/*

Copyright (C) 2007, 2008 Michael Weitzel

This file is part of Octave.

Octave is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by the
Free Software Foundation; either version 3 of the License, or (at your
option) any later version.

Octave is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
for more details.

You should have received a copy of the GNU General Public License
along with Octave; see the file COPYING.  If not, see
<http://www.gnu.org/licenses/>.

*/

/*
An implementation of the Reverse Cuthill-McKee algorithm (symrcm)

The implementation of this algorithm is based in the descriptions found in

@INPROCEEDINGS{,
	author = {E. Cuthill and J. McKee},
	title = {Reducing the Bandwidth of Sparse Symmetric Matrices},
	booktitle = {Proceedings of the 24th ACM National Conference},
	publisher = {Brandon Press},
	pages = {157 -- 172},
	location = {New Jersey},
	year = {1969}
}

@BOOK{,
	author = {Alan George and Joseph W. H. Liu},
	title = {Computer Solution of Large Sparse Positive Definite Systems},
	publisher = {Prentice Hall Series in Computational Mathematics},
	ISBN = {0-13-165274-5},
	year = {1981}
}

The algorithm represents a heuristic approach to the NP-complete minimum
bandwidth problem.

Written by Michael Weitzel <michael.weitzel@@uni-siegen.de>
                           <weitzel@@ldknet.org>
*/

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include "ov.h"
#include "defun-dld.h"
#include "error.h"
#include "gripes.h"
#include "utils.h"
#include "oct-locbuf.h"

#include "ov-re-mat.h"
#include "ov-re-sparse.h"
#include "ov-cx-sparse.h"
#include "oct-sparse.h"

// A node struct for the Cuthill-McKee algorithm
struct CMK_Node
{
  // the node's id (matrix row index)
  octave_idx_type id;
  // the node's degree
  octave_idx_type deg;
  // minimal distance to the root of the spanning tree
  octave_idx_type dist;
};

// A simple queue.
// Queues Q have a fixed maximum size N (rows,cols of the matrix) and are
// stored in an array. qh and qt point to queue head and tail.

// Enqueue operation (adds a node "o" at the tail)

inline static void 
Q_enq (CMK_Node *Q, octave_idx_type N, octave_idx_type& qt, const CMK_Node& o)
{	
  Q[qt] = o;
  qt = (qt + 1) % (N + 1);
}

// Dequeue operation (removes a node from the head)

inline static CMK_Node 
Q_deq (CMK_Node * Q, octave_idx_type N, octave_idx_type& qh)
{
  CMK_Node r = Q[qh];
  qh = (qh + 1) % (N + 1);
  return r;
}

// Predicate (queue empty)
#define Q_empty(Q, N, qh, qt)	((qh) == (qt))

// A simple, array-based binary heap (used as a priority queue for nodes)

// the left descendant of entry i
#define LEFT(i)		(((i) << 1) + 1)	// = (2*(i)+1)
// the right descendant of entry i
#define RIGHT(i)	(((i) << 1) + 2)	// = (2*(i)+2)
// the parent of entry i
#define PARENT(i)	(((i) - 1) >> 1)	// = floor(((i)-1)/2)

// Builds a min-heap (the root contains the smallest element). A is an array
// with the graph's nodes, i is a starting position, size is the length of A.

static void 
H_heapify_min (CMK_Node *A, octave_idx_type i, octave_idx_type size)
{
  octave_idx_type j = i;
  for (;;)
    {
      octave_idx_type l = LEFT(j);
      octave_idx_type r = RIGHT(j);

      octave_idx_type smallest;
      if (l < size && A[l].deg < A[j].deg)
	smallest = l;
      else
	smallest = j;

      if (r < size && A[r].deg < A[smallest].deg)
	smallest = r;

      if (smallest != j)
	{
	  CMK_Node tmp = A[j];
	  A[j] = A[smallest];
	  A[smallest] = tmp;
	  j = smallest;
	}
      else 
	break;
    }
}

// Heap operation insert. Running time is O(log(n))

static void 
H_insert (CMK_Node *H, octave_idx_type& h, const CMK_Node& o)
{
  octave_idx_type i = h++;

