= Packed \Data

Certain Ruby core methods deal with packing and unpacking data:

- \Method Array#pack:
  Formats each element in array +self+ into a binary string;
  returns that string.
- \Method String#unpack:
  Extracts data from string +self+,
  forming objects that become the elements of a new array;
  returns that array.
- \Method String#unpack1:
  Does the same, but unpacks and returns only the first extracted object.

Each of these methods accepts a string +template+,
consisting of zero or more _directive_ characters,
each followed by zero or more _modifier_ characters.

Examples (directive <tt>'C'</tt> specifies 'unsigned character'):

  [65].pack('C')      # => "A"  # One element, one directive.
  [65, 66].pack('CC') # => "AB" # Two elements, two directives.
  [65, 66].pack('C')  # => "A"  # Extra element is ignored.
  [65].pack('')       # => ""   # No directives.
  [65].pack('CC')               # Extra directive raises ArgumentError.

  'A'.unpack('C')   # => [65]      # One character, one directive.
  'AB'.unpack('CC') # => [65, 66]  # Two characters, two directives.
  'AB'.unpack('C')  # => [65]      # Extra character is ignored.
  'A'.unpack('CC')  # => [65, nil] # Extra directive generates nil.
  'AB'.unpack('')   # => []        # No directives.

The string +template+ may contain any mixture of valid directives
(directive <tt>'c'</tt> specifies 'signed character'):

  [65, -1].pack('cC')  # => "A\xFF"
  "A\xFF".unpack('cC') # => [65, 255]

The string +template+ may contain whitespace (which is ignored)
and comments, each of which begins with character <tt>'#'</tt>
and continues up to and including the next following newline:

  [0,1].pack("  C  #foo \n  C  ")    # => "\x00\x01"
  "\0\1".unpack("  C  #foo \n  C  ") # => [0, 1]

Any directive may be followed by either of these modifiers:

- <tt>'*'</tt> - The directive is to be applied as many times as needed:

    [65, 66].pack('C*') # => "AB"
    'AB'.unpack('C*')   # => [65, 66]

- Integer +count+ - The directive is to be applied +count+ times:

    [65, 66].pack('C2') # => "AB"
    [65, 66].pack('C3') # Raises ArgumentError.
    'AB'.unpack('C2')   # => [65, 66]
    'AB'.unpack('C3')   # => [65, 66, nil]

  Note: Directives in <tt>%w[A a Z m]</tt> use +count+ differently;
  see {String Directives}[rdoc-ref:packed_data.rdoc@String+Directives].

If elements don't fit the provided directive, only least significant bits are encoded:

    [257].pack("C").unpack("C") # => [1]

== Packing \Method

\Method Array#pack accepts optional keyword argument
+buffer+ that specifies the target string (instead of a new string):

  [65, 66].pack('C*', buffer: 'foo') # => "fooAB"

The method can accept a block:

  # Packed string is passed to the block.
  [65, 66].pack('C*') {|s| p s }    # => "AB"

== Unpacking Methods

Methods String#unpack and String#unpack1 each accept
an optional keyword argument +offset+ that specifies an offset
into the string:

  'ABC'.unpack('C*', offset: 1)  # => [66, 67]
  'ABC'.unpack1('C*', offset: 1) # => 66

Both methods can accept a block:

  # Each unpacked object is passed to the block.
  ret = []
  "ABCD".unpack("C*") {|c| ret << c }
  ret # => [65, 66, 67, 68]

  # The single unpacked object is passed to the block.
  'AB'.unpack1('C*') {|ele| p ele } # => 65

== \Integer Directives

Each integer directive specifies the packing or unpacking
for one element in the input or output array.

=== 8-Bit \Integer Directives

- <tt>'c'</tt> - 8-bit signed integer
  (like C <tt>signed char</tt>):

    [0, 1, 255].pack('c*')  # => "\x00\x01\xFF"
    s = [0, 1, -1].pack('c*') # => "\x00\x01\xFF"
    s.unpack('c*') # => [0, 1, -1]

- <tt>'C'</tt> - 8-bit unsigned integer
  (like C <tt>unsigned char</tt>):

    [0, 1, 255].pack('C*')    # => "\x00\x01\xFF"
    s = [0, 1, -1].pack('C*') # => "\x00\x01\xFF"
    s.unpack('C*')            # => [0, 1, 255]

=== 16-Bit \Integer Directives

- <tt>'s'</tt> - 16-bit signed integer, native-endian
  (like C <tt>int16_t</tt>):

