[GitHub] [arrow-rs] tustvold commented on a diff in pull request #2127: Lazily materialize the null buffer builder for all array builders.

[GitBox <gi...@apache.org>]

tustvold commented on code in PR #2127:
URL: https://github.com/apache/arrow-rs/pull/2127#discussion_r927919817


##########
arrow/src/array/builder/mod.rs:
##########
@@ -53,6 +54,7 @@ pub use generic_binary_builder::GenericBinaryBuilder;
 pub use generic_list_builder::GenericListBuilder;
 pub use generic_string_builder::GenericStringBuilder;
 pub use map_builder::MapBuilder;
+pub(self) use null_buffer_builder::NullBufferBuilder;

Review Comment:
   How is `pub(self) use` different from just `use`



##########
arrow/src/array/builder/null_buffer_builder.rs:
##########
@@ -0,0 +1,185 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+use crate::buffer::Buffer;
+
+use super::BooleanBufferBuilder;
+
+/// Builder for creating the null bit buffer.
+/// This builder only materializes the buffer when we append `false`.
+/// If you only append `true`s to the builder, what you get will be
+/// `None` when calling [`finish`](#method.finish).
+/// This optimization is **very** important for the performance.
+#[derive(Debug)]
+pub(super) struct NullBufferBuilder {
+    bitmap_builder: Option<BooleanBufferBuilder>,
+    len: usize,
+    capacity: usize,
+}
+
+impl NullBufferBuilder {
+    /// Creates a new empty builder.
+    /// `capacity` is the number of bits in the null buffer.
+    pub fn new(capacity: usize) -> Self {
+        Self {
+            bitmap_builder: None,
+            len: 0,
+            capacity,
+        }
+    }
+
+    /// Appends `n` `true`s into the builder.
+    pub fn append_n_true(&mut self, n: usize) {
+        if let Some(buf) = self.bitmap_builder.as_mut() {
+            buf.append_n(n, true)
+        }
+        self.len += n;
+    }
+
+    /// Appends a `true` into the builder.
+    #[inline]
+    pub fn append_true(&mut self) {
+        self.append_n_true(1);
+    }
+
+    /// Appends `n` `false`s into the builder.
+    pub fn append_n_false(&mut self, n: usize) {
+        self.materialize_if_needed();
+        self.bitmap_builder.as_mut().unwrap().append_n(n, false);
+        self.len += n;
+    }
+
+    /// Appends a `false` into the builder.
+    #[inline]
+    pub fn append_false(&mut self) {
+        self.append_n_false(1);
+    }
+
+    /// Appends a boolean value into the builder.
+    pub fn append(&mut self, v: bool) {
+        if v {
+            self.append_true()
+        } else {
+            self.append_false()
+        }
+    }
+
+    /// Appends a boolean slice into the builder.
+    pub fn append_slice(&mut self, slice: &[bool]) {
+        if slice.iter().any(|v| !v) {
+            self.materialize_if_needed()
+        }
+        if let Some(buf) = self.bitmap_builder.as_mut() {
+            buf.append_slice(slice)
+        }
+        self.len += slice.len();
+    }
+
+    /// Builds the null buffer and resets the builder.
+    /// Returns `None` if the builder only contains `true`s.
+    pub fn finish(&mut self) -> Option<Buffer> {
+        let buf = self.bitmap_builder.as_mut().map(|b| b.finish());

Review Comment:
   Could use take here 



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