# Copyright 2010, 2011, 2012, 2013, 2014 Kevin Ryde
# This file is part of Math-NumSeq.
#
# Math-NumSeq 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, or (at your option) any later
# version.
#
# Math-NumSeq 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 Math-NumSeq. If not, see <http://www.gnu.org/licenses/>.
package Math::NumSeq::Pronic;
use 5.004;
use strict;
use POSIX 'ceil';
use List::Util 'max';
use vars '$VERSION','@ISA';
$VERSION = 71;
use Math::NumSeq;
use Math::NumSeq::Base::IterateIth;
@ISA = ('Math::NumSeq::Base::IterateIth',
'Math::NumSeq');
# uncomment this to run the ### lines
#use Smart::Comments;
# use constant name => Math::NumSeq::__('Pronic Numbers');
use constant description => Math::NumSeq::__('The pronic numbers 0, 2, 6, 12, 20, 30, etc, etc, i*(i+1). These are twice the triangular numbers, and half way between perfect squares.');
use constant default_i_start => 0;
use constant characteristic_increasing => 1;
use constant characteristic_integer => 1;
use constant values_min => 0; # at i=0
use constant oeis_anum => 'A002378'; # pronic, starting from 0
sub rewind {
my ($self) = @_;
$self->{'i'} = $self->i_start;
}
sub seek_to_i {
my ($self, $i) = @_;
$self->{'i'} = $i;
}
sub seek_to_value {
my ($self, $value) = @_;
$self->seek_to_i($self->value_to_i_ceil($value));
}
sub ith {
my ($self, $i) = @_;
return $i*($i+1);
}
# [0,1,2,3,4],[0,2,6,12,20]
# N = (d^2 + d)
# = ($d**2 + $d)
# = (($d + 1)*$d)
# d = -1/2 + sqrt(1 * $n + 1/4)
# = (-1 + 2*sqrt(1 * $n + 1/4))/2
# = (-1 + sqrt(4 * $n + 1))/2
# = (sqrt(4*$n + 1) - 1)/2
sub pred {
my ($self, $value) = @_;
if ($value < 0) { return 0; }
my $i = $self->value_to_i_floor($value);
return ($value == $i*($i+1));
}
sub value_to_i {
my ($self, $value) = @_;
if ($value >= 0) {
my $int = int($value);
if ($value == $int) {
my $i = int((sqrt(4*$int + 1) - 1)/2);
if ($int == $self->ith($i)) {
return $i;
}
}
}
return undef;
}
sub value_to_i_floor {
my ($self, $value) = @_;
if ($value < 0) {
return 0;
}
return int((sqrt(4*int($value) + 1) - 1)/2);
}
sub value_to_i_ceil {
my ($self, $value) = @_;
my $i = $self->value_to_i_floor($value);
if ($self->ith($i) < $value) {
return $i+1;
} else {
return $i;
}
}
*value_to_i_estimate = \&value_to_i_floor;
1;
__END__
=for stopwords Ryde Math-NumSeq pronic ie
=head1 NAME
Math::NumSeq::Pronic -- pronic numbers
=head1 SYNOPSIS
use Math::NumSeq::Pronic;
my $seq = Math::NumSeq::Pronic->new;
my ($i, $value) = $seq->next;
=head1 DESCRIPTION
The pronic numbers i*(i+1),
0, 2, 6, 12, 20, 30, ...
starting i=0
These are twice the triangular numbers, and half way between the perfect
squares.
=head1 FUNCTIONS
See L<Math::NumSeq/FUNCTIONS> for behaviour common to all sequence classes.
=over 4
=item C<$seq = Math::NumSeq::Pronic-E<gt>new ()>
Create and return a new sequence object.
=back
=head2 Iterating
=over
=item C<$seq-E<gt>seek_to_i($i)>
Move the current sequence position to C<$i>. The next call to C<next()>
will return C<$i> and corresponding value.
=item C<$seq-E<gt>seek_to_value($value)>
Move the current sequence position so that C<next()> will give C<$value> on
the next call, or if C<$value> is not a pronic number then the next pronic
above C<$value>.
=back
=head2 Random Access
=over
=item C<$value = $seq-E<gt>ith($i)>
Return C<$i*($i+1)>.
=item C<$bool = $seq-E<gt>pred($value)>
Return true if C<$value> is a pronic number, ie. i*(i+1) for some i.
=item C<$i = $seq-E<gt>value_to_i_ceil($value)>
=item C<$i = $seq-E<gt>value_to_i_floor($value)>
Return the C<$i> index of C<$value>, rounding up or down if C<$value> is not
a pronic number.
=item C<$i = $seq-E<gt>value_to_i_estimate($value)>
Return an estimate of the i corresponding to C<$value>. value=i*(i+1) is
inverted by
$i = int( (sqrt(4*$value + 1) - 1)/2 )
=back
=head1 SEE ALSO
L<Math::NumSeq>,
L<Math::NumSeq::Squares>,
L<Math::NumSeq::Triangular>
=head1 HOME PAGE
L<http://user42.tuxfamily.org/math-numseq/index.html>
=head1 LICENSE
Copyright 2010, 2011, 2012, 2013, 2014 Kevin Ryde
Math-NumSeq 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, or (at your option) any later
version.
Math-NumSeq 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
Math-NumSeq. If not, see <http://www.gnu.org/licenses/>.
=cut