    [513, -514].pack('s*')      # => "\x01\x02\xFE\xFD"
    s = [513, 65022].pack('s*') # => "\x01\x02\xFE\xFD"
    s.unpack('s*')              # => [513, -514]

- <tt>'S'</tt> - 16-bit unsigned integer, native-endian
  (like C <tt>uint16_t</tt>):

    [513, -514].pack('S*')      # => "\x01\x02\xFE\xFD"
    s = [513, 65022].pack('S*') # => "\x01\x02\xFE\xFD"
    s.unpack('S*')              # => [513, 65022]

- <tt>'n'</tt> - 16-bit network integer, big-endian:

    s = [0, 1, -1, 32767, -32768, 65535].pack('n*')
    # => "\x00\x00\x00\x01\xFF\xFF\x7F\xFF\x80\x00\xFF\xFF"
    s.unpack('n*')
    # => [0, 1, 65535, 32767, 32768, 65535]

- <tt>'v'</tt> - 16-bit VAX integer, little-endian:

    s = [0, 1, -1, 32767, -32768, 65535].pack('v*')
    # => "\x00\x00\x01\x00\xFF\xFF\xFF\x7F\x00\x80\xFF\xFF"
    s.unpack('v*')
    # => [0, 1, 65535, 32767, 32768, 65535]

=== 32-Bit \Integer Directives

- <tt>'l'</tt> - 32-bit signed integer, native-endian
  (like C <tt>int32_t</tt>):

    s = [67305985, -50462977].pack('l*')
    # => "\x01\x02\x03\x04\xFF\xFE\xFD\xFC"
    s.unpack('l*')
    # => [67305985, -50462977]

- <tt>'L'</tt> - 32-bit unsigned integer, native-endian
  (like C <tt>uint32_t</tt>):

    s = [67305985, 4244504319].pack('L*')
    # => "\x01\x02\x03\x04\xFF\xFE\xFD\xFC"
    s.unpack('L*')
    # => [67305985, 4244504319]

- <tt>'N'</tt> - 32-bit network integer, big-endian:

    s = [0,1,-1].pack('N*')
    # => "\x00\x00\x00\x00\x00\x00\x00\x01\xFF\xFF\xFF\xFF"
    s.unpack('N*')
    # => [0, 1, 4294967295]

- <tt>'V'</tt> - 32-bit VAX integer, little-endian:

    s = [0,1,-1].pack('V*')
    # => "\x00\x00\x00\x00\x01\x00\x00\x00\xFF\xFF\xFF\xFF"
    s.unpack('v*')
    # => [0, 0, 1, 0, 65535, 65535]

=== 64-Bit \Integer Directives

- <tt>'q'</tt> - 64-bit signed integer, native-endian
  (like C <tt>int64_t</tt>):

    s = [578437695752307201, -506097522914230529].pack('q*')
    # => "\x01\x02\x03\x04\x05\x06\a\b\xFF\xFE\xFD\xFC\xFB\xFA\xF9\xF8"
    s.unpack('q*')
    # => [578437695752307201, -506097522914230529]

- <tt>'Q'</tt> - 64-bit unsigned integer, native-endian
  (like C <tt>uint64_t</tt>):

    s = [578437695752307201, 17940646550795321087].pack('Q*')
    # => "\x01\x02\x03\x04\x05\x06\a\b\xFF\xFE\xFD\xFC\xFB\xFA\xF9\xF8"
    s.unpack('Q*')
    # => [578437695752307201, 17940646550795321087]

=== Platform-Dependent \Integer Directives

- <tt>'i'</tt> - Platform-dependent width signed integer,
  native-endian (like C <tt>int</tt>):

    s = [67305985, -50462977].pack('i*')
    # => "\x01\x02\x03\x04\xFF\xFE\xFD\xFC"
    s.unpack('i*')
    # => [67305985, -50462977]

- <tt>'I'</tt> - Platform-dependent width unsigned integer,
  native-endian (like C <tt>unsigned int</tt>):

    s = [67305985, -50462977].pack('I*')
    # => "\x01\x02\x03\x04\xFF\xFE\xFD\xFC"
    s.unpack('I*')
    # => [67305985, 4244504319]

