fix: bal binary search cases

crates/state/src/bal/writes.rs

@@ -40,11 +40,15 @@ impl<T: PartialEq + Clone> BalWrites<T> {
             return self.get_linear_search(bal_index);
         }
         // else do binary search.
-        let index = self
+        let i = match self
             .writes
             .binary_search_by_key(&bal_index, |(index, _)| *index)
-            .ok()?;
-        Some(self.writes[index].1.clone())
+        {
+            Ok(i) => i,
+            Err(i) => i,
+        };
+        // only if i is not zero, we return the previous value.
+        (i != 0).then(|| self.writes[i - 1].1.clone())
     }
 
     /// Extend the builder with another builder.
@@ -145,4 +149,36 @@ mod tests {
         assert_eq!(bal_writes.get(3), Some(3));
         assert_eq!(bal_writes.get(4), Some(3));
     }
+
+    fn get_binary_search(threshold: BalIndex) {
+        // Construct test data up to (threshold - 1), skipping one key to simulate a gap.
+        let entries: Vec<_> = (0..threshold - 1)
+            .map(|i| (i, i + 1))
+            .chain(std::iter::once((threshold, threshold + 1)))
+            .collect();
+
+        let bal_writes = BalWrites::new(entries);
+
+        // Case 1: lookup before any entries
+        assert_eq!(bal_writes.get(0), None);
+
+        // Case 2: lookups for existing keys before the gap
+        for i in 1..threshold - 1 {
+            assert_eq!(bal_writes.get(i), Some(i));
+        }
+
+        // Case 3: lookup at the skipped key — should return the previous value
+        assert_eq!(bal_writes.get(threshold), Some(threshold - 1));
+
+        // Case 4: lookup after the skipped key — should return the next valid value
+        assert_eq!(bal_writes.get(threshold + 1), Some(threshold + 1));
+    }
+
+    #[test]
+    fn test_get_binary_search() {
+        get_binary_search(4);
+        get_binary_search(5);
+        get_binary_search(6);
+        get_binary_search(7);
+    }
 }