  H[i] = o;

  if (i == 0) 
    return;
  do
    {
      octave_idx_type p = PARENT(i);
      if (H[i].deg < H[p].deg)
	{
	  CMK_Node tmp = H[i];
	  H[i] = H[p];
	  H[p] = tmp;

	  i = p;
	}
      else 
	break;
    }
  while (i > 0);
}

// Heap operation remove-min. Removes the smalles element in O(1) and
// reorganizes the heap optionally in O(log(n))

inline static CMK_Node 
H_remove_min (CMK_Node *H, octave_idx_type& h, int reorg/*=1*/)
{
  CMK_Node r = H[0];
  H[0] = H[--h];
  if (reorg) 
    H_heapify_min(H, 0, h);
  return r;
}

// Predicate (heap empty)
#define H_empty(H, h)	((h) == 0)

// Helper function for the Cuthill-McKee algorithm. Tries to determine a
// pseudo-peripheral node of the graph as starting node.

static octave_idx_type 
find_starting_node (octave_idx_type N, const octave_idx_type *ridx, 
		    const octave_idx_type *cidx, const octave_idx_type *ridx2, 
		    const octave_idx_type *cidx2, octave_idx_type *D, 
		    octave_idx_type start)
{
  CMK_Node w;

  OCTAVE_LOCAL_BUFFER (CMK_Node, Q, N+1);
  boolNDArray btmp (dim_vector (1, N), false);
  bool *visit = btmp.fortran_vec ();

  octave_idx_type qh = 0;
  octave_idx_type qt = 0;
  CMK_Node x;
  x.id = start;
  x.deg = D[start];
  x.dist = 0;
  Q_enq (Q, N, qt, x);
  visit[start] = true;

  // distance level
  octave_idx_type level = 0;
  // current largest "eccentricity"
  octave_idx_type max_dist = 0;

  for (;;)
    {
      while (! Q_empty (Q, N, qh, qt))
	{
	  CMK_Node v = Q_deq (Q, N, qh);

	  if (v.dist > x.dist || (v.id != x.id && v.deg > x.deg))
	    x = v;

	  octave_idx_type i = v.id;

	  // add all unvisited neighbors to the queue
	  octave_idx_type j1 = cidx[i];
	  octave_idx_type j2 = cidx2[i];
	  while (j1 < cidx[i+1] || j2 < cidx2[i+1])
	    {
	      OCTAVE_QUIT;

	      if (j1 == cidx[i+1])
		{
		  octave_idx_type r2 = ridx2[j2++];
		  if (! visit[r2])
		    {
		      // the distance of node j is dist(i)+1
		      w.id = r2;
		      w.deg = D[r2];
		      w.dist = v.dist+1;
		      Q_enq (Q, N, qt, w);
		      visit[r2] = true;

		      if (w.dist > level)
			level = w.dist;
		    }
		}
	      else if (j2 == cidx2[i+1])
		{
		  octave_idx_type r1 = ridx[j1++];
		  if (! visit[r1])
		    {
		      // the distance of node j is dist(i)+1
		      w.id = r1;
		      w.deg = D[r1];
		      w.dist = v.dist+1;
		      Q_enq (Q, N, qt, w);
		      visit[r1] = true;

		      if (w.dist > level)
			level = w.dist;
		    }
		}
	      else
		{
		  octave_idx_type r1 = ridx[j1];
		  octave_idx_type r2 = ridx2[j2];
		  if (r1 <= r2)
		    {
		      if (! visit[r1])
			{
			  w.id = r1;
			  w.deg = D[r1];
			  w.dist = v.dist+1;
			  Q_enq (Q, N, qt, w);
			  visit[r1] = true;

			  if (w.dist > level)
			    level = w.dist;
			}
		      j1++;
		      if (r1 == r2)
			j2++;
		    }
		  else
		    {
		      if (! visit[r2])
			{
			  w.id = r2;
			  w.deg = D[r2];
			  w.dist = v.dist+1;
			  Q_enq (Q, N, qt, w);
			  visit[r2] = true;

			  if (w.dist > level)
			    level = w.dist;
			}
		      j2++;
		    }
		}
	    }
	} // finish of BFS

      if (max_dist < x.dist)
	{
	  max_dist = x.dist;

	  for (octave_idx_type i = 0; i < N; i++)
	    visit[i] = false;

	  visit[x.id] = true;
	  x.dist = 0;
	  qt = qh = 0;
	  Q_enq (Q, N, qt, x);
	}
      else
	break;
    }
  return x.id;
}

