Solve challenge 12.

src/bytes.rs

@@ -16,8 +16,8 @@ impl Bytes {
         String::from(std::str::from_utf8(&v).unwrap())
     }
 
-    pub fn first_block(&self, length: usize) -> Bytes {
-        Bytes(self.0[0..length].to_vec())
+    pub fn get_block(&self, block_index: usize, block_size: usize) -> Bytes {
+        Bytes(self.0[(block_index * block_size)..(block_index + 1) * block_size].to_vec())
     }
 
     pub fn random(length: usize) -> Bytes {