# json_document against other JSON libraries [`compare.py`](../compare.py) compares `json_document` with yyjson, simdjson and Boost.JSON. The programs here add sonic-cpp, RapidJSON and Glaze in C++, and sonic-rs, simd-json and serde_json in Rust, for the post [How 3.13.0 got faster](https://json.nlohmann.me/home/blog/how_3_13_0_got_faster/#how-it-compares). They are not built by CMake or run by CI. ## What is measured The programs compare **use cases**, not library functions. A library may implement a use case in every reasonable way (another API, SAX events, in-situ parsing, a struct binding); each such way is a *variant*, named `: ` (`yyjson: insitu (copy)`, `RapidJSON: SAX`, `nlohmann json_document: accept`). The report shows each library's fastest correct variant. `nlohmann json` (`basic_json`) and `nlohmann json_document` (with `json_view`) count as two libraries. | use case | what it does | |---|---| | `validate` | is this text valid JSON? returns a bool | | `sum` | visit every value, convert every number, touch every string and key; sum all numbers in document order | | `select` | read a few fields of every record (the fields of `bench_view.cpp`, e.g. `id`, `user.screen_name` and `retweet_count` of every status) | | `edit` | change some values of every record and write the document as compact JSON (see below) | | `minify` | text in, compact text out | | `write` | write a document parsed beforehand as compact JSON; only the writing is timed | The edits: | file | edit | |---|---| | `twitter.json` | for every element of `statuses`: `retweet_count` + 1, erase `entities`, add `"seen": true` | | status | the same edit on the one status | | `citm_catalog.json` | for every element of `performances`: `start` + 1, erase `seatMapImage`, add `"seen": true` | | `canada.json` | `features[0].properties.name` = `"Kanada"`; append `[0.5,0.25]` to the first ring (`geometry.coordinates[0]`) of every feature | | `jeopardy.json` | for every record: `value` = `null`, erase `air_date`, add `"seen": true` | The files are `twitter.json`, `citm_catalog.json`, `canada.json` (nativejson-benchmark), `jeopardy.json`, the first status of `twitter.json` (`status`; written by `json::dump()`, timed in batches of 200), and `twitter_min` and `citm_catalog_min`: `twitter.json` and `citm_catalog.json` with all whitespace outside strings removed, built in memory at startup by the same small function in both programs (`strip_ws`; the bytes inside strings are untouched). The minified files use the select and edit of their originals. ### Input contract The input is a `const std::string` holding the file, with at least 64 bytes of spare capacity (`reserve`), so libraries that need padding (simdjson) can use it without a copy. A variant that modifies the input (in-situ parsing: yyjson `YYJSON_READ_INSITU`, RapidJSON `ParseInsitu`, simd-json) copies it inside the timed call; its name says `(copy)`. Each timed call starts from the text and frees everything it built, except variants named `(reused)` or `(reused allocator)`, which keep a parser, document or allocator across calls (simdjson's reused parsers, `json_document::read()`, Boost.JSON's reset `parser` and `basic_parser`, yyjson's dynamic allocator, RapidJSON's user-buffer allocators, simd-json's buffers). These are reported but never chosen as a library's best; `summarize.py` reports each library's best reused variant separately. In Rust, the input is the file as bytes (`&[u8]`), so every library has to validate UTF-8 itself. ### Correctness rules Before timing, every variant runs on the file and must produce the right result: - `validate`: `true`. - `sum`: the same number of values, the same number of string and key bytes, and a sum of all numbers that is bit-identical to `json_document`'s (document order), so every number is converted exactly; variants that visit objects in another order (`nlohmann json: parse`, simd-json and serde_json values) are compared within 1e-9. - `select`: the same value as `json_document`. A variant that does not convert numbers (`(raw numbers)`, `(numbers: none)`) has to convert the numbers it selects exactly itself. - `edit`, `minify`, `write`: the output, read with `json::parse`, compares equal (`==`) to the expected document (for `edit`, the edit applied to a `json` value; floats compare exactly, so every float must be written exactly). In Rust, `serde_json::Value` (with `float_roundtrip`) is the reference. A variant that gives a wrong result on a file, or crashes on it, is printed as `excluded: | | ` and not timed. ### Validation levels Except for `write`, every variant