[ruby/base64] Enhanced Rdoc for Base64

(https://github.com/ruby/base64/pull/7)

* Enhanced Rdoc for Base64

* Enhanced RDoc for Base64

https://github.com/ruby/base64/commit/155c39a949

1 files changed, 310 insertions, 60 deletions

diff --git a/lib/base64.rb b/lib/base64.rb
index 15c0b297ce..59f66f8fd5 100644
--- a/lib/base64.rb
+++ b/lib/base64.rb
@@ -1,88 +1,330 @@
 # frozen_string_literal: true
 #
-# = base64.rb: methods for base64-encoding and -decoding strings
+# \Module \Base64 provides methods for:
 #
-
-# The Base64 module provides for the encoding (#encode64, #strict_encode64,
-# #urlsafe_encode64) and decoding (#decode64, #strict_decode64,
-# #urlsafe_decode64) of binary data using a Base64 representation.
+# - Encoding a binary string (containing non-ASCII characters)
+#   as a string of printable ASCII characters.
+# - Decoding such an encoded string.
 #
-# == Example
+# \Base64 is commonly used in contexts where binary data
+# is not allowed or supported:
 #
-# A simple encoding and decoding.
+# - Images in HTML or CSS files, or in URLs.
+# - Email attachments.
 #
-#     require "base64"
+# A \Base64-encoded string is about one-third larger that its source.
+# See the {Wikipedia article}[https://en.wikipedia.org/wiki/Base64]
+# for more information.
 #
-#     enc   = Base64.encode64('Send reinforcements')
-#                         # -> "U2VuZCByZWluZm9yY2VtZW50cw==\n"
-#     plain = Base64.decode64(enc)
-#                         # -> "Send reinforcements"
+# This module provides three pairs of encode/decode methods.
+# Your choices among these methods should depend on:
+#
+# - Which character set is to be used for encoding and decoding.
+# - Whether "padding" is to be used.
+# - Whether encoded strings are to contain newlines.
+#
+# Note: Examples on this page assume that the including program has executed:
+#
+#   require 'base64'
+#
+# == Encoding Character Sets
+#
+# A \Base64-encoded string consists only of characters from a 64-character set:
+#
+# - <tt>('A'..'Z')</tt>.
+# - <tt>('a'..'z')</tt>.
+# - <tt>('0'..'9')</tt>.
+# - <tt>=</tt>, the 'padding' character.
+# - Either:
+#   - <tt>%w[+ /]</tt>:
+#     {RFC-2045-compliant}[https://datatracker.ietf.org/doc/html/rfc2045];
+#     _not_ safe for URLs.
+#   - <tt>%w[- _]</tt>:
+#     {RFC-4648-compliant}[https://datatracker.ietf.org/doc/html/rfc4648];
+#     safe for URLs.
+#
+# If you are working with \Base64-encoded strings that will come from
+# or be put into URLs, you should choose this encoder-decoder pair
+# of RFC-4648-compliant methods:
+#
+# - Base64.urlsafe_encode64 and Base64.urlsafe_decode64.
+#
+# Otherwise, you may choose any of the pairs in this module,
+# including the pair above, or the RFC-2045-compliant pairs:
+#
+# - Base64.encode64 and Base64.decode64.
+# - Base64.strict_encode64 and Base64.strict_decode64.
+#
+# == Padding
+#
+# \Base64-encoding changes a triplet of input bytes
+# into a quartet of output characters.
+#
+# <b>Padding in Encode Methods</b>
+#
+# Padding -- extending an encoded string with zero, one, or two trailing
+# <tt>=</tt> characters -- is performed by methods Base64.encode64,
+# Base64.strict_encode64, and, by default, Base64.urlsafe_encode64:
+#
+#   Base64.encode64('s')                         # => "cw==\n"
+#   Base64.strict_encode64('s')                  # => "cw=="
+#   Base64.urlsafe_encode64('s')                 # => "cw=="
+#   Base64.urlsafe_encode64('s', padding: false) # => "cw"
+#
+# When padding is performed, the encoded string is always of length <em>4n</em>,
+# where +n+ is a non-negative integer:
+#
+# - Input bytes of length <em>3n</em> generate unpadded output characters
+#   of length <em>4n</em>:
+#
+#     # n = 1:  3 bytes => 4 characters.
+#     Base64.strict_encode64('123')      # => "MDEy"
+#     # n = 2:  6 bytes => 8 characters.
+#     Base64.strict_encode64('123456')   # => "MDEyMzQ1"
+#
+# - Input bytes of length <em>3n+1</em> generate padded output characters
+#   of length <em>4(n+1)</em>, with two padding characters at the end:
