# 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::Triangular;
use 5.004;
use strict;
use POSIX 'ceil';
use List::Util 'max';
use vars '$VERSION','@ISA';
$VERSION = 72;
use Math::NumSeq;
use Math::NumSeq::Base::IterateIth;
@ISA = ('Math::NumSeq::Base::IterateIth',
'Math::NumSeq');
# uncomment this to run the ### lines
# use Smart::Comments;
# use constant name => Math::NumSeq::__('Triangular Numbers');
use constant description => Math::NumSeq::__('The triangular numbers 0, 1, 3, 6, 10, 15, 21, 28, etc, i*(i+1)/2.');
use constant characteristic_increasing => 1;
use constant characteristic_integer => 1;
use constant default_i_start => 0;
use constant values_min => 0; # at i=0
#------------------------------------------------------------------------------
# cf A062828 gcd(2n,triangular(n))
#
use constant oeis_anum => 'A000217'; # starting from i=0 value=0
#------------------------------------------------------------------------------
sub ith {
my ($self, $i) = @_;
return $i*($i+1)/2;
}
# [1,2,3,4,5],[1,3,6,10,15]
# N = ((1/2*$d + 1/2)*$d)
# d = -1/2 + sqrt(2 * $n + 1/4)
# = (-1 + 2*sqrt(2 * $n + 1/4))/2
# = (sqrt(4*2*$n + 1) - 1)/2
# = (sqrt(8*$n + 1) - 1)/2
sub pred {
my ($self, $value) = @_;
### Triangular pred(): $value
if ($value < 0) { return 0; }
my $int = int($value);
if ($value != $int) { return 0; }
$int *= 2;
my $i = int((sqrt(4*$int + 1) - 1)/2);
### $int
### $i
### triangular: ($i+1)*$i/2
return ($int == ($i+1)*$i);
}
sub value_to_i {
my ($self, $value) = @_;
if ($value >= 0) {
my $int = int($value);
if ($value == $int) {
my $i = int((sqrt(8*$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(8*int($value) + 1) - 1)/2);
}
*value_to_i_estimate = \&value_to_i_floor;
sub value_to_i_ceil {
my ($self, $value) = @_;
### value_to_i_ceil(): $value
if ($value <= 0) {
return 0;
}
my $i = $self->value_to_i_floor($value);
if ($self->ith($i) < $value) {
return $i+1;
} else {
return $i;
}
}
1;
__END__
=for stopwords Ryde Math-NumSeq ie
=head1 NAME
Math::NumSeq::Triangular -- triangular numbers
=head1 SYNOPSIS
use Math::NumSeq::Triangular;
my $seq = Math::NumSeq::Triangular->new;
my ($i, $value) = $seq->next;
=head1 DESCRIPTION
The triangular numbers i*(i+1)/2,
0, 1, 3, 6, 10, 15, 21, 28, ...
starting i=0
The numbers are how many points are in an equilateral triangle of side i,
* i=1 1
* i=2 3
* *
*
* * i=3 6
* * *
*
* * i=4 10
* * *
* * * *
From a given i, the next value is formed by adding i+1, being a new row of
that length on the bottom of the triangle.
=head1 FUNCTIONS
See L<Math::NumSeq/FUNCTIONS> for behaviour common to all sequence classes.
=over 4
=item C<$seq = Math::NumSeq::Triangular-E<gt>new ()>
Create and return a new sequence object.
=back
=head2 Random Access
=over
=item C<$value = $seq-E<gt>ith($i)>
Return C<$i*($i+1)/2>.
=item C<$bool = $seq-E<gt>pred($value)>
Return true if C<$value> is a triangular number, ie. i*(i+1)/2 for some
integer i.
=item C<$i = $seq-E<gt>value_to_i_ceil($value)>
=item C<$i = $seq-E<gt>value_to_i_floor($value)>
Return the index i of C<$value>, or if C<$value> is not a triangular number
then the next higher for C<ceil> or lower for C<floor>.
=item C<$i = $seq-E<gt>value_to_i_estimate($value)>
Return an estimate of the i corresponding to C<$value>. This is
value=i*(i+1)/2 is inverted to
$i = int ((sqrt(8*$value + 1) - 1)/2)
=back
=head1 SEE ALSO
L<Math::NumSeq>,
L<Math::NumSeq::Pronic>,
L<Math::NumSeq::Squares>
=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