geometric-traits 0.2.3

//! Shared helper functions used by both the LAPJV and LAPMOD algorithms.
#![cfg(feature = "alloc")]
use alloc::vec::Vec;
use core::fmt::Debug;

use num_traits::AsPrimitive;

use crate::traits::{AssignmentState, DenseValuedMatrix2D, Finite, Number, TotalOrd, TryFromUsize};

/// Finds the minimum distance among the columns in `to_scan[lower_bound..]`,
/// rearranges them so that all minimum-distance columns come immediately after
/// `to_scan[lower_bound]`, and returns the new upper-bound index.
///
/// # Arguments
///
/// * `lower_bound`: The index from which to start scanning.
/// * `distances`: The distances vector indexed by column usize.
/// * `to_scan`: The slice of columns to scan (modified in place).
///
/// # Returns
///
/// The index one past the last column at the minimum distance.
pub(crate) fn find_minimum_distance<C, V>(
    lower_bound: usize,
    distances: &[V],
    to_scan: &mut [C],
) -> usize
where
    C: AsPrimitive<usize> + Copy,
    V: PartialOrd + Copy,
{
    debug_assert!(
        lower_bound < to_scan.len(),
        "We expected the lower bound to be less than the length of the to scan vector"
    );
    let mut upper_bound = lower_bound + 1;
    let column_index = to_scan[lower_bound];
    let mut minimum_distance = distances[column_index.as_()];

    for k in lower_bound + 1..to_scan.len() {
        let column_index = to_scan[k];
        let distance = distances[column_index.as_()];
        if distance <= minimum_distance {
            if distance < minimum_distance {
                upper_bound = lower_bound;
                minimum_distance = distance;
            }
            to_scan[k] = to_scan[upper_bound];
            to_scan[upper_bound] = column_index;
            upper_bound += 1;
        }
    }

    upper_bound
}

/// Backtracks along the predecessor chain to update the assignment after an
/// augmenting path has been found.
///
/// # Arguments
///
/// * `column_index`: The sink column found by the path search.
/// * `predecessors`: The predecessor map indexed by column usize.
/// * `assigned_rows`: Column → row assignment (modified in place).
/// * `assigned_columns`: Row → column assignment (modified in place).
/// * `start_row`: The unassigned row that started the augmenting path.
pub(crate) fn augmentation_backtrack<R, C>(
    mut column_index: C,
    predecessors: &[R],
    assigned_rows: &mut [AssignmentState<R>],
    assigned_columns: &mut [AssignmentState<C>],
    start_row: R,
) where
    R: Copy + Eq + AsPrimitive<usize>,
    C: Copy + AsPrimitive<usize>,
{
    let mut number_of_steps = 0usize;
    let max_steps = assigned_rows.len();

    loop {
        number_of_steps += 1;
        assert!(
            number_of_steps <= max_steps,
            "augmentation_backtrack detected a predecessor cycle"
        );

        let row_index = predecessors[column_index.as_()];

        assigned_rows[column_index.as_()] = AssignmentState::Assigned(row_index);

        // Root of the augmenting path: this row was intentionally unassigned
        // when the search started, so it has no previous column to follow.
        if row_index == start_row {
            assigned_columns[row_index.as_()] = AssignmentState::Assigned(column_index);
            break;
        }

        let AssignmentState::Assigned(old_column_index) = assigned_columns[row_index.as_()] else {
            unreachable!("We expected the assigned column to be in the assigned state");
        };

        assigned_columns[row_index.as_()] = AssignmentState::Assigned(column_index);
        column_index = old_column_index;
    }
}

/// Core loop of the augmenting row reduction phase, parameterised by the
/// function that computes the first and second minimum reduced costs for a row.
///
/// The `first_and_second_min` closure receives the row index and a shared slice
/// of the current column costs, so it can be called even while `column_costs`
/// is owned mutably by this function.
///
/// # Arguments
///
/// * `unassigned_rows`: The current list of unassigned rows (modified).
/// * `assigned_rows`: Column → row assignment (indexed by column, modified).
/// * `assigned_columns`: Row → column assignment (indexed by row, modified).
/// * `column_costs`: Current column dual variables (modified).
/// * `number_of_rows`: Total number of rows (used for iteration-count guard).
/// * `first_and_second_min`: Closure `(row, &col_costs) -> ((best_col,
///   best_val), (Option<second_col>, second_val))`.
pub(crate) fn augmenting_row_reduction_impl<R, C, V>(
    unassigned_rows: &mut Vec<R>,
    assigned_rows: &mut [AssignmentState<R>],
    assigned_columns: &mut [AssignmentState<C>],
    column_costs: &mut Vec<V>,
    number_of_rows: usize,
    mut first_and_second_min: impl FnMut(R, &[V]) -> ((C, V), (Option<C>, V)),
) where
    R: Copy + AsPrimitive<usize>,
    C: Copy + AsPrimitive<usize>,
    V: Number + TotalOrd,
{
    if unassigned_rows.is_empty() {
        return;
    }

