moonbitlang / core / 6921

90%

push

github

perf(json): presize the results of FromJson for Array, ArrayView and Map (#4303)

`FromJson for Array[X]` / `ArrayView[X]` built the result with an array
comprehension, which grows by pushing; the code's own TODO recorded the
~33% cost on nested arrays against the presized `mapi` it replaced.
Decode into `Array(capacity=a.length())` with push (a private
`decode_elements` shared by both), which keeps the shared `JsonPath`
node and leaves no uninitialised slots if an element fails. `FromJson
for Map[String, V]` now sizes the map from the object's length, as
`ToJson for Map` already does.

Release (json/from_json_bench_test.mbt), main -> this:

| bench                          | native        | js            | wasm-gc       |
|--------------------------------|---------------|---------------|---------------|
| Array[Array[Int]] 500x50       | 137 -> 102us  | 235 -> 234us  | 164 -> 133us  |
| Array[Double] n=10000          | 29.5 -> 26.1us| 35.3 -> 33.7us| 57.4 -> 35.6us|
| Map[String, Int] n=1000        | 66.0 -> 53.1us| 55.4 -> 42.4us| 38.4 -> 33.5us|

Codex review: "No blocking issues found. The helper preserves decoding order, error paths/messages, and ArrayView behavior; presizing resolves the TODO without exposing uninitialized elements."

Signed-off-by: Codex CLI <codex@openai.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

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