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.
All parsing code for C++11/STL generated by Kaitai Struct depends on the C++/STL runtime library. You have to install it before you can parse data.
For C++, the easiest way is to clone the runtime library sources and build them along with your project.
Using Kaitai Struct in C++/STL usually consists of 3 steps.
std::istream). One can open local file for that, or use existing std::string or char* buffer.
#include <fstream>
std::ifstream is("path/to/local/file.bin", std::ifstream::binary);
#include <sstream>
std::istringstream is(str);
#include <sstream>
const char buf[] = { ... };
std::string str(buf, sizeof buf);
std::istringstream is(str);
#include "kaitai/kaitaistream.h"
kaitai::kstream ks(&is);
vlq_base128_le_t data(&ks);
After that, one can get various attributes from the structure by invoking getter methods like:
data.value() // => Resulting unsigned value as normal integer
#pragma once
// This is a generated file! Please edit source .ksy file and use kaitai-struct-compiler to rebuild
#include "kaitai/kaitaistruct.h"
#include <stdint.h>
#include <memory>
#include <vector>
#if KAITAI_STRUCT_VERSION < 9000L
#error "Incompatible Kaitai Struct C++/STL API: version 0.9 or later is required"
#endif
/**
* 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:
*
* * DWARF debug file format, where it's dubbed "unsigned LEB128" or "ULEB128".
* <https://dwarfstd.org/doc/dwarf-2.0.0.pdf> - page 139
* * Google Protocol Buffers, where it's called "Base 128 Varints".
* <https://protobuf.dev/programming-guides/encoding/#varints>
* * Apache Lucene, where it's called "VInt"
* <https://lucene.apache.org/core/3_5_0/fileformats.html#VInt>
* * Apache Avro uses this as a basis for integer encoding, adding ZigZag on
* top of it for signed ints
* <https://avro.apache.org/docs/1.12.0/specification/#primitive-types-1>
*
* 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>.
*/
class vlq_base128_le_t : public kaitai::kstruct {
public:
class group_t;
vlq_base128_le_t(kaitai::kstream* p__io, kaitai::kstruct* p__parent = nullptr, vlq_base128_le_t* p__root = nullptr);
private:
void _read();
void _clean_up();
public:
~vlq_base128_le_t();
/**
* One byte group, clearly divided into 7-bit "value" chunk and 1-bit "continuation" flag.
*/
class group_t : public kaitai::kstruct {
public:
group_t(int32_t p_idx, uint64_t p_prev_interm_value, uint64_t p_multiplier, kaitai::kstream* p__io, vlq_base128_le_t* p__parent = nullptr, vlq_base128_le_t* p__root = nullptr);
private:
void _read();
void _clean_up();
public:
~group_t();
private:
bool f_interm_value;
uint64_t m_interm_value;
public:
uint64_t interm_value();
private:
bool m_has_next;
uint64_t m_value;
int32_t m_idx;
uint64_t m_prev_interm_value;
uint64_t m_multiplier;
vlq_base128_le_t* m__root;
vlq_base128_le_t* m__parent;
public:
/**
* If `true`, then we have more bytes to read.
*
* Since this implementation only supports serialized values up to 10
* bytes, this must be `false` in the 10th group (`groups[9]`).
*/
bool has_next() const { return m_has_next; }
/**
* The 7-bit (base128) numeric value chunk of this group
*
* Since this implementation only supports integer values up to 64 bits,
* the `value` in the 10th group (`groups[9]`) can only be `0` or `1`
* (otherwise the width of the represented value would be 65 bits or
* more, which is not supported).
