support empty input

Author: kunxian-xia

extensions/native/recursion/src/challenger/duplex.rs

@@ -80,17 +80,19 @@ impl<C: Config> DuplexChallengerVariable<C> {
     // Observes multiple elements from an array.
     // This is equivalent to calling `observe` multiple times, but more efficient.
     pub fn observe_slice_opt(&self, builder: &mut Builder<C>, arr: &Array<C, Felt<C::F>>) {
-        let next_pos = builder.poseidon2_multi_observe(&self.sponge_state, self.input_ptr, arr);
-
-        builder.assign(&self.input_ptr, self.io_empty_ptr + next_pos.clone());
-        builder.if_ne(next_pos, Usize::from(0)).then_or_else(
-            |builder| {
-                builder.assign(&self.output_ptr, self.io_empty_ptr);
-            },
-            |builder| {
-                builder.assign(&self.output_ptr, self.io_full_ptr);
-            },
-        );
+        builder.if_ne(arr.len(), Usize::from(0)).then(|builder| {
+            let next_pos = builder.poseidon2_multi_observe(&self.sponge_state, self.input_ptr, arr);
+
+            builder.assign(&self.input_ptr, self.io_empty_ptr + next_pos.clone());
+            builder.if_ne(next_pos, Usize::from(0)).then_or_else(
+                |builder| {
+                    builder.assign(&self.output_ptr, self.io_empty_ptr);
+                },
+                |builder| {
+                    builder.assign(&self.output_ptr, self.io_full_ptr);
+                },
+            );
+        });
     }
 
     fn sample(&self, builder: &mut Builder<C>) -> Felt<C::F> {

extensions/native/recursion/tests/recursion.rs

@@ -230,7 +230,7 @@ fn test_multi_observe() {
 }
 
 fn build_test_program<C: Config>(builder: &mut Builder<C>) {
-    let sample_lens: Vec<usize> = vec![10, 2, 1, 3, 20, 200, 400];
+    let sample_lens: Vec<usize> = vec![10, 2, 1, 0, 3, 20, 200, 400];
 
     let mut rng = create_seeded_rng();