Handle zero input in public bit-clearing helpers

src/lib.rs

@@ -1303,6 +1303,9 @@ pub struct Ones<'a> {
 }
 
 impl<'a> Ones<'a> {
+    /// Clears the least significant set bit and returns its index.
+    ///
+    /// If `n` is zero, returns the number of bits in a block and leaves it unchanged.
     #[inline]
     pub fn last_positive_bit_and_unset(n: &mut usize) -> usize {
         // Find the last set bit using x & -x
@@ -1312,7 +1315,7 @@ impl<'a> Ones<'a> {
         let position = last_bit.trailing_zeros();
 
         // Unset the last set bit
-        *n &= *n - 1;
+        *n &= n.wrapping_sub(1);
 
         position as usize
     }
@@ -1525,6 +1528,9 @@ pub struct IntoOnes {
 }
 
 impl IntoOnes {
+    /// Clears the least significant set bit and returns its index.
+    ///
+    /// If `n` is zero, returns the number of bits in a block and leaves it unchanged.
     #[inline]
     pub fn last_positive_bit_and_unset(n: &mut Block) -> usize {
         // Find the last set bit using x & -x
@@ -1534,7 +1540,7 @@ impl IntoOnes {
         let position = last_bit.trailing_zeros();
 
         // Unset the last set bit
-        *n &= *n - 1;
+        *n &= n.wrapping_sub(1);
 
         position as usize
     }

tests/tests.rs

@@ -1359,3 +1359,35 @@ fn clone() {
     assert_eq!(&fb, &fb_clone_from_same);
     assert_eq!(&fb, &fb_clone_from_bigger);
 }
+
+fn check_last_positive_bit_and_unset(unset: fn(&mut usize) -> usize) {
+    let mut value = 0;
+    assert_eq!(unset(&mut value), BITS);
+    assert_eq!(value, 0);
+
+    for bit in 0..BITS {
+        value = 1 << bit;
+        assert_eq!(unset(&mut value), bit);
+        assert_eq!(value, 0);
+    }
+
+    value = usize::MAX;
+    for bit in 0..BITS {
+        assert_eq!(unset(&mut value), bit);
+        assert_eq!(value.count_ones() as usize, BITS - bit - 1);
+    }
+    assert_eq!(unset(&mut value), BITS);
+    assert_eq!(value, 0);
+}
+
+#[test]
+#[cfg_attr(target_family = "wasm", wasm_bindgen_test)]
+fn ones_last_positive_bit_and_unset() {
+    check_last_positive_bit_and_unset(Ones::last_positive_bit_and_unset);
+}
+
+#[test]
+#[cfg_attr(target_family = "wasm", wasm_bindgen_test)]
+fn into_ones_last_positive_bit_and_unset() {
+    check_last_positive_bit_and_unset(IntoOnes::last_positive_bit_and_unset);
+}