/***
 * The Directed Sparse Graph Data Structure.
 * 
 * Definition:
 * A sparse graph is a graph G = (V, E) in which |E| = O(|V|).
 * A directed graph is a graph where each edge follow one direction only between any two vertices.
 * A weighted graph is a graph where each edge has a weight (zero weights mean there is no edge).
 * 
 * An adjacency-list weighted digraph representation. Shares a good deal of implemention details 
 * with the Directed Sparse version (DirectedSparseGraph<T>). Edges are instances of WeightedEdge<T> class. 
 * Implements both interfaces: IGraph<T> and IWeightedGraph<T>.
 */

using System;
using System.Collections.Generic;
using DataStructures.Common;
using DataStructures.Lists;

namespace DataStructures.Graphs
{
    public class DirectedWeightedSparseGraph<T> : IGraph<T>, IWeightedGraph<T> where T : IComparable<T>
    {
        /// <summary>
        /// INSTANCE VARIABLES
        /// </summary>
        private const long EMPTY_EDGE_VALUE = 0;
        protected virtual int _edgesCount { get; set; }
        protected virtual T _firstInsertedNode { get; set; }
        protected virtual Dictionary<T, DLinkedList<WeightedEdge<T>>> _adjacencyList { get; set; }


        /// <summary>
        /// CONSTRUCTOR
        /// </summary>
        public DirectedWeightedSparseGraph() : this(10) { }

        public DirectedWeightedSparseGraph(uint initialCapacity)
        {
            _edgesCount = 0;
            _adjacencyList = new Dictionary<T, DLinkedList<WeightedEdge<T>>>((int)initialCapacity);
        }


        /// <summary>
        /// Helper function. Returns edge object from source to destination, if exists; otherwise, null.
        /// </summary>
        protected virtual WeightedEdge<T> _tryGetEdge(T source, T destination)
        {
            WeightedEdge<T> edge = null;

            // Predicate
            var sourceToDestinationPredicate = new Predicate<WeightedEdge<T>>((item) => item.Source.IsEqualTo<T>(source) && item.Destination.IsEqualTo<T>(destination));

            // Try to find a match
            if(_adjacencyList.ContainsKey(source))
                _adjacencyList[source].TryFindFirst(sourceToDestinationPredicate, out edge);
            
            // Return!
            // Might return a null object.
            return edge;
        }

        /// <summary>
        /// Helper function. Checks if edge exist in graph.
        /// </summary>
        protected virtual bool _doesEdgeExist(T source, T destination)
        {
            return _tryGetEdge(source, destination) != null;
        }

        /// <summary>
        /// Helper function. Gets the weight of a directed edge.
        /// Presumes edge does already exist.
        /// </summary>
        private long _getEdgeWeight(T source, T destination)
        {
            return _tryGetEdge(source, destination).Weight;
        }


        /// <summary>
        /// Returns true, if graph is directed; false otherwise.
        /// </summary>
        public virtual bool IsDirected
        {
            get { return true; }
        }

        /// <summary>
        /// Returns true, if graph is weighted; false otherwise.
        /// </summary>
        public virtual bool IsWeighted
        {
            get { return true; }
        }

        /// <summary>
        /// Gets the count of vetices.
        /// </summary>
        public int EdgesCount
        {
            get { return _edgesCount; }
        }

        /// <summary>
        /// Gets the count of edges.
        /// </summary>
        public int VerticesCount
        {
            get { return _adjacencyList.Count; }
        }

        /// <summary>
        /// Returns the list of Vertices.
        /// </summary>
        public IEnumerable<T> Vertices
        {
            get
            {
                foreach (var vertex in _adjacencyList)
                    yield return vertex.Key;
            }
        }


        IEnumerable<IEdge<T>> IGraph<T>.Edges
        {
            get { return this.Edges; }
        }

        IEnumerable<IEdge<T>> IGraph<T>.IncomingEdges(T vertex)
        {
            return this.IncomingEdges(vertex);
        }

        IEnumerable<IEdge<T>> IGraph<T>.OutgoingEdges(T vertex)
        {
            return this.OutgoingEdges(vertex);
        }


        /// <summary>
        /// An enumerable collection of all directed weighted edges in graph.
        /// </summary>
        public virtual IEnumerable<WeightedEdge<T>> Edges
        {
            get
            {
                foreach (var vertex in _adjacencyList)
                    foreach (var edge in vertex.Value)
                        yield return edge;
            }
        }

        /// <summary>
        /// Get all incoming directed weighted edges to a vertex.
        /// </summary>
        public virtual IEnumerable<WeightedEdge<T>> IncomingEdges(T vertex)
        {
            if (!HasVertex(vertex))
                throw new KeyNotFoundException("Vertex doesn't belong to graph.");

            var predicate = new Predicate<WeightedEdge<T>>((edge) => edge.Destination.IsEqualTo(vertex));

            foreach(var adjacent in _adjacencyList.Keys)
            {
                WeightedEdge<T> incomingEdge = null;

                if (_adjacencyList[adjacent].TryFindFirst(predicate, out incomingEdge))
                    yield return incomingEdge;
            }//end-foreach
        }

