mpv/TOOLS/lib/Parse/Matroska/Element.pm

use 5.008;
use strict;
use warnings;

# ABSTRACT: a mid-level representation of an EBML element
package Parse::Matroska::Element;

use Carp;
use List::Util qw{first};

=head1 SYNOPSIS

    use Parse::Matroska::Reader;
    my $reader = Parse::Matroska::Reader->new($path);
    my $elem = $reader->read_element;

    print "ID: $elem->{elid}\n";
    print "Name: $elem->{name}\n";
    print "Length: $elem->{content_len}\n";
    print "Type: $elem->{type}\n";
    print "Child count: ", scalar(@{$elem->all_children}), "\n";
    if ($elem->{type} eq 'sub') {
        while (my $chld = $elem->next_child) {
            print "Child Name: $chld->{name}\n";
        }
    } else {
        print "Value: ", $elem->get_value, "\n";
    }

=head1 DESCRIPTION

Represents a single Matroska element as decoded by
L<Parse::Matroska::Reader>. This is essentially a hash
augmented with functions for delay-loading of binary
values and children elements.

=head1 NOTE

The API of this module is not yet considered stable.

=attr elid

The EBML Element ID, suitable for passing to
L<Parse::Matroska::Definitions/elem_by_hexid>.

=attr name

The EBML Element's name.

=attr type

The EBML Element's type. Can be C<uint>, C<sint>,
C<float>, C<ebml_id>, C<str> or C<binary>. See L</value>
for details.

Equivalent to
C<elem_by_hexid($elem-E<gt>{value})-E<gt>{valtype}>.

=attr value

The EBML Element's value. Should be obtained through
L</get_value>.

Is an unicode string if the L</type> is C<str>, that is,
the string has already been decoded by L<Encode/decode>.

Is C<undef> if the L</type> is C<binary> and the contents
were delay-loaded and not yet read. L</get_value> will
do the delayed load if needed.

Is an arrayref if the L</type> is C<sub>, containing
the children nodes that were already loaded.

Is a hashref if the L</type> is C<ebml_id>, containing
the referred element's information as defined in
L<Parse::Matroska::Definitions>. Calling
C<elem_by_hexid($elem-E<gt>{value}-E<gt>{elid})> will
return the same object as $elem->{value}.

=attr full_len

The entire length of this EBML Element, including
the header's.

=attr size_len

The length of the size marker. Used when calculating
L</full_len> from L</content_len>

=attr content_len

The length of the contents of this EBML Element,
which excludes the header.

=attr reader

A weakened reference to the associated
L<Parse::Matroska::Reader>.

=method new(%hash)

Creates a new Element initialized with the hash
given as argument.

=cut
sub new {
    my $class = shift;
    my $self = {};
    bless $self, $class;

    $self->initialize(@_);
    return $self;
}

=method initialize(%hash)

Called by L</new> on initialization.

=cut
sub initialize {
    my ($self, %args) = @_;
    for (keys %args) {
        $self->{$_} = $args{$_};
    }
    $self->{depth} = 0 unless $self->{depth};
}

=method skip

Called by the user to ignore the contents of this EBML node.
Needed when ignoring the children of a node.

=cut
sub skip {
    my ($self) = @_;
    my $reader = $self->{reader};
    return unless $reader; # we don't have to skip if there's no reader
    my $pos = $reader->getpos;
    croak "Too late to skip, reads were already done"
        if $pos ne $self->{data_pos};
    $reader->skip($self->{content_len});
}

=method get_value($keep_bin)

Returns the value contained by this EBML element.

If the element has children, returns an arrayref to
the children elements that were already encountered.

If the element's type is C<binary> and the value was
delay-loaded, does the reading now.

If $keep_bin is true, the delay-loaded data is kept
as the L</value>, otherwise, further calls to
C<get_value> will reread the data from the L</reader>.

=cut
sub get_value {
    my ($self, $keep_bin) = @_;

    return undef if $self->{type} eq 'skip';
    return $self->{value} if $self->{value};

    my $reader = $self->{reader} or
        croak "The associated Reader has been deleted";

    # delay-loaded 'binary'
    if ($self->{type} eq 'binary') {
        croak "Cannot seek in the current Reader" unless $self->{data_pos};
        # seek to the data position...
        $reader->setpos($self->{data_pos});
        # read the data, keeping it in value if requested
        if ($keep_bin) {
            $self->{value} = $reader->readlen($self->{content_len});
            return $self->{value};
        } else {
            return $reader->readlen($self->{content_len});
        }
    }
}

=method next_child($read_bin)

Builtin iterator; reads and returns the next child element.
Always returns undef if the type isn't C<sub>.

