Improve FixedEvaluator implementation

executor/src/witgen/jit/block_machine_processor.rs

@@ -188,7 +188,14 @@ impl<T: FieldElement> FixedEvaluator<T> for &BlockMachineProcessor<'_, T> {
     fn evaluate(&self, var: &AlgebraicReference, row_offset: i32) -> Option<T> {
         assert!(var.is_fixed());
         let values = self.fixed_data.fixed_cols[&var.poly_id].values_max_size();
-        let row = (row_offset + var.next as i32 + values.len() as i32) as usize % values.len();
+
+        // By assumption of the block machine, all fixed columns are cyclic with a period of <block_size>.
+        // An exception might be the first and last row.
+        assert!(row_offset >= -1);
+        assert!(self.block_size >= 1);
+        // The current row is guaranteed to be at least 1.
+        let current_row = (2 * self.block_size as i32 + row_offset) as usize;
+        let row = current_row + var.next as usize;
         Some(values[row])
     }
 }