Crate simd_json

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simd-json is a rust port of the simdjson c++ library. It follows most of the design closely with a few exceptions to make it better fit into the rust ecosystem.

Note: On x86 it will select the best SIMD featureset (avx2, or sse4.2) during runtime. If simd-json is compiled with SIMD support, it will disable runtime detection.


the goal of the rust port of simdjson is not to create a one to one copy, but to integrate the principles of the c++ library into a rust library that plays well with the rust ecosystem. As such we provide both compatibility with serde as well as parsing to a dom to manipulate data.


As a rule of thumb this library tries to get as close as possible to the performance of the c++ implementation, but some of the design decisions - such as parsing to a dom or a tape, weigh ergonomics over performance. In other places Rust makes it harder to achieve the same level of performance.


this library uses unsafe all over the place, and while it leverages quite a few test cases along with property based testing, please use this library with caution.

§Features comes with a number of features that can be toggled, the following features are intended for ‘user’ selection. Additional features in the Cargo.toml exist to work around cargo limitations.

§swar-number-parsing (default)

Enables a parsing method that will parse 8 digits at a time for floats - this is a common pattern but comes as a slight perf hit if all the floats have less then 8 digits.

§serde_impl (default)

Compatibility with serde. This allows to use to deserialize serde objects as well as serde compatibility of the different Value types. This can be disabled if serde is not used alongside simd-json.


Support for signed and unsigned 128 bit integer. This feature is disabled by default as 128 bit integers are rare in the wild and parsing them comes as a performance penalty due to extra logic and a changed memory layout.


The known-key feature changes hasher for the objects, from ahash to fxhash, ahash is faster at hashing and provides protection against DOS attacks by forcing multiple keys into a single hashing bucket. fxhash on the other hand allows for repeatable hashing results, that allows memorizing hashes for well know keys and saving time on lookups. In workloads that are heavy at accessing some well known keys this can be a performance advantage.


simd-json offers two main entry points for usage:

§Values API

The values API is a set of optimized DOM objects that allow parsed json to JSON data that has no known variable structure. simd-lite has two versions of this:

Borrowed Values

use simd_json;
let mut d = br#"{"some": ["key", "value", 2]}"#.to_vec();
let v: simd_json::BorrowedValue = simd_json::to_borrowed_value(&mut d).unwrap();

Owned Values

use simd_json;
let mut d = br#"{"some": ["key", "value", 2]}"#.to_vec();
let v: simd_json::OwnedValue = simd_json::to_owned_value(&mut d).unwrap();

§Serde Compatible API

use simd_json;
use serde_json::Value;

let mut d = br#"{"some": ["key", "value", 2]}"#.to_vec();
let v: Value = simd_json::serde::from_slice(&mut d).unwrap();



  • Reexport of Cow Reexport of Cow
  • Default trait imports;
  • serde related helper functions
  • simd-json JSON-DOM value



  • A struct to hold the buffers for the parser.
  • Deserializer struct to deserialize a JSON
  • Parser error




  • Fills a already existing tape from the input for later consumption
  • Creates a tape from the input for later consumption
  • Creates a tape from the input for later consumption

Type Aliases§