fix: avoid overflow when counting scalars (PROOF-906) (#38)

* avoid overflow

* fix clippy

* drop dead code

src/compute/fixed_msm.rs

@@ -4,6 +4,13 @@ use ark_ec::short_weierstrass::Affine;
 use rayon::prelude::*;
 use std::marker::PhantomData;
 
+fn count_scalars_per_output(scalars_len: usize, output_bit_table: &[u32]) -> u32 {
+    let bit_sum: usize = output_bit_table.iter().map(|s| *s as usize).sum();
+    let num_output_bytes = (bit_sum + 7) / 8;
+    assert!(scalars_len % num_output_bytes == 0);
+    (scalars_len / num_output_bytes).try_into().unwrap()
+}
+
 /// Handle to compute multi-scalar multiplications (MSMs) with pre-specified generators
 ///
 /// # Example 1 - compute an MSM using the handle
@@ -96,10 +103,7 @@ impl<T: CurveId> MsmHandle<T> {
     /// exponents for generator g_i with the output scalars packed contiguously and padded with zeros.
     pub fn packed_msm(&self, res: &mut [T], output_bit_table: &[u32], scalars: &[u8]) {
         let num_outputs = res.len() as u32;
-        let bit_sum: u32 = output_bit_table.iter().sum();
-        let num_output_bytes = (bit_sum + 7) / 8;
-        assert!(scalars.len() as u32 % num_output_bytes == 0);
-        let n = scalars.len() as u32 / num_output_bytes;
+        let n = count_scalars_per_output(scalars.len(), output_bit_table);
         unsafe {
             blitzar_sys::sxt_fixed_packed_multiexponentiation(
                 res.as_ptr() as *mut std::ffi::c_void,
@@ -235,3 +239,24 @@ impl<C: SwCurveConfig + Clone> SwMsmHandle for MsmHandle<ElementP2<C>> {
         });
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn we_can_count_the_number_of_scalars_per_output() {
+        let output_bit_table = [1];
+        let n = count_scalars_per_output(1, &output_bit_table);
+        assert_eq!(n, 1);
+
+        let output_bit_table = [14, 2];
+        let n = count_scalars_per_output(10, &output_bit_table);
+        assert_eq!(n, 5);
+
+        // we handle cases that overflow
+        let output_bit_table = [u32::MAX, 1];
+        let n = count_scalars_per_output((u32::MAX as usize) + 1, &output_bit_table);
+        assert_eq!(n, 8);
+    }
+}