Variable length quantity, unsigned integer, base128, big-endian: Lua 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: 0.7

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.

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

vlq_base128_be.lua

-- This is a generated file! Please edit source .ksy file and use kaitai-struct-compiler to rebuild
--
-- This file is compatible with Lua 5.3

local class = require("class")
require("kaitaistruct")
local utils = require("utils")

-- 
-- 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.
VlqBase128Be = class.class(KaitaiStruct)

function VlqBase128Be:_init(io, parent, root)
  KaitaiStruct._init(self, io)
  self._parent = parent
  self._root = root or self
  self:_read()
end

function VlqBase128Be:_read()
  self.groups = {}
  local i = 0
  while true do
    _ = VlqBase128Be.Group(self._io, self, self._root)
    self.groups[i + 1] = _
    if not(_.has_next) then
      break
    end
    i = i + 1
  end
end

VlqBase128Be.property.last = {}
function VlqBase128Be.property.last:get()
if self._m_last ~= nil then
  return self._m_last
end

self._m_last = (#self.groups - 1)
return self._m_last
end

-- 
-- Resulting value as normal integer.
VlqBase128Be.property.value = {}
function VlqBase128Be.property.value:get()
if self._m_value ~= nil then
  return self._m_value
end

self._m_value = (((((((self.groups[self.last + 1].value + utils.box_unwrap((self.last >= 1) and utils.box_wrap((self.groups[(self.last - 1) + 1].value << 7)) or (0))) + utils.box_unwrap((self.last >= 2) and utils.box_wrap((self.groups[(self.last - 2) + 1].value << 14)) or (0))) + utils.box_unwrap((self.last >= 3) and utils.box_wrap((self.groups[(self.last - 3) + 1].value << 21)) or (0))) + utils.box_unwrap((self.last >= 4) and utils.box_wrap((self.groups[(self.last - 4) + 1].value << 28)) or (0))) + utils.box_unwrap((self.last >= 5) and utils.box_wrap((self.groups[(self.last - 5) + 1].value << 35)) or (0))) + utils.box_unwrap((self.last >= 6) and utils.box_wrap((self.groups[(self.last - 6) + 1].value << 42)) or (0))) + utils.box_unwrap((self.last >= 7) and utils.box_wrap((self.groups[(self.last - 7) + 1].value << 49)) or (0)))
return self._m_value
end


-- 
-- One byte group, clearly divided into 7-bit "value" chunk and 1-bit "continuation" flag.
VlqBase128Be.Group = class.class(KaitaiStruct)

function VlqBase128Be.Group:_init(io, parent, root)
KaitaiStruct._init(self, io)
self._parent = parent
self._root = root or self
self:_read()
end

function VlqBase128Be.Group:_read()
self.b = self._io:read_u1()
end

-- 
-- If true, then we have more bytes to read.
VlqBase128Be.Group.property.has_next = {}
function VlqBase128Be.Group.property.has_next:get()
if self._m_has_next ~= nil then
  return self._m_has_next
end

self._m_has_next = (self.b & 128) ~= 0
return self._m_has_next
end

-- 
-- The 7-bit (base128) numeric value chunk of this group.
VlqBase128Be.Group.property.value = {}
function VlqBase128Be.Group.property.value:get()
if self._m_value ~= nil then
  return self._m_value
end

self._m_value = (self.b & 127)
return self._m_value
end