Merge pull request #256 from mir-protocol/fix_reduce

Small fixes in the `le_sum` and `reduce` gadgets

src/gadgets/split_base.rs

@@ -32,6 +32,18 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
         bits: impl ExactSizeIterator<Item = impl Borrow<BoolTarget>> + Clone,
     ) -> Target {
         let num_bits = bits.len();
+        if num_bits == 0 {
+            return self.zero();
+        } else if num_bits == 1 {
+            let mut bits = bits;
+            return bits.next().unwrap().borrow().target;
+        } else if num_bits == 2 {
+            let two = self.two();
+            let mut bits = bits;
+            let b0 = bits.next().unwrap().borrow().target;
+            let b1 = bits.next().unwrap().borrow().target;
+            return self.mul_add(two, b1, b0);
+        }
         debug_assert!(
             BaseSumGate::<2>::START_LIMBS + num_bits <= self.config.num_routed_wires,
             "Not enough routed wires."

src/util/reducing.rs

@@ -137,12 +137,6 @@ impl<const D: usize> ReducingFactorTarget<D> {
         acc
     }
 
-    /// Reduces a length `n` vector of `ExtensionTarget`s using `n/2` `ArithmeticExtensionGate`s.
-    /// It does this by batching two steps of Horner's method in each gate.
-    /// Here's an example with `n=4, alpha=2, D=1`:
-    /// 1st gate: 2  0 4  4 3  4 11 <- 2*0+4=4, 2*4+3=11
-    /// 2nd gate: 2 11 2 24 1 24 49 <- 2*11+2=24, 2*24+1=49
-    /// which verifies that `2.reduce([1,2,3,4]) = 49`.
     pub fn reduce<F>(
         &mut self,
         terms: &[ExtensionTarget<D>], // Could probably work with a `DoubleEndedIterator` too.
@@ -155,7 +149,7 @@ impl<const D: usize> ReducingFactorTarget<D> {
         self.count += l as u64;
 
         let mut terms_vec = terms.to_vec();
-        let mut acc = terms_vec.pop().unwrap();
+        let mut acc = builder.zero_extension();
         terms_vec.reverse();
 
         for x in terms_vec {