Add Witness as replacement of AllocatedScalar

`AllocatedScalar` may suggest to the users to evaluate runtime scalars
to build their gadgets. This is inconsistent because it might create
different circuit descriptions out of the same code.

`AllocatedScalar` was entirely removed in favor of `Witness`. The latter
is a simple index to the witness value in the composer.

Resolves #588

CHANGELOG.md

@@ -10,10 +10,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 - Add back benchmarks to the crate. [#555](https://github.com/dusk-network/plonk/issue/555)
+
 ### Changed
 
 - Change `StandardComposer` to `TurboComposer`. [#288](https://github.com/dusk-network/plonk/issue/288)
 - Change `Variable` usage in favor of `AllocatedScalar`. [#565]((https://github.com/dusk-network/plonk/issue/565))
+- Add `Witness` by removing `AllocatedScalar`. [#588]((https://github.com/dusk-network/plonk/issue/588))
+
 ### Fixed
 
 - Fix the document references and typos [#533](https://github.com/dusk-network/plonk/pull/533)

README.md

@@ -36,13 +36,13 @@ impl Circuit for TestCircuit {
         &mut self,
         composer: &mut TurboComposer,
     ) -> Result<(), Error> {
-        let a = composer.add_input(self.a);
-        let b = composer.add_input(self.b);
+        let a = composer.append_witness(self.a);
+        let b = composer.append_witness(self.b);
         // Make first constraint a + b = c
         composer.poly_gate(
             a,
             b,
-            composer.allocated_zero(),
+            composer.zero(),
             BlsScalar::zero(),
             BlsScalar::one(),
             BlsScalar::one(),
@@ -57,7 +57,7 @@ impl Circuit for TestCircuit {
         composer.poly_gate(
             a,
             b,
-            composer.allocated_zero(),
+            composer.zero(),
             BlsScalar::one(),
             BlsScalar::zero(),
             BlsScalar::zero(),
@@ -66,7 +66,7 @@ impl Circuit for TestCircuit {
             Some(-self.d),
         );
 
-        let e = composer.add_input(self.e);
+        let e = composer.append_witness(self.e);
         let scalar_mul_result = composer
             .fixed_base_scalar_mul(e, dusk_jubjub::GENERATOR_EXTENDED);
         // Apply the constrain

src/circuit.rs

@@ -137,9 +137,9 @@ impl VerifierData {
 ///         composer: &mut TurboComposer,
 ///     ) -> Result<(), Error> {
 ///         // Add fixed witness zero
-///         let zero = composer.add_witness_to_circuit_description(BlsScalar::zero());
-///         let a = composer.add_input(self.a);
-///         let b = composer.add_input(self.b);
+///         let zero = composer.zero();
+///         let a = composer.append_witness(self.a);
+///         let b = composer.append_witness(self.b);
 ///         // Make first constraint a + b = c
 ///         composer.poly_gate(
 ///             a,
@@ -168,7 +168,7 @@ impl VerifierData {
 ///             Some(-self.d),
 ///         );
 ///
-///         let e = composer.add_input(self.e);
+///         let e = composer.append_witness(self.e);
 ///         let scalar_mul_result =
 ///             composer.fixed_base_scalar_mul(
 ///                 e, dusk_jubjub::GENERATOR_EXTENDED,
@@ -376,13 +376,13 @@ mod tests {
             &mut self,
             composer: &mut TurboComposer,
         ) -> std::result::Result<(), Error> {
-            let a = composer.add_input(self.a);
-            let b = composer.add_input(self.b);
+            let a = composer.append_witness(self.a);
+            let b = composer.append_witness(self.b);
             // Make first constraint a + b = c
             composer.poly_gate(
                 a,
                 b,
-                composer.allocated_zero(),
+                composer.zero(),
                 BlsScalar::zero(),
                 BlsScalar::one(),
                 BlsScalar::one(),
@@ -397,7 +397,7 @@ mod tests {
             composer.poly_gate(
                 a,
                 b,
-                composer.allocated_zero(),
+                composer.zero(),
                 BlsScalar::one(),
                 BlsScalar::zero(),
                 BlsScalar::zero(),
@@ -406,7 +406,7 @@ mod tests {
                 Some(-self.d),
             );
 
-            let e = composer.add_input(self.e);
+            let e = composer.append_witness(self.e);
             let scalar_mul_result = composer
                 .fixed_base_scalar_mul(e, dusk_jubjub::GENERATOR_EXTENDED);
             // Apply the constrain