icydb-core 0.75.2

//! Module: executor::order
//! Responsibility: shared structural ordering helpers for executor row paths.
//! Does not own: planner order semantics or cursor wire validation.
//! Boundary: consumes planner-resolved order contracts and applies canonical ordering over slot-readable rows.

use crate::{
    db::{
        contracts::canonical_value_compare,
        cursor::{CursorBoundary, CursorBoundarySlot, apply_order_direction},
        query::plan::{
            ExpressionOrderTerm, OrderDirection, ResolvedOrder, ResolvedOrderValueSource,
        },
        scalar_expr::derive_expression_order_value,
    },
    value::Value,
};
use std::{borrow::Cow, cmp::Ordering};

///
/// OrderReadableRow
///
/// Structural executor row contract used by shared ordering logic.
/// Implementors expose slot-indexed values without re-entering typed entity
/// comparators in sort and cursor-boundary hot loops.
///

pub(in crate::db::executor) trait OrderReadableRow {
    /// Read one slot value for structural ordering and predicate evaluation.
    /// Structural row paths may return borrowed values so shared order/cursor
    /// helpers do not clone already-decoded slots in comparator hot loops.
    fn read_order_slot_cow(&self, slot: usize) -> Option<Cow<'_, Value>>;

    /// Read one slot value as an owned payload when a caller still needs to
    /// leave the borrowed structural-ordering boundary.
    fn read_order_slot(&self, slot: usize) -> Option<Value> {
        self.read_order_slot_cow(slot).map(Cow::into_owned)
    }
}

/// Apply canonical in-memory ordering with an optional bounded top-k window.
pub(in crate::db::executor) fn apply_structural_order_window<R>(
    rows: &mut Vec<R>,
    resolved_order: &ResolvedOrder,
    keep_count: Option<usize>,
) where
    R: OrderReadableRow,
{
    if let Some(keep_count) = keep_count
        && keep_count == 0
    {
        rows.clear();
        return;
    }

    if rows.len() <= 1 {
        return;
    }

    // Phase 1: cache resolved order values once per row so bounded selection
    // and final sort do not re-read sparse slots or re-run expression-order
    // derivation inside comparator hot loops.
    let mut cached_rows = std::mem::take(rows)
        .into_iter()
        .map(|row| {
            let cached_values = cache_order_values_from_row(&row, resolved_order);

            (row, cached_values)
        })
        .collect::<Vec<_>>();

    // Phase 2: retain only the bounded canonical window when pagination
    // exposes one, using the cached order keys instead of live row reads.
    if let Some(keep_count) = keep_count
        && cached_rows.len() > keep_count
    {
        cached_rows.select_nth_unstable_by(keep_count - 1, |left, right| {
            compare_cached_orderable_rows(left.1.as_slice(), right.1.as_slice(), resolved_order)
        });
        cached_rows.truncate(keep_count);
    }

    // Phase 3: sort the retained rows into final canonical order using the
    // precomputed key values.
    cached_rows.sort_by(|left, right| {
        compare_cached_orderable_rows(left.1.as_slice(), right.1.as_slice(), resolved_order)
    });
    rows.extend(cached_rows.into_iter().map(|(row, _)| row));
}

/// Compare one structural row against one cursor boundary under the canonical order contract.
pub(in crate::db::executor) fn compare_orderable_row_with_boundary<R>(
    row: &R,
    resolved_order: &ResolvedOrder,
    boundary: &CursorBoundary,
) -> Ordering
where
    R: OrderReadableRow,
{
    compare_structural_order_slots(resolved_order, |slot_index, field_index, direction| {
        let row_slot = order_value_from_row(row, field_index);
        let boundary_slot = boundary
            .slots
            .get(slot_index)
            .expect("cursor boundary must align with resolved order");

        apply_order_direction(
            compare_order_value_with_boundary(row_slot, boundary_slot),
            direction,
        )
    })
}

/// Materialize one cursor boundary directly from one already-decoded row under
/// the planner-frozen resolved order contract.
#[must_use]
pub(in crate::db::executor) fn cursor_boundary_from_orderable_row<R>(
    row: &R,
    resolved_order: &ResolvedOrder,
) -> CursorBoundary
where
    R: OrderReadableRow,
{
    CursorBoundary {
        slots: cache_order_values_from_row(row, resolved_order)
            .into_iter()
            .map(|value| match value {
                Some(value) => CursorBoundarySlot::Present(value),
                None => CursorBoundarySlot::Missing,
            })
            .collect(),
    }
}

