Add to_bytes to bigint

CHANGELOG.md

@@ -69,6 +69,7 @@ The main features of this release are:
 - #153 (ark-serialize) Add an impl of `CanonicalSerialize/Deserialize` for `Rc<T>`.
 - #157 (ark-ec) Speed up `variable_base_msm` by not relying on unnecessary normalization.
 - #158 (ark-serialize) Add an impl of `CanonicalSerialize/Deserialize` for `()`.
+- #166 (ark-ff) Add a `to_bytes()` method to `BigInt`.
 
 ### Bug fixes
 - #36 (ark-ec) In Short-Weierstrass curves, include an infinity bit in `ToConstraintField`.

ff/src/biginteger/macros.rs

@@ -168,9 +168,24 @@ macro_rules! bigint_impl {
 
             #[inline]
             fn to_bits(&self) -> Vec<bool> {
-                let mut res = Vec::with_capacity(256);
+                BigIteratorBE::new(self.0).collect::<Vec<_>>()
+            }
+
+            #[inline]
+            fn to_bytes(&self) -> Vec<u8> {
+                let mut res = Vec::with_capacity($num_limbs * 8);
+                let mut cur_byte = 0u8;
+                let mut bit_num_mod_8 = 0;
                 for b in BitIteratorBE::new(self.0) {
-                    res.push(b);
+                    cur_byte = (cur_byte * 2) + (b as u8);
+                    bit_num_mod_8 = (bit_num_mod_8 + 1) % 8;
+                    if bit_num_mod_8 == 0 {
+                        res.push(cur_byte);
+                        cur_byte = 0u8;
+                    }
+                }
+                if bit_num_mod_8 != 0 {
+                    res.push(cur_byte);
                 }
                 res
             }

ff/src/biginteger/mod.rs

@@ -98,10 +98,14 @@ pub trait BigInteger:
     /// array.
     fn from_bits(bits: &[bool]) -> Self;
 
-    /// Returns the bit representation in a big endian boolean array, without
-    /// leading zeros.
+    /// Returns the bit representation in a big endian boolean array,
+    /// with leading zeroes.
     fn to_bits(&self) -> Vec<bool>;
 
+    /// Returns the byte representation in a big endian byte array,
+    /// with leading zeros.
+    fn to_bytes(&self) -> Vec<u8>;
+
     /// Returns a vector for wnaf.
     fn find_wnaf(&self) -> Vec<i64>;