Construct `FpVar` from bits (#35)

src/bits/boolean.rs

@@ -1,4 +1,4 @@
-use ark_ff::{BitIteratorBE, Field, PrimeField};
+use ark_ff::{BitIteratorBE, Field, FpParameters, PrimeField};
 
 use crate::{fields::fp::FpVar, prelude::*, Assignment, ToConstraintFieldGadget, Vec};
 use ark_relations::r1cs::{
@@ -608,6 +608,56 @@ impl Boolean {
         }
     }
 
+    /// Convert a little-endian bitwise representation of a field element to `FpVar<F>`
+    #[tracing::instrument(target = "r1cs", skip(bits))]
+    pub fn le_bits_to_fp_var(bits: &[Self]) -> Result<FpVar<F>, SynthesisError>
+    where
+        F: PrimeField,
+    {
+        // Compute the value of the `FpVar` variable via double-and-add.
+        let mut value = None;
+        let cs = bits.cs();
+        // Assign a value only when `cs` is in setup mode, or if we are constructing
+        // a constant.
+        let should_construct_value = (!cs.is_in_setup_mode()) || bits.is_constant();
+        if should_construct_value {
+            let bits = bits.iter().map(|b| b.value().unwrap()).collect::<Vec<_>>();
+            let bytes = bits
+                .chunks(8)
+                .map(|c| {
+                    let mut value = 0u8;
+                    for (i, &bit) in c.iter().enumerate() {
+                        value += (bit as u8) << i;
+                    }
+                    value
+                })
+                .collect::<Vec<_>>();
+            value = Some(F::from_le_bytes_mod_order(&bytes));
+        }
+
+        if bits.is_constant() {
+            Ok(FpVar::constant(value.unwrap()))
+        } else {
+            let mut power = F::one();
+            // Compute a linear combination for the new field variable, again
+            // via double and add.
+            let mut combined_lc = LinearCombination::zero();
+            bits.iter().for_each(|b| {
+                combined_lc = &combined_lc + (power, b.lc());
+                power.double_in_place();
+            });
+            // Allocate the new variable as a SymbolicLc
+            let variable = cs.new_lc(combined_lc)?;
+            // If the number of bits is less than the size of the field,
+            // then we do not need to enforce that the element is less than
+            // the modulus.
+            if bits.len() >= F::Params::MODULUS_BITS as usize {
+                Self::enforce_in_field_le(bits)?;
+            }
+            Ok(crate::fields::fp::AllocatedFp::new(value, variable, cs.clone()).into())
+        }
+    }
+
     /// Enforces that `bits`, when interpreted as a integer, is less than
     /// `F::characteristic()`, That is, interpret bits as a little-endian
     /// integer, and enforce that this integer is "in the field Z_p", where
@@ -1737,4 +1787,37 @@ mod test {
         }
         Ok(())
     }
+
+    #[test]
+    fn test_bits_to_fp() -> Result<(), SynthesisError> {
+        use AllocationMode::*;
+        let rng = &mut ark_std::test_rng();
+        let cs = ConstraintSystem::<Fr>::new_ref();
+
+        let modes = [Input, Witness, Constant];
+        for &mode in modes.iter() {
+            for _ in 0..1000 {
+                let f = Fr::rand(rng);
+                let bits = BitIteratorLE::new(f.into_repr()).collect::<Vec<_>>();
+                let bits: Vec<_> =
+                    AllocVar::new_variable(cs.clone(), || Ok(bits.as_slice()), mode)?;
+                let f = AllocVar::new_variable(cs.clone(), || Ok(f), mode)?;
+                let claimed_f = Boolean::le_bits_to_fp_var(&bits)?;
+                claimed_f.enforce_equal(&f)?;
+            }
+
+            for _ in 0..1000 {
+                let f = Fr::from(u64::rand(rng));
+                let bits = BitIteratorLE::new(f.into_repr()).collect::<Vec<_>>();
+                let bits: Vec<_> =
+                    AllocVar::new_variable(cs.clone(), || Ok(bits.as_slice()), mode)?;
+                let f = AllocVar::new_variable(cs.clone(), || Ok(f), mode)?;
+                let claimed_f = Boolean::le_bits_to_fp_var(&bits)?;
+                claimed_f.enforce_equal(&f)?;
+            }
+            assert!(cs.is_satisfied().unwrap());
+        }
+
+        Ok(())
+    }
 }