Variable length quantity, unsigned integer, base128, big-endian: Ruby 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.

Usage

Runtime library

All parsing code for Ruby generated by Kaitai Struct depends on the Ruby runtime library. You have to install it before you can parse data.

The Ruby runtime library can be installed from RubyGems:

gem install kaitai-struct

Code

Parse a local file and get structure in memory:

data = VlqBase128Be.from_file("path/to/local/file.Variable length quantity, unsigned integer, base128, big-endian")

Or parse structure from a string of bytes:

bytes = "\x00\x01\x02..."
data = VlqBase128Be.new(Kaitai::Struct::Stream.new(bytes))

After that, one can get various attributes from the structure by invoking getter methods like:

data.value # => Resulting value as normal integer

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

vlq_base128_be.rb

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

require 'kaitai/struct/struct'

unless Gem::Version.new(Kaitai::Struct::VERSION) >= Gem::Version.new('0.9')
  raise "Incompatible Kaitai Struct Ruby API: 0.9 or later is required, but you have #{Kaitai::Struct::VERSION}"
end


##
# 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 VlqBase128Be < Kaitai::Struct::Struct
  def initialize(_io, _parent = nil, _root = self)
    super(_io, _parent, _root)
    _read
  end

  def _read
    @groups = []
    i = 0
    begin
      _ = Group.new(@_io, self, @_root)
      @groups << _
      i += 1
    end until !(_.has_next)
    self
  end

  ##
  # One byte group, clearly divided into 7-bit "value" chunk and 1-bit "continuation" flag.
  class Group < Kaitai::Struct::Struct
    def initialize(_io, _parent = nil, _root = self)
      super(_io, _parent, _root)
      _read
    end

    def _read
      @b = @_io.read_u1
      self
    end

    ##
    # If true, then we have more bytes to read
    def has_next
      return @has_next unless @has_next.nil?
      @has_next = (b & 128) != 0
      @has_next
    end

    ##
    # The 7-bit (base128) numeric value chunk of this group
    def value
      return @value unless @value.nil?
      @value = (b & 127)
      @value
    end
    attr_reader :b
  end
  def last
    return @last unless @last.nil?
    @last = (groups.length - 1)
    @last
  end

  ##
  # Resulting value as normal integer
  def value
    return @value unless @value.nil?
    @value = (((((((groups[last].value + (last >= 1 ? (groups[(last - 1)].value << 7) : 0)) + (last >= 2 ? (groups[(last - 2)].value << 14) : 0)) + (last >= 3 ? (groups[(last - 3)].value << 21) : 0)) + (last >= 4 ? (groups[(last - 4)].value << 28) : 0)) + (last >= 5 ? (groups[(last - 5)].value << 35) : 0)) + (last >= 6 ? (groups[(last - 6)].value << 42) : 0)) + (last >= 7 ? (groups[(last - 7)].value << 49) : 0))
    @value
  end
  attr_reader :groups
end