Module feature 

Per-feature latency classification + manifest.

A recipe’s optional features are not all the same kind of thing: some change the per-op hot path (a measured p99 claim), some are O(n) whole-structure ops (serialize, compaction) that cannot honestly carry a per-op sub-ms number, and some are pure capabilities with no latency delta (serde derives). This module lets the bench decide the category from a size sweep, and merge-writes the decision into a per-language manifest .subms/features/<lang>.json that the website renders.

The manifest is a public contract: the merge preserves any fields the harness does not own (yours, or another tool’s), touching only a feature’s perf rating + p99ByStage. Zero-dependency: the JSON value model, parser and serialiser here are hand-written, like the rest of the crate.

use subms::{SubMsFeatureCategory, classify_feature};

// A flat p99 across a 64x size sweep, well above the base op -> hot-path.
let sweep = [(1_024usize, 300u64), (65_536usize, 320u64)];
let (cat, _why) = classify_feature(&sweep, Some(50), None);
assert_eq!(cat, SubMsFeatureCategory::HotPath);

// p99 grows ~linearly with size -> structural (O(n)).
let sweep = [(1_024usize, 1_000u64), (65_536usize, 64_000u64)];
let (cat, _why) = classify_feature(&sweep, None, None);
assert_eq!(cat, SubMsFeatureCategory::Structural);

Structs

SubMsFeatureManifest

A per-language feature manifest (.subms/features/<lang>.json). Wraps a Json object so a load/mutate/save round-trip preserves every field the harness does not own - only a feature’s perf rating + p99ByStage are set.

SubMsStageClass

One stage of a feature, classified on its own measurements.

Enums

Json

A minimal JSON value. Objects keep insertion order (a Vec of pairs, not a map) so a merge preserves the on-disk key order and any unknown fields. Numbers keep their literal text so large integer p99 values round-trip exactly.

SubMsFeatureCategory

How a feature relates to the latency claim. The bench decides this from a size sweep; the string form is the manifest wire value + the UI enum.

SubMsP99Source

Which box a manifest’s p99ByStage figures were measured on.

Functions

classify_feature

Decide a feature’s category from a size sweep of (structure_size, p99_ns) measurements. base_p99_ns is the base op’s p99 for the no-delta check (None -> any flat, non-empty sweep reads as hot-path). override_category short-circuits the decision for a genuinely ambiguous feature (amortized, borderline-flat O(log n)); the reason records that it was overridden.

parse_json

Parse JSON text into a Json value. Returns an error string on malformed input. Std-only recursive descent; enough for manifest documents.

roll_up_stages

Roll per-stage categories into the one a feature publishes.