Trivial Description
This is interesting. I posted a link to a state-machine-based UTF8 parser/recognizer:
http://bjoern.hoehrmann.de/utf-8/decoder/dfa/
I spent some time thinking if the lookup table could be converted into a stateless computational function, which would avoid a table lookup (which in Java will cause an additional bounds check that will be hard to eliminate I think). This didn't turn out to be easy (it boils down to finding a simple function that would map a set of integers to its concrete permutation; a generalization of minimal perfect hashing).
But out of curiosity I though it'd be fun to compare how Lucene's codepoint counting compares to Java's built-in one (Decoder) and a sequence of if's.
I've put together a Caliper benchmark that processes 50 million unicode codepoints; one only ASCII, one Unicode. The results are interesting. On my win/I7:
implementation dataType ns linear runtime
LUCENE UNICODE 167359502.6 ===============
LUCENE ASCII 334015746.5 ==============================
NOLOOKUP_SWITCH UNICODE 154294141.8 =============
NOLOOKUP_SWITCH ASCII 119500892.8 ==========
NOLOOKUP_IF UNICODE 90149072.6 ========
NOLOOKUP_IF ASCII 29151411.4 ==
Disregard the switch lookup – it's for fun only. But a sequence of if's is significantly faster than the current Lucene's table lookup, especially on ASCII input. And now compare this to Java's built-in decoder...
JAVA UNICODE 5753930.1 =
JAVA ASCII 23.8 =
Yes, it's the same benchmark. Wtf? I realize buffers are partially native and probably so is utf8 decoder but by so much?! Again, to put it in context:
implementation dataType ns linear runtime
LUCENE UNICODE 167359502.6 ===============
LUCENE ASCII 334015746.5 ==============================
JAVA UNICODE 5753930.1 =
JAVA ASCII 23.8 =
NOLOOKUP_IF UNICODE 90149072.6 ========
NOLOOKUP_IF ASCII 29151411.4 ==
NOLOOKUP_SWITCH UNICODE 154294141.8 =============
NOLOOKUP_SWITCH ASCII 119500892.8 ==========
Wtf? The code is here if you want to experiment.
https://github.com/dweiss/utf8dfa
I realize the Java version needs to allocate a temporary space buffer but if these numbers hold for different VMs it may actually be worth it...