package algs42;
import stdlib.*;
import algs13.Queue;
import algs13.Stack;
/* ***********************************************************************
 *  Compilation:  javac GabowSCC.java
 *  Execution:    java GabowSCC V E
 *  Dependencies: Digraph.java Stack.java TransitiveClosure.java StdOut.java
 *
 *  Compute the strongly-connected components of a digraph using
 *  Gabow's algorithm (aka Cheriyan-Mehlhorn algorithm).
 *
 *  Runs in O(E + V) time.
 *
 *  % java GabowSCC tinyDG.txt
 *  5 components
 *  1
 *  0 2 3 4 5
 *  9 10 11 12
 *  6
 *  7 8
 *
 *************************************************************************/

public class XGabowSCC {

  private final boolean[] marked;        // marked[v] = has v been visited?
  private final int[] id;                // id[v] = id of strong component containing v
  private final int[] preorder;          // preorder[v] = preorder of v
  private int pre;                 // preorder number counter
  private int count;               // number of strongly-connected components
  private final Stack<Integer> stack1;
  private final Stack<Integer> stack2;


  public XGabowSCC(Digraph G) {
    marked = new boolean[G.V()];
    stack1 = new Stack<>();
    stack2 = new Stack<>();
    id = new int[G.V()];
    preorder = new int[G.V()];
    for (int v = 0; v < G.V(); v++) id[v] = -1;

    for (int v = 0; v < G.V(); v++) {
      if (!marked[v]) dfs(G, v);
    }

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

  private void dfs(Digraph G, int v) {
    marked[v] = true;
    preorder[v] = pre++;
    stack1.push(v);
    stack2.push(v);
    for (int w : G.adj(v)) {
      if (!marked[w]) dfs(G, w);
      else if (id[w] == -1) {
        while (preorder[stack2.peek()] > preorder[w])
          stack2.pop();
      }
    }

    // found strong component containing v
    if (stack2.peek() == v) {
      stack2.pop();
      int w;
      do {
        w = stack1.pop();
        id[w] = count;
      } while (w != v);
      count++;
    }
  }



  // 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];
  }

  // in which strongly connected component is vertex 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" };

    In in = new In(args[0]);
    Digraph G = DigraphGenerator.fromIn(in);
    XGabowSCC scc = new XGabowSCC(G);

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

    // compute list of vertices in each strong component
    @SuppressWarnings("unchecked")
    final
    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();
    }

  }

}