// Compare two cached structural ordering tuples according to the resolved
// canonical order without re-reading row slots inside the comparator.
fn compare_cached_orderable_rows(
    left: &[Option<Value>],
    right: &[Option<Value>],
    resolved_order: &ResolvedOrder,
) -> Ordering {
    compare_structural_order_slots(resolved_order, |slot_index, _field_index, direction| {
        let left_slot = left.get(slot_index).and_then(Option::as_ref);
        let right_slot = right.get(slot_index).and_then(Option::as_ref);

        debug_assert!(
            left.get(slot_index).is_some() && right.get(slot_index).is_some(),
            "cached order values must align with resolved order fields",
        );
        apply_order_direction(
            compare_cached_order_values(left_slot, right_slot),
            direction,
        )
    })
}

// Cache one row's order values once so sort/select hot loops can compare
// cheap owned key tuples instead of re-deriving them repeatedly.
fn cache_order_values_from_row<R>(row: &R, resolved_order: &ResolvedOrder) -> Vec<Option<Value>>
where
    R: OrderReadableRow,
{
    resolved_order
        .fields()
        .iter()
        .copied()
        .map(|field| match field.source() {
            ResolvedOrderValueSource::DirectField(slot) => row.read_order_slot(slot),
            ResolvedOrderValueSource::ExpressionLower(slot) => {
                derive_expression_order_row_value(row, slot, ExpressionOrderTerm::Lower(""))
            }
            ResolvedOrderValueSource::ExpressionUpper(slot) => {
                derive_expression_order_row_value(row, slot, ExpressionOrderTerm::Upper(""))
            }
        })
        .collect()
}

// Compare one structural ordering tuple by resolving slot pairs lazily in canonical field order.
fn compare_structural_order_slots<F>(
    resolved_order: &ResolvedOrder,
    mut compare_slot: F,
) -> Ordering
where
    F: FnMut(usize, ResolvedOrderValueSource, OrderDirection) -> Ordering,
{
    for (slot_index, field) in resolved_order.fields().iter().copied().enumerate() {
        let ordering = compare_slot(slot_index, field.source(), field.direction());
        if ordering != Ordering::Equal {
            return ordering;
        }
    }

    Ordering::Equal
}

// Borrow one slot-reader value through the shared ordering seam.
fn order_value_from_row(
    row: &dyn OrderReadableRow,
    source: ResolvedOrderValueSource,
) -> Option<Cow<'_, Value>> {
    match source {
        ResolvedOrderValueSource::DirectField(slot) => row.read_order_slot_cow(slot),
        ResolvedOrderValueSource::ExpressionLower(slot) => {
            derive_expression_order_row_value(row, slot, ExpressionOrderTerm::Lower(""))
                .map(Cow::Owned)
        }
        ResolvedOrderValueSource::ExpressionUpper(slot) => {
            derive_expression_order_row_value(row, slot, ExpressionOrderTerm::Upper(""))
                .map(Cow::Owned)
        }
    }
}

// Derive one owned expression-order value from one structural row slot.
fn derive_expression_order_row_value(
    row: &dyn OrderReadableRow,
    slot: usize,
    term: ExpressionOrderTerm<'_>,
) -> Option<Value> {
    let value = row.read_order_slot_cow(slot)?;

    derive_expression_order_value(term, value.as_ref())
}

// Compare two cached owned ordering values after key precomputation.
fn compare_cached_order_values(left: Option<&Value>, right: Option<&Value>) -> Ordering {
    match (left, right) {
        (None, None) => Ordering::Equal,
        (None, Some(_)) => Ordering::Less,
        (Some(_), None) => Ordering::Greater,
        (Some(left), Some(right)) => canonical_value_compare(left, right),
    }
}

// Compare one row-provided ordering value against one persisted cursor
// boundary slot without rebuilding the row side into an owned boundary slot.
fn compare_order_value_with_boundary(
    value: Option<Cow<'_, Value>>,
    boundary: &CursorBoundarySlot,
) -> Ordering {
    match (value, boundary) {
        (None, CursorBoundarySlot::Missing) => Ordering::Equal,
        (None, CursorBoundarySlot::Present(_)) => Ordering::Less,
        (Some(_), CursorBoundarySlot::Missing) => Ordering::Greater,
        (Some(value), CursorBoundarySlot::Present(boundary_value)) => {
            canonical_value_compare(value.as_ref(), boundary_value)
        }
    }
}

