package algs42;
import stdlib.*;
import algs13.Queue;
/* ***********************************************************************
 *  Compilation:  javac KosarajuSharirSCC.java
 *  Execution:    java KosarajuSharirSCC filename.txt
 *  Dependencies: Digraph.java TransitiveClosure.java StdOut.java In.java
 *  Data files:   http://algs4.cs.princeton.edu/42directed/tinyDG.txt
 *
 *  Compute the strongly-connected components of a digraph using the
 *  Kosaraju-Sharir algorithm.
 *
 *  Runs in O(E + V) time.
 *
 *  % java KosarajuSCC tinyDG.txt
 *  5 components
 *  1
 *  0 2 3 4 5
 *  9 10 11 12
 *  6
 *  7 8
 *
 *  % java KosarajuSharirSCC mediumDG.txt
 *  10 components
 *  21
 *  2 5 6 8 9 11 12 13 15 16 18 19 22 23 25 26 28 29 30 31 32 33 34 35 37 38 39 40 42 43 44 46 47 48 49
 *  14
 *  3 4 17 20 24 27 36
 *  41
 *  7
 *  45
 *  1
 *  0
 *  10
 *
 *************************************************************************/

public class XKosarajuSharirReverseSCC {
  private final boolean[] marked;     // marked[v] = has vertex v been visited?
  private final int[] id;             // id[v] = id of strong component containing v
  private int count;                  // number of strongly-connected components


  public XKosarajuSharirReverseSCC(Digraph G) {

    // compute reverse postorder of graph
    Digraph R = G.reverse ();
    DepthFirstOrder dfs = new DepthFirstOrder(G);

    // run DFS on G.reverse(), using reverse postorder to guide calculation
    marked = new boolean[G.V()];
    id = new int[G.V()];
    for (int v : dfs.reversePost()) {
      if (!marked[v]) {
        dfs(R, v);
        count++;
      }
    }

    // check that id[] gives strong components
    assert check(G);
  }

  // DFS on graph G
  private void dfs(Digraph G, int v) {
    marked[v] = true;
    id[v] = count;
    for (int w : G.adj(v)) {
      if (!marked[w]) dfs(G, w);
    }
  }

  // return the number of strongly connected components
  public int count() { return count; }

  // are v and w strongly connected?
  public boolean stronglyConnected(int v, int w) {
    return id[v] == id[w];
  }

  // id of strong component containing v
  public int id(int v) {
    return id[v];
  }

  // does the id[] array contain the strongly connected components?
  private boolean check(Digraph G) {
    TransitiveClosure tc = new TransitiveClosure(G);
    for (int v = 0; v < G.V(); v++) {
      for (int w = 0; w < G.V(); w++) {
        if (stronglyConnected(v, w) != (tc.reachable(v, w) && tc.reachable(w, v)))
          return false;
      }
    }
    return true;
  }

  public static void main(String[] args) {
    //args = new String[] { "data/tinyDG.txt" };
    args = new String[] { "data/mediumDG.txt" };

    In in = new In(args[0]);
    Digraph G = DigraphGenerator.fromIn(in);
    XKosarajuSharirReverseSCC scc = new XKosarajuSharirReverseSCC(G);
    if (!scc.check(G)) throw new Error ();

    // number of connected components
    int M = scc.count();
    StdOut.println(M + " components");

    // compute list of vertices in each strong component
    @SuppressWarnings("unchecked")
    Queue<Integer>[] components = new Queue[M];
    for (int i = 0; i < M; i++) {
      components[i] = new Queue<>();
    }
    for (int v = 0; v < G.V(); v++) {
      components[scc.id(v)].enqueue(v);
    }

    // print results
    for (int i = 0; i < M; i++) {
      for (int v : components[i]) {
        StdOut.print(v + " ");
      }
      StdOut.println();
    }

  }

}