MeaningOfLights_dB2GraphQL_Net/src/DatabaseSchemaReader/Utilities/TopologicalSorter.cs
using System;

namespace DatabaseSchemaReader.Utilities
{
    /// <summary>
    /// Sorts a graph by the dependencies.
    /// </summary>
    /// <remarks>
    /// Copied directly from http://tawani.blogspot.com/2009/02/topological-sorting-and-cyclic.html
    /// (in turn from a Java example).
    /// </remarks>
    class TopologicalSorter
    {

        private readonly int[] _vertices; // list of vertices  
        private readonly int[,] _matrix; // adjacency matrix  
        private int _numVerts; // current number of vertices  
        private readonly int[] _sortedArray;

        public TopologicalSorter(int size)
        {
            _vertices = new int[size];
            _matrix = new int[size, size];
            for (int i = 0; i < size; i++)
                for (int j = 0; j < size; j++)
                    _matrix[i, j] = 0;
            _sortedArray = new int[size]; // sorted vert labels  
        }

        public int AddVertex(int vertex)
        {
            _vertices[_numVerts++] = vertex;
            return _numVerts - 1;
        }

        public void AddEdge(int start, int end)
        {
            _matrix[start, end] = 1;
        }

        public int[] Sort() // toplogical sort  
        {
            while (_numVerts > 0) // while vertices remain,  
            {
                // get a vertex with no successors, or -1  
                int currentVertex = NoSuccessors();
                if (currentVertex == -1) // must be a cycle                  
                    throw new InvalidOperationException("Graph has cycles");

                // insert vertex label in sorted array (start at end)  
                _sortedArray[_numVerts - 1] = _vertices[currentVertex];

                DeleteVertex(currentVertex); // delete vertex  
            }

            // vertices all gone; return sortedArray  
            return _sortedArray;
        }

        // returns vert with no successors (or -1 if no such verts)  
        private int NoSuccessors()
        {
            for (int row = 0; row < _numVerts; row++)
            {
                bool isEdge = false; // edge from row to column in adjMat  
                for (int col = 0; col < _numVerts; col++)
                {
                    if (_matrix[row, col] > 0) // if edge to another,  
                    {
                        isEdge = true;
                        break; // this vertex has a successor try another  
                    }
                }
                if (!isEdge) // if no edges, has no successors  
                    return row;
            }
            return -1; // no  
        }

        private void DeleteVertex(int delVert)
        {
            // if not last vertex, delete from vertexList  
            if (delVert != _numVerts - 1)
            {
                for (int j = delVert; j < _numVerts - 1; j++)
                    _vertices[j] = _vertices[j + 1];

                for (int row = delVert; row < _numVerts - 1; row++)
                    MoveRowUp(row, _numVerts);

                for (int col = delVert; col < _numVerts - 1; col++)
                    MoveColLeft(col, _numVerts - 1);
            }
            _numVerts--; // one less vertex  
        }

        private void MoveRowUp(int row, int length)
        {
            for (int col = 0; col < length; col++)
                _matrix[row, col] = _matrix[row + 1, col];
        }

        private void MoveColLeft(int col, int length)
        {
            for (int row = 0; row < length; row++)
                _matrix[row, col] = _matrix[row, col + 1];
        }

    }
}