add LetterSet::{to,from}_index

src/letterset.rs

@@ -64,6 +64,17 @@ impl LetterSet {
         self.0 == 0
     }
 
+    pub const fn to_index(self) -> usize {
+        self.0 as usize
+    }
+
+    pub fn from_index(index: usize) -> Option<Self> {
+        u32::try_from(index)
+            .ok()
+            .filter(|n| *n <= Self::FULL.0)
+            .map(Self)
+    }
+
     const fn letter_index(letter: u8) -> Option<u8> {
         if letter.is_ascii_alphabetic() {
             let lower = letter.to_ascii_lowercase();
@@ -203,4 +214,25 @@ mod test {
         assert!(!AB.is_empty());
         assert!(!BC.is_empty());
     }
+
+    #[test]
+    fn to_index() {
+        assert_eq!(LetterSet::EMPTY.to_index(), 0);
+        assert_eq!(A.to_index(), 1);
+        assert_eq!(B.to_index(), 2);
+        assert_eq!(AB.to_index(), 3);
+        assert_eq!(C.to_index(), 4);
+        assert_eq!(AC.to_index(), 5);
+        assert_eq!(BC.to_index(), 6);
+        assert_eq!(ABC.to_index(), 7);
+        assert_eq!(LetterSet::FULL.to_index(), (1 << 26) - 1);
+    }
+
+    #[test]
+    fn from_index() {
+        for ls in [LetterSet::EMPTY, A, B, C, AB, AC, BC, ABC, LetterSet::EMPTY] {
+            assert_eq!(LetterSet::from_index(ls.to_index()), Some(ls));
+        }
+        assert_eq!(LetterSet::from_index(1 << 26), None);
+    }
 }