File Coverage

blib/lib/Perl/Critic/Policy/Variables/RequireNegativeIndices.pm

Criterion	Covered	Total	%
statement	101	103	98.0
branch	59	80	73.7
condition	11	18	61.1
subroutine	19	19	100.0
pod	4	5	80.0
total	194	225	86.2

package Perl::Critic::Policy::Variables::RequireNegativeIndices;

use 5.010001;
use strict;
use warnings;
use Readonly;

use Perl::Critic::Utils qw{ :severities };
use parent 'Perl::Critic::Policy';

our $VERSION = '1.148';

#-----------------------------------------------------------------------------

Readonly::Scalar my $DESC => q{Negative array index should be used};
Readonly::Scalar my $EXPL => [ 88 ];

#-----------------------------------------------------------------------------

sub supported_parameters { return ()                          }
sub default_severity     { return $SEVERITY_HIGH              }
sub default_themes       { return qw( core maintenance pbp )  }
sub applies_to           { return 'PPI::Structure::Subscript' }

#-----------------------------------------------------------------------------

sub violates {
    my ( $self, $elem, $doc ) = @_;

    return if $elem->braces ne '[]';
    my ($name, $isref) = _is_bad_index( $elem );
    return if ( !$name );
    return if !_is_array_name( $elem, $name, $isref );
    return $self->violation( $DESC, $EXPL, $elem );
}

Readonly::Scalar my $MAX_EXPRESSION_COMPLEXITY => 4;

sub _is_bad_index {
    # return (varname, 0|1) if this could be a violation
    my ( $elem ) = @_;

    my @children = $elem->schildren();
    return if @children != 1; # too complex
    return if !$children[0]->isa( 'PPI::Statement::Expression'); # too complex

    # This is the expression elements that compose the array indexing
    my @expr = $children[0]->schildren();
    return if !@expr || @expr > $MAX_EXPRESSION_COMPLEXITY;
    my ($name, $isref, $isindex) = _is_bad_var_in_index(\@expr);
    return if !$name;
    return $name, $isref if !@expr && $isindex;
    return if !_is_minus_number(@expr);
    return $name, $isref;
}

sub _is_bad_var_in_index {
    # return (varname, isref=0|1, isindex=0|1) if this could be a violation
    my ( $expr ) = @_;

    if ( $expr->[0]->isa('PPI::Token::ArrayIndex') ) {
        # [$#arr]
        return _arrayindex($expr);
    }
    elsif ( $expr->[0]->isa('PPI::Token::Cast') ) {
        # [$#{$arr} ...] or [$#$arr ...] or [@{$arr} ...] or [@$arr ...]
        return _cast($expr);
    }
    elsif ($expr->[0]->isa('PPI::Token::Symbol')) {
        # [@arr ...]
        return _symbol($expr);
    }

    return;
}

sub _arrayindex {
    # return (varname, isref=0|1, isindex=0|1) if this could be a violation
    my ( $expr ) = @_;
    my $arrindex = shift @{$expr};
    if ($arrindex->content =~ m/\A \$[#] (.*) \z /xms) { # What else could it be???
       return $1, 0, 1;
    }
    return;
}

sub _cast {
    # return (varname, isref=0|1, isindex=0|1) if this could be a violation
    my ( $expr ) = @_;
    my $cast = shift @{$expr};
    if ( $cast eq q{$#} || $cast eq q{@} ) { ## no critic(RequireInterpolationOfMetachars)
        my $isindex = $cast eq q{$#} ? 1 : 0;  ## no critic(RequireInterpolationOfMetachars)
        my $arrvar = shift @{$expr};
        if ($arrvar->isa('PPI::Structure::Block')) {
            # look for [$#{$arr} ...] or [@{$arr} ...]
            my @blockchildren = $arrvar->schildren();
            return if @blockchildren != 1;
            return if !$blockchildren[0]->isa('PPI::Statement');
            my @ggg = $blockchildren[0]->schildren;
            return if @ggg != 1;
            return if !$ggg[0]->isa('PPI::Token::Symbol');
            if ($ggg[0] =~ m/\A \$ (.*) \z/xms) {
                return $1, 1, $isindex;
            }
        }
        elsif ( $arrvar->isa('PPI::Token::Symbol') ) {
           # look for [$#$arr ...] or [@$arr ...]
           if ($arrvar =~ m/\A \$ (.*) \z/xms) {
              return $1, 1, $isindex;
           }
        }
    }
    return;
}