        /// <summary>
        /// Get all outgoing directed weighted edges from a vertex.
        /// </summary>
        public virtual IEnumerable<WeightedEdge<T>> OutgoingEdges(T vertex)
        {
            if (!HasVertex(vertex))
                throw new KeyNotFoundException("Vertex doesn't belong to graph.");
            
            foreach(var edge in _adjacencyList[vertex])
                yield return edge;
        }


        /// <summary>
        /// Obsolete. Another AddEdge function is implemented with a weight parameter.
        /// </summary>
        [Obsolete("Use the AddEdge method with the weight parameter.")]
        public bool AddEdge(T source, T destination)
        {
            throw new NotImplementedException();
        }

        /// <summary>
        /// Connects two vertices together with a weight, in the direction: first->second.
        /// </summary>
        public bool AddEdge(T source, T destination, long weight)
        {
            // Check existence of nodes, the validity of the weight value, and the non-existence of edge
            if (weight == EMPTY_EDGE_VALUE)
                return false;
            if (!HasVertex(source) || !HasVertex(destination))
                return false;
            if (_doesEdgeExist(source, destination))
                return false;

            // Add edge from source to destination
            var edge = new WeightedEdge<T>(source, destination, weight);
            _adjacencyList[source].Append(edge);

            // Increment edges count
            ++_edgesCount;

            return true;
        }

        /// <summary>
        /// Removes edge, if exists, from source to destination.
        /// </summary>
        public virtual bool RemoveEdge(T source, T destination)
        {
            // Check existence of nodes and non-existence of edge
            if (!HasVertex(source) || !HasVertex(destination))
                return false;

            // Try get edge
            var edge = _tryGetEdge(source, destination);

            // Return false if edge doesn't exists
            if (edge == null)
                return false;

            // Remove edge from source to destination
            _adjacencyList[source].Remove(edge);

            // Decrement the edges count
            --_edgesCount;

            return true;
        }

        public bool UpdateEdgeWeight(T source, T destination, long weight)
        {
            // Check existence of vertices and validity of the weight value
            if (weight == EMPTY_EDGE_VALUE)
                return false;
            if (!HasVertex(source) || !HasVertex(destination))
                return false;

            foreach (var edge in _adjacencyList[source])
            {
                if (edge.Destination.IsEqualTo(destination))
                {
                    edge.Weight = weight;
                    return true;
                }
            }

            return false;
        }

        /// <summary>
        /// Get edge object from source to destination.
        /// </summary>
        public virtual WeightedEdge<T> GetEdge(T source, T destination)
        {
            if (!HasVertex(source) || !HasVertex(destination))
                throw new KeyNotFoundException("Either one of the vertices or both of them don't exist.");

            var edge = _tryGetEdge(source, destination);

            // Check the existence of edge
            if (edge == null)
                throw new Exception("Edge doesn't exist.");

            // Try get edge
            return edge;
        }

        /// <summary>
        /// Returns the edge weight from source to destination.
        /// </summary>
        public virtual long GetEdgeWeight(T source, T destination)
        {
            return GetEdge(source, destination).Weight;
        }

        /// <summary>
        /// Add a collection of vertices to the graph.
        /// </summary>
        public virtual void AddVertices(IList<T> collection)
        {
            if (collection == null)
                throw new ArgumentNullException();

            foreach (var vertex in collection)
                AddVertex(vertex);
        }

        /// <summary>
        /// Add vertex to the graph
        /// </summary>
        public virtual bool AddVertex(T vertex)
        {
            if (_adjacencyList.ContainsKey(vertex))
                return false;

            if (_adjacencyList.Count == 0)
                _firstInsertedNode = vertex;

            _adjacencyList.Add(vertex, new DLinkedList<WeightedEdge<T>>());

            return true;
        }

        /// <summary>
        /// Removes the specified vertex from graph.
        /// </summary>
        public virtual bool RemoveVertex(T vertex)
        {
            // Check existence of vertex
            if (!_adjacencyList.ContainsKey(vertex))
                return false;

            // Subtract the number of edges for this vertex from the total edges count
            _edgesCount = _edgesCount - _adjacencyList[vertex].Count;

            // Remove vertex from graph
            _adjacencyList.Remove(vertex);

            // Remove destination edges to this vertex
            foreach (var adjacent in _adjacencyList)
            {
                var edge = _tryGetEdge(adjacent.Key, vertex);

                if (edge != null)
                {
                    adjacent.Value.Remove(edge);

                    // Decrement the edges count.
                    --_edgesCount;
                }
            }

            return true;
        }

        /// <summary>
        /// Checks whether there is an edge from source to destination.
        /// </summary>
        public virtual bool HasEdge(T source, T destination)
        {
            return (_adjacencyList.ContainsKey(source) && _adjacencyList.ContainsKey(destination) && _doesEdgeExist(source, destination));
        }

