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:
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.
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.
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_be_t data(&ks);
After that, one can get various attributes from the structure by invoking getter methods like:
data.value() // => Resulting 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 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.
*/
class vlq_base128_be_t : public kaitai::kstruct {
public:
class group_t;
vlq_base128_be_t(kaitai::kstream* p__io, kaitai::kstruct* p__parent = nullptr, vlq_base128_be_t* p__root = nullptr);
private:
void _read();
void _clean_up();
public:
~vlq_base128_be_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(kaitai::kstream* p__io, vlq_base128_be_t* p__parent = nullptr, vlq_base128_be_t* p__root = nullptr);
private:
void _read();
void _clean_up();
public:
~group_t();
private:
bool m_has_next;
uint64_t m_value;
vlq_base128_be_t* m__root;
vlq_base128_be_t* m__parent;
public:
/**
* If true, then we have more bytes to read
*/
bool has_next() const { return m_has_next; }
/**
* The 7-bit (base128) numeric value chunk of this group
*/
uint64_t value() const { return m_value; }
vlq_base128_be_t* _root() const { return m__root; }
vlq_base128_be_t* _parent() const { return m__parent; }
};
private:
bool f_last;
int32_t m_last;
public:
int32_t last();
private:
bool f_value;
uint64_t m_value;
public:
/**
* Resulting value as normal integer
*/
uint64_t value();
private:
std::unique_ptr<std::vector<std::unique_ptr<group_t>>> m_groups;
vlq_base128_be_t* m__root;
kaitai::kstruct* m__parent;
public:
std::vector<std::unique_ptr<group_t>>* groups() const { return m_groups.get(); }
vlq_base128_be_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_be.h"
vlq_base128_be_t::vlq_base128_be_t(kaitai::kstream* p__io, kaitai::kstruct* p__parent, vlq_base128_be_t* p__root) : kaitai::kstruct(p__io) {
m__parent = p__parent;
m__root = this;
m_groups = nullptr;
f_last = false;
f_value = false;
_read();
}
void vlq_base128_be_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(m__io, this, m__root);
m_groups->push_back(std::move(std::unique_ptr<group_t>(_)));
i++;
} while (!(!(_->has_next())));
}
}
vlq_base128_be_t::~vlq_base128_be_t() {
_clean_up();
}
void vlq_base128_be_t::_clean_up() {
}
vlq_base128_be_t::group_t::group_t(kaitai::kstream* p__io, vlq_base128_be_t* p__parent, vlq_base128_be_t* p__root) : kaitai::kstruct(p__io) {
m__parent = p__parent;
m__root = p__root;
_read();
}
void vlq_base128_be_t::group_t::_read() {
m_has_next = m__io->read_bits_int_be(1);
m_value = m__io->read_bits_int_be(7);
}
vlq_base128_be_t::group_t::~group_t() {
_clean_up();
}
void vlq_base128_be_t::group_t::_clean_up() {
}
int32_t vlq_base128_be_t::last() {
if (f_last)
return m_last;
m_last = (groups()->size() - 1);
f_last = true;
return m_last;
}
uint64_t vlq_base128_be_t::value() {
if (f_value)
return m_value;
m_value = static_cast<uint64_t>((((((((groups()->at(last())->value() + ((last() >= 1) ? ((groups()->at((last() - 1))->value() << 7)) : (0))) + ((last() >= 2) ? ((groups()->at((last() - 2))->value() << 14)) : (0))) + ((last() >= 3) ? ((groups()->at((last() - 3))->value() << 21)) : (0))) + ((last() >= 4) ? ((groups()->at((last() - 4))->value() << 28)) : (0))) + ((last() >= 5) ? ((groups()->at((last() - 5))->value() << 35)) : (0))) + ((last() >= 6) ? ((groups()->at((last() - 6))->value() << 42)) : (0))) + ((last() >= 7) ? ((groups()->at((last() - 7))->value() << 49)) : (0))));
f_value = true;
return m_value;
}