+#
+#     # n = 1:  4 bytes => 8 characters.
+#     Base64.strict_encode64('1234')     # => "MDEyMw=="
+#     # n = 2:  7 bytes => 12 characters.
+#     Base64.strict_encode64('1234567')  # => "MDEyMzQ1Ng=="
+#
+# - Input bytes of length <em>3n+2</em> generate padded output characters
+#   of length <em>4(n+1)</em>, with one padding character at the end:
+#
+#     # n = 1:  5 bytes => 8 characters.
+#     Base64.strict_encode64('12345')    # => "MDEyMzQ="
+#     # n = 2:  8 bytes => 12 characters.
+#     Base64.strict_encode64('12345678') # => "MDEyMzQ1Njc="
+#
+# When padding is suppressed, for a positive integer <em>n</em>:
+#
+# - Input bytes of length <em>3n</em> generate unpadded output characters
+#   of length <em>4n</em>:
+#
+#     # n = 1:  3 bytes => 4 characters.
+#     Base64.urlsafe_encode64('123', padding: false)      # => "MDEy"
+#     # n = 2:  6 bytes => 8 characters.
+#     Base64.urlsafe_encode64('123456', padding: false)   # => "MDEyMzQ1"
+#
+# - Input bytes of length <em>3n+1</em> generate unpadded output characters
+#   of length <em>4n+2</em>, with two padding characters at the end:
+#
+#     # n = 1:  4 bytes => 6 characters.
+#     Base64.urlsafe_encode64('1234', padding: false)     # => "MDEyMw"
+#     # n = 2:  7 bytes => 10 characters.
+#     Base64.urlsafe_encode64('1234567', padding: false)  # => "MDEyMzQ1Ng"
+#
+# - Input bytes of length <em>3n+2</em> generate unpadded output characters
+#   of length <em>4n+3</em>, with one padding character at the end:
+#
+#     # n = 1:  5 bytes => 7 characters.
+#     Base64.urlsafe_encode64('12345', padding: false)    # => "MDEyMzQ"
+#     # m = 2:  8 bytes => 11 characters.
+#     Base64.urlsafe_encode64('12345678', padding: false) # => "MDEyMzQ1Njc"
+#
+# <b>Padding in Decode Methods</b>
+#
+# All of the \Base64 decode methods support (but do not require) padding.
+#
+# \Method Base64.decode64 does not check the size of the padding:
+#
+#   Base64.decode64("MDEyMzQ1Njc") # => "01234567"
+#   Base64.decode64("MDEyMzQ1Njc=") # => "01234567"
+#   Base64.decode64("MDEyMzQ1Njc==") # => "01234567"
+#
+# \Method Base64.strict_decode64 strictly enforces padding size:
+#
+#   Base64.strict_decode64("MDEyMzQ1Njc")   # Raises ArgumentError
+#   Base64.strict_decode64("MDEyMzQ1Njc=")  # => "01234567"
+#   Base64.strict_decode64("MDEyMzQ1Njc==") # Raises ArgumentError
+#
+# \Method Base64.urlsafe_decode64 allows padding in +str+,
+# which if present, must be correct:
+# see {Padding}[Base64.html#module-Base64-label-Padding], above:
+#
+#   Base64.urlsafe_decode64("MDEyMzQ1Njc") # => "01234567"
+#   Base64.urlsafe_decode64("MDEyMzQ1Njc=") # => "01234567"
+#   Base64.urlsafe_decode64("MDEyMzQ1Njc==") # Raises ArgumentError.
+#
+# == Newlines
+#
+# An encoded string returned by Base64.encode64 or Base64.urlsafe_encode64
+# has an embedded newline character
+# after each 60-character sequence, and, if non-empty, at the end:
+#
+#   # No newline if empty.
+#   encoded = Base64.encode64("\x00" *  0)
+#   encoded.index("\n") # => nil
+#
+#   # Newline at end of short output.
+#   encoded = Base64.encode64("\x00" *  1)
+#   encoded.size        # => 4
+#   encoded.index("\n") # => 4
+#
+#   # Newline at end of longer output.
+#   encoded = Base64.encode64("\x00" * 45)
+#   encoded.size        # => 60
+#   encoded.index("\n") # => 60
+#
+#   # Newlines embedded and at end of still longer output.
+#   encoded = Base64.encode64("\x00" * 46)
+#   encoded.size                          # => 65
+#   encoded.rindex("\n")                  # => 65
+#   encoded.split("\n").map {|s| s.size } # => [60, 4]
+#
+# The string to be encoded may itself contain newlines,
+# which are encoded as \Base64:
+#
+#      #   Base64.encode64("\n\n\n") # => "CgoK\n"
+#    s = "This is line 1\nThis is line 2\n"
+#    Base64.encode64(s) # => "VGhpcyBpcyBsaW5lIDEKVGhpcyBpcyBsaW5lIDIK\n"
 #
-# The purpose of using base64 to encode data is that it translates any
-# binary data into purely printable characters.
-
 module Base64
 