// Calculates the node's degrees. This means counting the non-zero elements
// in the symmetric matrix' rows. This works for non-symmetric matrices 
// as well.

static octave_idx_type 
calc_degrees (octave_idx_type N, const octave_idx_type *ridx, 
	      const octave_idx_type *cidx, octave_idx_type *D)
{
  octave_idx_type max_deg = 0;

  for (octave_idx_type i = 0; i < N; i++) 
    D[i] = 0;

  for (octave_idx_type j = 0; j < N; j++)
    {
      for (octave_idx_type i = cidx[j]; i < cidx[j+1]; i++)
	{
	  OCTAVE_QUIT;
	  octave_idx_type k = ridx[i];
	  // there is a non-zero element (k,j)
	  D[k]++;
	  if (D[k] > max_deg) 
	    max_deg = D[k];
	  // if there is no element (j,k) there is one in
	  // the symmetric matrix:
	  if (k != j)
	    {
	      bool found = false;
	      for (octave_idx_type l = cidx[k]; l < cidx[k + 1]; l++)
		{
		  OCTAVE_QUIT;

		  if (ridx[l] == j)
		    {
		      found = true;
		      break;
		    }
		  else if (ridx[l] > j)
		    break;
		}

	      if (! found)
		{
		  // A(j,k) == 0
		  D[j]++;
		  if (D[j] > max_deg) 
		    max_deg = D[j];
		}
	    }
	}
    }
  return max_deg;
}

// Transpose of the structure of a square sparse matrix

static void
transpose (octave_idx_type N, const octave_idx_type *ridx, 
	   const octave_idx_type *cidx, octave_idx_type *ridx2, 
	   octave_idx_type *cidx2)
{
  octave_idx_type nz = cidx[N];

  OCTAVE_LOCAL_BUFFER (octave_idx_type, w, N + 1);
  for (octave_idx_type i = 0; i < N; i++)
    w[i] = 0;
  for (octave_idx_type i = 0; i < nz; i++)
    w[ridx[i]]++;
  nz = 0;
  for (octave_idx_type i = 0; i < N; i++)
    {
      OCTAVE_QUIT;
      cidx2[i] = nz;
      nz += w[i];
      w[i] = cidx2[i];
    }
  cidx2[N] = nz;
  w[N] = nz;

  for (octave_idx_type j = 0; j < N; j++)
    for (octave_idx_type k = cidx[j]; k < cidx[j + 1]; k++)
      {
	OCTAVE_QUIT;
	octave_idx_type q = w [ridx[k]]++;
	ridx2[q] = j;
      }
}

// An implementation of the Cuthill-McKee algorithm.
DEFUN_DLD (symrcm, args, ,
  "-*- texinfo -*-\n\
@deftypefn {Loadable Function} {@var{p} =} symrcm (@var{S})\n\
Symmetric reverse Cuthill-McKee permutation of @var{S}.\n\
Return a permutation vector @var{p} such that\n\
@code{@var{S} (@var{p}, @var{p})} tends to have its diagonal elements\n\
closer to the diagonal than @var{S}.  This is a good preordering for LU\n\
or Cholesky factorization of matrices that come from 'long, skinny'\n\
problems.  It works for both symmetric and asymmetric @var{S}.\n\
\n\
The algorithm represents a heuristic approach to the NP-complete\n\
bandwidth minimization problem.  The implementation is based in the\n\
descriptions found in\n\
\n\
E. Cuthill, J. McKee: Reducing the Bandwidth of Sparse Symmetric\n\
Matrices. Proceedings of the 24th ACM National Conference, 157--172\n\
1969, Brandon Press, New Jersey.\n\
\n\
Alan George, Joseph W. H. Liu: Computer Solution of Large Sparse\n\
Positive Definite Systems, Prentice Hall Series in Computational\n\
Mathematics, ISBN 0-13-165274-5, 1981.\n\
\n\
@seealso{colperm, colamd, symamd}\n\
@end deftypefn")
{
  octave_value retval;
  int nargin = args.length ();

  if (nargin != 1)
    {
      print_usage ();
      return retval;
    }

  octave_value arg = args(0);