is also probed with four malformed inputs derived from each file: the file truncated by one byte (after trailing whitespace), the file followed by ` ]x` (trailing garbage), a stray `,` before the first `}` (before the last `]` in a file without objects), and a `0xFF` byte inside a string value (the first in the second half of the file, or the last one; never a key, so that a variant that accepts the byte still finds the keys it looks up; a file without string values gets no `0xFF` probe). The probes run in a child process, so a crash is caught. The result is the variant's *validation level*, printed in its cell (`sonic-cpp: Document [no UTF-8] 380.1us`): | level | meaning | |---|---| | `full` | rejects all four probes | | `no UTF-8` | rejects the structural probes, accepts the `0xFF` byte | | `partial` | accepts the truncated input, the trailing garbage or the stray comma, or crashes on any probe | | `n/a` | `write`: there is no input to validate (counts as `full`) | What a variant accepts is printed as `level: | | | `. A variant's level in the summary is the worst it got for any file. UTF-8 validation is enabled where a library has a switch (RapidJSON `kParseValidateEncodingFlag`; the `(no encoding check)` variants leave it off). sonic-cpp 1.0.2 cannot validate UTF-8 at all; serde_json does not validate UTF-8 in the parts it skips (`IgnoredAny`, unknown struct fields) unless the input is checked with `std::str::from_utf8` first (the `from_utf8 + ...` variants). The sum and select reference values are printed as `check ... reference:` lines; `summarize.py` warns if the C++ and Rust logs disagree. The four probes do not cover everything a parser can get wrong. `--conformance ` runs every `validate` variant on every file of [JSONTestSuite](https://github.com/nst/JSONTestSuite)'s `test_parsing` (json_test_data has it as `nst_json_testsuite2/test_parsing`) and prints `conformance: | | accepted|rejected|crashed`; a `y_` file must be accepted, an `n_` file rejected, and an `i_` file may go either way. bench_all runs each call in a child process (a call that hangs for 60 s counts as crashed); a stack overflow aborts bench_rs, so `conformance-rs.sh` runs it one file per process and, for a file that aborts, one variant per process (`CONF_VARIANT`). The trailing garbage probe was added after this check: Glaze reads anything after the value unless `validate_trailing_whitespace` is set. ### Variants Level on the Mac in brackets where it is not `full` (`P` = partial, `U` = no UTF-8); every `write` variant is `n/a`. | library | validate | sum | select | edit | minify | write | |---|---|---|---|---|---|---| | nlohmann json_document | `accept`, `parse`, `read (reused)` | `parse`, `read (reused)` | `parse`, `structs (from_json)`, `read (reused)` | `editable`, `editable (source numbers)` (`json_editable_document`: set/erase, `dump` with `number_format::source`) | `parse + dump`, `parse + dump (source numbers)`, `read + dump (reused)` | `dump`, `dump (source numbers)` | | nlohmann json | `accept`, `parse` | `parse`, `sax` | `parse`, `structs (from_json)` | `parse`, `ordered_json` | `parse + dump` | `dump` | | yyjson | `read`, `insitu (copy)`, `read (raw numbers)`, `read (reused allocator)` | `read`, `insitu (copy)`, `read (reused allocator)` | `read`, `insitu (copy)`, `read (raw numbers)`, `read (reused allocator)` | `mut copy`, `insitu (copy) + mut copy` (`yyjson_doc_mut_copy`) | `read + write`, `insitu (copy) + write`, `read (raw numbers) + write`, `read + write (reused allocator)` | `write` | | simdjson | `DOM`, `DOM (reused)` | `DOM`, `DOM (reused)`, `On-Demand`, `On-Demand (reused)` | `DOM`, `DOM (reused)`, `On-Demand` [P], `On-Demand (reused)` [P], `On-Demand structs` [P], `On-Demand (key selectors)` [P] | n/a (no mutable document) | `DOM + to_string`, `DOM (reused) + to_string`, `minify` [P] | `to_string` | | RapidJSON | `SAX` (null handler), `Document`, `insitu (copy)`, `SAX (raw numbers)`, `Document (reused allocator)` | `Document`, `SAX`, `insitu (copy)`, `Document (reused allocator)` | `Document`, `insitu (copy)`, `Document (raw numbers)`, `Document (reused allocator)` | `Document`, `insitu (copy)` | `Document + Writer`, `SAX to Writer`, `SAX to Writer (raw numbers)`, `Document + Writer (reused allocator)` | `Writer` | | RapidJSON, `(no encoding check)` | the same [U] | the same [U] | the same [U] | the same [U] | the same [U] | | | sonic-cpp | `Document` [U] | `Document` [U] | `Document` [U] | `Document` [U] | `Document + Serialize` [U] | `Serialize` | | Glaze | `validate_json`, `generic` [P], `generic (double)` [P], `generic (strict)`, `generic (double, strict)` | `generic` [P], `generic (double)` [P], `generic (strict)`, `generic (double, strict)`, `lazy_json` [P], `lazy_json (string_view)` (x) | `structs` [P], `structs (validate_skipped)`, `generic` [P], `generic (double)` [P], `generic (strict)`, `generic (double, strict)`, `lazy_json` [P], `lazy_json (string_view)` (x) | `generic` [P], `generic (double)` (x), `generic (strict)`, `generic (double, strict)` (x) | `generic` [P], `generic (double)` (x), `generic (strict)`, `generic (double, strict)` (x), `minify_json` [P] | `generic`, `generic (double)` (x), `generic (strict)`, `generic (double, strict)` (x) | | Boost.JSON | `SAX` (`basic_parser`), `parse`, `SAX (numbers: none)`, `parse (numbers: none)`, `SAX (reused)`, `parser (reused)` | `parse`, `SAX`, `SAX (reused)`, `parser (reused)` | `parse`, `parse (numbers: none)` (x), `parser (reused)`, `parse_into structs` | `parse` | `parse + serialize`, `parser (reused) + serialize` | `serialize` | | serde_json | `IgnoredAny` [U], `from_utf8 + IgnoredAny`, `Value` | `visitor`, `Value` | `structs` [U], `from_utf8 + structs`, `Value` | `Value` | `Value` | `Value` | | sonic-rs | `IgnoredAny`, `Value` | `visitor`, `Value` | `structs`, `Value` | `Value` | `Value` | `Value` | | simd-json | `tape (copy)`, `borrowed (copy)`, `tape (reused)` | `visitor (copy)`, `borrowed (copy)`, `borrowed (reused)` | `structs (copy)`, `borrowed (copy)` | `owned (copy)`, `borrowed (copy)` | `borrowed (copy)` | `borrowed` | (x): excluded for a wrong result on some file, and so in the summary for every file: `generic (double)` loses the 64-bit ids of `twitter.json` when it writes them; `lazy_json (string_view)` reads the raw bytes of a string, so escaped strings have the wrong length (`sum`, and the `"` in 19,077 `jeopardy.json` categories in `select`); `numbers: none` reads every number as 0. `structs` read into types that declare only the fields `select` uses and hold the same data: nlohmann/json's `from_json()` of the `NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE` macro (from a `json` value or from a `json_document`'s view) reads into `nsel`, where the arrays and objects that are only counted (`seatCategories`, the values of `events`, `coordinates`) are a `count_only` holding their size. Glaze reads into the same types (reflected; `count_only` has a reader that skips and counts the elements; `error_on_unknown_keys = false`, and with `validate_skipped` to validate what it skips), and so does simdjson On-Demand (`get()` with `tag_invoke`, fields in document order, `count_elements()`/`count_fields()`). serde derive reads into the same fields with a `Count` that counts with `IgnoredAny` (and a `BTreeMap`, as `std::map`). Boost.JSON's `parse_into` has no type that skips any value, so its counted members only count what it converts into them: `seatCategories` into a counting sequence of an empty described struct, `coordinates` into three levels of counting sequences down to `double`, and the `events` values into an empty described struct, so their members are not counted. `key selectors` are simdjson 5.0's `object::for_each<"k1", ...>`, which walks each object once and matches keys with a compile-time perfect hash. `visitor` is a serde `DeserializeSeed` that sums without building a value; `SAX` and `sax` are the libraries' event interfaces with a summing (or, for `validate`, an empty) handler. RapidJSON parses with `kParseFullPrecisionFlag` (correctly rounded floats; without it, sums and written floats are wrong) and Boost.JSON with `number_precision::precise`, as every other library rounds correctly by default. Variants named `(raw numbers)` or `(numbers: none)` use a library's mode that validates numbers without converting them (yyjson `YYJSON_READ_NUMBER_AS_RAW`, RapidJSON `kParseNumbersAsStringsFlag`, Boost.JSON `number_precision::none`), as `json_document` converts a number only when it is read. In `select` they convert the selected numbers themselves (`std::from_chars`); Boost.JSON's `none` leaves no number text in its DOM, so it cannot. yyjson writes a raw number verbatim, so `read (raw numbers) + write` is exact. sonic-cpp 1.0.2 has no parse flags, and Glaze's generic DOM has no raw number mode (`validate_json` converts no number anyway). `(reused allocator)`: yyjson's `yyjson_alc_dyn_new()`, RapidJSON's `MemoryPoolAllocator`s over user buffers for the values and the parse stack; Boost.JSON's `parser (reused)` builds its value in a `monotonic_resource` over a buffer kept across calls. These buffers grow after a call that needed more, so the timed calls