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str

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A SIMD optimized fixed-length string class along with an adaptive hash table for fast searching

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## Str
`Str` is a char array wrapper providing some frequently used operations in the most efficient way(supporting AVX512 SIMD optimization), including string comparisons and conversion to/from integers.

## StrHash
`StrHash` is an adaptive open addressing hash table template taking `Str` as key and providing a find function in the most efficient way. It's adaptive in that it can extract features from the keys contained in the table and train its hashing parameters dynamically to distribute the keys for avoiding collision.

`StrHash` is actually a subclass of `std::map`, so user can use whatever funcitons it provides to modify the table, and then call `doneModify` to train the table and `fastFind` to find keys in the table. Note that `doneModify` is pretty slow so it's not efficient to modify the table frequently between `fastFind`. It's recommended that `clear` be called immediately after `doneModify` if only `fastFind` is needed afterwards, so some memory can be saved.

`StrHash` currently supports 7 hash functions and one of which can be selected using template parameter `HashFunc`:
* 0: djb ver1(default)
* 1: djb ver2
* 2: sax
* 3: fnv
* 4: oat
* 5: murmur
* 6: int(for integer keys)

User can also add other hash functions himself.

`StrHash` is also suitable to have integers(such as uint32_t or uint64_t) as key for searching. Define `StrHash<8, Value, NullV, 6>`
for uint64_t and `StrHash<4, Value, NullV, 6>` for uint32_t, see `benchfindint.cc` for detailed usage.

## Benchmark
Tests show that `StrHash` is 7x faster than `std::unordered_map` and 3x faster than other open addressing hash table implementations such as `tsl::hopscotch_map`, `tsl::robin_map`, `robin_hood::unordered_map` and `google::dense_hash_map`.

`Str`'s `operator==` and `compare` is 2x faster than `strncmp`/`memcmp` or those of `std::string`.

`Str`'s `fromi` and `toi` is 10x faster than `stoi`/`strtol`/`to_string`/`sprintf`.

`benchfindstr.cc` tests the performance of multiple string search solutions using the same data set. The data set contains the KRX option issue codes of Feb 2019 that we are interested in and are to be inserted into the table, and the first 1000 option issue codes we received from the market data(which are mostly of Feb 2019 but some are of other months) and are to be searched in the table.
In `benchfindstr.cc`: 
* `bench_hash<0~5>` compair the performance of different hash functions `StrHash` supports.
* `bench_hash` vs other searching solutions shows how `StrHash` is faster than others.
* `bench_map` vs `bench_string_map` and `bench_bsearch` vs `bench_string_bsearch` show how `Str` is faster than `std::string`.

`benchfindint.cc` tests the performance of multiple integer search solutions in similar way to `benchfindstr.cc`. The data set contains the SHFE instrument No of type uint64_t. Here `bench_hash6` should be the most suitable method.

`benchcmp.cc` tests string comparison operations.

`benchnum.cc` tests conversions to/from integers.

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