- <tt>'j'</tt> - Pointer-width signed integer, native-endian
  (like C <tt>intptr_t</tt>):

    s = [67305985, -50462977].pack('j*')
    # => "\x01\x02\x03\x04\x00\x00\x00\x00\xFF\xFE\xFD\xFC\xFF\xFF\xFF\xFF"
    s.unpack('j*')
    # => [67305985, -50462977]

- <tt>'J'</tt> - Pointer-width unsigned integer, native-endian
  (like C <tt>uintptr_t</tt>):

    s = [67305985, 4244504319].pack('J*')
    # => "\x01\x02\x03\x04\x00\x00\x00\x00\xFF\xFE\xFD\xFC\x00\x00\x00\x00"
    s.unpack('J*')
    # => [67305985, 4244504319]

=== Other \Integer Directives

- <tt>'U'</tt> - UTF-8 character:

    s = [4194304].pack('U*')
    # => "\xF8\x90\x80\x80\x80"
    s.unpack('U*')
    # => [4194304]

- <tt>'w'</tt> - BER-encoded integer
  (see {BER encoding}[https://en.wikipedia.org/wiki/X.690#BER_encoding]):

    s = [1073741823].pack('w*')
    # => "\x83\xFF\xFF\xFF\x7F"
    s.unpack('w*')
    # => [1073741823]

=== Modifiers for \Integer Directives

For the following directives, <tt>'!'</tt> or <tt>'_'</tt> modifiers may be
suffixed as underlying platform’s native size.

- <tt>'i'</tt>, <tt>'I'</tt> - C <tt>int</tt>, always native size.
- <tt>'s'</tt>, <tt>'S'</tt> - C <tt>short</tt>.
- <tt>'l'</tt>, <tt>'L'</tt> - C <tt>long</tt>.
- <tt>'q'</tt>, <tt>'Q'</tt> - C <tt>long long</tt>, if available.
- <tt>'j'</tt>, <tt>'J'</tt> - C <tt>intptr_t</tt>, always native size.

Native size modifiers are silently ignored for always native size directives.

The endian modifiers also may be suffixed in the directives above:

- <tt>'>'</tt> - Big-endian.
- <tt>'<'</tt> - Little-endian.

== \Float Directives

Each float directive specifies the packing or unpacking
for one element in the input or output array.

=== Single-Precision \Float Directives

- <tt>'F'</tt> or <tt>'f'</tt> - Native format:

    s = [3.0].pack('F') # => "\x00\x00@@"
    s.unpack('F')       # => [3.0]

- <tt>'e'</tt> - Little-endian:

    s = [3.0].pack('e') # => "\x00\x00@@"
    s.unpack('e')       # => [3.0]

- <tt>'g'</tt> - Big-endian:

    s = [3.0].pack('g') # => "@@\x00\x00"
    s.unpack('g')       # => [3.0]

=== Double-Precision \Float Directives

- <tt>'D'</tt> or <tt>'d'</tt> - Native format:

    s = [3.0].pack('D') # => "\x00\x00\x00\x00\x00\x00\b@"
    s.unpack('D')       # => [3.0]

- <tt>'E'</tt> - Little-endian:

    s = [3.0].pack('E') # => "\x00\x00\x00\x00\x00\x00\b@"
    s.unpack('E')       # => [3.0]

- <tt>'G'</tt> - Big-endian:

    s = [3.0].pack('G') # => "@\b\x00\x00\x00\x00\x00\x00"
    s.unpack('G')       # => [3.0]

A float directive may be infinity or not-a-number:

  inf = 1.0/0.0                  # => Infinity
  [inf].pack('f')                # => "\x00\x00\x80\x7F"
  "\x00\x00\x80\x7F".unpack('f') # => [Infinity]

  nan = inf/inf                  # => NaN
  [nan].pack('f')                # => "\x00\x00\xC0\x7F"
  "\x00\x00\xC0\x7F".unpack('f') # => [NaN]

== \String Directives

Each string directive specifies the packing or unpacking
for one byte in the input or output string.

