Introduce mld_polymat_get_row() helper function

- Add mld_polymat_get_row() to retrieve matrix row pointer
- Update mld_polyvec_matrix_pointwise_montgomery() to use helper

Addresses #738 (steps 2-3 of #736)

Signed-off-by: Hanno Becker <beckphan@amazon.co.uk>

Author: hanno-becker

mldsa/mldsa_native.S

@@ -273,6 +273,7 @@
 /* mldsa/src/polyvec.h */
 #undef MLD_POLYVEC_H
 #undef mld_polymat
+#undef mld_polymat_get_row
 #undef mld_polyvec_matrix_expand
 #undef mld_polyvec_matrix_pointwise_montgomery
 #undef mld_polyveck

mldsa/mldsa_native.c

@@ -269,6 +269,7 @@
 /* mldsa/src/polyvec.h */
 #undef MLD_POLYVEC_H
 #undef mld_polymat
+#undef mld_polymat_get_row
 #undef mld_polyvec_matrix_expand
 #undef mld_polyvec_matrix_pointwise_montgomery
 #undef mld_polyveck

mldsa/src/polyvec.c

@@ -66,6 +66,12 @@ __contract__(
 #endif /* !MLD_USE_NATIVE_NTT_CUSTOM_ORDER */
 }
 
+MLD_INTERNAL_API
+const mld_polyvecl *mld_polymat_get_row(const mld_polymat *mat,
+                                        unsigned int row)
+{
+  return &mat->vec[row];
+}
 
 MLD_INTERNAL_API
 void mld_polyvec_matrix_expand(mld_polymat *mat,
@@ -172,7 +178,8 @@ void mld_polyvec_matrix_pointwise_montgomery(mld_polyveck *t,
                      array_abs_bound(t->vec[k0].coeffs, 0, MLDSA_N, MLDSA_Q)))
   )
   {
-    mld_polyvecl_pointwise_acc_montgomery(&t->vec[i], &mat->vec[i], v);
+    const mld_polyvecl *row = mld_polymat_get_row(mat, i);
+    mld_polyvecl_pointwise_acc_montgomery(&t->vec[i], row, v);
   }
 
   mld_assert_abs_bound_2d(t->vec, MLDSA_K, MLDSA_N, MLDSA_Q);

mldsa/src/polyvec.h

@@ -753,6 +753,21 @@ __contract__(
     array_bound(p->vec[k1].coeffs, 0, MLDSA_N, -(1<<(MLDSA_D-1)) + 1, (1<<(MLDSA_D-1)) + 1)))
 );
 
+#define mld_polymat_get_row MLD_NAMESPACE_KL(polymat_get_row)
+/*************************************************
+ * Name:        mld_polymat_get_row
+ *
+ * Description: Retrieve a pointer to a specific row of the matrix.
+ *
+ * Arguments:   - const mld_polymat *mat: pointer to matrix
+ *              - unsigned int row: row index (must be < MLDSA_K)
+ *
+ * Returns pointer to the row (mld_polyvecl)
+ **************************************************/
+MLD_INTERNAL_API
+const mld_polyvecl *mld_polymat_get_row(const mld_polymat *mat,
+                                        unsigned int row);
+
 #define mld_polyvec_matrix_expand MLD_NAMESPACE_KL(polyvec_matrix_expand)
 /*************************************************
  * Name:        mld_polyvec_matrix_expand