vortex_array/arrays/list/

array.rs

// SPDX-License-Identifier: Apache-2.0
// SPDX-FileCopyrightText: Copyright the Vortex contributors

use std::sync::Arc;

use num_traits::AsPrimitive;
use vortex_dtype::{DType, match_each_integer_ptype, match_each_native_ptype};
use vortex_error::{VortexExpect, VortexResult, vortex_bail, vortex_ensure};

use crate::arrays::{ListVTable, PrimitiveVTable};
use crate::compute::{min_max, sub_scalar};
use crate::stats::ArrayStats;
use crate::validity::Validity;
use crate::{Array, ArrayRef, IntoArray};

/// A list array that stores variable-length lists of elements, similar to `Vec<Vec<T>>`.
///
/// This mirrors the Apache Arrow List array encoding and provides efficient storage
/// for nested data where each row contains a list of elements of the same type.
///
/// ## Data Layout
///
/// The list array uses an offset-based encoding:
/// - **Elements array**: A flat array containing all list elements concatenated together
/// - **Offsets array**: Integer array where `offsets[i]` is an (inclusive) start index into
///   the **elements** and `offsets[i+1]` is the (exclusive) stop index for the `i`th list.
/// - **Validity**: Optional mask indicating which lists are null
///
/// This allows for excellent cascading compression of the elements and offsets, as similar values
/// are clustered together and the offsets have a predictable pattern and small deltas between
/// consecutive elements.
///
/// ## Offset Semantics
///
/// - Offsets must be non-nullable integers (i32, i64, etc.)
/// - Offsets array has length `n+1` where `n` is the number of lists
/// - List `i` contains elements from `elements[offsets[i]..offsets[i+1]]`
/// - Offsets must be monotonically increasing
///
/// # Examples
///
/// ```
/// use vortex_array::arrays::{ListArray, PrimitiveArray};
/// use vortex_array::validity::Validity;
/// use vortex_array::IntoArray;
/// use vortex_buffer::buffer;
/// use std::sync::Arc;
///
/// // Create a list array representing [[1, 2], [3, 4, 5], []]
/// let elements = buffer![1i32, 2, 3, 4, 5].into_array();
/// let offsets = buffer![0u32, 2, 5, 5].into_array(); // 3 lists
///
/// let list_array = ListArray::try_new(
///     elements.into_array(),
///     offsets.into_array(),
///     Validity::NonNullable,
/// ).unwrap();
///
/// assert_eq!(list_array.len(), 3);
///
/// // Access individual lists
/// let first_list = list_array.list_elements_at(0);
/// assert_eq!(first_list.len(), 2); // [1, 2]
///
/// let third_list = list_array.list_elements_at(2);
/// assert!(third_list.is_empty()); // []
/// ```
#[derive(Clone, Debug)]
pub struct ListArray {
    pub(super) dtype: DType,
    pub(super) elements: ArrayRef,
    pub(super) offsets: ArrayRef,
    pub(super) validity: Validity,
    pub(super) stats_set: ArrayStats,
}

impl ListArray {
    /// Creates a new [`ListArray`].
    ///
    /// # Panics
    ///
    /// Panics if the provided components do not satisfy the invariants documented
    /// in [`ListArray::new_unchecked`].
    pub fn new(elements: ArrayRef, offsets: ArrayRef, validity: Validity) -> Self {
        Self::try_new(elements, offsets, validity).vortex_expect("ListArray new")
    }

    /// Constructs a new `ListArray`.
    ///
    /// See [`ListArray::new_unchecked`] for more information.
    ///
    /// # Errors
    ///
    /// Returns an error if the provided components do not satisfy the invariants documented in
    /// [`ListArray::new_unchecked`].
    pub fn try_new(
        elements: ArrayRef,
        offsets: ArrayRef,
        validity: Validity,
    ) -> VortexResult<Self> {
        Self::validate(&elements, &offsets, &validity)?;

        // SAFETY: validate ensures all invariants are met.
        Ok(unsafe { Self::new_unchecked(elements, offsets, validity) })
    }

    /// Creates a new [`ListArray`] without validation from these components:
    ///
    /// * `elements` is a flat array containing all list elements concatenated.
    /// * `offsets` is an integer array where `offsets[i]` is the start index for list `i`.
    /// * `validity` holds the null values.
    ///
    /// # Safety
    ///
    /// The caller must ensure all of the following invariants are satisfied:
    ///
    /// - Offsets must be a non-nullable integer array.
    /// - Offsets must have at least one element (even for empty lists, it should contain \[0\]).
    /// - Offsets must be sorted (monotonically increasing).
    /// - All offset values must be non-negative.
    /// - The maximum offset must not exceed `elements.len()`.
    /// - If validity is an array, its length must equal `offsets.len() - 1`.
    pub unsafe fn new_unchecked(elements: ArrayRef, offsets: ArrayRef, validity: Validity) -> Self {
        #[cfg(debug_assertions)]
        Self::validate(&elements, &offsets, &validity)
            .vortex_expect("[Debug Assertion]: Invalid `ListViewArray` parameters");

        Self {
            dtype: DType::List(Arc::new(elements.dtype().clone()), validity.nullability()),
            elements,
            offsets,
            validity,
            stats_set: Default::default(),
        }
    }