   VERSION = "0.1.1"
 
   module_function
 
-  # Returns the Base64-encoded version of +bin+.
-  # This method complies with RFC 2045.
-  # Line feeds are added to every 60 encoded characters.
+  # Returns a string containing the RFC-2045-compliant \Base64-encoding of +bin+.
+  #
+  # Per RFC 2045, the returned string may contain the URL-unsafe characters
+  # <tt>+</tt> or <tt>/</tt>;
+  # see {Encoding Character Set}[Base64.html#module-Base64-label-Encoding+Character+Sets] above:
+  #
+  #   Base64.encode64("\xFB\xEF\xBE") # => "++++\n"
+  #   Base64.encode64("\xFF\xFF\xFF") # => "////\n"
+  #
+  # The returned string may include padding;
+  # see {Padding}[Base64.html#module-Base64-label-Padding] above.
   #
-  #    require 'base64'
-  #    Base64.encode64("Now is the time for all good coders\nto learn Ruby")
+  #   Base64.encode64('*') # => "Kg==\n"
   #
-  # <i>Generates:</i>
+  # The returned string ends with a newline character, and if sufficiently long
+  # will have one or more embedded newline characters;
+  # see {Newlines}[Base64.html#module-Base64-label-Newlines] above:
+  #
+  #   Base64.encode64('*') # => "Kg==\n"
+  #   Base64.encode64('*' * 46)
+  #   # => "KioqKioqKioqKioqKioqKioqKioqKioqKioqKioqKioqKioqKioqKioqKioq\nKg==\n"
+  #
+  # The string to be encoded may itself contain newlines,
+  # which will be encoded as ordinary \Base64:
+  #
+  #   Base64.encode64("\n\n\n") # => "CgoK\n"
+  #   s = "This is line 1\nThis is line 2\n"
+  #   Base64.encode64(s) # => "VGhpcyBpcyBsaW5lIDEKVGhpcyBpcyBsaW5lIDIK\n"
   #
-  #    Tm93IGlzIHRoZSB0aW1lIGZvciBhbGwgZ29vZCBjb2RlcnMKdG8gbGVhcm4g
-  #    UnVieQ==
   def encode64(bin)
     [bin].pack("m")
   end
 
