impl lazily compute for null count when it is expansive to calc it ea…

…gerly.

arrow-buffer/src/buffer/null.rs

@@ -15,35 +15,66 @@
 // specific language governing permissions and limitations
 // under the License.
 
+use std::sync::atomic::{AtomicI64, Ordering};
+
 use crate::bit_iterator::{BitIndexIterator, BitIterator, BitSliceIterator};
 use crate::buffer::BooleanBuffer;
 use crate::{Buffer, MutableBuffer};
 
+const UNINITIALIZED_NULL_COUNT: i64 = -1;
+
+#[derive(Debug)]
+pub enum NullCount {
+    Eager(usize),
+    Lazy(AtomicI64),
+}
+
+impl Clone for NullCount {
+    fn clone(&self) -> Self {
+        match self {
+            Self::Eager(v) => Self::Eager(*v),
+            Self::Lazy(v) => {
+                let v = v.load(Ordering::Relaxed);
+                Self::Lazy(AtomicI64::new(v))
+            }
+        }
+    }
+}
+
 /// A [`BooleanBuffer`] used to encode validity for arrow arrays
 ///
 /// As per the [Arrow specification], array validity is encoded in a packed bitmask with a
 /// `true` value indicating the corresponding slot is not null, and `false` indicating
 /// that it is null.
 ///
 /// [Arrow specification]: https://arrow.apache.org/docs/format/Columnar.html#validity-bitmaps
-#[derive(Debug, Clone, Eq, PartialEq)]
+#[derive(Debug, Clone)]
 pub struct NullBuffer {
     buffer: BooleanBuffer,
-    null_count: usize,
+    null_count: NullCount,
+}
+
+impl PartialEq for NullBuffer {
+    fn eq(&self, other: &Self) -> bool {
+        self.buffer == other.buffer
+    }
 }
 
+impl Eq for NullBuffer {}
+
 impl NullBuffer {
     /// Create a new [`NullBuffer`] computing the null count
     pub fn new(buffer: BooleanBuffer) -> Self {
-        let null_count = buffer.len() - buffer.count_set_bits();
+        // Expensive to calc the null count, we should lazily compute it when
+        let null_count = NullCount::Lazy(AtomicI64::new(UNINITIALIZED_NULL_COUNT));
         Self { buffer, null_count }
     }
 
     /// Create a new [`NullBuffer`] of length `len` where all values are null
     pub fn new_null(len: usize) -> Self {
         Self {
             buffer: BooleanBuffer::new_unset(len),
-            null_count: len,
+            null_count: NullCount::Eager(len),
         }
     }
 
@@ -53,7 +84,7 @@ impl NullBuffer {
     pub fn new_valid(len: usize) -> Self {
         Self {
             buffer: BooleanBuffer::new_set(len),
-            null_count: 0,
+            null_count: NullCount::Eager(0),
         }
     }
 
@@ -63,7 +94,10 @@ impl NullBuffer {
     ///
     /// `buffer` must contain `null_count` `0` bits
     pub unsafe fn new_unchecked(buffer: BooleanBuffer, null_count: usize) -> Self {
-        Self { buffer, null_count }
+        Self {
+            buffer,
+            null_count: NullCount::Eager(null_count),
+        }
     }
 
     /// Computes the union of the nulls in two optional [`NullBuffer`]
@@ -81,9 +115,12 @@ impl NullBuffer {
 
     /// Returns true if all nulls in `other` also exist in self
     pub fn contains(&self, other: &NullBuffer) -> bool {
-        if other.null_count == 0 {
-            return true;
+        if let NullCount::Eager(v) = &other.null_count {
+            if *v == 0 {
+                return true;
+            }
         }
+
         let lhs = self.inner().bit_chunks().iter_padded();
         let rhs = other.inner().bit_chunks().iter_padded();
         lhs.zip(rhs).all(|(l, r)| (l & !r) == 0)
@@ -106,9 +143,17 @@ impl NullBuffer {
                 crate::bit_util::set_bit(buffer.as_mut(), i * count + j)
             }
         }
+
+        let null_count = if let NullCount::Eager(v) = &self.null_count {
+            NullCount::Eager(v * count)
+        } else {
+            // TODO: not sure about if it is better to load the atomic and attempt to reuse the compute result
+            NullCount::Lazy(AtomicI64::new(UNINITIALIZED_NULL_COUNT))
+        };
+
         Self {
             buffer: BooleanBuffer::new(buffer.into(), 0, capacity),
-            null_count: self.null_count * count,
+            null_count,
         }
     }
 
@@ -131,9 +176,20 @@ impl NullBuffer {
     }
 
     /// Returns the null count for this [`NullBuffer`]
-    #[inline]
     pub fn null_count(&self) -> usize {
-        self.null_count
+        match &self.null_count {
+            NullCount::Eager(v) => *v,
+            NullCount::Lazy(v) => {
+                let cached_null_count = v.load(Ordering::Acquire);
+                if cached_null_count != UNINITIALIZED_NULL_COUNT {
+                    return cached_null_count as usize;
+                }
+
+                let computed_null_count = self.buffer.len() - self.buffer.count_set_bits();
+                v.store(computed_null_count as i64, Ordering::Release);
+                computed_null_count
+            }
+        }
     }
 
     /// Returns `true` if the value at `idx` is not null
@@ -189,8 +245,10 @@ impl NullBuffer {
         &self,
         f: F,
     ) -> Result<(), E> {
-        if self.null_count == self.len() {
-            return Ok(());
+        if let NullCount::Eager(v) = &self.null_count {
+            if *v == self.len() {
+                return Ok(());
+            }
         }
         self.valid_indices().try_for_each(f)
     }