Issue31optimizeitoa

[miloyip]

Fixes #31 Optimize integer-to-string conversion in GenericWriter

In the beginning, some tests are written in misctest.cpp. And the performance is nearly the same to the other thirdparty implementations. However, it is quite difficult to further develop and optimize, so I created a side project https://github.com/miloyip/itoa-benchmark .

Currently RapidJSON adopt the branchlut implementation. However, it may subject to be changed in future if better algorithm is developed. The API is very simple. It can be replaced by other implementation easily.

Full specialization of Writer for StringBuffer is added. Additional Push() and Pop() member functions are added for StringBuffer in order to prevent memory copying.

PerfTest results are not obvious because the sample JSON do not contain a lot of integers.
https://gist.github.com/miloyip/1326371e39652acf21d5

Before:
[       OK ] RapidJson.Writer_NullStream (676 ms)
[       OK ] RapidJson.Writer_StringBuffer (1144 ms)

After:
[       OK ] RapidJson.Writer_NullStream (638 ms)
[       OK ] RapidJson.Writer_StringBuffer (1098 ms)

However, when just using Writer::WriteXXX(), the results are obvious:

[       OK ] Misc.itoa_Writer_StringBuffer (180 ms)
[       OK ] Misc.itoa_Writer1_StringBuffer (289 ms)
[       OK ] Misc.itoa64_Writer_StringBuffer (247 ms)
[       OK ] Misc.itoa64_Writer1_StringBuffer (705 ms)

Writer is the new implementation. Writer1 is the original naive implementation.

However, these performance tests are not rigorous enough. I would like to create a native json benchmark project in the future.

[miloyip]

Hum... So the specialization hide an error that is not covered by unit tests.