-  # Returns the Base64-decoded version of +str+.
-  # This method complies with RFC 2045.
-  # Characters outside the base alphabet are ignored.
-  #
-  #   require 'base64'
-  #   str = 'VGhpcyBpcyBsaW5lIG9uZQpUaGlzIG' +
-  #         'lzIGxpbmUgdHdvClRoaXMgaXMgbGlu' +
-  #         'ZSB0aHJlZQpBbmQgc28gb24uLi4K'
-  #   puts Base64.decode64(str)
-  #
-  # <i>Generates:</i>
-  #
-  #    This is line one
-  #    This is line two
-  #    This is line three
-  #    And so on...
+  # Returns a string containing the decoding of an RFC-2045-compliant
+  # \Base64-encoded string +str+:
+  #
+  #   s = "VGhpcyBpcyBsaW5lIDEKVGhpcyBpcyBsaW5lIDIK\n"
+  #   Base64.decode64(s) # => "This is line 1\nThis is line 2\n"
+  #
+  # Non-\Base64 characters in +str+ are ignored;
+  # see {Encoding Character Set}[Base64.html#module-Base64-label-Encoding+Character+Sets] above:
+  # these include newline characters and characters <tt>-</tt> and <tt>/</tt>:
+  #
+  #   Base64.decode64("\x00\n-_") # => ""
+  #
+  # Padding in +str+ (even if incorrect) is ignored:
+  #
+  #   Base64.decode64("MDEyMzQ1Njc")   # => "01234567"
+  #   Base64.decode64("MDEyMzQ1Njc=")  # => "01234567"
+  #   Base64.decode64("MDEyMzQ1Njc==") # => "01234567"
+  #
   def decode64(str)
     str.unpack1("m")
   end
 
-  # Returns the Base64-encoded version of +bin+.
-  # This method complies with RFC 4648.
-  # No line feeds are added.
+  # Returns a string containing the RFC-2045-compliant \Base64-encoding of +bin+.
+  #
+  # Per RFC 2045, the returned string may contain the URL-unsafe characters
+  # <tt>+</tt> or <tt>/</tt>;
+  # see {Encoding Character Set}[Base64.html#module-Base64-label-Encoding+Character+Sets] above:
+  #
+  #   Base64.strict_encode64("\xFB\xEF\xBE") # => "++++\n"
+  #   Base64.strict_encode64("\xFF\xFF\xFF") # => "////\n"
+  #
+  # The returned string may include padding;
+  # see {Padding}[Base64.html#module-Base64-label-Padding] above.
+  #
+  #   Base64.strict_encode64('*') # => "Kg==\n"
+  #
+  # The returned string will have no newline characters, regardless of its length;
+  # see {Newlines}[Base64.html#module-Base64-label-Newlines] above:
+  #
+  #   Base64.strict_encode64('*') # => "Kg=="
+  #   Base64.strict_encode64('*' * 46)
+  #   # => "KioqKioqKioqKioqKioqKioqKioqKioqKioqKioqKioqKioqKioqKioqKioqKg=="
+  #
+  # The string to be encoded may itself contain newlines,
+  # which will be encoded as ordinary \Base64:
+  #
+  #   Base64.strict_encode64("\n\n\n") # => "CgoK"
+  #   s = "This is line 1\nThis is line 2\n"
+  #   Base64.strict_encode64(s) # => "VGhpcyBpcyBsaW5lIDEKVGhpcyBpcyBsaW5lIDIK"
+  #
   def strict_encode64(bin)
     [bin].pack("m0")
   end
 
-  # Returns the Base64-decoded version of +str+.
-  # This method complies with RFC 4648.
-  # ArgumentError is raised if +str+ is incorrectly padded or contains
-  # non-alphabet characters.  Note that CR or LF are also rejected.
+  # Returns a string containing the decoding of an RFC-2045-compliant
+  # \Base64-encoded string +str+:
+  #
+  #   s = "VGhpcyBpcyBsaW5lIDEKVGhpcyBpcyBsaW5lIDIK"
+  #   Base64.strict_decode64(s) # => "This is line 1\nThis is line 2\n"
+  #
+  # Non-\Base64 characters in +str+ not allowed;
+  # see {Encoding Character Set}[Base64.html#module-Base64-label-Encoding+Character+Sets] above:
+  # these include newline characters and characters <tt>-</tt> and <tt>/</tt>:
+  #
+  #   Base64.strict_decode64("\n") # Raises ArgumentError
+  #   Base64.strict_decode64('-')  # Raises ArgumentError
+  #   Base64.strict_decode64('_')  # Raises ArgumentError
+  #
+  # Padding in +str+, if present, must be correct:
+  #
+  #   Base64.strict_decode64("MDEyMzQ1Njc")   # Raises ArgumentError
+  #   Base64.strict_decode64("MDEyMzQ1Njc=")  # => "01234567"
+  #   Base64.strict_decode64("MDEyMzQ1Njc==") # Raises ArgumentError
+  #
   def strict_decode64(str)
     str.unpack1("m0")
   end
 