*/
uint64_t value() const { return m_value; }
int32_t idx() const { return m_idx; }
uint64_t prev_interm_value() const { return m_prev_interm_value; }
uint64_t multiplier() const { return m_multiplier; }
vlq_base128_le_t* _root() const { return m__root; }
vlq_base128_le_t* _parent() const { return m__parent; }
};
private:
bool f_len;
int32_t m_len;
public:
int32_t len();
private:
bool f_value;
uint64_t m_value;
public:
/**
* Resulting unsigned value as normal integer
*/
uint64_t value();
private:
bool f_sign_bit;
uint64_t m_sign_bit;
public:
uint64_t sign_bit();
private:
bool f_value_signed;
int64_t m_value_signed;
public:
int64_t value_signed();
private:
std::unique_ptr<std::vector<std::unique_ptr<group_t>>> m_groups;
vlq_base128_le_t* m__root;
kaitai::kstruct* m__parent;
public:
std::vector<std::unique_ptr<group_t>>* groups() const { return m_groups.get(); }
vlq_base128_le_t* _root() const { return m__root; }
kaitai::kstruct* _parent() const { return m__parent; }
};
// This is a generated file! Please edit source .ksy file and use kaitai-struct-compiler to rebuild
#include "vlq_base128_le.h"
#include "kaitai/exceptions.h"
vlq_base128_le_t::vlq_base128_le_t(kaitai::kstream* p__io, kaitai::kstruct* p__parent, vlq_base128_le_t* p__root) : kaitai::kstruct(p__io) {
m__parent = p__parent;
m__root = this;
m_groups = nullptr;
f_len = false;
f_value = false;
f_sign_bit = false;
f_value_signed = false;
_read();
}
void vlq_base128_le_t::_read() {
m_groups = std::unique_ptr<std::vector<std::unique_ptr<group_t>>>(new std::vector<std::unique_ptr<group_t>>());
{
int i = 0;
group_t* _;
do {
_ = new group_t(i, ((i != 0) ? (groups()->at((i - 1))->interm_value()) : (0)), ((i != 0) ? (((i == 9) ? (9223372036854775808ULL) : ((groups()->at((i - 1))->multiplier() * 128)))) : (1)), m__io, this, m__root);
m_groups->push_back(std::move(std::unique_ptr<group_t>(_)));
i++;
} while (!(!(_->has_next())));
}
}
vlq_base128_le_t::~vlq_base128_le_t() {
_clean_up();
}
void vlq_base128_le_t::_clean_up() {
}
vlq_base128_le_t::group_t::group_t(int32_t p_idx, uint64_t p_prev_interm_value, uint64_t p_multiplier, kaitai::kstream* p__io, vlq_base128_le_t* p__parent, vlq_base128_le_t* p__root) : kaitai::kstruct(p__io) {
m__parent = p__parent;
m__root = p__root;
m_idx = p_idx;
m_prev_interm_value = p_prev_interm_value;
m_multiplier = p_multiplier;
f_interm_value = false;
_read();
}
void vlq_base128_le_t::group_t::_read() {
m_has_next = m__io->read_bits_int_be(1);
if (!(has_next() == ((idx() == 9) ? (false) : (has_next())))) {
throw kaitai::validation_not_equal_error<bool>(((idx() == 9) ? (false) : (has_next())), has_next(), _io(), std::string("/types/group/seq/0"));
}
m_value = m__io->read_bits_int_be(7);
if (!(value() <= static_cast<uint64_t>(((idx() == 9) ? (1) : (127))))) {
throw kaitai::validation_greater_than_error<uint64_t>(static_cast<uint64_t>(((idx() == 9) ? (1) : (127))), value(), _io(), std::string("/types/group/seq/1"));
}
}
vlq_base128_le_t::group_t::~group_t() {
_clean_up();
}
void vlq_base128_le_t::group_t::_clean_up() {
}
uint64_t vlq_base128_le_t::group_t::interm_value() {
if (f_interm_value)
return m_interm_value;
m_interm_value = static_cast<uint64_t>((prev_interm_value() + (value() * multiplier())));
f_interm_value = true;
return m_interm_value;
}
int32_t vlq_base128_le_t::len() {
if (f_len)
return m_len;
m_len = groups()->size();
f_len = true;
return m_len;
}
uint64_t vlq_base128_le_t::value() {
if (f_value)
return m_value;
m_value = groups()->back()->interm_value();
f_value = true;
return m_value;
}
uint64_t vlq_base128_le_t::sign_bit() {
if (f_sign_bit)
return m_sign_bit;
m_sign_bit = static_cast<uint64_t>(((len() == 10) ? (9223372036854775808ULL) : ((groups()->back()->multiplier() * 64))));
f_sign_bit = true;
return m_sign_bit;
}
int64_t vlq_base128_le_t::value_signed() {
if (f_value_signed)
return m_value_signed;
m_value_signed = (( ((sign_bit() > 0) && (value() >= sign_bit())) ) ? (-(static_cast<int64_t>((sign_bit() - (value() - sign_bit()))))) : (static_cast<int64_t>(value())));
f_value_signed = true;
return m_value_signed;
}