#  Copyright (c) 1997-2015
#  Ewgenij Gawrilow, Michael Joswig (Technische Universitaet Berlin, Germany)
#  http://www.polymake.org
#
#  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 2, or (at your option) any
#  later version: http://www.gnu.org/licenses/gpl.txt.
#
#  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.
#-------------------------------------------------------------------------------

sub term_sign($$){
   my ($s,$t) = @_;
   my $s_it = entire($s);
   my $t_it = entire($t);
   my $count = 0;
   my $ind = 0;
   while( $s_it && $t_it){
      if($$s_it==$$t_it){return 0;}
      if($$s_it<$$t_it){++$ind; ++$s_it;}
      else{$count += $s->size()-$ind; ++$t_it;} 
   }
   return (-1)**$count;
}

# @category Producing an ideal from scratch
# Generates the ideal of all Grassmann-Plücker relations of dxd minors of an dxn matrix.
# For the algorithm see Sturmfels: Algorithms in invariant theory, Springer, 2nd ed., 2008
# @param Int d
# @param Int n
# @return Ideal the Grassmann-Plücker ideal
user_function pluecker_ideal($$){
   my ($d,$n) = @_;
   my $m = new Int(binomial($n,$d));
   my $subsets = new Array< Set<Int> >($m);
   my $HM = new HashMap<Set,Int>;
   my $weights = new Vector<Int>($m);
   my $i=0;
   my @v = map{ 
      $subsets->[$i]=$_;
      $HM->{new Set($_)}=$i;
      my $w = 2**$n;
      for my $j (@$_){
         $w += 2**$j; # this is not a optimal choice
         }
      $weights->[$i++]= $w ;
      "p_(".join(",",@$_).")";
      } all_subsets_of_k($d,0..$n-1);
      # variable order in M2:
      # my @vecs = sort { $a cmp $b } map { new Vector<Int>(reverse(@$_)) } all_subsets_of_k($d,0..$n-1);
   my @var = monomials(scalar(@v));
   my @GB;
   for my $i (0..$m-2){
      for my $j ($i+1..$m-1){
         next if ( ($subsets->[$i]*$subsets->[$j])->size() > $d-2 );
         my $array = new Array<Int>($d+1);
         my $ind = 0;
         my $x = new Set();
         my $y = new Set();
         my $i_it = entire($subsets->[$i]);
         my $j_it = entire($subsets->[$j]);
         while ($i_it && $j_it && $$i_it <= $$j_it) {
            $x += $$i_it;
            $array->[$ind++] += $$j_it;
            ++$i_it; ++$j_it;
         }
         next if (!$i_it || !$j_it);
         $array->[$ind++] += $$j_it;
         $array->[$ind++] += $$i_it;
         ++$i_it; ++$j_it;
         while ($i_it && $j_it) {
            $y += $$j_it;
            $array->[$ind++] += $$i_it;
            ++$i_it; ++$j_it;
         }
         my $poly = new Polynomial(scalar(@v));
         my $set = new Set(0..$array->size-1);
         map{
               my $temp1=new Array<Int>([@{$array}[@{$_}]]);
               my $temp2=new Array<Int>([@{$array}[@{($set-new Set(@{$_}))}]]);
               $poly += term_sign($x,$temp2)*term_sign($temp1,$temp2)*term_sign($temp1,$y)*
                        $var[$HM->{$x+new Set($temp2)}]*$var[$HM->{$y+new Set($temp1)}]; 
            } all_subsets_of_k($x->size()+1,@{$set});
         push @GB, $poly;
      }
   }
   return new ideal::Ideal("GROEBNER.ORDER_VECTOR"=>$weights,"GROEBNER.BASIS"=>\@GB,GENERATORS=>\@GB);
}

# Local Variables:
# mode: perl
# cperl-indent-level: 3
# indent-tabs-mode:nil
# End:
