Fix the hex encoder

src/hex.rs

@@ -55,11 +55,12 @@ pub trait HexEncodable<T> {
 pub struct HexEncoder<T> {
     input: T,
     current_byte: u8,
+    num_nibbles: u8,
 }
 
 impl<T> HexEncoder<T> {
     pub fn new(input: T) -> HexEncoder<T> {
-        HexEncoder { input: input, current_byte: 0 }
+        HexEncoder { input: input, current_byte: 0, num_nibbles: 0 }
     }
 }
 
@@ -67,12 +68,13 @@ impl<'a, T> Iterator for HexEncoder<T> where T: Iterator<Item=u8> + 'a {
     type Item = char;
 
     fn next(&mut self) -> Option<Self::Item> {
-        if self.current_byte == 0 {
+        if self.num_nibbles == 0 {
             // Get a new byte.
             match self.input.next() {
                 Some(byte) => {
                     // Swap the order of the nibbles to make the code below nicer.
                     self.current_byte = (byte << 4) + (byte >> 4);
+                    self.num_nibbles = 2;
                 }
                 None => return None
             }
@@ -81,6 +83,7 @@ impl<'a, T> Iterator for HexEncoder<T> where T: Iterator<Item=u8> + 'a {
         match char::from_digit((self.current_byte & 0xF) as u32, 16) {
             Some(ch) => {
                 self.current_byte = self.current_byte >> 4;
+                self.num_nibbles -= 1;
                 return Some(ch);
             },
             None => panic!("This will never happen???")
@@ -107,4 +110,32 @@ mod tests {
         let decoded: Vec<u8> = "313233".chars().hex_decoded().collect();
         assert_eq!(decoded, vec![0x31u8, 0x32u8, 0x33u8]);
     }
+
+    #[test]
+    fn cryptopals12_decode() {
+        let a = "1c0111001f010100061a024b53535009181c";
+        let out: Vec<u8> = a.chars().hex_decoded().collect();
+        assert_eq!(out, vec![0x1c, 0x01, 0x11, 0x00, 0x1f, 0x01, 0x01, 0x00,
+                             0x06, 0x1a, 0x02, 0x4b, 0x53, 0x53, 0x50, 0x09,
+                             0x18, 0x1c]);
+    }
+
+    #[test]
+    fn each_byte_encodes_to_two_characters() {
+        let input: String = vec![0x06].into_iter().hex_encoded().collect();
+        assert_eq!(input, "06");
+
+        // This was an actual bug, the last nibble was getting eaten. :(
+        let input: String = vec![0x20].into_iter().hex_encoded().collect();
+        assert_eq!(input, "20");
+    }
+
+    #[test]
+    fn reciprocity_of_encode_and_decode() {
+        let input = "746865206b";
+        let output: String = input.chars()
+            .hex_decoded().inspect(|x| println!("{:08b}", x))
+            .hex_encoded().collect();
+        assert_eq!(input, output);
+    }
 }