Remove while loop in constant case for sub_op

crates/emulated/src/field_ops.rs

@@ -9,7 +9,7 @@ use bellpepper_core::num::{AllocatedNum, Num};
 use bellpepper_core::{ConstraintSystem, LinearCombination, SynthesisError};
 use ff::{PrimeField, PrimeFieldBits};
 use num_bigint::BigInt;
-use num_traits::{One, Zero};
+use num_traits::One;
 
 use crate::field_element::{EmulatedFieldElement, EmulatedFieldParams, EmulatedLimbs};
 use crate::util::{bigint_to_scalar, decompose, recompose};
@@ -435,11 +435,11 @@ where
         if a.is_constant() && b.is_constant() {
             let a_int = BigInt::from(a);
             let b_int = BigInt::from(b);
-            let mut res_int = a_int - b_int;
-            while res_int < BigInt::zero() {
-                res_int += P::modulus();
-            }
-            res_int = res_int.rem(P::modulus());
+            let res_int = if a_int > b_int {
+                (a_int - b_int).rem(P::modulus())
+            } else {
+                P::modulus() - (b_int - a_int).rem(P::modulus())
+            };
             return Ok(Self::from(&res_int));
         }