fn logical_nulls()

in arrow-array/src/array/union_array.rs [777:893]
93 lines of code
11 McCabe index (conditional complexity)

    fn logical_nulls(&self) -> Option<NullBuffer> {
        let fields = match self.data_type() {
            DataType::Union(fields, _) => fields,
            _ => unreachable!(),
        };

        if fields.len() <= 1 {
            return self
                .fields
                .iter()
                .flatten()
                .map(Array::logical_nulls)
                .next()
                .flatten();
        }

        let logical_nulls = self.fields_logical_nulls();

        if logical_nulls.is_empty() {
            return None;
        }

        let fully_null_count = logical_nulls
            .iter()
            .filter(|(_, nulls)| nulls.null_count() == nulls.len())
            .count();

        if fully_null_count == fields.len() {
            if let Some((_, exactly_sized)) = logical_nulls
                .iter()
                .find(|(_, nulls)| nulls.len() == self.len())
            {
                return Some(exactly_sized.clone());
            }

            if let Some((_, bigger)) = logical_nulls
                .iter()
                .find(|(_, nulls)| nulls.len() > self.len())
            {
                return Some(bigger.slice(0, self.len()));
            }

            return Some(NullBuffer::new_null(self.len()));
        }

        let boolean_buffer = match &self.offsets {
            Some(_) => self.gather_nulls(logical_nulls),
            None => {
                // Choose the fastest way to compute the logical nulls
                // Gather computes one null per iteration, while the others work on 64 nulls chunks,
                // but must also compute selection masks, which is expensive,
                // so it's cost is the number of selection masks computed per chunk
                // Since computing the selection mask gets auto-vectorized, it's performance depends on which simd feature is enabled
                // For gather, the cost is the threshold where masking becomes slower than gather, which is determined with benchmarks
                // TODO: bench on avx512f(feature is still unstable)
                let gather_relative_cost = if cfg!(target_feature = "avx2") {
                    10
                } else if cfg!(target_feature = "sse4.1") {
                    3
                } else if cfg!(target_arch = "x86") || cfg!(target_arch = "x86_64") {
                    // x86 baseline includes sse2
                    2
                } else {
                    // TODO: bench on non x86
                    // Always use gather on non benchmarked archs because even though it may slower on some cases,
                    // it's performance depends only on the union length, without being affected by the number of fields
                    0
                };

                let strategies = [
                    (SparseStrategy::Gather, gather_relative_cost, true),
                    (
                        SparseStrategy::MaskAllFieldsWithNullsSkipOne,
                        fields.len() - 1,
                        fields.len() == logical_nulls.len(),
                    ),
                    (
                        SparseStrategy::MaskSkipWithoutNulls,
                        logical_nulls.len(),
                        true,
                    ),
                    (
                        SparseStrategy::MaskSkipFullyNull,
                        fields.len() - fully_null_count,
                        true,
                    ),
                ];

                let (strategy, _, _) = strategies
                    .iter()
                    .filter(|(_, _, applicable)| *applicable)
                    .min_by_key(|(_, cost, _)| cost)
                    .unwrap();

                match strategy {
                    SparseStrategy::Gather => self.gather_nulls(logical_nulls),
                    SparseStrategy::MaskAllFieldsWithNullsSkipOne => {
                        self.mask_sparse_all_with_nulls_skip_one(logical_nulls)
                    }
                    SparseStrategy::MaskSkipWithoutNulls => {
                        self.mask_sparse_skip_without_nulls(logical_nulls)
                    }
                    SparseStrategy::MaskSkipFullyNull => {
                        self.mask_sparse_skip_fully_null(logical_nulls)
                    }
                }
            }
        };

        let null_buffer = NullBuffer::from(boolean_buffer);

        if null_buffer.null_count() > 0 {
            Some(null_buffer)
        } else {
            None
        }
    }