Returns undef at the end of the iterator and resets itself to
point to the first element; so calling L</next_child($read_bin)>
after the iterator returned C<undef> will return the first child.

The optional C<$read_bin> parameter has the children elements
not delay-load their value if their type is C<binary>.

If all children elements have already been read, return
each element in-order as would be given by
L</all_children($recurse,$read_bin)>.

=cut
sub next_child {
    my ($self, $read_bin) = @_;
    return unless $self->{type} eq 'sub';

    if ($self->{_all_children_read}) {
        my $idx = $self->{_last_child} ||= 0;
        if ($idx == @{$self->{value}}) {
            # reset the iterator, returning undef once
            $self->{_last_child} = 0;
            return;
        }
        my $ret = $self->{value}->[$idx];

        ++$idx;
        $self->{_last_child} = $idx;
        return $ret;
    }

    my $len = defined $self->{remaining_len}
        ? $self->{remaining_len}
        : $self->{content_len};

    if ($len == 0) {
        # we've read all children; switch into $self->{value} iteration mode
        $self->{_all_children_read} = 1;
        # return undef since the iterator will reset
        return;
    }

    $self->{pos_offset} ||= 0;
    my $pos = $self->{data_pos};
    my $reader = $self->{reader} or croak "The associated reader has been deleted";
    $reader->setpos($pos);
    $reader->{fh}->seek($self->{pos_offset}, 1) if $pos;

    my $chld = $reader->read_element($read_bin);
    return undef unless defined $chld;
    $self->{pos_offset} += $chld->{full_len};

    $self->{remaining_len} = $len - $chld->{full_len};

    if ($self->{remaining_len} < 0) {
        croak "Child elements consumed $self->{remaining_len} more bytes than parent $self->{name} contained";
    }

    $chld->{depth} = $self->{depth} + 1;
    $self->{value} ||= [];

    push @{$self->{value}}, $chld;

    return $chld;
}

=method all_children($recurse,$read_bin)

Calls L</populate_children($recurse,$read_bin)> on self
and returns an arrayref with the children nodes.

Both C<$recurse> and C<$read_bin> are optional and default
to false.

=cut
sub all_children {
    my ($self, $recurse, $read_bin) = @_;
    $self->populate_children($recurse, $read_bin);
    return $self->{value};
}

=method children_by_name($name)

Searches in the already read children elements for all
elements with the EBML name C<$name>. Returns an array
containing all found elements. On scalar context,
returns only the first element found.

Croaks if the element's C<type> isn't C<sub>.

=cut
sub children_by_name {
    my ($self, $name) = @_;
    return unless defined wantarray; # don't do work if work isn't wanted
    croak "Element can't have children" unless $self->{type} eq 'sub';

    my @found = grep { $_->{name} eq $name } @{$self->{value}};
    return @found       if wantarray;         # list
    return shift @found if defined wantarray; # scalar
}

=method populate_children($recurse,$read_bin)

Populates the internal array of children elements, that is,
requests that the associated L<Matroska::Parser::Reader> reads
all children elements. Returns itself.

Returns false if the element's C<type> isn't C<sub>.

If C<$recurse> is provided and is true, the method will call
itself in the children elements with the same parameters it
received; this will build a full EBML tree.

If C<$read_bin> is provided and is true, disables delay-loading
of the contents of C<binary>-type nodes, reading the contents
to memory.

If both C<$recurse> and C<$read_bin> are true, entire EBML trees
can be loaded without requiring seeks, thus behaving correctly
on unseekable streams. If C<$read_bin> is false, the entire EBML
tree is still loaded, but calling L</get_value> on C<binary>-type
nodes will produce an error on unseekable streams.

=cut
sub populate_children {
    my ($self, $recurse, $read_bin) = @_;

    return unless $self->{type} eq 'sub';

    if (@{$self->{value}} && $recurse) {
        # only recurse
        foreach (@{$self->{value}}) {
            $_->populate_children($recurse, $read_bin);
        }
        return $self;
    }

    while (my $chld = $self->next_child($read_bin)) {
        $chld->populate_children($recurse, $read_bin) if $recurse;
    }

    return $self;
}

1;