Variable length quantity, unsigned integer, base128, big-endian: Perl parsing library

A variable-length unsigned integer using base128 encoding. 1-byte groups consist of 1-bit flag of continuation and 7-bit value chunk, and are ordered "most significant group first", i.e. in "big-endian" manner.

This particular encoding is specified and used in:

  • Standard MIDI file format
  • ASN.1 BER encoding
  • RAR 5.0 file format

More information on this encoding is available at https://en.wikipedia.org/wiki/Variable-length_quantity

This particular implementation supports serialized values to up 8 bytes long.

KS implementation details

License: CC0-1.0
Minimal Kaitai Struct required: 0.9

References

This page hosts a formal specification of Variable length quantity, unsigned integer, base128, big-endian using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.

Perl source code to parse Variable length quantity, unsigned integer, base128, big-endian

VlqBase128Be.pm

Download 

# This is a generated file! Please edit source .ksy file and use kaitai-struct-compiler to rebuild

use strict;
use warnings;
use IO::KaitaiStruct 0.011_000;

########################################################################
package VlqBase128Be;

our @ISA = 'IO::KaitaiStruct::Struct';

sub from_file {
    my ($class, $filename) = @_;
    my $fd;

    open($fd, '<', $filename) or return undef;
    binmode($fd);
    return new($class, IO::KaitaiStruct::Stream->new($fd));
}

sub new {
    my ($class, $_io, $_parent, $_root) = @_;
    my $self = IO::KaitaiStruct::Struct->new($_io);

    bless $self, $class;
    $self->{_parent} = $_parent;
    $self->{_root} = $_root || $self;

    $self->_read();

    return $self;
}

sub _read {
    my ($self) = @_;

    $self->{groups} = [];
    {
        my $_it;
        do {
            $_it = VlqBase128Be::Group->new($self->{_io}, $self, $self->{_root});
            push @{$self->{groups}}, $_it;
        } until (!($_it->has_next()));
    }
}

sub last {
    my ($self) = @_;
    return $self->{last} if ($self->{last});
    $self->{last} = scalar(@{$self->groups()}) - 1;
    return $self->{last};
}

sub value {
    my ($self) = @_;
    return $self->{value} if ($self->{value});
    $self->{value} = (((((((@{$self->groups()}[$self->last()]->value() + ($self->last() >= 1 ? @{$self->groups()}[$self->last() - 1]->value() << 7 : 0)) + ($self->last() >= 2 ? @{$self->groups()}[$self->last() - 2]->value() << 14 : 0)) + ($self->last() >= 3 ? @{$self->groups()}[$self->last() - 3]->value() << 21 : 0)) + ($self->last() >= 4 ? @{$self->groups()}[$self->last() - 4]->value() << 28 : 0)) + ($self->last() >= 5 ? @{$self->groups()}[$self->last() - 5]->value() << 35 : 0)) + ($self->last() >= 6 ? @{$self->groups()}[$self->last() - 6]->value() << 42 : 0)) + ($self->last() >= 7 ? @{$self->groups()}[$self->last() - 7]->value() << 49 : 0));
    return $self->{value};
}

sub groups {
    my ($self) = @_;
    return $self->{groups};
}

########################################################################
package VlqBase128Be::Group;

our @ISA = 'IO::KaitaiStruct::Struct';

sub from_file {
    my ($class, $filename) = @_;
    my $fd;

    open($fd, '<', $filename) or return undef;
    binmode($fd);
    return new($class, IO::KaitaiStruct::Stream->new($fd));
}

sub new {
    my ($class, $_io, $_parent, $_root) = @_;
    my $self = IO::KaitaiStruct::Struct->new($_io);

    bless $self, $class;
    $self->{_parent} = $_parent;
    $self->{_root} = $_root;

    $self->_read();

    return $self;
}

sub _read {
    my ($self) = @_;

    $self->{has_next} = $self->{_io}->read_bits_int_be(1);
    $self->{value} = $self->{_io}->read_bits_int_be(7);
}

sub has_next {
    my ($self) = @_;
    return $self->{has_next};
}

sub value {
    my ($self) = @_;
    return $self->{value};
}

1;