>
			{
				int2 d = ( here - destination ).Abs();
				int diag = Math.Min( d.X, d.Y );
				int straight = Math.Abs( d.X - d.Y );
				return 1.5 * diag + straight;
			};
		}
	}

	struct CellInfo
	{
		public double MinCost;
		public int2 Path;
		public bool Seen;

		public CellInfo( double minCost, int2 path, bool seen )
		{
			MinCost = minCost;
			Path = path;
			Seen = seen;
		}
	}

	struct PathDistance : IComparable<PathDistance>
	{
		public double EstTotal;
		public int2 Location;

		public PathDistance(double estTotal, int2 location)
		{
			EstTotal = estTotal;
			Location = location;
		}

		public int CompareTo(PathDistance other)
		{
			return Math.Sign(EstTotal - other.EstTotal);
		}
	}
}