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

public class XTarjanSCC {

	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[] low;               // low[v] = low number of v
	private int pre;                 // preorder number counter
	private int count;               // number of strongly-connected components
	private final Stack<Integer> stack;


	public XTarjanSCC(Digraph G) {
		marked = new boolean[G.V()];
		stack = new Stack<>();
		id = new int[G.V()];
		low = new int[G.V()];
		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;
		low[v] = pre++;
		int min = low[v];
		stack.push(v);
		for (int w : G.adj(v)) {
			if (!marked[w]) dfs(G, w);
			if (low[w] < min) min = low[w];
		}
		if (min < low[v]) { low[v] = min; return; }
		int w;
		do {
			w = stack.pop();
			id[w] = count;
			low[w] = G.V();
		} 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);
		XTarjanSCC scc = new XTarjanSCC(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();
		}

	}

}