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+## Copyright (C) 2013 Kai T. Ohlhus <k.ohlhus@gmail.com>
+## Copyright (C) 2014 Eduardo Ramos Fernández <eduradical951@gmail.com>
+## 
+## 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/>.
+
+## -*- texinfo -*-
+## @deftypefn  {Function File} ichol (@var{A}, @var{opts})
+## @deftypefnx {Function File} {@var{L} =} ichol (@var{A}, @var{opts})
+##
+## @code{@var{L} = ichol (@var{A})} performs the incomplete Cholesky
+## factorization of A with zero-fill.
+##
+## @code{@var{L} = ichol (@var{A}, @var{opts})} performs the incomplete Cholesky
+## factorization of A with options specified by opts.
+##
+## By default, ichol references the lower triangle of A and produces lower
+## triangular factors.
+##
+## The factor given by this routine may be useful as a preconditioner for a
+## system of linear equations being solved by iterative methods such as
+## PCG (Preconditioned conjugate gradient).
+##
+## ichol works only for sparse square matrices.
+##
+## The fields of @var{opts} must be named exactly as shown below. You can
+## include any number of these fields in the structure and define them in any
+## order. Any additional fields are ignored. Names and specifiers are case
+## sensitive.
+##
+## @table @asis
+## @item type
+## Type of factorization.
+## String indicating which flavor of incomplete Cholesky to perform. Valid
+## values of this field are @samp{nofill} and @samp{ict}. The
+## @samp{nofill} variant performs incomplete Cholesky with zero-fill [IC(0)].
+## The @samp{ict} variant performs incomplete Cholesky with threshold dropping
+## [ICT]. The default value is @samp{nofill}.
+##
+## @item droptol
+## Drop tolerance when type is @samp{ict}.
+## Nonnegative scalar used as a drop tolerance when performing ICT. Elements
+## which are smaller in magnitude than a local drop tolerance are dropped from
+## the resulting factor except for the diagonal element which is never dropped.
+## The local drop tolerance at step j of the factorization is
+## @code{norm (@var{A}(j:end, j), 1) * droptol}. @samp{droptol} is ignored if
+## @samp{type} is @samp{nofill}. The default value is 0.
+##
+## @item michol
+## Indicates whether to perform modified incomplete Cholesky.
+## Indicates whether or not modified incomplete Cholesky [MIC] is performed.
+## The field may be @samp{on} or @samp{off}. When performing MIC, the diagonal
+## is compensated for dropped elements to enforce the relationship
+## @code{@var{A} * @var{e} = @var{L} * @var{L}' * @var{e}} where
+## @code{@var{e} = ones (size (@var{A}, 2), 1))}. The default value is
+## @samp{off}.
+##
+## @item diagcomp
+## Perform compensated incomplete Cholesky with the specified coefficient.
+## Real nonnegative scalar used as a global diagonal shift @code{@var{alpha}}
+## in forming the incomplete Cholesky factor. That is, instead of performing
+## incomplete Cholesky on @code{@var{A}}, the factorization of
+## @code{@var{A} + @var{alpha} * diag (diag (@var{A}))} is formed. The default
+## value is 0.
+##
+## @item shape
+## Determines which triangle is referenced and returned.
+## Valid values are @samp{upper} and @samp{lower}. If @samp{upper} is specified,
+## only the upper triangle of @code{@var{A}} is referenced and @code{@var{R}}
+## is constructed such that @code{@var{A}} is approximated by
+## @code{@var{R}' * @var{R}}. If @samp{lower} is specified, only the lower
+## triangle of @code{@var{A}} is referenced and @code{@var{L}} is constructed
+## such that @code{@var{A}} is approximated by @code{@var{L} * @var{L}'}. The
+## default value is @samp{lower}.
+## @end table
+##
+## EXAMPLES
+##
+## The following problem demonstrates how to factorize a sample symmetric
+## positive definite matrix with the full Cholesky decomposition and with the
+## incomplete one.