  // the parameter of the matrix is converted into a sparse matrix 
  //(if necessary)
  octave_idx_type *cidx;
  octave_idx_type *ridx;
  SparseMatrix Ar;
  SparseComplexMatrix Ac;

  if (arg.is_real_type ())
    {
      Ar = arg.sparse_matrix_value ();
      // Note cidx/ridx are const, so use xridx and xcidx...
      cidx = Ar.xcidx ();
      ridx = Ar.xridx ();
    }
  else
    {
      Ac = arg.sparse_complex_matrix_value ();
      cidx = Ac.xcidx ();
      ridx = Ac.xridx ();
    }

  if (error_state)
    return retval;

  octave_idx_type nr = arg.rows ();
  octave_idx_type nc = arg.columns ();

  if (nr != nc)
    {
      gripe_square_matrix_required ("symrcm");
      return retval;
    }

  if (nr == 0 && nc == 0)
    return octave_value (NDArray (dim_vector (1, 0)));

  // sizes of the heaps
  octave_idx_type s = 0;

  // head- and tail-indices for the queue
  octave_idx_type qt = 0, qh = 0;
  CMK_Node v, w;
  // dimension of the matrix
  octave_idx_type N = nr;

  OCTAVE_LOCAL_BUFFER (octave_idx_type, cidx2, N + 1);
  OCTAVE_LOCAL_BUFFER (octave_idx_type, ridx2, cidx[N]);
  transpose (N, ridx, cidx, ridx2, cidx2);

  // the permutation vector
  NDArray P (dim_vector (1, N));

  // compute the node degrees
  OCTAVE_LOCAL_BUFFER (octave_idx_type, D, N);
  octave_idx_type max_deg = calc_degrees (N, ridx, cidx, D);

  // if none of the nodes has a degree > 0 (a matrix of zeros)
  // the return value corresponds to the identity permutation
  if (max_deg == 0)
    {
      for (octave_idx_type i = 0; i < N; i++) 
	P(i) = i;
      return octave_value (P);
    }

  // a heap for the a node's neighbors. The number of neighbors is
  // limited by the maximum degree max_deg:
  OCTAVE_LOCAL_BUFFER (CMK_Node, S, max_deg);

  // a queue for the BFS. The array is always one element larger than
  // the number of entries that are stored.
  OCTAVE_LOCAL_BUFFER (CMK_Node, Q, N+1);

  // a counter (for building the permutation)
  octave_idx_type c = -1;

  // upper bound for the bandwidth (=quality of solution)
  // initialize the bandwidth of the graph with 0. B contains the
  // the maximum of the theoretical lower limits of the subgraphs
  // bandwidths.
  octave_idx_type B = 0;

  // mark all nodes as unvisited; with the exception of the nodes
  // that have degree==0 and build a CC of the graph.
	
  boolNDArray btmp (dim_vector (1, N), false);
  bool *visit = btmp.fortran_vec ();

  do
    {
      // locate an unvisited starting node of the graph
      octave_idx_type i;
      for (i = 0; i < N; i++)
	if (! visit[i]) 
	  break;

      // locate a probably better starting node
      v.id = find_starting_node (N, ridx, cidx, ridx2, cidx2, D, i);
		
      // mark the node as visited and enqueue it (a starting node
      // for the BFS). Since the node will be a root of a spanning
      // tree, its dist is 0.
      v.deg = D[v.id];
      v.dist = 0;
      visit[v.id] = true;
      Q_enq (Q, N, qt, v);

