Merge pull request #2174 from ljedrz/perf/better_ToBits_allocs

Reserve the exact capacity in more ToBits impls

fields/src/fp_256.rs

@@ -583,6 +583,10 @@ impl<P: Fp256Parameters> ToBits for Fp256<P> {
         self.write_bits_le(vec);
         vec[initial_len..].reverse();
     }
+
+    fn num_bits() -> Option<usize> {
+        Some(256)
+    }
 }
 
 impl<P: Fp256Parameters> ToBytes for Fp256<P> {

fields/src/fp_384.rs

@@ -612,6 +612,10 @@ impl<P: Fp384Parameters> ToBits for Fp384<P> {
         self.write_bits_le(vec);
         vec[initial_len..].reverse();
     }
+
+    fn num_bits() -> Option<usize> {
+        Some(384)
+    }
 }
 
 impl<P: Fp384Parameters> ToBytes for Fp384<P> {

utilities/src/bits.rs

@@ -41,17 +41,22 @@ pub trait ToBits: Sized {
 
     /// Returns `self` as a boolean array in little-endian order.
     fn to_bits_le(&self) -> Vec<bool> {
-        let mut bits = Vec::with_capacity(32);
+        let mut bits = Vec::new();
         self.write_bits_le(&mut bits);
         bits
     }
 
     /// Returns `self` as a boolean array in big-endian order.
     fn to_bits_be(&self) -> Vec<bool> {
-        let mut bits = Vec::with_capacity(32);
+        let mut bits = Vec::new();
         self.write_bits_be(&mut bits);
         bits
     }
+
+    /// An optional indication of how many bits an object can be represented with.
+    fn num_bits() -> Option<usize> {
+        None
+    }
 }
 
 pub trait FromBits: Sized {
@@ -144,7 +149,6 @@ macro_rules! impl_bits_for_integer {
             /// Returns `self` as a boolean array in little-endian order.
             #[inline]
             fn write_bits_le(&self, vec: &mut Vec<bool>) {
-                vec.reserve(<$int>::BITS as usize);
                 let mut value = *self;
                 for _ in 0..<$int>::BITS {
                     vec.push(value & 1 == 1);
@@ -158,6 +162,10 @@ macro_rules! impl_bits_for_integer {
                 let reversed = self.reverse_bits();
                 reversed.write_bits_le(vec);
             }
+
+            fn num_bits() -> Option<usize> {
+                Some(<$int>::BITS as usize)
+            }
         }
 
         impl FromBits for $int {
@@ -266,6 +274,10 @@ impl<C: ToBits> ToBits for &[C] {
     /// A helper method to return a concatenated list of little-endian bits.
     #[inline]
     fn write_bits_le(&self, vec: &mut Vec<bool>) {
+        if let Some(num_bits) = C::num_bits() {
+            vec.reserve(num_bits * self.len());
+        }
+
         for elem in self.iter() {
             elem.write_bits_le(vec);
         }
@@ -274,6 +286,10 @@ impl<C: ToBits> ToBits for &[C] {
     /// A helper method to return a concatenated list of big-endian bits.
     #[inline]
     fn write_bits_be(&self, vec: &mut Vec<bool>) {
+        if let Some(num_bits) = C::num_bits() {
+            vec.reserve(num_bits * self.len());
+        }
+
         for elem in self.iter() {
             elem.write_bits_be(vec);
         }