M r1cs-std/src/fields/fp/cmp.rs → r1cs-std/src/fields/fp/cmp.rs

@@ -8,52 +8,11 @@ use core::cmp::Ordering;
 use r1cs_core::{ConstraintSystem, SynthesisError};
 
 impl<F: PrimeField> FpGadget<F> {
-    fn process_cmp_inputs<CS: ConstraintSystem<F>>(
-        mut cs: CS,
-        a: &FpGadget<F>,
-        b: &FpGadget<F>,
-        ordering: Ordering,
-        should_also_check_equality: bool,
-    ) -> Result<(FpGadget<F>, FpGadget<F>), SynthesisError> {
-        let left;
-        let right;
-        match ordering {
-            Ordering::Less => {
-                left = a;
-                right = b;
-            },
-            Ordering::Greater => {
-                left = b;
-                right = a;
-            },
-            Ordering::Equal => {
-                return Err(SynthesisError::Unsatisfiable);
-            },
-        };
-        let right_for_check = if should_also_check_equality {
-            right.add_constant(cs.ns(|| "plus one"), &F::one())?
-        } else {
-            right.clone()
-        };
-
-        Ok((left.clone(), right_for_check))
-    }
-
-    fn check_smaller_than_mod_minus_one_div_two<CS: ConstraintSystem<F>>(
-        mut cs: CS,
-        a: &FpGadget<F>,
-    ) -> Result<(), SynthesisError> {
-        let a_bits = a.to_bits(cs.ns(|| "a to bits"))?;
-        Boolean::enforce_smaller_or_equal_than::<_, _, F, _>(
-            cs.ns(|| "enforce smaller than modulus minus one div two"),
-            &a_bits,
-            F::modulus_minus_one_div_two(),
-        )?;
-
-        Ok(())
-    }
-
-    /// this function verifies a and b are <= (p-1)/2
+    /// This function enforces the ordering between `self` and `b`. The
+    /// constraint system will not be satisfied otherwise. If `self` should
+    /// also be checked for equality, e.g. `a <= b` instead of `a < b`, set
+    /// `should_also_check_quality` to `true`. This variant verifies `a` and `b`
+    /// are `<= (p-1)/2`.
     pub fn enforce_cmp<CS: ConstraintSystem<F>>(
         &self,
         mut cs: CS,
@@ -71,7 +30,11 @@ )?;
         Self::enforce_smaller_than_unchecked(cs.ns(|| "enforce smaller than"), &left, &right)
     }
 
-    /// this function assumes a and b are known to be <= (p-1)/2
+    /// This function enforces the ordering between `self` and `b`. The
+    /// constraint system will not be satisfied otherwise. If `self` should
+    /// also be checked for equality, e.g. `a <= b` instead of `a < b`, set
+    /// `should_also_check_quality` to `true`. This variant assumes `a` and `b`
+    /// are `<= (p-1)/2` and does not generate constraints to verify that.
     pub fn enforce_cmp_unchecked<CS: ConstraintSystem<F>>(
         &self,
         mut cs: CS,
@@ -89,7 +52,12 @@ )?;
         Self::enforce_smaller_than(cs.ns(|| "enforce smaller than"), &left, &right)
     }
 
-    /// this function verifies a and b are <= (p-1)/2
+    /// This function checks the ordering between `self` and `b`. It outputs a
+    /// `Boolean` that contains the result - `1` if true, `0` otherwise. The
+    /// constraint system will be satisfied in any case. If `self` should
+    /// also be checked for equality, e.g. `a <= b` instead of `a < b`, set
+    /// `should_also_check_quality` to `true`. This variant verifies `a` and `b`
+    /// are `<= (p-1)/2`.
     pub fn is_cmp<CS: ConstraintSystem<F>>(
         &self,
         mut cs: CS,
@@ -107,7 +75,12 @@ )?;
         Self::is_smaller_than(cs.ns(|| "enforce smaller than"), &left, &right)
     }
 