      // lower bound for the bandwidth of a subgraph
      // keep a "level" in the spanning tree (= min. distance to the
      // root) for determining the bandwidth of the computed
      // permutation P
      octave_idx_type Bsub = 0;
      // min. dist. to the root is 0
      octave_idx_type level = 0;
      // the root is the first/only node on level 0
      octave_idx_type level_N = 1;
	
      while (! Q_empty (Q, N, qh, qt))
	{
	  v = Q_deq (Q, N, qh);
	  i = v.id;

	  c++;

	  // for computing the inverse permutation P where
	  // A(inv(P),inv(P)) or P'*A*P is banded
	  //	     P(i) = c;
			
	  // for computing permutation P where
	  // A(P(i),P(j)) or P*A*P' is banded
	  P(c) = i;

	  // put all unvisited neighbors j of node i on the heap
	  s = 0;
	  octave_idx_type j1 = cidx[i];
	  octave_idx_type j2 = cidx2[i];

	  OCTAVE_QUIT;
	  while (j1 < cidx[i+1] || j2 < cidx2[i+1])
	    {
	      OCTAVE_QUIT;
	      if (j1 == cidx[i+1])
		{
		  octave_idx_type r2 = ridx2[j2++];
		  if (! visit[r2])
		    {
		      // the distance of node j is dist(i)+1
		      w.id = r2;
		      w.deg = D[r2];
		      w.dist = v.dist+1;
		      H_insert(S, s, w);
		      visit[r2] = true;
		    }
		}
	      else if (j2 == cidx2[i+1])
		{
		  octave_idx_type r1 = ridx[j1++];
		  if (! visit[r1])
		    {
		      w.id = r1;
		      w.deg = D[r1];
		      w.dist = v.dist+1;
		      H_insert(S, s, w);
		      visit[r1] = true;
		    }
		}
	      else
		{
		  octave_idx_type r1 = ridx[j1];
		  octave_idx_type r2 = ridx2[j2];
		  if (r1 <= r2)
		    {
		      if (! visit[r1])
			{
			  w.id = r1;
			  w.deg = D[r1];
			  w.dist = v.dist+1;
			  H_insert(S, s, w);
			  visit[r1] = true;
			}
		      j1++;
		      if (r1 == r2)
			j2++;
		    }
		  else
		    {
		      if (! visit[r2])
			{
			  w.id = r2;
			  w.deg = D[r2];
			  w.dist = v.dist+1;
			  H_insert(S, s, w);
			  visit[r2] = true;
			}
		      j2++;
		    }
		}
	    }

	  // add the neighbors to the queue (sorted by node degree)
	  while (! H_empty (S, s))
	    {
	      OCTAVE_QUIT;

	      // locate a neighbor of i with minimal degree in O(log(N))
	      v = H_remove_min(S, s, 1);
	
	      // entered the BFS a new level?
	      if (v.dist > level)
		{
		  // adjustment of bandwith:
		  // "[...] the minimum bandwidth that
		  // can be obtained [...] is the
		  //  maximum number of nodes per level"
		  if (Bsub < level_N)
		    Bsub = level_N;
	
		  level = v.dist;
		  // v is the first node on the new level
		  level_N = 1;
		}
	      else
		{
		  // there is no new level but another node on
		  // this level:
		  level_N++;
		}
	
	      // enqueue v in O(1)
	      Q_enq (Q, N, qt, v);
	    }
	
	  // synchronize the bandwidth with level_N once again:
	  if (Bsub < level_N)
	    Bsub = level_N;
	}
      // finish of BFS. If there are still unvisited nodes in the graph
      // then it is split into CCs. The computed bandwidth is the maximum
      // of all subgraphs. Update:
      if (Bsub > B)
	B = Bsub;
    }
  // are there any nodes left?
  while (c+1 < N);

  // compute the reverse-ordering
  s = N / 2 - 1;
  for (octave_idx_type i = 0, j = N - 1; i <= s; i++, j--)
    {
      double tmp = P.elem(i);
      P.elem(i) = P.elem(j);
      P.elem(j) = tmp;
    }

  // increment all indices, since Octave is not C
  return octave_value (P+1);
}