sub _symbol {
    # return (varname, isref=0|1, isindex=0|1) if this could be a violation
    my ( $expr ) = @_;
    my $arrvar = shift @{$expr};
    if ($arrvar =~ m/\A \@ (.*) \z/xms) {
       return $1, 0, 0;
    }
    return;
}

sub _is_minus_number {  # return true if @expr looks like "- n"
    my @expr = @_;

    return if !@expr;

    return if @expr != 2;

    my $op = shift @expr;
    return if !$op->isa('PPI::Token::Operator');
    return if $op ne q{-};

    my $number = shift @expr;
    return if !$number->isa('PPI::Token::Number');

    return 1;
}

sub _is_array_name {  # return true if name and isref matches
    my ( $elem, $name, $isref ) = @_;

    my $sib = $elem->sprevious_sibling;
    return if !$sib;

    if ($sib->isa('PPI::Token::Operator') && $sib eq '->') {
        return if ( !$isref );
        $isref = 0;
        $sib = $sib->sprevious_sibling;
        return if !$sib;
    }

    return if !$sib->isa('PPI::Token::Symbol');
    return if $sib !~ m/\A \$ \Q$name\E \z/xms;

    my $cousin = $sib->sprevious_sibling;
    return if $isref ^ _is_dereferencer( $cousin );
    return if $isref && _is_dereferencer( $cousin->sprevious_sibling );

    return $elem;
}

sub _is_dereferencer { # must return 0 or 1, not undef
    my $elem = shift;

    return 0 if !$elem;
    return 1 if $elem->isa('PPI::Token::Operator') && $elem eq '->';
    return 1 if $elem->isa('PPI::Token::Cast');
    return 0;
}

1;

#-----------------------------------------------------------------------------

__END__

=pod

=for stopwords performant

=head1 NAME

Perl::Critic::Policy::Variables::RequireNegativeIndices - Negative array index should be used.


=head1 AFFILIATION

This Policy is part of the core L<Perl::Critic|Perl::Critic>
distribution.


=head1 DESCRIPTION

Perl treats a negative array subscript as an offset from the end. Given
this, the preferred way to get the last element is C<$x[-1]>, not
C<$x[$#x]> or C<$x[@x-1]>, and the preferred way to get the next-to-last
is C<$x[-2]>, not C<$x[$#x-1> or C<$x[@x-2]>.

The biggest argument against the non-preferred forms is that B<their
semantics change> when the computed index becomes negative. If C<@x>
contains at least two elements, C<$x[$#x-1]> and C<$x[@x-2]> are
equivalent to C<$x[-2]>. But if it contains a single element,
C<$x[$#x-1]> and C<$x[@x-2]> are both equivalent to C<$x[-1]>. Simply
put, the preferred form is more likely to do what you actually want.

As Conway points out, the preferred forms also perform better, are more
readable, and are easier to maintain.

This policy notices all of the simple forms of the above problem, but
does not recognize any of these more complex examples:

    $some->[$data_structure]->[$#{$some->[$data_structure]} -1];
    my $ref = \@arr; $ref->[$#arr];


=head1 CONFIGURATION

This Policy is not configurable except for the standard options.


=head1 AUTHOR

Chris Dolan <[e-mail]>


=head1 COPYRIGHT

Copyright (c) 2006-2011 Chris Dolan.

This program is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.

=cut

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