## Copyright 2015 Oliver Heimlich
##
## This program 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.
##
## This program 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 this program; if not, see <http://www.gnu.org/licenses/>.

## -*- texinfo -*-
## @documentencoding UTF-8
## @deftypemethod {@@infsup} {[@var{L}, @var{U}] = } lu (@var{A})
## @deftypemethodx {@@infsup} {[@var{L}, @var{U}, @var{P}] = } lu (@var{A})
##
## Compute the LU decomposition of @var{A}.
##
## @var{A} will be a subset of @var{L} * @var{U} with lower triangular matrix
## @var{L} and upper triangular matrix @var{U}.
##
## The result is returned in a permuted form, according to the optional return
## value @var{P}.
##
## Accuracy: The result is a valid enclosure.
## @seealso{@@infsup/qr, @@infsup/chol}
## @end deftypemethod

## Author: Oliver Heimlich
## Keywords: interval
## Created: 2014-10-31

function [L, U, P] = lu (x)

  if (nargin ~= 1)
    print_usage ();
    return
  endif
  if (not (isa (x, "infsup")))
    x = infsup (x);
  endif

  if (isscalar (x))
    L = P = eye (size (x));
    U = x;
    return
  endif

  ## x must be square
  assert (issquare (x.inf), ...
          "operator \: nonconformant arguments, X is not square");

  n = rows (x.inf);

  if (nargout < 3)
    P = eye (n);
  else
    ## Compute P such that the computation of L below will not fail because of
    ## division by zero.  P * x should not have zeros in its main diagonal.
    ## The computation of P is a greedy heuristic algorithm, which I developed
    ## for the implementation of this function.
    P = zeros (n);
    migU = mig (x);
    magU = mag (x);
    ## Empty intervals are as bad as intervals containing only zero.
    migU (isnan (migU)) = 0;
    magU (isnan (magU)) = 0;
    for i = 1 : n
      ## Choose next pivot in one of the columns with the fewest mig (U) > 0.
      columnrating = sum (migU > 0, 1);
      ## Don't choose used columns
      columnrating(max (migU, [], 1) == inf) = inf;

      ## Use first possible column
      possiblecolumns = columnrating == min (columnrating);
      column = find (possiblecolumns, 1);

      if (columnrating(column) >= 1)
        ## Greedy: Use only intervals that do not contain zero.
        possiblerows = migU(:, column) > 0;
      else
        ## Only intervals left which contain zero. Try to use an interval
        ## that additionally contains other numbers.
        possiblerows = migU(:, column) >= 0 & magU(:, column) > 0;
        if (not (max (possiblerows)))
          ## All possible intervals contain only zero.
          possiblerows = migU(:, column) >= 0;
        endif
      endif

      if (sum (possiblerows) == 1)
        ## Short-ciruit: Take the only remaining useful row
        row = find (possiblerows, 1);
      else
        ## There are several good choices, take the one that will hopefully
        ## not hinder the choice of further pivot elements.
        ## That is, the row with the least mig (U) > 0.

        rowrating = sum (migU > 0, 2);
        ## We weight the rating in the columns with few mig (U) > 0 in
        ## order to prevent problems during the choice of pivots in the
        ## near future.
        rowrating += 0.5 * sum (migU(:, possiblecolumns) > 0, 2);
        rowrating (not (possiblerows)) = inf;
        row = find (rowrating == min (rowrating), 1);
      endif

       # assert (0 <= migU (row, column) && migU (row, column) < inf);

      P(row, column) = 1;

      ## In mig (U), for the choice of further pivots:
      ##  - mark used columns with inf
      ##  - mark used rows in unused columns with -inf
      migU(row, :) -= inf;
      migU(isnan (migU)) = inf;
      migU(:, column) = inf;
    endfor
  endif

  ## Initialize L and U
  L = infsup (eye (n));
  U = permute (P, x);

  ## Compute L and U
  varidx.type = rowstart.type = Urefrow.type = Urow.type = "()";
  for i = 1 : (n - 1)
    varidx.subs = {i, i};
    Urefrow.subs = {i, i : n};
    ## Go through rows of the remaining matrix
    for k = (i + 1) : n
      rowstart.subs = {k, i};
      ## Compute L
      Lcurrentelement = mulrev (subsref (U, varidx), subsref (U, rowstart));
      L = subsasgn (L, rowstart, Lcurrentelement);
      ## Go through columns of the remaining matrix
      Urow.subs = {k, i : n};

      ## Compute U
      minuend = subsref (U, Urow);
      subtrahend = Lcurrentelement .* subsref (U, Urefrow);
      U = subsasgn (U, Urow, minuend - subtrahend);
    endfor
  endfor

  ## Cleanup U
  U.inf = triu (U.inf);
  U.sup = triu (U.sup);

endfunction

## Apply permutation matrix P to an interval matrix: B = P * A.
## This is much faster than a matrix product, because the matrix product would
## use a lot of dot products.
function B = permute (P, A)
  ## Note: [B.inf, B.sup] = deal (P * A.inf, P * A.sup) is not possible,
  ## because empty or unbound intervals would create NaNs during
  ## multiplication with P.

  B = A;
  for i = 1 : rows (P)
    targetrow = find (P(i, :) == 1, 1);
    B.inf(targetrow, :) = A.inf(i, :);
    B.sup(targetrow, :) = A.sup(i, :);
  endfor
endfunction

%!test
%! [l, u] = lu (infsup (magic (3)));
%! assert (l == infsup ({1, 0, 0; .375, 1, 0; .5, "68/37", 1}));, ...
%! assert (subset (u, infsup ({8, 1, 6; 0, 4.625, 4.75; 0, 0, "-0x1.3759F2298375Bp3"}, ...
%!                            {8, 1, 6; 0, 4.625, 4.75; 0, 0, "-0x1.3759F22983759p3"})));
%!test
%! A = magic (3);
%! A([1, 5, 9]) = 0;
%! [l, u, p] = lu (infsup (A));
%! assert (p, [0, 0, 1; 1, 0, 0; 0, 1, 0]);
%! assert (l == infsup ({1, 0, 0; "4/3", 1, 0; 0, "1/9", 1}));
%! assert (subset (u, infsup ({3, 0, 7; 0, 9, "-0x1.2AAAAAAAAAAACp3"; 0, 0, "0x1.C25ED097B425Ep2"}, ...
%!                            {3, 0, 7; 0, 9, "-0x1.2AAAAAAAAAAAAp3"; 0, 0, "0x1.C25ED097B426p2"})));