=== Binary \String Directives

- <tt>'A'</tt> - Arbitrary binary string (space padded; count is width);
  +nil+ is treated as the empty string:

    ['foo'].pack('A')    # => "f"
    ['foo'].pack('A*')   # => "foo"
    ['foo'].pack('A2')   # => "fo"
    ['foo'].pack('A4')   # => "foo "
    [nil].pack('A')      # => " "
    [nil].pack('A*')     # => ""
    [nil].pack('A2')     # => "  "
    [nil].pack('A4')     # => "    "

    "foo\0".unpack('A')      # => ["f"]
    "foo\0".unpack('A4')     # => ["foo"]
    "foo\0bar".unpack('A10') # => ["foo\x00bar"] # Reads past "\0".
    "foo ".unpack('A')       # => ["f"]
    "foo ".unpack('A4')      # => ["foo"]
    "foo".unpack('A4')       # => ["foo"]

    russian = "\u{442 435 441 442}" # => "тест"
    russian.size         # => 4
    russian.bytesize     # => 8
    [russian].pack('A')  # => "\xD1"
    [russian].pack('A*') # => "\xD1\x82\xD0\xB5\xD1\x81\xD1\x82"
    russian.unpack('A')  # => ["\xD1"]
    russian.unpack('A2') # => ["\xD1\x82"]
    russian.unpack('A4') # => ["\xD1\x82\xD0\xB5"]
    russian.unpack('A*') # => ["\xD1\x82\xD0\xB5\xD1\x81\xD1\x82"]

- <tt>'a'</tt> - Arbitrary binary string (null padded; count is width):

    ["foo"].pack('a')    # => "f"
    ["foo"].pack('a*')   # => "foo"
    ["foo"].pack('a2')   # => "fo"
    ["foo\0"].pack('a4') # => "foo\x00"
    [nil].pack('a')      # => "\x00"
    [nil].pack('a*')     # => ""
    [nil].pack('a2')     # => "\x00\x00"
    [nil].pack('a4')     # => "\x00\x00\x00\x00"

    "foo\0".unpack('a')     # => ["f"]
    "foo\0".unpack('a4')    # => ["foo\x00"]
    "foo ".unpack('a4')     # => ["foo "]
    "foo".unpack('a4')      # => ["foo"]
    "foo\0bar".unpack('a4') # => ["foo\x00"] # Reads past "\0".

- <tt>'Z'</tt> - Same as <tt>'a'</tt>,
  except that null is added or ignored with <tt>'*'</tt>:

    ["foo"].pack('Z*')   # => "foo\x00"
    [nil].pack('Z*')     # => "\x00"

    "foo\0".unpack('Z*')    # => ["foo"]
    "foo".unpack('Z*')      # => ["foo"]
    "foo\0bar".unpack('Z*') # => ["foo"] # Does not read past "\0".

=== Bit \String Directives

- <tt>'B'</tt> - Bit string (high byte first):

    ['11111111' + '00000000'].pack('B*') # => "\xFF\x00"
    ['10000000' + '01000000'].pack('B*') # => "\x80@"

    ['1'].pack('B0') # => ""
    ['1'].pack('B1') # => "\x80"
    ['1'].pack('B2') # => "\x80\x00"
    ['1'].pack('B3') # => "\x80\x00"
    ['1'].pack('B4') # => "\x80\x00\x00"
    ['1'].pack('B5') # => "\x80\x00\x00"
    ['1'].pack('B6') # => "\x80\x00\x00\x00"

    "\xff\x00".unpack("B*") # => ["1111111100000000"]
    "\x01\x02".unpack("B*") # => ["0000000100000010"]

    "".unpack("B0")     # => [""]
    "\x80".unpack("B1") # => ["1"]
    "\x80".unpack("B2") # => ["10"]
    "\x80".unpack("B3") # => ["100"]

- <tt>'b'</tt> - Bit string (low byte first):

    ['11111111' + '00000000'].pack('b*') # => "\xFF\x00"
    ['10000000' + '01000000'].pack('b*') # => "\x01\x02"

    ['1'].pack('b0') # => ""
    ['1'].pack('b1') # => "\x01"
    ['1'].pack('b2') # => "\x01\x00"
    ['1'].pack('b3') # => "\x01\x00"
    ['1'].pack('b4') # => "\x01\x00\x00"
    ['1'].pack('b5') # => "\x01\x00\x00"
    ['1'].pack('b6') # => "\x01\x00\x00\x00"

    "\xff\x00".unpack("b*") # => ["1111111100000000"]
    "\x01\x02".unpack("b*") # => ["1000000001000000"]

    "".unpack("b0")     # => [""]
    "\x01".unpack("b1") # => ["1"]
    "\x01".unpack("b2") # => ["10"]
    "\x01".unpack("b3") # => ["100"]

=== Hex \String Directives

- <tt>'H'</tt> - Hex string (high nibble first):