+##
+## @example
+## A = [ 0.37, -0.05,  -0.05,  -0.07;
+##      -0.05,  0.116,  0.0,   -0.05;
+##      -0.05,  0.0,    0.116, -0.05;
+##      -0.07, -0.05,  -0.05,   0.202];
+## A = sparse(A);
+## nnz(tril (A))
+## ans =  9
+## L = chol(A, "lower");
+## nnz (L)
+## ans =  10
+## norm (A - L * L', 'fro') / norm (A, 'fro')
+## ans =  1.1993e-16
+## opts.type = 'nofill';
+## L = ichol(A,opts);
+## nnz (L)
+## ans =  9
+## norm (A - L * L', 'fro') / norm (A, 'fro')
+## ans =  0.019736
+## @end example
+##
+## Another example for decomposition is finite difference matrix to solve a
+## boundary value problem on the unit square.
+##
+## @example
+## nx = 400; ny = 200;
+## hx = 1 / (nx + 1); hy = 1 / (ny + 1);
+## Dxx = spdiags ([ones(nx, 1), -2 * ones(nx, 1), ones(nx, 1)], [-1 0 1 ], nx, nx) / (hx ^ 2);
+## Dyy = spdiags ([ones(ny, 1), -2 * ones(ny, 1), ones(ny, 1)], [-1 0 1 ], ny, ny) / (hy ^ 2);
+## A = -kron (Dxx, speye (ny)) - kron (speye (nx), Dyy);
+## nnz (tril (A))
+## ans =  239400
+## opts.type = 'nofill';
+## L = ichol (A, opts);
+## nnz (tril (A))
+## ans =  239400
+## norm (A - L * L', 'fro') / norm (A, 'fro')
+## ans =  0.062327
+## @end example
+##
+## References for the implemented algorithms:
+##
+## [1] Saad, Yousef. "Preconditioning Techniques." Iterative Methods for Sparse Linear
+## Systems. PWS Publishing Company, 1996.
+##
+## [2] Jones, Mark T. and Plassmann, Paul E.: An Improved Incomplete Cholesky
+## Factorization (1992).
+## @end deftypefn
+
+function [L] = ichol (A, opts)
+
+  if ((nargin > 2) || (nargin < 1) || (nargout > 1))
+    print_usage ();
+  endif
+
+  % Check input matrix
+  if (isempty (A) || ~issparse(A) || ~issquare (A))
+    error ("ichol: Input A must be a non-empty sparse square matrix");
+  endif
+
+  % Check input structure, otherwise set default values
+  if (nargin == 2)
+    if (~isstruct (opts))
+      error ("ichol: Input \"opts\" must be a valid structure.");
+    endif
+  else
+    opts = struct ();
+  endif
+
+  if (~isfield (opts, "type"))
+    opts.type = "nofill"; % set default
+  else
+    type = tolower (getfield (opts, "type"));
+    if ((strcmp (type, "nofill") == 0)
+        && (strcmp (type, "ict") == 0))
+      error ("ichol: Invalid field \"type\" in input structure.");
+    else
+      opts.type = type;
+    endif
+  endif
+
+  if (~isfield (opts, "droptol"))
+    opts.droptol = 0; % set default
+  else
+    if (~isscalar (opts.droptol) || (opts.droptol < 0))
+      error ("ichol: Invalid field \"droptol\" in input structure.");
+    endif
+  endif
+
+  michol = "";
+  if (~isfield (opts, "michol"))
+    opts.michol = "off"; % set default
+  else
+    michol = tolower (getfield (opts, "michol"));
+    if ((strcmp (michol, "off") == 0) 
+        && (strcmp (michol, "on") == 0))
+      error ("ichol: Invalid field \"michol\" in input structure.");
+    else
+      opts.michol = michol;
+    endif
+  endif
+
+  if (~isfield (opts, "diagcomp"))
+    opts.diagcomp = 0; % set default
+  else
+    if (~isscalar (opts.diagcomp) || (opts.diagcomp < 0))
+      error ("ichol: Invalid field \"diagcomp\" in input structure.");
+    endif
+  endif
+
+  if (~isfield (opts, "shape"))
+    opts.shape = "lower"; % set default
+  else