    let mut current_unassigned_row_index = 0;
    let mut updated_number_of_unassigned_rows = 0;
    let mut number_of_iterations = 0;
    let original_number_of_unassigned_rows = unassigned_rows.len();

    while current_unassigned_row_index < original_number_of_unassigned_rows {
        let unassigned_row_index = unassigned_rows[current_unassigned_row_index];
        current_unassigned_row_index += 1;
        number_of_iterations += 1;

        // We determine the first and second minimum reduced costs of the row.
        // The immutable borrow of column_costs ends when this call returns.
        let (
            (mut first_minimum_column_index, first_minimum_value),
            (second_minimum_column_index, second_minimum_value),
        ) = first_and_second_min(unassigned_row_index, column_costs.as_slice());

        let mut row_index = assigned_rows[first_minimum_column_index.as_()];

        if number_of_iterations < current_unassigned_row_index * number_of_rows {
            if first_minimum_value < second_minimum_value {
                column_costs[first_minimum_column_index.as_()] -=
                    second_minimum_value - first_minimum_value;
            } else if let (AssignmentState::Assigned(_), Some(second_minimum_column_index)) =
                (row_index, second_minimum_column_index)
            {
                first_minimum_column_index = second_minimum_column_index;
                row_index = assigned_rows[first_minimum_column_index.as_()];
            }
            if let AssignmentState::Assigned(assigned_row) = row_index {
                if first_minimum_value < second_minimum_value {
                    current_unassigned_row_index -= 1;
                    unassigned_rows[current_unassigned_row_index] = assigned_row;
                } else {
                    unassigned_rows[updated_number_of_unassigned_rows] = assigned_row;
                    updated_number_of_unassigned_rows += 1;
                }
            }
        } else if let AssignmentState::Assigned(assigned_row) = row_index {
            unassigned_rows[updated_number_of_unassigned_rows] = assigned_row;
            updated_number_of_unassigned_rows += 1;
        }

        // We update the assigned row with the new column index.
        assigned_rows[first_minimum_column_index.as_()] =
            AssignmentState::Assigned(unassigned_row_index);
        // We update the assigned column with the new row index.
        assigned_columns[unassigned_row_index.as_()] =
            AssignmentState::Assigned(first_minimum_column_index);
    }

    unassigned_rows.truncate(updated_number_of_unassigned_rows);
}

/// Shared Dijkstra-style frontier scan for dense LAP solvers (LAPJV and
/// Hungarian).
///
/// Expands the frontier `to_scan[lower_bound..upper_bound]` by relaxing
/// distances through each frontier column's assigned row.  Returns
/// `Some(col)` immediately if an unassigned column is reached at the
/// current minimum distance.
#[allow(clippy::too_many_arguments)]
#[inline]
pub(crate) fn dense_scan<M: DenseValuedMatrix2D + ?Sized>(
    lower_bound_ref: &mut usize,
    upper_bound_ref: &mut usize,
    to_scan: &mut [M::ColumnIndex],
    distances: &mut [M::Value],
    predecessors: &mut [M::RowIndex],
    assigned_rows: &[AssignmentState<M::RowIndex>],
    column_costs: &[M::Value],
    matrix: &M,
) -> Option<M::ColumnIndex>
where
    M::Value: Number + Finite + TotalOrd,
{
    let mut lower_bound = *lower_bound_ref;
    let mut upper_bound = *upper_bound_ref;

    while lower_bound != upper_bound {
        let column_index = to_scan[lower_bound];
        lower_bound += 1;
        let AssignmentState::Assigned(row_index) = assigned_rows[column_index.as_()] else {
            unreachable!("Frontier column must be assigned during scan");
        };
        let minimum_distance = distances[column_index.as_()];
        let initial_reduced_cost = matrix.value((row_index, column_index))
            - column_costs[column_index.as_()]
            - minimum_distance;

        let current_upper_bound = upper_bound;
        for k in current_upper_bound..to_scan.len() {
            let col = to_scan[k];
            let reduced_cost =
                matrix.value((row_index, col)) - column_costs[col.as_()] - initial_reduced_cost;
            if reduced_cost < distances[col.as_()] {
                distances[col.as_()] = reduced_cost;
                predecessors[col.as_()] = row_index;
                if reduced_cost == minimum_distance {
                    if assigned_rows[col.as_()].is_unassigned() {
                        *lower_bound_ref = lower_bound;
                        *upper_bound_ref = upper_bound;
                        return Some(col);
                    }
                    to_scan[k] = to_scan[upper_bound];
                    to_scan[upper_bound] = col;
                    upper_bound += 1;
                }
            }
        }
    }

