big to little endian, some more tests

eth2/utils/ssz/src/encode.rs

@@ -70,13 +70,13 @@ impl SszStream {
 /// Encode some length into a ssz size prefix.
 ///
 /// The ssz size prefix is 4 bytes, which is treated as a continuious
-/// 32bit big-endian integer.
+/// 32bit little-endian integer.
 pub fn encode_length(len: usize, length_bytes: usize) -> Vec<u8> {
     assert!(length_bytes > 0); // For sanity
     assert!((len as usize) < 2usize.pow(length_bytes as u32 * 8));
     let mut header: Vec<u8> = vec![0; length_bytes];
     for (i, header_byte) in header.iter_mut().enumerate() {
-        let offset = (length_bytes - i - 1) * 8;
+        let offset = (length_bytes - (length_bytes - i)) * 8;
         *header_byte = ((len >> offset) & 0xff) as u8;
     }
     header
@@ -95,15 +95,27 @@ mod tests {
     #[test]
     fn test_encode_length_4_bytes() {
         assert_eq!(encode_length(0, LENGTH_BYTES), vec![0; 4]);
-        assert_eq!(encode_length(1, LENGTH_BYTES), vec![0, 0, 0, 1]);
-        assert_eq!(encode_length(255, LENGTH_BYTES), vec![0, 0, 0, 255]);
-        assert_eq!(encode_length(256, LENGTH_BYTES), vec![0, 0, 1, 0]);
+        assert_eq!(encode_length(1, LENGTH_BYTES), vec![1, 0, 0, 0]);
+        assert_eq!(encode_length(255, LENGTH_BYTES), vec![255, 0, 0, 0]);
+        assert_eq!(encode_length(256, LENGTH_BYTES), vec![0, 1, 0, 0]);
         assert_eq!(
             encode_length(4294967295, LENGTH_BYTES), // 2^(3*8) - 1
             vec![255, 255, 255, 255]
         );
     }
 
+    #[test]
+    fn test_encode_lower_length() {
+        assert_eq!(encode_length(0, LENGTH_BYTES - 2), vec![0; 2]);
+        assert_eq!(encode_length(1, LENGTH_BYTES - 2), vec![1, 0]);
+    }
+
+    #[test]
+    fn test_encode_higher_length() {
+        assert_eq!(encode_length(0, LENGTH_BYTES + 2), vec![0; 6]);
+        assert_eq!(encode_length(1, LENGTH_BYTES + 2), vec![1, 0, 0, 0, 0, 0]);
+    }
+
     #[test]
     #[should_panic]
     fn test_encode_length_4_bytes_panic() {
@@ -117,8 +129,42 @@ mod tests {
         stream.append_vec(&test_vec);
         let ssz = stream.drain();
 
-        assert_eq!(ssz.len(), 4 + (12 * 2));
-        assert_eq!(ssz[0..4], *vec![0, 0, 0, 24]);
-        assert_eq!(ssz[4..6], *vec![1, 0]);
+        assert_eq!(ssz.len(), LENGTH_BYTES + (12 * 2));
+        assert_eq!(ssz[0..4], *vec![24, 0, 0, 0]);
+        assert_eq!(ssz[4..6], *vec![0, 1]);        
+    }
+
+    #[test]
+    fn test_encode_mixed_prefixed() {
+        let test_vec: Vec<u16> = vec![100, 200];
+        let test_value: u8 = 5;
+        
+        let mut stream = SszStream::new();
+        stream.append_vec(&test_vec);
+        stream.append(&test_value);
+        let ssz = stream.drain();
+
+        assert_eq!(ssz.len(), LENGTH_BYTES + (2 * 2) + 1);
+        assert_eq!(ssz[0..4], *vec![4, 0, 0, 0]);
+        assert_eq!(ssz[4..6], *vec![100, 0]);
+        assert_eq!(ssz[6..8], *vec![200, 0]);
+        assert_eq!(ssz[8], 5);
+    }
+
+     #[test]
+    fn test_encode_mixed_postfixed() {
+        let test_value: u8 = 5;
+        let test_vec: Vec<u16> = vec![100, 200];        
+        
+        let mut stream = SszStream::new();
+        stream.append(&test_value);
+        stream.append_vec(&test_vec);
+        let ssz = stream.drain();
+
+        assert_eq!(ssz.len(), 1 + LENGTH_BYTES + (2 * 2));
+        assert_eq!(ssz[0], 5);
+        assert_eq!(ssz[1..5], *vec![4, 0, 0, 0]);
+        assert_eq!(ssz[5..7], *vec![100, 0]);
+        assert_eq!(ssz[7..9], *vec![200, 0]);
     }
 }