        /// <summary>
        /// Checks whether a vertex exists in the graph
        /// </summary>
        public virtual bool HasVertex(T vertex)
        {
            return _adjacencyList.ContainsKey(vertex);
        }

        /// <summary>
        /// Returns the neighbours doubly-linked list for the specified vertex.
        /// </summary>
        public virtual DLinkedList<T> Neighbours(T vertex)
        {
            if (!HasVertex(vertex))
                return null;

            var neighbors = new DLinkedList<T>();
            var adjacents = _adjacencyList[vertex];

            foreach (var adjacent in adjacents)
                neighbors.Append(adjacent.Destination);

            return neighbors;
        }

        /// <summary>
        /// Returns the neighbours of a vertex as a dictionary of nodes-to-weights.
        /// </summary>
        public Dictionary<T, long> NeighboursMap(T vertex)
        {
            if (!HasVertex(vertex))
                return null;

            var neighbors = _adjacencyList[vertex];
            var map = new Dictionary<T, long>(neighbors.Count);

            foreach (var adjacent in neighbors)
                map.Add(adjacent.Destination, adjacent.Weight);

            return map;
        }

        /// <summary>
        /// Returns the degree of the specified vertex.
        /// </summary>
        public virtual int Degree(T vertex)
        {
            if (!HasVertex(vertex))
                throw new KeyNotFoundException();

            return _adjacencyList[vertex].Count;
        }

        /// <summary>
        /// Returns a human-readable string of the graph.
        /// </summary>
        public virtual string ToReadable()
        {
            string output = string.Empty;

            foreach (var node in _adjacencyList)
            {
                var adjacents = string.Empty;

                output = String.Format("{0}\r\n{1}: [", output, node.Key);

                foreach (var adjacentNode in node.Value)
                    adjacents = String.Format("{0}{1}({2}), ", adjacents, adjacentNode.Destination, adjacentNode.Weight);

                if (adjacents.Length > 0)
                    adjacents = adjacents.TrimEnd(new char[] { ',', ' ' });

                output = String.Format("{0}{1}]", output, adjacents);
            }

            return output;
        }

        /// <summary>
        /// A depth first search traversal of the graph starting from the first inserted node.
        /// Returns the visited vertices of the graph.
        /// </summary>
        public virtual IEnumerable<T> DepthFirstWalk()
        {
            return DepthFirstWalk(_firstInsertedNode);
        }

        /// <summary>
        /// A depth first search traversal of the graph, starting from a specified vertex.
        /// Returns the visited vertices of the graph.
        /// </summary>
        public virtual IEnumerable<T> DepthFirstWalk(T source)
        {
            // Check for existence of source
            if (VerticesCount == 0)
                return new ArrayList<T>(0);
            if (!HasVertex(source))
                throw new KeyNotFoundException("The source vertex doesn't exist.");

            var visited = new HashSet<T>();
            var stack = new DataStructures.Lists.Stack<T>();
            var listOfNodes = new ArrayList<T>(VerticesCount);

            stack.Push(source);

            while (!stack.IsEmpty)
            {
                var current = stack.Pop();

                if (!visited.Contains(current))
                {
                    listOfNodes.Add(current);
                    visited.Add(current);

                    foreach (var adjacent in Neighbours(current))
                        if (!visited.Contains(adjacent))
                            stack.Push(adjacent);
                }
            }

            return listOfNodes;
        }

        /// <summary>
        /// A breadth first search traversal of the graphstarting from the first inserted node.
        /// Returns the visited vertices of the graph.
        /// </summary>
        public virtual IEnumerable<T> BreadthFirstWalk()
        {
            return BreadthFirstWalk(_firstInsertedNode);
        }

        /// <summary>
        /// A breadth first search traversal of the graph, starting from a specified vertex.
        /// Returns the visited vertices of the graph.
        /// </summary>
        public virtual IEnumerable<T> BreadthFirstWalk(T source)
        {
            // Check for existence of source
            if (VerticesCount == 0)
                return new ArrayList<T>(0);
            if (!HasVertex(source))
                throw new KeyNotFoundException("The source vertex doesn't exist.");

            var visited = new HashSet<T>();
            var queue = new DataStructures.Lists.Queue<T>();
            var listOfNodes = new ArrayList<T>(VerticesCount);

            listOfNodes.Add(source);
            visited.Add(source);

            queue.Enqueue(source);

            while (!queue.IsEmpty)
            {
                var current = queue.Dequeue();
                var neighbors = Neighbours(current);

                foreach (var adjacent in neighbors)
                {
                    if (!visited.Contains(adjacent))
                    {
                        listOfNodes.Add(adjacent);
                        visited.Add(adjacent);
                        queue.Enqueue(adjacent);
                    }
                }
            }

            return listOfNodes;
        }

        /// <summary>
        /// Clear this graph.
        /// </summary>
        public virtual void Clear()
        {
            _edgesCount = 0;
            _adjacencyList.Clear();
        }

    }

}