do not allocate. `generic (double)` is `glz::generic`, which stores every number as a double. The Glaze `(strict)` variants read with `validate_trailing_whitespace` (else Glaze accepts anything after the value) and write with `escape_control_characters` (else Glaze writes a string's control characters, e.g. U+0001, unescaped, which is not JSON; `gsoc-2018.json` has some). | library | version | notes | |---|---|---| | `json_document`, `json` | this checkout | | | yyjson | 0.13.0 | | | simdjson | 5.0.8 | | | sonic-cpp | 1.0.2 | | | RapidJSON | master 24b5e7a8b27f (2025-02-05) | | | Glaze | 9.1.0 | `generic_u64` and `generic` (owning DOMs, member order kept) | | Boost.JSON | 1.92.0 | monotonic resource | | sonic-rs, simd-json, serde_json | see `rust/Cargo.lock` | serde_json with `float_roundtrip` | SHA-256 of the downloads: ``` 55f8a675397c505f9bee6c160e1c26b0085868f185d04d7a7727531fdbff4bd4 glaze-v9.1.0.tar.gz 30edf9e47c417f653546c39295fdb66ba5f89fb37eb3780122c6ee66605e5837 sonic-cpp v1.0.2.tar.gz 2d2601a82d2d3b7e143a3c8d43ef616671391034bc46891a9816b79cf2d3e7a8 rapidjson 24b5e7a8b27f42fa16b96fc70aade9106cf7102f.tar.gz 34e0f62a2bc11ab20d601e8ca1cc2b2079503aa45119a19133d89d19b94a0fae yyjson 0.13.0.tar.gz 356cc3a2d6a3d0d45bf5eba813e58ef4f2f6d9cc2a25d8aada2563bc13755789 simdjson-v5.0.8.tar.gz c4a3b310ddd2472416e091067166b0713be97c63f38c212c484ada022fd296ce boost_1_92_0.tar.gz ``` ### Timing Every variant of a use case runs interleaved in every round, each timed call after an untimed one, and the best round is reported (the method of `bench_view.cpp`). The order of the variants changes in every round (a shuffle with a fixed seed; the first round keeps the listed order): on the Mac, a variant's time can depend on which variant ran before it, by up to 15 %, and a fixed order would give each variant the same neighbour in every round. `jeopardy.json` (50 MB) gets a fifth of the rounds (at least 3, at most the given number). ## Building and running All libraries except yyjson and simdjson are used header-only. With the downloads unpacked into `DL` (`compare.py --download` fetches yyjson and Boost into its build directory, and simdjson 4.6.11, so take simdjson 5.0.8 from its release archive): ```sh cc -O3 -DNDEBUG -c $DL/yyjson-0.13.0/src/yyjson.c -o yyjson.o c++ -std=c++17 -O3 -DNDEBUG -c $DL/simdjson-5.0.8/singleheader/simdjson.cpp -o simdjson.o c++ -std=c++23 -O3 -DNDEBUG -DRAPIDJSON_NEON \ -I ../../../../include -I $DL/yyjson-0.13.0/src -I $DL/simdjson-5.0.8/singleheader -I $DL/boost_1_92_0 \ -I $DL/glaze-9.1.0/include -I $DL/rapidjson/include -I $DL/sonic/include \ bench_all.cpp yyjson.o simdjson.o -o bench_all ./bench_all 30 > mac-cpp-1.log cd rust && cargo build --release --locked && ./target/release/bench_rs 30 > ../mac-rs-1.log python3 summarize.py mac-cpp-*.log --rust mac-rs-*.log --json mac.json ``` With GCC 16 and Homebrew Clang on macOS, build the three steps with the same flags plus `--sysroot=$(xcrun --show-sdk-path)` (`gcc-16`, `g++-16`) or `-isysroot $(xcrun --show-sdk-path)` (`/opt/homebrew/opt/llvm/bin/clang`, `clang++`). The key selectors and simdjson's `tag_invoke` need C++20, so `bench_all.cpp` needs `-std=c++20` or newer. Both programs take ` [rounds] [file] [use case]`: `./bench_all 30 canada edit` times only the edit of `canada.json`; `""` stands for all files; the file names are `twitter`, `citm_catalog`, `canada`, `jeopardy`, `status`, `twitter_min` and `citm_catalog_min`. The checks cover only the selected files and use cases. bench_all uses `fork()` for the probes and the crash check (macOS, Linux). `--files [rounds] [file] [use case]` runs `validate`, `sum`, `minify` and `write` (`select` and `edit` are defined only for the files above) on every `*.json` file of a directory, named by their stem; files under 500 KB are timed in batches of about 500 KB of input. It is meant for files the libraries were not tuned on, such as the `jsonexamples` of [simdjson-data](https://github.com/simdjson/simdjson-data) without `twitter.json`, `citm_catalog.json` and `canada.json`. `summarize.py` takes such logs as well. On x86-64, use `-march=haswell -DRAPIDJSON_SSE42` instead of `-DRAPIDJSON_NEON` (sonic-cpp needs AVX2), pin the process to one core, and time one file per process. Glaze 9 needs `std::expected`, which Clang 18 and older cannot use from GCC 13's standard library (Clang 19 and newer can); `-DBENCH_NO_GLAZE=1` builds without it. For Rust, build with `RUSTFLAGS="-C target-cpu=haswell"` to match `-march=haswell`. ## Reading the results A log line holds the file, the use case, and one cell per variant: `| : []