src/circuit.rs

 
 #[derive(Debug, Clone)]
 pub struct Circuit {
-    pub input_bits: Vec<bool>, // Input wires
     pub gates: Vec<Gate>,      // All gates
 }
 
 impl Circuit {
-    pub fn eval(self) -> bool {
+    pub fn eval(self, input_bits: Vec<bool>,)  -> bool {
         let mut evaluated_gates = vec![];
 
         for gate in self.gates {
-            let result = gate.eval(&self.input_bits, &evaluated_gates);
+            let result = gate.eval(&input_bits, &evaluated_gates);
             evaluated_gates.push(result);
         }
 
         This method should create a circuit that outputs 1 if the first number A (encoded in the first n
         bits) is greater than the second number B (encoded in the next n bits) .
     */
-    pub fn compare_n_bit_numbers(input_bits: Vec<bool>, n: usize) -> Self {
-        if input_bits.len() < 2 * n {
-            panic!(
-                "Expected input_bits to be of at least length {}, but it was {}",
-                2 * n,
-                input_bits.len()
-            )
-        }
+    pub fn compare_n_bit_numbers( n: usize) -> Self {
 
         /*
             base case n=1: 1-bit:
         */
 
         let gates = create_n_bit_comparator_gates(n);
-        return Circuit { input_bits, gates };
+        return Circuit { gates };
     }
 }
 
 fn create_n_bit_comparator_gates(n: usize) -> Vec<Gate> {
-    let mut indices: Vec<usize> = vec![0; n];
-    let mut all_gates: Vec<Gate> = Vec::with_capacity(1+3*n);
-    let mut and_gate_indices: Vec<usize> = vec![0; n];
-
-    for curr in 0..n {
-        // Gate(A_current > B_current)
-        let gt_gate = Gate::new(GateType::Bigger, vec![], vec![curr, curr + n]);
-        let gt_gate_index = all_gates.len();
-        all_gates.push(gt_gate);
-
-        // print!("( ({}) ", format!("{} > {}", format!("A{}", n-1-curr), format!("B{}", n-1-curr)));
-
-        let mut current_bit_gate_indices: Vec<usize> = Vec::with_capacity(curr+1);
-        current_bit_gate_indices.push(gt_gate_index);
-
-        // Bit to the left of curr. The one at array-index 0 doesn't have one.
-        if curr != 0 {
-            // Gate(A_i = B_i)
-            let eq_gate = Gate::new(GateType::Equal, vec![], vec![curr - 1, curr - 1 + n]);
-            // remember which key-index (see below) this bit belongs to.
-            indices[n - curr] = all_gates.len();
-            all_gates.push(eq_gate);
-        }
+    let mut all_gates: Vec<Gate> = Vec::with_capacity(2+3*(n-1));
+    let mut eq_gate_indices: Vec<usize> = Vec::with_capacity(n-1);
+    let mut or_gate_input_indices: Vec<usize> = Vec::with_capacity(n);
 
-        for i in 0..curr {
-            // Translate i to the key'th bit position because the array index does not match the bit index
-            // i.e.
-            //
-            //  let input_bits = vec![
-            //         A2     A1    A0    <- A_key
-            //          0      1     2    <- i iterates all array indices less than curr
-            //        false, false, true,
-            //
-            //         B2     B1    B0
-            //          3      4     5
-            //        false, true, false
-            //  ];
-            let key = n - 1 - i;
-
-            // print!("&& ({}) ", format!("{} = {}", format!("A{}", key), format!("B{}", key)));
-
-            // Index of Gate(A_i = B_i). 
-            // You could maybe find a solution without the indices vector but it's insanely cheap anyway and maybe even better
-            let eq_gate_index = indices[key];
-            current_bit_gate_indices.push(eq_gate_index);
-        }
+    const EMPTY_VEC: Vec<usize> = Vec::new();
 
-        // The AND spanning all gates for this bit
-        let and_gate_index = all_gates.len();
-        let and_gate = Gate::new(GateType::And, current_bit_gate_indices, vec![]);
+    all_gates.push(Gate::new(GateType::Bigger, EMPTY_VEC, vec![0, n]));
+    or_gate_input_indices.push(0);
 
-        // println!(")");
-        all_gates.push(and_gate);
-        and_gate_indices.push(and_gate_index);
+    for curr in 1..n {
+        // Gate(A_curr > B_curr)
+        let mut and_gate_input_indices = vec!(all_gates.len());
+        all_gates.push(Gate::new(GateType::Bigger, EMPTY_VEC, vec![curr, curr + n]));
+
+        // Gate(A_curr-1 = B_curr-1)
+        eq_gate_indices.push(all_gates.len());
+        all_gates.push(Gate::new(GateType::Equal, EMPTY_VEC, vec![curr - 1, curr - 1 + n]));
+
+        and_gate_input_indices.extend(eq_gate_indices.iter());
+
+        // The AND spanning all gates for this bit
+        or_gate_input_indices.push(all_gates.len());
+        all_gates.push(Gate::new(GateType::And, and_gate_input_indices, EMPTY_VEC));
     }
 
     // the OR spanning all ANDs
-    let or_gate = Gate::new(GateType::Or, and_gate_indices, vec![]);
-    all_gates.push(or_gate);
+    all_gates.push(Gate::new(GateType::Or, or_gate_input_indices, EMPTY_VEC));
 
     return all_gates;
 }

src/main.rs

     input_bits.extend_from_slice(&b_bits);
 
     // Build and evaluate the comparison circuit
-    let circuit = Circuit::compare_n_bit_numbers(input_bits, 256);
-    let circuit_result = circuit.eval();
+    let circuit = Circuit::compare_n_bit_numbers(256);
+    let circuit_result = circuit.eval(input_bits);
 
     let a_int = BigUint::from_bytes_le(&a.to_bytes());
     let b_int = BigUint::from_bytes_le(&b.to_bytes());

tests/test.rs

     use rand::rngs::OsRng;
     #[test]
     fn test_scalar_comparison_via_circuit() {
+        let circuit = Circuit::compare_n_bit_numbers(256);
+
         for _ in 0..100 {
             let a = Scalar::random(&mut OsRng);
             let b = Scalar::random(&mut OsRng);
             input_bits.extend_from_slice(&a_bits);
             input_bits.extend_from_slice(&b_bits);
 
-            // Build and evaluate the comparison circuit
-            let circuit = Circuit::compare_n_bit_numbers(input_bits, 256);
-            let circuit_result = circuit.eval();
+            // Evaluate the comparison circuit
+            let circuit_result = circuit.clone().eval(input_bits);
 
             // Compare expected result using BigUint
             let a_int = BigUint::from_bytes_le(&a.to_bytes());