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                $false_negatives += 1;
            }
        }, $pred_label->unpdl, $label->unpdl);
        my $precision;
        my $recall;
        if($true_positives + $false_positives > 0)
        {
            $precision = $true_positives / ($true_positives + $false_positives);
        }
        else
        {
            $precision = 0;
        }
        if($true_positives + $false_negatives > 0)
        {
            $recall = $true_positives / ($true_positives +  $false_negatives);
        }
        else
        {
            $recall = 0;
        }
        my $f1_score;
        if($precision + $recall > 0)
        {
            $f1_score = 2 * $precision * $recall / ($precision + $recall);
        }
        else
        {
            $f1_score = 0;
        }
        $self->sum_metric($self->sum_metric + $f1_score);
        $self->num_inst($self->num_inst + 1);
    }, $labels, $preds);
}

package AI::MXNet::Perplexity;
use Mouse;
use AI::MXNet::Base;
extends 'AI::MXNet::EvalMetric';
has '+name'        => (default => 'Perplexity');
has 'ignore_label' => (is => 'ro', isa => 'Maybe[Int]');
has 'axis'         => (is => 'ro', isa => 'Int', default => -1);
around BUILDARGS => sub {
    my $orig  = shift;
    my $class = shift;
    return $class->$orig(ignore_label => $_[0]) if @_ == 1;
    return $class->$orig(@_);
};

=head1 NAME

    AI::MXNet::Perplexity
=cut

=head1 DESCRIPTION

    Calculate perplexity.

    Parameters
    ----------
    ignore_label : int or undef
        index of invalid label to ignore when
        counting. usually should be -1. Include
        all entries if undef.
    axis : int (default -1)
        The axis from prediction that was used to
        compute softmax. By default uses the last
        axis.
=cut

method update(ArrayRef[AI::MXNet::NDArray] $labels, ArrayRef[AI::MXNet::NDArray] $preds)
{
    AI::MXNet::Metric::check_label_shapes($labels, $preds);
    my ($loss, $num) = (0, 0);
    zip(sub {
        my ($label, $pred) = @_;
        my $label_shape = $label->shape;
        my $pred_shape  = $pred->shape;
        assert(
            (product(@{ $label_shape }) == product(@{ $pred_shape })/$pred_shape->[-1]),
            "shape mismatch: (@$label_shape) vs. (@$pred_shape)"
        );
        $label = $label->as_in_context($pred->context)->reshape([$label->size]);
        $pred = AI::MXNet::NDArray->pick($pred, $label->astype('int32'), { axis => $self->axis });
        if(defined $self->ignore_label)
        {
            my $ignore = ($label == $self->ignore_label);
            $num -= $ignore->sum->asscalar;
            $pred = $pred*(1-$ignore) + $ignore;
        }
        $loss -= $pred->maximum(1e-10)->log->sum->asscalar;
        $num  += $pred->size;
    }, $labels, $preds);
    $self->sum_metric($self->sum_metric + $loss);
    $self->num_inst($self->num_inst + $num);
}

method get()
{
    return ($self->name, exp($self->sum_metric / $self->num_inst));
}

####################
# REGRESSION METRICS
####################

# Calculate Mean Absolute Error loss
package AI::MXNet::MAE;
use Mouse;
use AI::MXNet::Base;
extends 'AI::MXNet::EvalMetric';
has '+name'   => (default => 'mae');

method update(ArrayRef[AI::MXNet::NDArray] $labels, ArrayRef[AI::MXNet::NDArray] $preds)
{
    AI::MXNet::Metric::check_label_shapes($labels, $preds);
    zip(sub {
        my ($label, $pred) = @_;
        $label = $label->aspdl;
        $pred =  $pred->aspdl;
        if($label->ndims == 1)
        {
            $label = $label->reshape(1, $label->shape->at(0));
        }