[GitHub] [arrow] tustvold commented on a change in pull request #7076: ARROW-8659: [Rust] ListBuilder allocate with_capacity

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

tustvold commented on a change in pull request #7076:
URL: https://github.com/apache/arrow/pull/7076#discussion_r418650048



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File path: rust/arrow/src/array/builder.rs
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@@ -527,11 +527,18 @@ pub struct ListBuilder<T: ArrayBuilder> {
 impl<T: ArrayBuilder> ListBuilder<T> {
     /// Creates a new `ListArrayBuilder` from a given values array builder
     pub fn new(values_builder: T) -> Self {
-        let mut offsets_builder = Int32BufferBuilder::new(values_builder.len() + 1);
+        let capacity = values_builder.len();
+        Self::with_capacity(values_builder, capacity)
+    }
+
+    /// Creates a new `ListArrayBuilder` from a given values array builder
+    /// `append` may be called up to `capacity` times without triggering reallocation

Review comment:
       I went with something I hope was in the spirit of what you wanted, as technically it allocates for an extra offset and also pre-allocates the bitmap builder. Let me know if you'd prefer I stuck with your exact wording




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