import java.util.LinkedList;
import java.util.Queue;

public class GraphAdjacentMatrix_TopologicalSort_BFS {

    private boolean adjMatrix[][];
    private int numVertices;
    private boolean visited[];

    public GraphAdjacentMatrix_TopologicalSort_BFS(int numVertices) {
        this.numVertices = numVertices;
        adjMatrix = new boolean[numVertices][numVertices];
        visited = new boolean[numVertices];
    }

    public void setEdge(int i, int j) {
        adjMatrix[i][j] = true;
//        adjMatrix[j][i] = true; // directed
    }

    public void delEdge(int i, int j) {
        adjMatrix[i][j] = false;
//        adjMatrix[j][i] = false; // directed
    }

    public boolean isEdge(int i, int j) {
        return adjMatrix[i][j];
    }

    public int first(int v1){
        boolean[] ary = this.adjMatrix[v1];
        int firstNeighborIndex = this.numVertices; // return n if none
        for (int i = 0; i < ary.length; i++) {
            if (ary[i] != false){
                firstNeighborIndex = i;
                break;
            }
        }
        return firstNeighborIndex;
    }

    public int next(int v1, int v2){
        boolean[] ary = this.adjMatrix[v1];
        int nextNeighborIndex = this.numVertices; // return n if none
        for (int i = v2 + 1; i < ary.length; i++) {
            if (ary[i] != false){
                nextNeighborIndex = i;
                break;
            }
        }
        return nextNeighborIndex; // return n if none
    }

    // we are processing an "directed" graph here
    public void topsort_BFS(){
        int[] prerequisiteCount = new int[this.numVertices];
        Queue<Integer> queue = new LinkedList<>();

        // add prerequisites - process every edge
        for (int v = 0; v < this.numVertices; v++) {
            for (int nextV = this.first(v); nextV < this.numVertices; nextV = this.next(v, nextV)) {
                prerequisiteCount[nextV]++;
            }
        }

        // add vertices that has no prerequisites now
        for (int v = 0; v < this.numVertices; v++) {
            if (prerequisiteCount[v] == 0) {
                queue.add(v);
            }
        }

        while(queue.peek() != null){
            Integer currV = queue.remove();
            Previsit(adjMatrix, currV);
            for (int nextV = this.first(currV); nextV < this.numVertices; nextV = this.next(currV, nextV)) {
                prerequisiteCount[nextV]--; // becuase the current vertes v is being proccessed/taken already
                if (prerequisiteCount[nextV] == 0) {
                    queue.add(nextV); // it means no prerequisite left, so we are good to go with this course
                }
            }
            Postvisit(adjMatrix, currV);
        }
    }

    private void Previsit(boolean[][] graph, int v) {
        System.out.println("Previsit: " + v);
    }

    private void Postvisit(boolean[][] graph, int v) {
        System.out.println("Postvisit: " + v);
    }

    public String toString() {
        StringBuilder s = new StringBuilder();
        for (int i = 0; i < numVertices; i++) {
            s.append(i + ": ");
            for (boolean j : adjMatrix[i]) {
                s.append((j?1:0) + " ");
            }
            s.append("\n");
        }
        return s.toString();
    }

    public static void main(String[] args) {
        GraphAdjacentMatrix_TopologicalSort_BFS gam = new GraphAdjacentMatrix_TopologicalSort_BFS(8);

        // directed graph
        gam.setEdge(1, 2);
        gam.setEdge(1, 3);
        gam.setEdge(2, 6);
        gam.setEdge(2, 5);
        gam.setEdge(2, 4);
        gam.setEdge(3, 4);
        gam.setEdge(4, 5);
        gam.setEdge(5, 7);

        gam.topsort_BFS();

        System.out.println();
    }

}