# SATD KERNELS KNOWLEDGE BASE
**Generated:** 2026-04-17
## OVERVIEW
Sum of Absolute Transformed Differences kernels. Hadamard-transformed block matching metric.
## STRUCTURE
```
src/satd/
├── rust.rs # Scalar implementation
├── avx2.rs # AVX2 SIMD implementation
├── tests.rs # Test generator macro (get_satd_tests!)
└── satd.rs # Module root, dispatch
```
## WHERE TO LOOK
| Scalar SATD | `rust.rs` | Baseline reference with Hadamard transform |
| AVX2 SATD | `avx2.rs` | SIMD-optimized Hadamard + SAD, compiled with `avx2` feature |
| Test coverage | `tests.rs` | `get_satd_tests!` macro generates tests for both backends |
| Dispatch | `satd.rs` | `#[cfg]` + cpudetect-backed runtime gate |
## CONVENTIONS
- Standard `rust.rs` + `avx2.rs` + `tests.rs` pattern.
- `avx2` feature gates AVX2 backend compilation; `--no-default-features` keeps scalar-only coverage.
- Runtime dispatch uses `cpudetect::x86_64::is_x86_64_v3_compatible()` at the boundary.
- Hadamard transform must produce identical results in scalar and AVX2 paths.
- Test generator `get_satd_tests!($module)` covers both backends.
- Both `u8` and `u16` pixel types tested.
- Used in DCT-based search modes (mode 3, 4) alongside SAD.
## ANTI-PATTERNS
- Diverging Hadamard results between scalar and AVX2 (validated by `verify_asm!`).
- Incorrect transform ordering (rows before columns, or vice versa).
- Missing overflow handling for 16-bit accumulators.