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    my $distribution => 'ArrayRef[Num]';

  my $error_ratio = $self->evaluate_error_ratio(
    classifier => $classifier,
    distribution => $distribution,
  );
  return log((1 - $error_ratio) / $error_ratio) / 2;
}

sub classify {
  args_pos
    my $self,
    my $feature => 'Any';
  Carp::croak 'Training phase is undone yet.' unless $self->trained;
  $self->final_classifier->classify($feature);
}

sub construct_hardest_distribution {
  args
    my $self,
    my $classifier => 'CodeRef',
    my $previous_distribution => 'ArrayRef[Num]',
    my $training_set => 'ArrayRef[HashRef]',
    my $weight => 'Num';

  my @distribution = map {
    my $training_data = $training_set->[$_];
    $previous_distribution->[$_]
      * exp(-$weight * $training_data->{label}
              * $classifier->($training_data->{feature}));
  } 0 .. $#$previous_distribution;
  my $partition_function = List::Util::sum(@distribution);
  [ map { $_ / $partition_function } @distribution ];
}

sub evaluate_error_ratio {
  args
    my $self,
    my $classifier => 'CodeRef',
    my $distribution => 'ArrayRef[Num]';

  my $accuracy = 0;
  for my $i (0 .. $#$distribution) {
    my $training_data = $self->training_set->[$i];
    if ($classifier->($training_data->{feature}) == $training_data->{label}) {
      $accuracy += $distribution->[$i];
    }
  }
  return 1 - $accuracy;
}

sub final_classifier {
  args my $self;
  Carp::croak 'The classifier is not trained' unless $self->trained;
  return $self->{final_classifier};
}

sub train {
  args
    my $self,
    my $num_iterations => 'Int',
    my $training_set => +{ isa => 'ArrayRef', optional => 1 },
    my $weak_classifier_generator => +{ isa => 'CodeRef', optional => 1 };

  $training_set //= $self->training_set
    // Carp::croak('Given no training set.');
  $weak_classifier_generator //= $self->weak_classifier_generator
    // Carp::croak('Given no weak classifier generator.');
  my $num_training_set = @$training_set;

  # Initial distribution is uniform.
  my $distribution = [ (1 / $num_training_set) x $num_training_set ];

  my ($weak_classifier, $weight);
  my @weak_classifiers;
  while ($num_iterations--) {
    # Construct a weak classifier which classifies data on the distribution.
    $weak_classifier = $weak_classifier_generator->(
      distribution => $distribution,
      training_set => $training_set,
    );
    $weight = $self->calculate_classifier_weight(
      classifier => $weak_classifier,
      distribution => $distribution,
    );
    push @weak_classifiers, +{
      classifier => $weak_classifier,
      weight => $weight,
    };
  } continue {
    $distribution = $self->construct_hardest_distribution(
      classifier => $weak_classifier,
      previous_distribution => $distribution,
      training_set => $training_set,
      weight => $weight,
    );
  }

  return $self->{final_classifier} = Algorithm::AdaBoost::Classifier->new(
    weak_classifiers => \@weak_classifiers,
  );
}

sub trained { exists shift->{final_classifier} }

sub training_set { shift->{training_set} }

sub weak_classifier_generator { shift->{weak_classifier_generator} }

1;
__END__

=head1 NAME

Algorithm::AdaBoost - AdaBoost learning algorithm

=head1 SYNOPSIS

  use Algorithm::AdaBoost;

  # Training phase.
  my $learner = Alogrithm::AdaBoost->new(
    training_set => [
      +{ feature => [...], label => 1, },
      +{ feature => [...], label => -1, },
      +{ feature => [...], label => -1, },
      ...
    ],
    weak_classifier_generator => \&my_poor_learning_algorithm,
  );
  $learner->train(num_iterations => 1_000);

  # Now you have a boost-ed classifier (Algorithm::AdaBoost::Classifier).
  my $classifier = $learner->final_classifier;
  given ($classifier->classify([...])) {
    when ($_ > 0) { say 'The data belongs to class 1.' }
    when ($_ < 0) { say 'The data belongs to class 2.' }
    default { warn 'The data cannot be classified.' }
  }

=head1 DESCRIPTION

AdaBoost is a machine learning algorithm proposed by Freund and Schapire.
Using an arbitrary binary classification algorithm, The algorithm can construct a more accurate classifier (i.e. it is a meta-algorithm).

=head1 METHODS

=head2 new

Constructor. You can specify 2 optional attributes:

=over 2

=item training_set

An ArrayRef which is used as a training data set.

Each item is a HashRef having 2 keys: C<feature> and C<label>. C<feature> is a arbitrary input that classifier accepts and C<label> is a expected output label (C<+1> or C<-1>).

=item weak_classifier_generator

A CodeRef which is expected to generate a binary classifier function.

When the function is called, 2 named parameters are specified like this:

  my $classifier = $generator->(
     distribution => [...],
     training_set => [...],
  );

C<distribution> is an ArrayRef which each item is a probability of corresponding item in C<training_set>. i.e. C<distribution> is P(X = t_i) where t_i is i-th item in C<training_set>.

The generated classifier is expected to be a CodeRef which takes 1 argument (value of C<feature>) and return C<+1> or C<-1> as a output label.

=back

Either of both can be overriden temporarily with parameters for C<train>.

=head2 classify

Shorthand for C<< $learner->final_classifier->classify >>.

=head2 final_classifier

Returns the last constructed classifier.

=head2 train

Constructs a stronger classifier from given training set and weak learning algorithm.

This method takes 1 mandatory parameter:

=over 2

=item num_iterations

Specifies how many training iterations to be excuted (i.e., how many weak classifiers to be generated).

=back

and 2 optional parameters:

=over 2

=item training_set

=item weak_classifier_generator

=back

If the optional parameters are ommited, parameters specified to C<new> are used as defaults. If constructor parameters are ommited too, an exception will be raised.

=head2 trained

True if C<train> method have called, false otherwise.

=head1 AUTHOR

Koichi SATOH E<lt>sekia@cpan.orgE<gt>

=head1 SEE ALSO

L<A Short Introduction to Boosting|http://www.site.uottawa.ca/~stan/csi5387/boost-tut-ppr.pdf>

=head1 LICENSE

The MIT License

Copyright (C) 2012 by Koichi SATOH

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modi...

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDE...

=cut