Update layouts.rs

src/circuit/ops/layouts.rs

@@ -75,20 +75,37 @@ pub fn is_closest_to<F: PrimeField + TensorType + PartialOrd + std::hash::Hash>(
     let l1_distance_1 = l1_distance(config, region, &[values[1].clone(), reference[0].clone()])?;
     let l1_distance_2 = l1_distance(config, region, &[values[2].clone(), reference[0].clone()])?;
 
-    let is_closest_to_0 = less(config, region, &[l1_distance_0.clone(), l1_distance_1])?;
-    let is_closest_to_1 = less(config, region, &[l1_distance_0, l1_distance_2])?;
+    // we need to account for rounding up or down so it is a <= and not a < comparison but only one of the distances can be less than or equal to the reference
+    // else we could have two distances that are equal to the reference,
+    // which could result in soundness issues if we're looking for a unique solution in monotically increasing distance functions
+    let is_equal_0 = equals(
+        config,
+        region,
+        &[l1_distance_0.clone(), l1_distance_1.clone()],
+    )?;
+    let is_closest_to_0 = less_equal(config, region, &[l1_distance_0.clone(), l1_distance_1])?;
+    let is_equal_1 = equals(
+        config,
+        region,
+        &[l1_distance_0.clone(), l1_distance_2.clone()],
+    )?;
+    let is_closest_to_1 = less_equal(config, region, &[l1_distance_0, l1_distance_2])?;
+
+    let is_closest = and(config, region, &[is_closest_to_0, is_closest_to_1])?;
+    let both_equal = and(config, region, &[is_equal_0, is_equal_1])?;
+    let not_both_equal = not(config, region, &[both_equal])?;
 
-    let is_closest_to = and(config, region, &[is_closest_to_0, is_closest_to_1])?;
+    let is_closest = and(config, region, &[is_closest, not_both_equal])?;
 
-    let mut comparison_unit = create_constant_tensor(integer_rep_to_felt(1), is_closest_to.len());
+    let mut comparison_unit = create_constant_tensor(integer_rep_to_felt(1), is_closest.len());
 
-    comparison_unit.reshape(is_closest_to.dims())?;
+    comparison_unit.reshape(is_closest.dims())?;
     // assigned unit
     let assigned_unit = region.assign(&config.custom_gates.inputs[1], &comparison_unit)?;
     region.increment(assigned_unit.len());
 
     // assert that the result is 1
-    enforce_equality(config, region, &[is_closest_to, assigned_unit])?;
+    enforce_equality(config, region, &[is_closest, assigned_unit])?;
 
     Ok(())
 }