From a6bd540a6c16bcd6a69969a5c0e2c8808ecf7935 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tancr=C3=A8de=20Lepoint?=
 <tlepoint@users.noreply.github.com>
Date: Tue, 27 Jan 2026 20:33:42 -0500
Subject: [PATCH] Optimize dot product accumulation

---
 crates/fhe/src/bfv/ops/dot_product.rs | 78 ++++++++++++++++++++-------
 1 file changed, 58 insertions(+), 20 deletions(-)

diff --git a/crates/fhe/src/bfv/ops/dot_product.rs b/crates/fhe/src/bfv/ops/dot_product.rs
index f51aed13..c08cf902 100644
--- a/crates/fhe/src/bfv/ops/dot_product.rs
+++ b/crates/fhe/src/bfv/ops/dot_product.rs
@@ -2,7 +2,7 @@ use std::cmp::min;
 
 use fhe_math::rq::{Poly, Representation, dot_product as poly_dot_product, traits::TryConvertFrom};
 use itertools::{Itertools, izip};
-use ndarray::{Array, Array2};
+use ndarray::Array2;
 
 use crate::{
     Error, Result,
@@ -105,21 +105,28 @@ where
             level: ct_first.level,
         })
     } else {
-        let mut acc = Array::zeros((ct_first.len(), ctx.moduli().len(), ct_first.par.degree()));
+        let moduli_len = ctx.moduli().len();
+        let degree = ct_first.par.degree();
+        let coeffs_len = moduli_len * degree;
+        let mut acc = vec![0u128; ct_first.len() * coeffs_len];
         for (ciphertext, plaintext) in izip!(ct, pt) {
             let pt_coefficients = plaintext.poly_ntt.coefficients();
-            for (mut acci, ci) in izip!(acc.outer_iter_mut(), ciphertext.iter()) {
+            let pt_coefficients = pt_coefficients.as_slice().ok_or_else(|| {
+                Error::DefaultError("Plaintext coefficients are not contiguous".to_string())
+            })?;
+            for (part_idx, ci) in ciphertext.iter().enumerate() {
                 let ci_coefficients = ci.coefficients();
-                for (mut accij, cij, pij) in izip!(
-                    acci.outer_iter_mut(),
-                    ci_coefficients.outer_iter(),
-                    pt_coefficients.outer_iter()
-                ) {
+                let ci_coefficients = ci_coefficients.as_slice().ok_or_else(|| {
+                    Error::DefaultError("Ciphertext coefficients are not contiguous".to_string())
+                })?;
+                let acc_offset = part_idx * coeffs_len;
+                for modulus_idx in 0..moduli_len {
+                    let coeff_offset = modulus_idx * degree;
                     unsafe {
                         fma(
-                            accij.as_slice_mut().unwrap(),
-                            cij.as_slice().unwrap(),
-                            pij.as_slice().unwrap(),
+                            &mut acc[acc_offset + coeff_offset..acc_offset + coeff_offset + degree],
+                            &ci_coefficients[coeff_offset..coeff_offset + degree],
+                            &pt_coefficients[coeff_offset..coeff_offset + degree],
                         )
                     }
                 }
@@ -128,15 +135,18 @@ where
 
         // Reduce
         let mut c = Vec::with_capacity(ct_first.len());
-        for acci in acc.outer_iter() {
-            let mut coeffs = Array2::zeros((ctx.moduli().len(), ct_first.par.degree()));
-            for (mut outij, accij, q) in izip!(
-                coeffs.outer_iter_mut(),
-                acci.outer_iter(),
-                ctx.moduli_operators()
-            ) {
-                for (outij_coeff, accij_coeff) in izip!(outij.iter_mut(), accij.iter()) {
-                    unsafe { *outij_coeff = q.reduce_u128_vt(*accij_coeff) }
+        for part_idx in 0..ct_first.len() {
+            let mut coeffs = Array2::zeros((moduli_len, degree));
+            let coeffs_slice = coeffs.as_slice_mut().ok_or_else(|| {
+                Error::DefaultError("Output coefficients are not contiguous".to_string())
+            })?;
+            let acc_slice = &acc[part_idx * coeffs_len..(part_idx + 1) * coeffs_len];
+            for (modulus_idx, q) in ctx.moduli_operators().iter().enumerate() {
+                let coeff_offset = modulus_idx * degree;
+                let out_slice = &mut coeffs_slice[coeff_offset..coeff_offset + degree];
+                let acc_coeffs = &acc_slice[coeff_offset..coeff_offset + degree];
+                for (out_coeff, acc_coeff) in out_slice.iter_mut().zip(acc_coeffs.iter()) {
+                    unsafe { *out_coeff = q.reduce_u128_vt(*acc_coeff) }
                 }
             }
             c.push(Poly::try_convert_from(
@@ -201,4 +211,32 @@ mod tests {
         }
         Ok(())
     }
+
+    #[test]
+    fn test_dot_product_scalar_step_by() -> Result<(), Box<dyn Error>> {
+        let mut rng = rng();
+        let params = BfvParameters::default_arc(1, 16);
+        let sk = SecretKey::random(&params, &mut rng);
+        let ct = (0..8)
+            .map(|_| {
+                let v = params.plaintext.random_vec(params.degree(), &mut rng);
+                let pt = Plaintext::try_encode(&v, Encoding::simd(), &params).unwrap();
+                sk.try_encrypt(&pt, &mut rng).unwrap()
+            })
+            .collect_vec();
+        let pt = (0..8)
+            .map(|_| {
+                let v = params.plaintext.random_vec(params.degree(), &mut rng);
+                Plaintext::try_encode(&v, Encoding::simd(), &params).unwrap()
+            })
+            .collect_vec();
+
+        let r = dot_product_scalar(ct.iter().step_by(2), pt.iter().step_by(2))?;
+
+        let mut expected = Ciphertext::zero(&params);
+        izip!(ct.iter().step_by(2), pt.iter().step_by(2))
+            .for_each(|(cti, pti)| expected += &(cti * pti));
+        assert_eq!(r, expected);
+        Ok(())
+    }
 }