    /// Validates the components that would be used to create a [`ListArray`].
    ///
    /// This function checks all the invariants required by [`ListArray::new_unchecked`].
    pub fn validate(
        elements: &dyn Array,
        offsets: &dyn Array,
        validity: &Validity,
    ) -> VortexResult<()> {
        // Offsets must have at least one element
        vortex_ensure!(
            !offsets.is_empty(),
            "Offsets must have at least one element, [0] for an empty list"
        );

        // Offsets must be of integer type, and cannot go lower than 0.
        vortex_ensure!(
            offsets.dtype().is_int() && !offsets.dtype().is_nullable(),
            "offsets have invalid type {}",
            offsets.dtype()
        );

        // We can safely unwrap the DType as primitive now
        let offsets_ptype = offsets.dtype().as_ptype();

        // Offsets must be sorted (but not strictly sorted, zero-length lists are allowed)
        if let Some(is_sorted) = offsets.statistics().compute_is_sorted() {
            vortex_ensure!(is_sorted, "offsets must be sorted");
        } else {
            vortex_bail!("offsets must report is_sorted statistic");
        }

        // Validate that offsets min is non-negative, and max does not exceed the length of
        // the elements array.
        if let Some(min_max) = min_max(offsets)? {
            match_each_integer_ptype!(offsets_ptype, |P| {
                let max_offset = P::try_from(offsets.scalar_at(offsets.len() - 1))
                    .vortex_expect("Offsets type must fit offsets values");

                #[allow(clippy::absurd_extreme_comparisons, unused_comparisons)]
                {
                    if let Some(min) = min_max.min.as_primitive().as_::<P>() {
                        vortex_ensure!(
                            min >= 0 && min <= max_offset,
                            "offsets minimum {min} outside valid range [0, {max_offset}]"
                        );
                    }

                    if let Some(max) = min_max.max.as_primitive().as_::<P>() {
                        vortex_ensure!(
                            max >= 0 && max <= max_offset,
                            "offsets maximum {max} outside valid range [0, {max_offset}]"
                        )
                    }
                }

                vortex_ensure!(
                    max_offset
                        <= P::try_from(elements.len())
                            .vortex_expect("Offsets type must be able to fit elements length"),
                    "Max offset {max_offset} is beyond the length of the elements array {}",
                    elements.len()
                );
            })
        } else {
            // TODO(aduffy): fallback to slower validation pathway?
            vortex_bail!(
                "offsets array with encoding {} must support min_max compute function",
                offsets.encoding_id()
            );
        };

        // If a validity array is present, it must be the same length as the ListArray
        if let Some(validity_len) = validity.maybe_len() {
            vortex_ensure!(
                validity_len == offsets.len() - 1,
                "validity with size {validity_len} does not match array size {}",
                offsets.len() - 1
            );
        }

        Ok(())
    }

    /// Returns the offset at the given index from the list array.
    ///
    /// Panics if the index is out of bounds.
    pub fn offset_at(&self, index: usize) -> usize {
        assert!(
            index <= self.len(),
            "Index {index} out of bounds 0..={}",
            self.len()
        );

        self.offsets()
            .as_opt::<PrimitiveVTable>()
            .map(|p| match_each_native_ptype!(p.ptype(), |P| { p.as_slice::<P>()[index].as_() }))
            .unwrap_or_else(|| {
                self.offsets()
                    .scalar_at(index)
                    .as_primitive()
                    .as_::<usize>()
                    .vortex_expect("index must fit in usize")
            })
    }

    /// Returns the elements of the list scalar at the given index of the list array.
    pub fn list_elements_at(&self, index: usize) -> ArrayRef {
        let start = self.offset_at(index);
        let end = self.offset_at(index + 1);
        self.elements().slice(start..end)
    }

    /// Returns elements of the list array referenced by the offsets array.
    ///
    /// This is useful for discarding any potentially unused parts of the underlying `elements`
    /// child array.
    pub fn sliced_elements(&self) -> ArrayRef {
        let start = self.offset_at(0);
        let end = self.offset_at(self.len());
        self.elements().slice(start..end)
    }

    /// Returns the offsets array.
    pub fn offsets(&self) -> &ArrayRef {
        &self.offsets
    }

    /// Returns the elements array.
    pub fn elements(&self) -> &ArrayRef {
        &self.elements
    }

    /// Create a copy of this array by adjusting `offsets` to start at `0` and removing elements not
    /// referenced by the `offsets`.
    pub fn reset_offsets(&self, recurse: bool) -> VortexResult<Self> {
        let mut elements = self.sliced_elements();
        if recurse && elements.is_canonical() {
            elements = elements.to_canonical().compact()?.into_array();
        } else if recurse && let Some(child_list_array) = elements.as_opt::<ListVTable>() {
            elements = child_list_array.reset_offsets(recurse)?.into_array();
        }

        let offsets = self.offsets();
        let first_offset = offsets.scalar_at(0);
        let adjusted_offsets = sub_scalar(offsets, first_offset)?;

        Self::try_new(elements, adjusted_offsets, self.validity.clone())
    }
}