File Coverage

blib/lib/Math/NumSeq/Pronic.pm

Criterion	Covered	Total	%
statement	67	68	98.5
branch	11	12	91.6
condition			n/a
subroutine	21	21	100.0
pod	8	8	100.0
total	107	109	98.1

# 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 .

package Math::NumSeq::Pronic;
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::__('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__