[GitHub] [arrow-rs] tustvold commented on a diff in pull request #3954: Add ListBuilder::append_value (#3949)

["tustvold (via GitHub)" <gi...@apache.org>]

tustvold commented on code in PR #3954:
URL: https://github.com/apache/arrow-rs/pull/3954#discussion_r1148590506


##########
arrow-array/src/builder/generic_list_builder.rs:
##########
@@ -125,11 +125,97 @@ where
     /// Panics if the length of [`Self::values`] exceeds `OffsetSize::MAX`
     #[inline]
     pub fn append(&mut self, is_valid: bool) {
-        self.offsets_builder
-            .append(OffsetSize::from_usize(self.values_builder.len()).unwrap());
+        self.offsets_builder.append(self.next_offset());
         self.null_buffer_builder.append(is_valid);
     }
 
+    /// Returns the next offset
+    ///
+    /// # Panics
+    ///
+    /// Panics if the length of [`Self::values`] exceeds `OffsetSize::MAX`
+    #[inline]
+    fn next_offset(&self) -> OffsetSize {
+        OffsetSize::from_usize(self.values_builder.len()).unwrap()
+    }
+
+    /// Append a value to this [`GenericListBuilder`]
+    ///
+    /// ```
+    /// # use arrow_array::builder::{Int32Builder, ListBuilder};
+    /// # use arrow_array::cast::AsArray;
+    /// # use arrow_array::{Array, Int32Array};
+    /// # use arrow_array::types::Int32Type;
+    /// let mut builder = ListBuilder::new(Int32Builder::new());
+    ///
+    /// builder.append_value([Some(1), Some(2), Some(3)]);
+    /// builder.append_value([]);
+    /// builder.append_value([None]);
+    ///
+    /// let array = builder.finish();
+    /// assert_eq!(array.len(), 3);
+    ///
+    /// assert_eq!(array.value_offsets(), &[0, 3, 3, 4]);
+    /// let values = array.values().as_primitive::<Int32Type>();
+    /// assert_eq!(values, &Int32Array::from(vec![Some(1), Some(2), Some(3), None]));
+    /// ```
+    ///
+    /// This is an alternative API to appending directly to [`Self::values`] and
+    /// delimiting the result with [`Self::append`]
+    ///
+    /// ```
+    /// # use arrow_array::builder::{Int32Builder, ListBuilder};
+    /// # use arrow_array::cast::AsArray;
+    /// # use arrow_array::{Array, Int32Array};
+    /// # use arrow_array::types::Int32Type;
+    /// let mut builder = ListBuilder::new(Int32Builder::new());
+    ///
+    /// builder.values().append_value(1);
+    /// builder.values().append_value(2);
+    /// builder.values().append_value(3);
+    /// builder.append(true);
+    /// builder.append(true);
+    /// builder.values().append_null();
+    /// builder.append(true);
+    ///
+    /// let array = builder.finish();
+    /// assert_eq!(array.len(), 3);
+    ///
+    /// assert_eq!(array.value_offsets(), &[0, 3, 3, 4]);
+    /// let values = array.values().as_primitive::<Int32Type>();
+    /// assert_eq!(values, &Int32Array::from(vec![Some(1), Some(2), Some(3), None]));
+    /// ```
+    #[inline]
+    pub fn append_value<I, V>(&mut self, i: I)
+    where
+        T: Extend<Option<V>>,
+        I: IntoIterator<Item = Option<V>>,
+    {
+        self.extend(std::iter::once(Some(i)))

Review Comment:
   At least in some basic experiments LLVM seems to be able to optimise this as one would hope
   
   https://rust.godbolt.org/z/zcxzqzf59



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