A variable-length unsigned/signed integer using base128 encoding. 1-byte groups consist of 1-bit flag of continuation and 7-bit value chunk, and are ordered "least significant group first", i.e. in "little-endian" manner.
This particular encoding is specified and used in:
More information on this encoding is available at https://en.wikipedia.org/wiki/LEB128
This particular implementation supports integer values up to 64 bits (i.e. the
maximum unsigned value supported is 2**64 - 1
), which implies that serialized
values can be up to 10 bytes in length.
If the most significant 10th byte (groups[9]
) is present, its has_next
must be false
(otherwise we would have 11 or more bytes, which is not
supported) and its value
can be only 0
or 1
(because a 9-byte VLQ can
represent 9 * 7 = 63
bits already, so the 10th byte can only add 1 bit,
since only integers up to 64 bits are supported). These restrictions are
enforced by this implementation. They were inspired by the Protoscope tool,
see https://github.com/protocolbuffers/protoscope/blob/8e7a6aafa2c9958527b1e0747e66e1bfff045819/writer.go#L644-L648.
This page hosts a formal specification of Variable length quantity, unsigned/signed integer, base128, little-endian using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.
# This is a generated file! Please edit source .ksy file and use kaitai-struct-compiler to rebuild
use strict;
use warnings;
use IO::KaitaiStruct 0.009_000;
########################################################################
package VlqBase128Le;
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} = ();
do {
$_ = VlqBase128Le::Group->new($self->{_io}, $self, $self->{_root});
push @{$self->{groups}}, $_;
} until (!($_->has_next()));
}
sub len {
my ($self) = @_;
return $self->{len} if ($self->{len});
$self->{len} = scalar(@{$self->groups()});
return $self->{len};
}
sub value {
my ($self) = @_;
return $self->{value} if ($self->{value});
$self->{value} = @{$self->groups()}[-1]->interm_value();
return $self->{value};
}
sub sign_bit {
my ($self) = @_;
return $self->{sign_bit} if ($self->{sign_bit});
$self->{sign_bit} = ($self->len() == 10 ? 9223372036854775808 : (@{$self->groups()}[-1]->multiplier() * 64));
return $self->{sign_bit};
}
sub value_signed {
my ($self) = @_;
return $self->{value_signed} if ($self->{value_signed});
$self->{value_signed} = ( (($self->sign_bit() > 0) && ($self->value() >= $self->sign_bit())) ? -(($self->sign_bit() - ($self->value() - $self->sign_bit()))) : $self->value());
return $self->{value_signed};
}
sub groups {
my ($self) = @_;
return $self->{groups};
}
########################################################################
package VlqBase128Le::Group;
our @ISA = 'IO::KaitaiStruct::Struct';
sub from_file {
my ($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;;
$self->_read();
return $self;
}
sub _read {
my ($self) = @_;
$self->{has_next} = $self->{_io}->read_bits_int_be(1);
$self->{value} = $self->{_io}->read_bits_int_be(7);
}
sub interm_value {
my ($self) = @_;
return $self->{interm_value} if ($self->{interm_value});
$self->{interm_value} = ($self->prev_interm_value() + ($self->value() * $self->multiplier()));
return $self->{interm_value};
}
sub has_next {
my ($self) = @_;
return $self->{has_next};
}
sub value {
my ($self) = @_;
return $self->{value};
}
sub idx {
my ($self) = @_;
return $self->{idx};
}
sub prev_interm_value {
my ($self) = @_;
return $self->{prev_interm_value};
}
sub multiplier {
my ($self) = @_;
return $self->{multiplier};
}
1;