Issue #10025 has been reported by Tomas Hoger.

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Bug #10025: Incorrect wrapping of base64 output of Array.pack()
https://bugs.ruby-lang.org/issues/10025

* Author: Tomas Hoger
* Status: Open
* Priority: Low
* Assignee: 
* Category: core
* Target version: 
* ruby -v: ruby 2.0.0p353 (2013-11-22 revision 43784) [x86_64-linux]
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN
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String format directive `m` for Array `pack()` is documented as:

~~~
   m         | String  | base64 encoded string (see RFC 2045, count is width)
             |         | (if count is 0, no line feed are added, see RFC 4648)
~~~

http://www.ruby-doc.org/core-2.1.2/Array.html#method-i-pack

While the description of the meaning of count argument is rather limited, it seems it's supposed to mean the maximum length of the line in the output before line break is added.  However, that's not what actually happens:

~~~
$ ruby -e 'print ["a"*40].pack("m20")'
YWFhYWFhYWFhYWFhYWFhYWFh
YWFhYWFhYWFhYWFhYWFhYWFh
YWFhYQ==
~~~

In this example, output lines have 24 characters.  To have 20 character long output lines, `m15` has to be specified:

~~~
$ ruby -e 'print ["a"*40].pack("m15")'
YWFhYWFhYWFhYWFhYWFh
YWFhYWFhYWFhYWFhYWFh
YWFhYWFhYWFhYQ==
~~~

This is caused by the following in `pack_pack()`:

~~~
len = len / 3 * 3;
~~~

https://github.com/ruby/ruby/blob/dd5d029/pack.c#L832

This looks like a typo / thinko.  Base64 encoding produces 4 bytes of output for every 3 bytes of input.  Hence to get output line of length N, encoding should process N / 4 * 3 input bytes before inserting line break.  The `len` argument passed to `encodes()` is the number of input bytes to process to generate one output line.

The same applies to UU-encoding (the `u` format), with the difference that every line starts with and additional character specifying line length.  Hence even with the above fixed, `u20` would produces lines with 21 characters.



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