Niels Lohmann
8daec2b596
Speed up json_view's parser with SIMD scanning and a hash table for large objects. Long runs of string bytes are scanned 16 bytes at a time with NEON (AArch64, GCC and Clang) and SSE2 (x86-64), both baseline instruction sets. Keys keep 16 table checks before the vector loop, because their lengths repeat from record to record; string values get 8, because their lengths vary more. Non-ASCII text is validated 16 bytes at a time with simdjson's "lookup4" check (Keiser and Lemire, 2021), with NEON on AArch64 and, on x86-64, with SSSE3. SSSE3 is not part of baseline x86-64, so the check is compiled for SSSE3 with a function attribute and used only where CPUID reports it, which all x86-64 CPUs since about 2011 do; the answer is cached in a statically initialized atomic, so there is no guard of a local static and no global constructor. The same input is accepted either way. JSON_VIEW_NO_SIMD selects the portable code. On x86-64, string runs are now checked vector-first: one SSE2 compare from the first byte finds the end of most keys and short values, instead of a branch per byte for the first 8-16 bytes. AArch64 keeps the byte-wise steps, where a NEON mask costs more and the branches predict well. Entering an object or array no longer stalls: open() stores the parent's frame field by field instead of building it on the stack and reading it back with wider loads, which waited for the narrower stores to retire. Objects with 128 members or more get an open-addressing hash table built when the object closes, so operator[], at(), find(), contains(), count(), value(), and JSON pointers take constant time on average in such objects; of duplicate keys, the first is kept, as for the linear search. The idea comes from Boost.JSON. simdjson is credited in simd.hpp's SPDX block, the README, and license.md. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2.1 KiB
nlohmann::basic_json_view::find
iterator find(string_view_t key) const;
iterator find(const char* key) const;
iterator find(const string_t& key) const;
Finds a member with key key -- the first one, should the key occur more than once (see
Notes on duplicate keys). If the value is not an object, or no member has this key,
end() is returned.
Parameters
key(in)- key value of the element to search for
Return value
An iterator to the member with key key, or end() if there is none or the value is not an object.
Exception safety
No-throw guarantee: this function never throws exceptions.
Complexity
Linear in the number of members: as for ordered_json, members are compared one after
another, in document order, stopping at the first match. Each comparison first checks the key's length -- already
known from the index, without reading the key bytes -- before comparing its content.
Objects with 128 or more members get a hash index while parsing, so that a lookup in them takes constant time on
average.
Notes
Unlike BasicJsonType::find, which always returns #!cpp end() for a non-object type, this
also does so for a discarded view -- there is no separate "invalid" iterator to return.
Examples
??? example
The example below scans a batch of events for those that carry an optional `user_id` field, using `find()`
instead of [`operator[]`](operator[].md) (which would throw for the events that are not objects at all) or
[`contains()`](contains.md) followed by a second lookup. Events without a match are never materialized.
```cpp
--8<-- "examples/basic_json_view__find.cpp"
```
Output:
```json
--8<-- "examples/basic_json_view__find.output"
```
See also
- count - returns the number of occurrences of a key
- contains - checks whether a key exists
BasicJsonType::find- the corresponding function ofbasic_json
Version history
- Added in version 3.13.0.