fixed naming (zero --> nonzero), and other fixes

src/gadgets/insert.rs

@@ -1,33 +1,39 @@
 use crate::circuit_builder::CircuitBuilder;
 use crate::field::extension_field::target::ExtensionTarget;
 use crate::field::extension_field::Extendable;
-use crate::generator::EqualsZeroGenerator;
+use crate::generator::NonzeroTestGenerator;
 use crate::target::Target;
 
 impl<F: Extendable<D>, const D: usize> CircuitBuilder<F, D> {
-    /// Evaluates to 1 if `x` equals zero, 0 otherwise.
-    pub fn is_zero(&mut self, x: Target) -> Target {
+    /// Evaluates to 0 if `x` equals zero, 1 otherwise.
+    pub fn is_nonzero(&mut self, x: Target) -> Target {
+        // Dummy variable.
         let m = self.add_virtual_target();
+
+        // The prover sets the dummy variable to 0 if x == 0 and to 1/x otherwise.
+        self.add_generator(NonzeroTestGenerator {
+            to_test: x,
+            dummy: m,
+        });
+
+        // Evaluates to (0) * (0) = 0 if x == 0 and (x) * (1/x) = 1 otherwise.
         let y = self.mul(x, m);
 
+        // Enforce that (1 - y) * x == 0.
         let one = self.one();
         let diff = self.sub(one, y);
         let prod = self.mul(diff, x);
         self.assert_zero(prod);
 
-        self.add_generator(EqualsZeroGenerator {
-            to_test: x,
-            dummy: m,
-            is_zero: y,
-        });
-
         y
     }
 
     /// Evaluates to 1 if `x` and `y` are equal, 0 otherwise.
     pub fn is_equal(&mut self, x: Target, y: Target) -> Target {
         let difference = self.sub(x, y);
-        self.is_zero(difference)
+        let not_equal = self.is_nonzero(difference);
+        let one = self.one();
+        self.sub(one, not_equal)
     }
 
     /// Inserts a `Target` in a vector at a non-deterministic index. This is done by rotating to the

src/generator.rs

@@ -132,13 +132,12 @@ impl<F: Field> SimpleGenerator<F> for RandomValueGenerator {
 }
 
 /// A generator for testing if a value equals zero
-pub(crate) struct EqualsZeroGenerator {
+pub(crate) struct NonzeroTestGenerator {
     pub(crate) to_test: Target,
     pub(crate) dummy: Target,
-    pub(crate) is_zero: Target,
 }
 
-impl<F: Field> SimpleGenerator<F> for EqualsZeroGenerator {
+impl<F: Field> SimpleGenerator<F> for NonzeroTestGenerator {
     fn dependencies(&self) -> Vec<Target> {
         vec![self.to_test]
     }
@@ -146,12 +145,6 @@ impl<F: Field> SimpleGenerator<F> for EqualsZeroGenerator {
     fn run_once(&self, witness: &PartialWitness<F>) -> PartialWitness<F> {
         let to_test_value = witness.get_target(self.to_test);
 
-        let is_zero_value = if to_test_value == F::ZERO {
-            F::ZERO
-        } else {
-            F::ONE
-        };
-
         let dummy_value = if to_test_value == F::ZERO {
             F::ONE
         } else {
@@ -160,7 +153,6 @@ impl<F: Field> SimpleGenerator<F> for EqualsZeroGenerator {
 
         let mut witness = PartialWitness::new();
         witness.set_target(self.dummy, dummy_value);
-        witness.set_target(self.is_zero, is_zero_value);
         witness
     }
 }