The minimum length of an encoded-word is actually the pure encoding
overhead plus the length of the `output-charset` plus the minimum unit
of encoded text, which is 4 for B-encoding and (for simplicity) 3 for
Q-encoding. We also cater to the possibility that we need further
encoded words, which would be split by the `line-break-chars` followed
by a space character. Obviously, the former `out_charset_len + 12` is
too simplistic and wrong in the given case (where the magic number
would be 13).
These simplifications are somewhat wasteful, but iconv_mime_encode()
with Q-encoding is wasteful anyway (see bug 66828[1]), and the proper
solution to convert the whole input to the desired output charset
upfront, and applying the encoding afterwards appears too much a change
for the stable releases.
[1] <https://bugs.php.net/66828>
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