encapsulate sparse API (#352)

Co-authored-by: Hanting Zhang <winston@lurk-lab.com>

src/bellpepper/r1cs.rs

@@ -112,9 +112,9 @@ fn add_constraint<S: PrimeField>(
 ) {
   let (A, B, C, nn) = X;
   let n = **nn;
-  assert_eq!(n + 1, A.indptr.len(), "A: invalid shape");
-  assert_eq!(n + 1, B.indptr.len(), "B: invalid shape");
-  assert_eq!(n + 1, C.indptr.len(), "C: invalid shape");
+  assert_eq!(n, A.num_rows(), "A: invalid shape");
+  assert_eq!(n, B.num_rows(), "B: invalid shape");
+  assert_eq!(n, C.num_rows(), "C: invalid shape");
 
   let add_constraint_component = |index: Index, coeff: &S, M: &mut SparseMatrix<S>| {
     // we add constraints to the matrix only if the associated coefficient is non-zero

src/r1cs/sparse.rs

@@ -13,6 +13,7 @@ use ff::PrimeField;
 use itertools::Itertools as _;
 use rand_core::{CryptoRng, RngCore};
 use rayon::prelude::*;
+use ref_cast::RefCast;
 use serde::{Deserialize, Serialize};
 
 /// CSR format sparse matrix, We follow the names used by scipy.
@@ -31,6 +32,11 @@ pub struct SparseMatrix<F: PrimeField> {
   pub cols: usize,
 }
 
+/// Wrapper type for encode rows of [`SparseMatrix`]
+#[derive(Debug, Clone, RefCast)]
+#[repr(transparent)]
+pub struct RowData([usize; 2]);
+
 /// [`SparseMatrix`]s are often large, and this helps with cloning bottlenecks
 impl<F: PrimeField> Clone for SparseMatrix<F> {
   fn clone(&self) -> Self {
@@ -111,6 +117,30 @@ impl<F: PrimeField> SparseMatrix<F> {
     Self::new(&matrix, rows, cols)
   }
 
+  /// Returns an iterator into the rows
+  pub fn iter_rows(&self) -> impl Iterator<Item = &RowData> {
+    self
+      .indptr
+      .windows(2)
+      .map(|ptrs| RowData::ref_cast(ptrs.try_into().unwrap()))
+  }
+
+  /// Returns a parallel iterator into the rows
+  pub fn par_iter_rows(&self) -> impl IndexedParallelIterator<Item = &RowData> {
+    self
+      .indptr
+      .par_windows(2)
+      .map(|ptrs| RowData::ref_cast(ptrs.try_into().unwrap()))
+  }
+
+  /// Retrieves the data for row slice [i..j] from `row`.
+  /// [`RowData`] **must** be created from unmodified `self` previously to guarentee safety.
+  pub fn get_row(&self, row: &RowData) -> impl Iterator<Item = (&F, &usize)> {
+    self.data[row.0[0]..row.0[1]]
+      .iter()
+      .zip_eq(&self.indices[row.0[0]..row.0[1]])
+  }
+
   /// Retrieves the data for row slice [i..j] from `ptrs`.
   /// We assume that `ptrs` is indexed from `indptrs` and do not check if the
   /// returned slice is actually a valid row.
@@ -226,6 +256,14 @@ impl<F: PrimeField> SparseMatrix<F> {
       nnz: *self.indptr.last().unwrap(),
     }
   }
+
+  pub fn num_rows(&self) -> usize {
+    self.indptr.len() - 1
+  }
+
+  pub fn num_cols(&self) -> usize {
+    self.cols
+  }
 }
 
 /// Iterator for sparse matrix

src/spartan/batched.rs

@@ -513,13 +513,11 @@ impl<E: Engine, EE: EvaluationEngineTrait<E>> BatchedRelaxedR1CSSNARKTrait<E>
                           r_y: &[E::Scalar]|
      -> Vec<E::Scalar> {
       let evaluate_with_table =
-        // TODO(@winston-h-zhang): review
         |M: &SparseMatrix<E::Scalar>, T_x: &[E::Scalar], T_y: &[E::Scalar]| -> E::Scalar {
-          M.indptr
-            .par_windows(2)
+          M.par_iter_rows()
             .enumerate()
-            .map(|(row_idx, ptrs)| {
-              M.get_row_unchecked(ptrs.try_into().unwrap())
+            .map(|(row_idx, row)| {
+              M.get_row(row)
                 .map(|(val, col_idx)| T_x[row_idx] * T_y[*col_idx] * val)
                 .sum::<E::Scalar>()
             })

src/spartan/mod.rs

@@ -174,8 +174,9 @@ fn compute_eval_table_sparse<E: Engine>(
   assert_eq!(rx.len(), S.num_cons);
 
   let inner = |M: &SparseMatrix<E::Scalar>, M_evals: &mut Vec<E::Scalar>| {
-    for (row_idx, ptrs) in M.indptr.windows(2).enumerate() {
-      for (val, col_idx) in M.get_row_unchecked(ptrs.try_into().unwrap()) {
+    for (row_idx, row) in M.iter_rows().enumerate() {
+      for (val, col_idx) in M.get_row(row) {
+        // TODO(@winston-h-zhang): Parallelize? Will need more complicated locking
         M_evals[*col_idx] += rx[row_idx] * val;
       }
     }

src/spartan/snark.rs

@@ -351,11 +351,10 @@ impl<E: Engine, EE: EvaluationEngineTrait<E>> RelaxedR1CSSNARKTrait<E> for Relax
      -> Vec<E::Scalar> {
       let evaluate_with_table =
         |M: &SparseMatrix<E::Scalar>, T_x: &[E::Scalar], T_y: &[E::Scalar]| -> E::Scalar {
-          M.indptr
-            .par_windows(2)
+          M.par_iter_rows()
             .enumerate()
-            .map(|(row_idx, ptrs)| {
-              M.get_row_unchecked(ptrs.try_into().unwrap())
+            .map(|(row_idx, row)| {
+              M.get_row(row)
                 .map(|(val, col_idx)| T_x[row_idx] * T_y[*col_idx] * val)
                 .sum::<E::Scalar>()
             })