-  # Returns the Base64-encoded version of +bin+.
-  # This method complies with ``Base 64 Encoding with URL and Filename Safe
-  # Alphabet'' in RFC 4648.
-  # The alphabet uses '-' instead of '+' and '_' instead of '/'.
-  # Note that the result can still contain '='.
-  # You can remove the padding by setting +padding+ as false.
+  # Returns the RFC-4648-compliant \Base64-encoding of +bin+.
+  #
+  # Per RFC 4648, the returned string will not contain the URL-unsafe characters
+  # <tt>+</tt> or <tt>/</tt>,
+  # but instead may contain the URL-safe characters
+  # <tt>-</tt> and <tt>_</tt>;
+  # see {Encoding Character Set}[Base64.html#module-Base64-label-Encoding+Character+Sets] above:
+  #
+  #   Base64.urlsafe_encode64("\xFB\xEF\xBE") # => "----"
+  #   Base64.urlsafe_encode64("\xFF\xFF\xFF") # => "____"
+  #
+  # By default, the returned string may have padding;
+  # see {Padding}[Base64.html#module-Base64-label-Padding], above:
+  #
+  #   Base64.urlsafe_encode64('*') # => "Kg=="
+  #
+  # Optionally, you can suppress padding:
+  #
+  #   Base64.urlsafe_encode64('*', padding: false) # => "Kg"
+  #
+  # The returned string will have no newline characters, regardless of its length;
+  # see {Newlines}[Base64.html#module-Base64-label-Newlines] above:
+  #
+  #   Base64.urlsafe_encode64('*') # => "Kg=="
+  #   Base64.urlsafe_encode64('*' * 46)
+  #   # => "KioqKioqKioqKioqKioqKioqKioqKioqKioqKioqKioqKioqKioqKioqKioqKg=="
+  #
   def urlsafe_encode64(bin, padding: true)
     str = strict_encode64(bin)
     str.chomp!("==") or str.chomp!("=") unless padding
@@ -90,14 +332,22 @@ module Base64
     str
   end
 
-  # Returns the Base64-decoded version of +str+.
-  # This method complies with ``Base 64 Encoding with URL and Filename Safe
-  # Alphabet'' in RFC 4648.
-  # The alphabet uses '-' instead of '+' and '_' instead of '/'.
-  #
-  # The padding character is optional.
-  # This method accepts both correctly-padded and unpadded input.
-  # Note that it still rejects incorrectly-padded input.
+  # Returns the decoding of an RFC-4648-compliant \Base64-encoded string +str+:
+  #
+  # +str+ may not contain non-Base64 characters;
+  # see {Encoding Character Set}[Base64.html#module-Base64-label-Encoding+Character+Sets] above:
+  #
+  #   Base64.urlsafe_decode64('+')  # Raises ArgumentError.
+  #   Base64.urlsafe_decode64('/')  # Raises ArgumentError.
+  #   Base64.urlsafe_decode64("\n") # Raises ArgumentError.
+  #
+  # Padding in +str+, if present, must be correct:
+  # see {Padding}[Base64.html#module-Base64-label-Padding], above:
+  #
+  #   Base64.urlsafe_decode64("MDEyMzQ1Njc") # => "01234567"
+  #   Base64.urlsafe_decode64("MDEyMzQ1Njc=") # => "01234567"
+  #   Base64.urlsafe_decode64("MDEyMzQ1Njc==") # Raises ArgumentError.
+  #
   def urlsafe_decode64(str)
     # NOTE: RFC 4648 does say nothing about unpadded input, but says that
     # "the excess pad characters MAY also be ignored", so it is inferred that