-    /// this function assumes a and b are known to be <= (p-1)/2
+    /// This function checks the ordering between `self` and `b`. It outputs a
+    /// `Boolean` that contains the result - `1` if true, `0` otherwise. The
+    /// constraint system will be satisfied in any case. If `self` should
+    /// also be checked for equality, e.g. `a <= b` instead of `a < b`, set
+    /// `should_also_check_quality` to `true`. This variant assumes `a` and `b`
+    /// are `<= (p-1)/2` and does not generate constraints to verify that.
     pub fn is_cmp_unchecked<CS: ConstraintSystem<F>>(
         &self,
         mut cs: CS,
@@ -125,7 +98,54 @@ )?;
         Self::is_smaller_than_unchecked(cs.ns(|| "enforce smaller than"), &left, &right)
     }
 
-    /// this function verifies a and b are <= (p-1)/2
+    fn process_cmp_inputs<CS: ConstraintSystem<F>>(
+        mut cs: CS,
+        a: &FpGadget<F>,
+        b: &FpGadget<F>,
+        ordering: Ordering,
+        should_also_check_equality: bool,
+    ) -> Result<(FpGadget<F>, FpGadget<F>), SynthesisError> {
+        let left;
+        let right;
+        match ordering {
+            Ordering::Less => {
+                left = a;
+                right = b;
+            },
+            Ordering::Greater => {
+                left = b;
+                right = a;
+            },
+            Ordering::Equal => {
+                return Err(SynthesisError::Unsatisfiable);
+            },
+        };
+        let right_for_check = if should_also_check_equality {
+            right.add_constant(cs.ns(|| "plus one"), &F::one())?
+        } else {
+            right.clone()
+        };
+
+        Ok((left.clone(), right_for_check))
+    }
+
+    // Helper function to enforce `a <= (p-1)/2`.
+    fn check_smaller_than_mod_minus_one_div_two<CS: ConstraintSystem<F>>(
+        mut cs: CS,
+        a: &FpGadget<F>,
+    ) -> Result<(), SynthesisError> {
+        let a_bits = a.to_bits(cs.ns(|| "a to bits"))?;
+        Boolean::enforce_smaller_or_equal_than::<_, _, F, _>(
+            cs.ns(|| "enforce smaller than modulus minus one div two"),
+            &a_bits,
+            F::modulus_minus_one_div_two(),
+        )?;
+
+        Ok(())
+    }
+
+    /// Helper function to check `a < b` and output a result bit. This function
+    /// verifies `a` and `b` are `<= (p-1)/2`.
     fn is_smaller_than<CS: ConstraintSystem<F>>(
         mut cs: CS,
         a: &FpGadget<F>,
@@ -136,7 +156,9 @@ Self::check_smaller_than_mod_minus_one_div_two(cs.ns(|| "check b in range"), b)?;
         Self::is_smaller_than_unchecked(cs.ns(|| "enforce smaller than"), a, b)
     }
 
-    /// this function assumes a and b are known to be <= (p-1)/2
+    /// Helper function to check `a < b` and output a result bit. This function
+    /// assumes `a` and `b` are `<= (p-1)/2` and does not generate constraints
+    /// to verify that.
     fn is_smaller_than_unchecked<CS: ConstraintSystem<F>>(
         mut cs: CS,
         a: &FpGadget<F>,
@@ -150,7 +172,8 @@ let d_bits_len = d_bits.len();
         Ok(d_bits[d_bits_len - 1])
     }
 
-    /// this function verifies a and b are <= (p-1)/2
+    /// Helper function to enforce `a < b`. This function verifies `a` and `b`
+    /// are `<= (p-1)/2`.
     fn enforce_smaller_than<CS: ConstraintSystem<F>>(
         mut cs: CS,
         a: &FpGadget<F>,
@@ -161,7 +184,8 @@ Self::check_smaller_than_mod_minus_one_div_two(cs.ns(|| "check b in range"), b)?;
         Self::enforce_smaller_than_unchecked(cs.ns(|| "enforce smaller than"), a, b)
     }
 
-    /// this function assumes a and b are known to be <= (p-1)/2
+    /// Helper function to enforce `a < b`. This function assumes `a` and `b`
+    /// are `<= (p-1)/2` and does not generate constraints to verify that.
     fn enforce_smaller_than_unchecked<CS: ConstraintSystem<F>>(
         mut cs: CS,
         a: &FpGadget<F>,