    ['10ef'].pack('H*')    # => "\x10\xEF"
    ['10ef'].pack('H0')    # => ""
    ['10ef'].pack('H3')    # => "\x10\xE0"
    ['10ef'].pack('H5')    # => "\x10\xEF\x00"

    ['fff'].pack('H3')    # => "\xFF\xF0"
    ['fff'].pack('H4')    # => "\xFF\xF0"
    ['fff'].pack('H5')    # => "\xFF\xF0\x00"
    ['fff'].pack('H6')    # => "\xFF\xF0\x00"
    ['fff'].pack('H7')    # => "\xFF\xF0\x00\x00"
    ['fff'].pack('H8')    # => "\xFF\xF0\x00\x00"

    "\x10\xef".unpack('H*')    # => ["10ef"]
    "\x10\xef".unpack('H0')    # => [""]
    "\x10\xef".unpack('H1')    # => ["1"]
    "\x10\xef".unpack('H2')    # => ["10"]
    "\x10\xef".unpack('H3')    # => ["10e"]
    "\x10\xef".unpack('H4')    # => ["10ef"]
    "\x10\xef".unpack('H5')    # => ["10ef"]

- <tt>'h'</tt> - Hex string (low nibble first):

    ['10ef'].pack('h*') # => "\x01\xFE"
    ['10ef'].pack('h0') # => ""
    ['10ef'].pack('h3') # => "\x01\x0E"
    ['10ef'].pack('h5') # => "\x01\xFE\x00"

    ['fff'].pack('h3') # => "\xFF\x0F"
    ['fff'].pack('h4') # => "\xFF\x0F"
    ['fff'].pack('h5') # => "\xFF\x0F\x00"
    ['fff'].pack('h6') # => "\xFF\x0F\x00"
    ['fff'].pack('h7') # => "\xFF\x0F\x00\x00"
    ['fff'].pack('h8') # => "\xFF\x0F\x00\x00"

    "\x01\xfe".unpack('h*') # => ["10ef"]
    "\x01\xfe".unpack('h0') # => [""]
    "\x01\xfe".unpack('h1') # => ["1"]
    "\x01\xfe".unpack('h2') # => ["10"]
    "\x01\xfe".unpack('h3') # => ["10e"]
    "\x01\xfe".unpack('h4') # => ["10ef"]
    "\x01\xfe".unpack('h5') # => ["10ef"]

=== Pointer \String Directives

- <tt>'P'</tt> - Pointer to a structure (fixed-length string):

    s = ['abc'].pack('P')  # => "\xE0O\x7F\xE5\xA1\x01\x00\x00"
    s.unpack('P*')         # => ["abc"]
    ".".unpack("P")        # => []
    ("\0" * 8).unpack("P") # => [nil]
    [nil].pack("P")        # => "\x00\x00\x00\x00\x00\x00\x00\x00"

- <tt>'p'</tt> - Pointer to a null-terminated string:

    s = ['abc'].pack('p')  # => "(\xE4u\xE5\xA1\x01\x00\x00"
    s.unpack('p*')         # => ["abc"]
    ".".unpack("p")        # => []
    ("\0" * 8).unpack("p") # => [nil]
    [nil].pack("p")        # => "\x00\x00\x00\x00\x00\x00\x00\x00"

=== Other \String Directives

- <tt>'M'</tt> - Quoted printable, MIME encoding;
  text mode, but input must use LF and output LF;
  (see {RFC 2045}[https://www.ietf.org/rfc/rfc2045.txt]):

    ["a b c\td \ne"].pack('M') # => "a b c\td =\n\ne=\n"
    ["\0"].pack('M')           # => "=00=\n"

    ["a"*1023].pack('M') == ("a"*73+"=\n")*14+"a=\n"     # => true
    ("a"*73+"=\na=\n").unpack('M') == ["a"*74]           # => true
    (("a"*73+"=\n")*14+"a=\n").unpack('M') == ["a"*1023] # => true

    "a b c\td =\n\ne=\n".unpack('M')    # => ["a b c\td \ne"]
    "=00=\n".unpack('M')    # => ["\x00"]

    "pre=31=32=33after".unpack('M') # => ["pre123after"]
    "pre=\nafter".unpack('M')       # => ["preafter"]
    "pre=\r\nafter".unpack('M')     # => ["preafter"]
    "pre=".unpack('M')              # => ["pre="]
    "pre=\r".unpack('M')            # => ["pre=\r"]
    "pre=hoge".unpack('M')          # => ["pre=hoge"]
    "pre==31after".unpack('M')      # => ["pre==31after"]
    "pre===31after".unpack('M')     # => ["pre===31after"]

