Optimise .fill() throughput (#43)

src/lib.rs

@@ -444,18 +444,26 @@ impl Rng {
     }
 
     /// Fill a byte slice with random data.
+    #[inline]
     pub fn fill(&self, slice: &mut [u8]) {
-        // Filling the buffer in chunks of 8 is much faster.
-        let mut chunks = slice.chunks_exact_mut(8);
+        // We fill the slice by chunks of 8 bytes, or one block of
+        // WyRand output per new state.
+        let mut chunks = slice.chunks_exact_mut(core::mem::size_of::<u64>());
         for chunk in chunks.by_ref() {
-            let n = self.gen_u64();
+            let n = self.gen_u64().to_ne_bytes();
             // Safe because the chunks are always 8 bytes exactly.
-            let bytes: &mut [u8; 8] = chunk.try_into().unwrap();
-            *bytes = n.to_ne_bytes();
+            chunk.copy_from_slice(&n);
         }
 
-        for item in chunks.into_remainder() {
-            *item = self.u8(..);
+        let remainder = chunks.into_remainder();
+
+        // Any remainder will always be less than 8 bytes.
+        if !remainder.is_empty() {
+            // Generate one last block of 8 bytes of entropy
+            let n = self.gen_u64().to_ne_bytes();
+
+            // Use the remaining length to copy from block
+            remainder.copy_from_slice(&n[..remainder.len()]);
         }
     }