+    shape = tolower (getfield (opts, "shape"));
+    if ((strcmp (shape, "lower") == 0) 
+        && (strcmp (shape, "upper") == 0))
+      error ("ichol: Invalid field \"shape\" in input structure.");
+    else
+      opts.shape = shape;
+    endif
+  endif
+
+  % Prepare input for specialized ICHOL
+  A_in = [];
+  if (opts.diagcomp > 0)
+    A += opts.diagcomp * diag (diag (A));
+  endif
+  if (strcmp (opts.shape, "upper") == 1)
+    disp("entro");
+    A_in = triu (A);
+    A_in = A_in';
+
+  else
+    A_in = tril (A);
+  endif
+
+  % Delegate to specialized ICHOL
+  switch (opts.type)
+    case "nofill"
+      L  = ichol0 (A_in,  opts.michol);
+    case "ict"
+      L = icholt (A_in, opts.droptol, opts.michol);
+    otherwise
+      printf ("The input structure is invalid.\n");
+  endswitch
+
+  if (strcmp (opts.shape, "upper") == 1)
+    L = L';
+  endif
+  
+
+endfunction
+
+%!shared A1, A2
+%! A1 = [ 0.37, -0.05,  -0.05,  -0.07;
+%!      -0.05,  0.116,  0.0,   -0.05;
+%!      -0.05,  0.0,    0.116, -0.05;
+%!      -0.07, -0.05,  -0.05,   0.202];
+%! A1 = sparse(A1);
+%! nx = 400; ny = 200;
+%! hx = 1 / (nx + 1); hy = 1 / (ny + 1);
+%! Dxx = spdiags ([ones(nx, 1), -2 * ones(nx, 1), ones(nx, 1)], [-1 0 1 ], nx, nx) / (hx ^ 2);
+%! Dyy = spdiags ([ones(ny, 1), -2 * ones(ny, 1), ones(ny, 1)], [-1 0 1 ], ny, ny) / (hy ^ 2);
+%! A2 = -kron (Dxx, speye (ny)) - kron (speye (nx), Dyy);
+%!
+%!test
+%!error ichol ([]);
+%!error ichol (0);
+%!error ichol (-0);
+%!error ichol (1);
+%!error ichol (-1);
+%!error ichol (i);
+%!error ichol (-i);
+%!error ichol (1 + 1i);
+%!error ichol (1 - 1i);
+%!error ichol (sparse (0));
+%!error ichol (sparse (-0));
+%!error ichol (sparse (-1));
+%!error ichol (sparse (-1));
+%!
+%!test
+%! opts.type = "nofill";
+%! opts.michol = "off";
+%! assert (nnz (tril (A1)), nnz (ichol (A1, opts)));
+%! assert (nnz (tril (A2)), nnz (ichol (A2, opts)));
+%!
+%!test
+%! opts.type = "nofill";
+%! opts.michol = "off";
+%! L = ichol (A1, opts);
+%! assert (norm (A1 - L * L', 'fro') / norm (A1, 'fro'), 0.01, 0.01);
+%! L = ichol (A2, opts);
+%! assert (norm (A2 - L * L', 'fro') / norm (A2, 'fro'), 0.06, 0.01);
+%!
+%%!test
+%%! opts.type = "nofill";
+%%! opts.michol = "off";
+%%! opts.shape = "upper";
+%%! U = ichol (A1, opts);
+%%! assert (norm (A1 - U' * U, 'fro') / norm (A1, 'fro'), 0.01, 0.01);
+%!
+%!test
+%! opts.type = "nofill";
+%! opts.michol = "off";
+%! opts.shape = "lower";
+%! L = ichol (A1, opts);
+%! assert (norm (A1 - L * L', 'fro') / norm (A1, 'fro'), 0.01, 0.01);
+%!
+%!test
+%! opts.type = "nofill";
+%! opts.michol = "on";
+%! L = ichol (A1, opts);
+%! assert (norm (A1 - L * L', 'fro') / norm (A1, 'fro'), 0.02, 0.01);
+%!
+%!test
+%! opts.type = "nofill";
+%! opts.michol = "on";
+%! opts.diagcomp = 3e-3;
+%! L = ichol (A1, opts);
+%! assert (norm (A1 - L * L', 'fro') / norm (A1, 'fro'), 0.02, 0.01);
+%!
+%!test
+%! opts.type = "ict";
+%! opts.michol = "off";
+%! opts.droptol = 1e-4;
+%! L = ichol (A1, opts);
+%! assert (norm (A1 - L * L', 'fro') / norm (A1, 'fro'), eps, eps);
+%!
+%!test
+%! opts.type = "ict";
+%! opts.michol = "off";
+%! opts.droptol = 1e-4;
+%! L = ichol (A2, opts);
+%! assert (norm (A2 - L * L', 'fro') / norm (A2, 'fro'), 5e-4, 5e-4);