rlp uses insert_slice

src/lib.rs

@@ -5,6 +5,7 @@ pub mod hash;
 pub mod uint;
 pub mod bytes;
 pub mod rlp;
+pub mod vector;
 
 #[test]
 fn it_works() {

src/rlp.rs

@@ -48,6 +48,7 @@ use std::cell::Cell;
 use std::collections::LinkedList;
 use std::error::Error as StdError;
 use bytes::{ToBytes, FromBytes, FromBytesError};
+use vector::InsertSlice;
 
 /// rlp container
 #[derive(Debug)]
@@ -455,16 +456,9 @@ impl BasicEncoder {
     }
 
     /// inserts list prefix at given position
-    /// TODO: optimise it, so it does not copy an array
+    /// TODO: optimise it further?
     fn insert_list_len_at_pos(&mut self, len: usize, pos: usize) -> () {
-        // new bytes
-        let mut res: Vec<u8> = vec![];
-        // reserve a space equal at least current space + space for length
-        res.reserve(self.bytes.len() + 1);
-        {
-            let (before_slice, after_slice) = self.bytes.split_at(pos); 
-            res.extend(before_slice);
-
+        let mut res = vec![];
         match len {
             0...55 => res.push(0xc0u8 + len as u8),
             _ => {
@@ -473,9 +467,7 @@ impl BasicEncoder {
             }
         };
 
-            res.extend(after_slice);
-        }
-        self.bytes = res;
+        self.bytes.insert_slice(pos, &res);
     }
 
     /// get encoded value

src/vector.rs

@@ -0,0 +1,34 @@
+
+use std::ptr;
+
+pub trait InsertSlice<T> {
+    fn insert_slice(&mut self, index: usize, elements: &[T]);
+}
+
+/// based on `insert` function implementation from standard library
+impl<T> InsertSlice<T> for Vec<T> {
+    fn insert_slice(&mut self, index: usize, elements: &[T]) {
+        let e_len = elements.len();
+        if e_len == 0 {
+            return;
+        }
+
+        let len = self.len();
+        assert!(index <= len);
+
+        // space for the new element
+        self.reserve(e_len);
+
+        unsafe {
+            {
+                let p = self.as_mut_ptr().offset(index as isize);
+                let ep = elements.as_ptr().offset(0);
+                // shift everything by e_len, to make space
+                ptr::copy(p, p.offset(e_len as isize), len - index);
+                // write new element
+                ptr::copy(ep, p, e_len); 
+            }
+            self.set_len(len + e_len);
+        }
+    }
+}