# Feature: SIMD Acceleration
### Scope
- **Purpose**: Provide opt-in hardware-accelerated string processing via the `simd` feature flag, enabling vectorized delimiter search and byte counting where the target platform supports it.
- **Responsibility**: Documents the SIMD acceleration feature boundary, activation mechanism, and behavioral guarantees, linking to algorithmic detail and invariant documents.
- **In Scope**: The `simd` Cargo feature flag, SIMD-dependent optional dependencies, the vectorized delimiter search path, graceful scalar fallback.
- **Out of Scope**: Algorithm-level implementation detail (`algorithm/001`); split configuration model (`feature/001`); public API surface (`api/001`).
### Design
SIMD acceleration is entirely opt-in: callers activate it by enabling the `simd` Cargo feature. When the feature is absent the crate compiles and operates identically using scalar code paths.
When the feature is enabled, four additional dependencies become active: a byte-search library, a multi-pattern search library, a byte-counting library, and a caching primitive for compiled pattern objects. These libraries provide vectorized scanning that the splitting and delimiter-search internals can delegate to when the input meets the size threshold.
The SIMD path and the scalar path produce identical results. Correctness is not conditional on feature availability — only throughput changes. This guarantee is an invariant and is documented separately in `invariant/003_simd_fallback_contract.md`.
### Sources
- [src/simd.rs](../../src/simd.rs) — SIMD feature gating and platform dispatch
- [src/string/split/simd.rs](../../src/string/split/simd.rs) — Vectorized delimiter search within the split path
### Algorithms
- [001_simd_delimiter_search.md](../algorithm/001_simd_delimiter_search.md) — SIMD delimiter search algorithmic design
### Invariants
- [002_feature_gating_contract.md](../invariant/002_feature_gating_contract.md) — Feature opt-in gating invariant
- [003_simd_fallback_contract.md](../invariant/003_simd_fallback_contract.md) — Scalar fallback correctness guarantee