Speed up quoted printable encoding

[bossek]

Encoding to quoted printable was quite slow in cases when calling
string:substr and subsequent lists:concat for inserting soft newline was
used too often. New implementation uses only fast head operations.

[bossek]

For comparing before/after performance I used following:

Mail = fun(Body) ->
 {<<"multipart">>, <<"alternative">>,
  [{<<"From">>, <<"test@example.org">>}, {<<"To">>, <<"test@example.org">>}, {<<"Subject">>, <<"test">>}, {<<"Mime-Version">>, <<"1.0">>}], #{},
  [{"text", "html", [{<<"Content-Type">>, <<"text/html; charset=\"utf-8\"">>}, {<<"Content-Transfer-Encoding">>, <<"quoted-printable">>}],
  #{content_type_params => [{<<"charset">>, <<"utf-8">>}], disposition => <<"inline">>}, Body}]}
end.

timer:tc(fun() -> mimemail:encode(Mail(unicode:characters_to_binary(string:copies(" ř", 100000))), []) end).
timer:tc(fun() -> mimemail:encode(Mail(binary:list_to_bin(string:copies("a a", 100000))), []) end).

Both calls takes seconds before improvement and hundreds/tens of milliseconds after.

[seriyps]

Nice, thanks!

[seriyps]

minor: I think you can do "\n\r=" ++ Str and compiler will be able to optimize it to what your code looks like now

[bossek]

You are right! Should I change it before merge or keep it as it is?

[seriyps]

I'm fine as it is now.

[bossek]

OK, thanks!

[mworrell]

Good work, thank you!

[mworrell]

@seriyps Ready to be merged?

[seriyps]

@mworrell yes

[mworrell]

@bossek Merging now, thank you for your contribution!

[bossek]

@mworrell Glad to help :-)