///
/// TESTS
///

#[cfg(test)]
mod tests {
    use super::*;
    use crate::db::query::plan::ResolvedOrderField;
    use std::{borrow::Cow, cell::Cell, rc::Rc};

    struct TestRow {
        slots: Vec<Option<Value>>,
    }

    impl TestRow {
        fn new(slots: Vec<Option<Value>>) -> Self {
            Self { slots }
        }
    }

    impl OrderReadableRow for TestRow {
        fn read_order_slot_cow(&self, slot: usize) -> Option<Cow<'_, Value>> {
            self.slots
                .get(slot)
                .and_then(Option::as_ref)
                .map(Cow::Borrowed)
        }
    }

    struct CountingRow {
        reads: Rc<Cell<usize>>,
        slots: Vec<Option<Value>>,
    }

    impl CountingRow {
        fn new(reads: Rc<Cell<usize>>, slots: Vec<Option<Value>>) -> Self {
            Self { reads, slots }
        }
    }

    impl OrderReadableRow for CountingRow {
        fn read_order_slot_cow(&self, slot: usize) -> Option<Cow<'_, Value>> {
            self.reads.set(self.reads.get().saturating_add(1));
            self.slots
                .get(slot)
                .and_then(Option::as_ref)
                .map(Cow::Borrowed)
        }
    }

    fn resolved_order(fields: &[(usize, OrderDirection)]) -> ResolvedOrder {
        ResolvedOrder::new(
            fields
                .iter()
                .map(|(field_index, direction)| {
                    ResolvedOrderField::new(
                        ResolvedOrderValueSource::direct_field(*field_index),
                        *direction,
                    )
                })
                .collect(),
        )
    }

    #[test]
    fn apply_structural_order_sorts_rows_by_resolved_slots() {
        let mut rows = vec![
            TestRow::new(vec![Some(Value::Uint(3))]),
            TestRow::new(vec![Some(Value::Uint(1))]),
            TestRow::new(vec![Some(Value::Uint(2))]),
        ];

        apply_structural_order_window(
            &mut rows,
            &resolved_order(&[(0, OrderDirection::Asc)]),
            None,
        );

        let ordered = rows
            .into_iter()
            .map(|row| row.read_order_slot(0))
            .collect::<Vec<_>>();
        assert_eq!(
            ordered,
            vec![
                Some(Value::Uint(1)),
                Some(Value::Uint(2)),
                Some(Value::Uint(3))
            ]
        );
    }

    #[test]
    fn apply_structural_order_bounded_keeps_smallest_rows_in_canonical_order() {
        let mut rows = vec![
            TestRow::new(vec![Some(Value::Uint(4))]),
            TestRow::new(vec![Some(Value::Uint(2))]),
            TestRow::new(vec![Some(Value::Uint(3))]),
            TestRow::new(vec![Some(Value::Uint(1))]),
        ];

        apply_structural_order_window(
            &mut rows,
            &resolved_order(&[(0, OrderDirection::Asc)]),
            Some(2),
        );

        let ordered = rows
            .into_iter()
            .map(|row| row.read_order_slot(0))
            .collect::<Vec<_>>();
        assert_eq!(ordered, vec![Some(Value::Uint(1)), Some(Value::Uint(2))]);
    }

    #[test]
    fn compare_orderable_row_with_boundary_respects_desc_direction() {
        let row = TestRow::new(vec![Some(Value::Uint(7))]);
        let boundary = CursorBoundary {
            slots: vec![CursorBoundarySlot::Present(Value::Uint(5))],
        };

        let ordering = compare_orderable_row_with_boundary(
            &row,
            &resolved_order(&[(0, OrderDirection::Desc)]),
            &boundary,
        );

        assert_eq!(ordering, Ordering::Less);
    }

    #[test]
    fn apply_structural_order_window_caches_slot_reads_once_per_row() {
        let left_reads = Rc::new(Cell::new(0));
        let middle_reads = Rc::new(Cell::new(0));
        let right_reads = Rc::new(Cell::new(0));
        let mut rows = vec![
            CountingRow::new(left_reads.clone(), vec![Some(Value::Uint(3))]),
            CountingRow::new(middle_reads.clone(), vec![Some(Value::Uint(1))]),
            CountingRow::new(right_reads.clone(), vec![Some(Value::Uint(2))]),
        ];

        apply_structural_order_window(
            &mut rows,
            &resolved_order(&[(0, OrderDirection::Asc)]),
            Some(2),
        );

        assert_eq!(left_reads.get(), 1);
        assert_eq!(middle_reads.get(), 1);
        assert_eq!(right_reads.get(), 1);
    }
}