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use 5.12.0;
use warnings;
package Chess::960 0.003;
# ABSTRACT: a Chess960 starting position generator

use Carp ();

#pod =head1 OVERVIEW
#pod
#pod L<Chess960|https://en.wikipedia.org/wiki/Chess960> is a chess variant invented
#pod by Bobby Fischer, designed to somewhat reduce the value of memorization to
#pod play, while retaining key properties of the game such as castling and one
#pod bishop per color.
#pod
#pod Chess::960 generates random starting positions for a Chess960 game.
#pod
#pod   use Chess::960;
#pod
#pod   my $fen = Chess::960->new->fen; # Forsyth-Edwards notation of position
#pod
#pod   my $pos = Chess::960->new->generate_position; # simple data structure
#pod
#pod   my $pos = Chess::960->new->generate_position(123); # get position by number
#pod
#pod =cut

my @BRIGHT = qw(1 3 5 7);
my @DARK   = qw(0 2 4 6);

my @KNIGHTS = (
  [ 0, 1 ],
  [ 0, 2 ],
  [ 0, 3 ],
  [ 0, 4 ],
  [ 1, 2 ],
  [ 1, 3 ],
  [ 1, 4 ],
  [ 2, 3 ],
  [ 2, 4 ],
  [ 3, 4 ],
);

#pod =method new
#pod
#pod The constructor for Chess::960 does not, at present, take any argument.  In the
#pod future, it may take arguments to pick different mappings between positions
#pod and numbers.
#pod
#pod =cut

sub new {
  my ($class) = @_;
  bless {} => $class;
}

#pod =method generate_position
#pod
#pod   my $pos = $c960->generate_position($num);
#pod
#pod This returns a starting description, described by a hash.  If C<$num> is not
#pod provided, a random position will be returned.  If a value for C<$num> that
#pod isn't an integer between 0 and 959 is provided, an exception will be raised.
#pod
#pod Position 518 in the default mapping is the traditional chess starting position.
#pod
#pod The returned hashref has two entries:
#pod
#pod   number - the number of the generated position
#pod   rank   - an eight-element arrayref giving the pieces' positions
#pod            elements are characters in [BQNRK]
#pod
#pod =cut

sub generate_position {
  my ($self, $num) = @_;
  $num //= int rand 960;

  Carp::confess("starting position number must be between 0 and 959")
    unless defined $num && $num =~ /\A[0-9]{1,3}\z/ && $num >= 0 && $num <= 959;

  my $b1 = $num % 4;
  my $b2 = int( $num / 4 ) % 4;

  my $k  = int( $num / 96 );
  my $q  = ($num / 16) % 6;

  my @rank = (undef) x 8;
  $rank[ $BRIGHT[ $b1 ] ] = 'B';
  $rank[ $DARK[   $b2 ] ] = 'B';

  my @empty;

  @empty = grep { ! $rank[$_] } keys @rank;
  $rank[ $empty[ $q ] ] = 'Q';

  @empty = grep { ! $rank[$_] } keys @rank;
  @rank[ @empty[ @{ $KNIGHTS[$k] } ] ] = qw(N N);

  @empty = grep { ! $rank[$_] } keys @rank;
  @rank[ @empty ] = qw(R K R);

  return { number => $num, rank => \@rank };
}

#pod =method fen
#pod
#pod This method returns a
#pod L<FEN|https://en.wikipedia.org/wiki/Forsyth%E2%80%93Edwards_Notation>-format
#pod string describing the complete starting position of the board.  For example:
#pod
#pod   rnbbqkrn/pppppppp/8/8/8/8/PPPPPPPP/RNBBQKRN w KQkq - 0 1
#pod
#pod =cut