We take advantage of the fact that no one is able to modify the phrase
databases shipped with the binary (guranteed by macOS's integrity check
for notarized apps), and we can simply pre-sort the phrases in the
database files.
With this change, we can speed up McBopomofo's language model loading
during the app initialization by about 500-800x on a 2018 Intel MacBook
Pro. The LM loading used to take 300-400 ms, but now it's done within a
sub-millisecond range (0.5-0.6 ms). Microbenchmarking shows that
ParselessLM is about 16000x faster than FastLM. We amortize the latency
during the query time, and even by deferring the parsing, ParselessLM is
only ~1.5x slower than FastLM, and both LM classes serve queries unedr 6
microseconds (that's 0.006 ms), which means the tradeoff only
contributes to neglible overall latency.
This PR requires some small changes to the phrase db cooking scripts.
Python 3 is now used and the (value, reading, score) tuples are
rearranged to (reading, value, score) and sorted by reading ("key"). A
header is added to the phrase databases to call out the fact that these
are pre-sorted.
clang-format is used to apply WebKit C++ style to the new code. This
also applies to KeyValueBlobReader that was added recently.
Microbenchmark result below:
```
---------------------------------------------------------------------
Benchmark Time CPU Iterations
---------------------------------------------------------------------
BM_ParselessLMOpenClose 17710 ns 17199 ns 33422
BM_FastLMOpenClose 376520248 ns 367526500 ns 2
BM_ParselessLMFindUnigrams 5967 ns 5899 ns 113729
BM_FastLMFindUnigrams 2268 ns 2265 ns 307038
```
Lukhnos Liu