Fix range test panic

Test asserts that the provided amount of bits are always even.
Added test to check for that panic.

README.md

@@ -22,7 +22,7 @@ use rand_core::OsRng;
 // Implement a circuit that checks:
 // 1) a + b = c where C is a PI
 // 2) a < 2^6
-// 3) b < 2^5
+// 3) b < 2^4
 // 4) a * b = d where D is a PI
 // 5) JubJub::GENERATOR * e(JubJubScalar) = f where F is a Public Input
 #[derive(Debug, Default)]
@@ -51,7 +51,7 @@ impl Circuit for TestCircuit {
 
         // Check that a and b are in range
         composer.component_range(a, 6);
-        composer.component_range(b, 5);
+        composer.component_range(b, 4);
 
         // Make second constraint a * b = d
         let constraint =

src/composer.rs

@@ -915,18 +915,17 @@ pub trait Composer: Sized + Index<Witness, Output = BlsScalar> {
     }
 
     /// Adds a range-constraint gate that checks and constrains a [`Witness`]
-    /// to be encoded in at most `num_bits`, which means that it will be within
-    /// the range `[0, 2^num_bits[`.
+    /// to be encoded in at most `num_bits`, which means that the value of the
+    /// [`Witness`] will be within the range `[0, 2^num_bits[`.
     ///
-    /// This function adds min(1, `num_bits/4`) gates to the circuit description
-    /// in order to add the range constraint.
+    /// This function adds (num_bits - 1)/8 + 9 gates to the circuit
+    /// description, when num_bits > 0,
+    /// and 7 gates when num_bits = 0
     ///
     ///# Panics
-    /// This function will panic if the num_bits specified is not even, ie.
-    /// `num_bits % 2 != 0`.
+    /// This function will panic if the num_bits specified is not even.
     fn component_range(&mut self, witness: Witness, num_bits: usize) {
-        // number of bits must be even
-        debug_assert_eq!(num_bits % 2, 0);
+        assert_eq!(num_bits % 2, 0, "number of bits must be even");
 
         // if num_bits = 0 constrain witness to 0
         if num_bits == 0 {

tests/range.rs

@@ -98,29 +98,6 @@ fn range() {
     let circuit = TestCircuit::new(a, bits);
     check_unsatisfied_circuit(&prover, &circuit, rng, &msg);
 
-    // Test bits = 4
-    //
-    // Compile new circuit descriptions for the prover and verifier
-    let bits = 4;
-    let a = BlsScalar::from(15);
-    let circuit = TestCircuit::new(a, bits);
-    let (prover, verifier) =
-        Compiler::compile_with_circuit(&pp, label, &circuit)
-            .expect("Circuit should compile");
-
-    // Test:
-    // 15 < 2^4
-    let msg = "Verification of a satisfied circuit should pass";
-    let circuit = TestCircuit::new(a, bits);
-    check_satisfied_circuit(&prover, &verifier, &pi, &circuit, rng, &msg);
-
-    // Test fails:
-    // 16 !< 2^4
-    let msg = "Proof creation of an unsatisfied circuit should fail";
-    let a = BlsScalar::from(16);
-    let circuit = TestCircuit::new(a, bits);
-    check_unsatisfied_circuit(&prover, &circuit, rng, &msg);
-
     // Test bits = 74
     //
     // Compile new circuit descriptions for the prover and verifier
@@ -173,4 +150,15 @@ fn range() {
     let a = -BlsScalar::one();
     let circuit = TestCircuit::new(a, bits);
     check_satisfied_circuit(&prover, &verifier, &pi, &circuit, rng, &msg);
+
+    // Test with odd bits = 55
+    //
+    // Compilation is expected to panic
+    let bits = 55;
+    let a = BlsScalar::pow_of_2(74) - BlsScalar::one();
+    let circuit = TestCircuit::new(a, bits);
+    let result = std::panic::catch_unwind(|| {
+        Compiler::compile_with_circuit::<TestCircuit>(&pp, label, &circuit)
+    });
+    assert!(result.is_err());
 }