perf(sponge): optimize KeccakDuplexSponge absorb() method (#41)

Author: nougzarm

src/duplex_sponge/keccak.rs

@@ -97,14 +97,16 @@ impl DuplexSpongeInterface for KeccakDuplexSponge {
         self.squeeze_index = self.rate;
 
         while !input.is_empty() {
-            if self.absorb_index < self.rate {
-                self.state.state[self.absorb_index] = input[0];
-                self.absorb_index += 1;
-                input = &input[1..];
-            } else {
+            if self.absorb_index == self.rate {
                 self.state.permute();
                 self.absorb_index = 0;
             }
+
+            let chunk_size = usize::min(self.rate - self.absorb_index, input.len());
+            let dest = &mut self.state.state[self.absorb_index..self.absorb_index + chunk_size];
+            dest.copy_from_slice(&input[..chunk_size]);
+            self.absorb_index += chunk_size;
+            input = &input[chunk_size..];
         }
     }