function [p,q,r,s,cc,rr] = cs_dmperm (A,seed)                               %#ok
%CS_DMPERM maximum matching or Dulmage-Mendelsohn permutation.
%   p = cs_dmperm(A) finds a maximum matching p such that p(j) = i if column j
%   is matched to row i, or 0 if column j is unmatched.  If A is square and
%   full structural rank, p is a row permutation and A(p,:) has a zero-free
%   diagonal.  The structural rank of A is sprank(A) = sum(p>0).
%
%   [p,q,r,s,cc,rr] = cs_dmperm(A) finds the Dulmage-Mendelsohn decomposition
%   of A.  p and q are permutation vectors.  cc and rr are vectors of length 5.
%   C = A(p,q) is split into a 4-by-4 set of coarse blocks:
%
%       A11 A12 A13 A14
%        0  0   A23 A24
%        0  0    0  A34
%        0  0    0  A44
%
%   where A12, A23, and A34 are square with zero-free diagonals.  The columns of
%   A11 are the unmatched columns, and the rows of A44 are the unmatched rows.
%   Any of these blocks can be empty.  In the "coarse" decomposition, the
%   (i,j)th block is C(rr(i):rr(i+1)-1,cc(j):cc(j+1)-1).  In terms of a linear
%   system, [A11 A12] is the underdetermined part of the system (it is always
%   rectangular and with more columns and rows, or 0-by-0), A23 is the well-
%   determined part of the system (it is always square), and [A34 ; A44] is
%   the over-determined part of the system (it is always rectangular with more
%   rows than columns, or 0-by-0).
%
%   The structural rank of A is sprank(A) = rr(4)-1, which is an upper bound on
%   the numerical rank of A.  sprank(A) = rank(full(sprand(A))) with probability
%   1 in exact arithmetic.
%
%   The A23 submatrix is further subdivided into block upper triangular form
%   via the "fine" decomposition (the strongly-connected components of A23).
%   If A is square and structurally non-singular, A23 is the entire matrix.
%
%   C(r(i):r(i+1)-1,s(j):s(j+1)-1) is the (i,j)th block of the fine
%   decomposition.  The (1,1) block is the rectangular block [A11 A12], unless
%   this block is 0-by-0.  The (b,b) block is the rectangular block [A34 ; A44],
%   unless this block is 0-by-0, where b = length(r)-1.  All other blocks of the
%   form C(r(i):r(i+1)-1,s(i):s(i+1)-1) are diagonal blocks of A23, and are
%   square with a zero-free diagonal.
%
%   The matching algorithm used in cs_dmperm can take a very long time
%   in rare cases.  This can be avoided by exploiting a randomized algorithm,
%   with cs_dmperm(A,seed).  If seed=0, the non-randomized algorithm is used
%   (columns are considered in order 1:n).  If seed=-1, columns are considered
%   in reverse order.  Otherwise, the columns are considered in a random order,
%   using seed as the random number generator seed.  Try cs_dmpmerm(A,1) or
%   cs_dmperm(A,rand), for a randomized order, for example.  Seed defaults to 0.
%
%
%   Example:
%       Prob = ssget ('HB/west0479') ; A = Prob.A ;  cspy (A) ;
%       p = cs_dmperm (A) ;
%       cspy (A (p,:)) ;
%       [p q r s cc rr] = cs_dmperm (A) ;
%       cspy (A (p,q)) ;
%       cs_dmspy (A) ;
%
%   See also CS_DMSPY, CS_DMSOL, DMPERM, SPRANK, CS_RANDPERM, RAND

% CXSparse, Copyright (c) 2006-2022, Timothy A. Davis. All Rights Reserved.
% SPDX-License-Identifier: LGPL-2.1+

error ('cs_dmperm mexFunction not found') ;
