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chore: add SAFETY docs to unsafe blocks #162

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chore: add SAFETY docs to unsafe blocks #162

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8 changes: 8 additions & 0 deletions crates/whir/src/utils.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -144,6 +144,10 @@ fn prepare_evals_for_fft_unpacked<A: Copy + Send + Sync>(
let block_index = i % dft_n_cols;
let offset_in_block = i / dft_n_cols;
let src_index = ((block_index << log_block_size) + offset_in_block) >> log_inv_rate;
// SAFETY: src_index = ((block_index * block_size) + offset_in_block) >> log_inv_rate.
// block_index < dft_n_cols = n_blocks (folding_factor power-of-2),
// offset_in_block < block_size = full_len / n_blocks,
// so the numerator < full_len, and right-shifting by log_inv_rate gives < evals.len().
unsafe { *evals.get_unchecked(src_index) }
})
.collect()
Expand Down Expand Up @@ -171,10 +175,14 @@ fn prepare_evals_for_fft_packed_extension<EF: ExtensionField<PF<EF>>>(
let src_index = ((block_index << log_block_size) + offset_in_block) >> log_inv_rate;
let packed_src_index = src_index >> log_packing;
let offset_in_packing = src_index & packing_mask;
// SAFETY: src_index < evals.len() << log_packing (same argument as unpacked variant),
// so packed_src_index = src_index >> log_packing < evals.len().
let packed = unsafe { evals.get_unchecked(packed_src_index) };
let unpacked: &[PFPacking<EF>] = packed.as_basis_coefficients_slice();
EF::from_basis_coefficients_fn(|i| unsafe {
// SAFETY: i < EF::D (extension degree), which equals unpacked.len() by construction.
let u: &PFPacking<EF> = unpacked.get_unchecked(i);
// SAFETY: offset_in_packing = src_index & packing_mask < packing width.
*u.as_slice().get_unchecked(offset_in_packing)
})
})
Expand Down