- <tt>'m'</tt> - Base64 encoded string;
  count specifies input bytes between each newline,
  rounded down to nearest multiple of 3;
  if count is zero, no newlines are added;
  (see {RFC 4648}[https://www.ietf.org/rfc/rfc4648.txt]):

    [""].pack('m')             # => ""
    ["\0"].pack('m')           # => "AA==\n"
    ["\0\0"].pack('m')         # => "AAA=\n"
    ["\0\0\0"].pack('m')       # => "AAAA\n"
    ["\377"].pack('m')         # => "/w==\n"
    ["\377\377"].pack('m')     # => "//8=\n"
    ["\377\377\377"].pack('m') # => "////\n"

    "".unpack('m')       # => [""]
    "AA==\n".unpack('m') # => ["\x00"]
    "AAA=\n".unpack('m') # => ["\x00\x00"]
    "AAAA\n".unpack('m') # => ["\x00\x00\x00"]
    "/w==\n".unpack('m') # => ["\xFF"]
    "//8=\n".unpack('m') # => ["\xFF\xFF"]
    "////\n".unpack('m') # => ["\xFF\xFF\xFF"]
    "A\n".unpack('m')    # => [""]
    "AA\n".unpack('m')   # => ["\x00"]
    "AA=\n".unpack('m')  # => ["\x00"]
    "AAA\n".unpack('m')  # => ["\x00\x00"]

    [""].pack('m0')             # => ""
    ["\0"].pack('m0')           # => "AA=="
    ["\0\0"].pack('m0')         # => "AAA="
    ["\0\0\0"].pack('m0')       # => "AAAA"
    ["\377"].pack('m0')         # => "/w=="
    ["\377\377"].pack('m0')     # => "//8="
    ["\377\377\377"].pack('m0') # => "////"

    "".unpack('m0')     # => [""]
    "AA==".unpack('m0') # => ["\x00"]
    "AAA=".unpack('m0') # => ["\x00\x00"]
    "AAAA".unpack('m0') # => ["\x00\x00\x00"]
    "/w==".unpack('m0') # => ["\xFF"]
    "//8=".unpack('m0') # => ["\xFF\xFF"]
    "////".unpack('m0') # => ["\xFF\xFF\xFF"]

- <tt>'u'</tt> - UU-encoded string:

    [""].pack("u")        # => ""
    ["a"].pack("u")       # => "!80``\n"
    ["aaa"].pack("u")     # => "#86%A\n"

    "".unpack("u")        # => [""]
    "#86)C\n".unpack("u") # => ["abc"]

== Offset Directives

- <tt>'@'</tt> - Begin packing at the given byte offset;
  for packing, null fill or shrink if necessary:

    [1, 2].pack("C@0C")     # => "\x02"
    [1, 2].pack("C@1C")     # => "\x01\x02"
    [1, 2].pack("C@5C")     # => "\x01\x00\x00\x00\x00\x02"
    [*1..5].pack("CCCC@2C") # => "\x01\x02\x05"

  For unpacking, cannot to move to outside the string:

    "\x01\x00\x00\x02".unpack("C@3C") # => [1, 2]
    "\x00".unpack("@1C")              # => [nil]
    "\x00".unpack("@2C")              # Raises ArgumentError.

- <tt>'X'</tt> - For packing, shrink for the given byte offset:

    [0, 1, 2].pack("CCXC")    # => "\x00\x02"
    [0, 1, 2].pack("CCX2C")   # => "\x02"

  For unpacking; rewind unpacking position for the given byte offset:

    "\x00\x02".unpack("CCXC") # => [0, 2, 2]

  Cannot to move to outside the string:

    [0, 1, 2].pack("CCX3C")   # Raises ArgumentError.
    "\x00\x02".unpack("CX3C") # Raises ArgumentError.

- <tt>'x'</tt> - Begin packing at after the given byte offset;
  for packing, null fill if necessary:

    [].pack("x0")                # => ""
    [].pack("x")                 # => "\x00"
    [].pack("x8")                # => "\x00\x00\x00\x00\x00\x00\x00\x00"

  For unpacking, cannot to move to outside the string:

    "\x00\x00\x02".unpack("CxC") # => [0, 2]
    "\x00\x00\x02".unpack("x3C") # => [nil]
    "\x00\x00\x02".unpack("x4C") # Raises ArgumentError
