feat(gadgets): add multipack::pack_bits

src/gadgets/multipack.rs

@@ -1,7 +1,7 @@
 //! Helpers for packing vectors of bits into scalar field elements.
 
 use super::boolean::Boolean;
-use super::num::Num;
+use super::num::{AllocatedNum, Num};
 use super::Assignment;
 use crate::{ConstraintSystem, SynthesisError};
 use ff::{Field, PrimeField, ScalarEngine};
@@ -71,6 +71,36 @@ pub fn compute_multipacking<E: ScalarEngine>(bits: &[bool]) -> Vec<E::Fr> {
     result
 }
 
+/// Takes a sequence of booleans and exposes them as a single compact Num.
+pub fn pack_bits<E, CS>(mut cs: CS, bits: &[Boolean]) -> Result<AllocatedNum<E>, SynthesisError>
+where
+    E: ScalarEngine,
+    CS: ConstraintSystem<E>,
+{
+    let mut num = Num::<E>::zero();
+    let mut coeff = E::Fr::one();
+    for bit in bits.iter().take(E::Fr::CAPACITY as usize) {
+        num = num.add_bool_with_coeff(CS::one(), &bit, coeff);
+
+        coeff.double();
+    }
+
+    let alloc_num = AllocatedNum::alloc(cs.namespace(|| "input"), || {
+        num.get_value()
+            .ok_or_else(|| SynthesisError::AssignmentMissing)
+    })?;
+
+    // num * 1 = input
+    cs.enforce(
+        || "packing constraint",
+        |_| num.lc(E::Fr::one()),
+        |lc| lc + CS::one(),
+        |lc| lc + alloc_num.get_variable(),
+    );
+
+    Ok(alloc_num)
+}
+
 #[test]
 fn test_multipacking() {
     use crate::bls::Bls12;