Implement Ord, Hash and Default for Expansions iter (#49)

src/expansions.rs

@@ -29,7 +29,7 @@ struct Ambiguity {
 }
 
 /// Unambiguous DNA expansion produced by [`Expansions`] iterator.
-#[derive(Clone, PartialEq, Eq)]
+#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct Expansion(Arc<[Nucleotide]>);
 
 impl Expansions {
@@ -56,6 +56,13 @@ impl Expansions {
             began: false,
         }
     }
+
+    pub fn empty() -> Self {
+        // TODO: Possibly make this skip allocation by reusing Arc?
+        let mut this = Self::new(&[]);
+        this.next(); // `this` contains a single empty expansion; skip it
+        this
+    }
 }
 
 impl Iterator for Expansions {
@@ -100,6 +107,12 @@ impl Iterator for Expansions {
     }
 }
 
+impl Default for Expansions {
+    fn default() -> Self {
+        Self::empty()
+    }
+}
+
 impl std::fmt::Debug for Expansions {
     fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
         let dna = self.buf.iter().map(|&nuc| nuc.into()).collect();
@@ -273,6 +286,13 @@ mod test {
         assert_eq!(expansions, expected);
     }
 
+    #[test]
+    fn empty_expansions() {
+        let mut empty = Expansions::empty();
+        assert_eq!(empty.size_hint(), (0, Some(0)));
+        assert_eq!(empty.next(), None);
+    }
+
     #[test]
     fn debug_expansion() {
         let expansion = Expansion(Arc::from(dna("ATCG").as_slice()));