add apply_validity function to utf8array struct

src/array/utf8/mod.rs

@@ -515,6 +515,17 @@ impl<O: Offset> Utf8Array<O> {
     {
         MutableUtf8Array::<O>::try_from_trusted_len_iter(iter).map(|x| x.into())
     }
+
+    /// Applies a function `f` to the validity of this array.
+    ///
+    /// This is an API to leverage clone-on-write
+    /// # Panics
+    /// This function panics if the function `f` modifies the length of the [`Bitmap`].
+    pub fn apply_validity<F: FnOnce(Bitmap) -> Bitmap>(&mut self, f: F) {
+        if let Some(validity) = std::mem::take(&mut self.validity) {
+            self.set_validity(Some(f(validity)))
+        }
+    }
 }
 
 impl<O: Offset> Array for Utf8Array<O> {
@@ -579,3 +590,25 @@ impl<O: Offset> Default for Utf8Array<O> {
         Utf8Array::new(data_type, Default::default(), Default::default(), None)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_apply_validity() {
+        let mut array = Utf8Array::<i32>::from([Some("Red"), Some("Green"), Some("Blue")]);
+        array.set_validity(Some([true, true, true].into()));
+
+        array.apply_validity(|bitmap| {
+            let mut mut_bitmap = bitmap.into_mut().right().unwrap();
+            mut_bitmap.set(1, false);
+            mut_bitmap.set(2, false);
+            mut_bitmap.into()
+        });
+
+        assert!(array.is_valid(0));
+        assert!(!array.is_valid(1));
+        assert!(!array.is_valid(2));
+    }
+}