    *lower_bound_ref = lower_bound;
    *upper_bound_ref = upper_bound;

    None
}

/// Shared Dijkstra-style shortest-path search for dense LAP solvers
/// (LAPJV and Hungarian).
///
/// Initialises distances from `start_row_index`, then alternates between
/// `find_minimum_distance` and `dense_scan` until an unassigned column is
/// reached.  Finally updates dual variables (`column_costs`) for all
/// settled columns.  Returns the sink column.
#[inline]
pub(crate) fn dense_find_path<M: DenseValuedMatrix2D + ?Sized>(
    start_row_index: M::RowIndex,
    to_scan: &mut [M::ColumnIndex],
    predecessors: &mut [M::RowIndex],
    distances: &mut [M::Value],
    assigned_rows: &[AssignmentState<M::RowIndex>],
    column_costs: &mut [M::Value],
    matrix: &M,
) -> M::ColumnIndex
where
    M::Value: Number + Finite + TotalOrd,
{
    let mut lower_bound: usize = 0;
    let mut upper_bound: usize = 0;
    let mut n_ready: usize = 0;

    for (column_index, (col_to_scan, (predecessor, distance))) in matrix
        .column_indices()
        .zip(to_scan.iter_mut().zip(predecessors.iter_mut().zip(distances.iter_mut())))
    {
        *predecessor = start_row_index;
        *col_to_scan = column_index;
        *distance =
            matrix.value((start_row_index, column_index)) - column_costs[column_index.as_()];
    }

    let column_index = 'outer: loop {
        if lower_bound == upper_bound {
            n_ready = lower_bound;
            upper_bound = find_minimum_distance(lower_bound, distances, to_scan);

            for &col in &to_scan[lower_bound..upper_bound] {
                if assigned_rows[col.as_()].is_unassigned() {
                    break 'outer col;
                }
            }
        }

        if let Some(col) = dense_scan::<M>(
            &mut lower_bound,
            &mut upper_bound,
            to_scan,
            distances,
            predecessors,
            assigned_rows,
            column_costs,
            matrix,
        ) {
            break 'outer col;
        }
    };

    // Use the sink column's actual Dijkstra distance as reference.
    let minimum_distance = distances[column_index.as_()];
    for &col in &to_scan[0..n_ready] {
        column_costs[col.as_()] += distances[col.as_()] - minimum_distance;
    }

    column_index
}

/// Converts the column→row assignment vector into a `Vec<(R, C)>` of
/// `(row, column)` pairs.
///
/// Shared by LAPJV, Hungarian, and LAPMOD inner structs.
pub(crate) fn assignments_from_assigned_rows<R, C>(
    assigned_rows: Vec<AssignmentState<R>>,
    capacity: usize,
) -> Vec<(R, C)>
where
    R: Copy,
    C: TryFromUsize,
    <C as TryFrom<usize>>::Error: Debug,
{
    let mut assignments: Vec<(R, C)> = Vec::with_capacity(capacity);
    for (col_usize, state) in assigned_rows.into_iter().enumerate() {
        let AssignmentState::Assigned(row) = state else {
            unreachable!("Every column should be assigned after augmentation");
        };
        assignments.push((row, C::try_from_usize(col_usize).unwrap()));
    }
    assignments
}

#[cfg(all(test, feature = "alloc"))]
mod tests {
    use super::*;

    #[test]
    fn test_find_minimum_distance_groups_equal_minima() {
        let distances = vec![5.0_f64, 1.0, 1.0, 3.0];
        let mut to_scan = vec![0usize, 1, 2, 3];
        let upper_bound = find_minimum_distance(0, &distances, &mut to_scan);

        assert_eq!(upper_bound, 2);
        assert_eq!(to_scan[0], 1);
        assert_eq!(to_scan[1], 2);
    }

    #[test]
    fn test_find_minimum_distance_resets_on_strict_smaller_minimum() {
        let distances = vec![4.0_f64, 3.0, 1.0];
        let mut to_scan = vec![0usize, 1, 2];
        let upper_bound = find_minimum_distance(0, &distances, &mut to_scan);

        assert_eq!(upper_bound, 1);
        assert_eq!(to_scan[0], 2);
    }

    #[test]
    fn test_find_minimum_distance_keeps_single_frontier_when_first_is_best() {
        let distances = vec![1.0_f64, 5.0, 6.0];
        let mut to_scan = vec![0usize, 1, 2];
        let upper_bound = find_minimum_distance(0, &distances, &mut to_scan);

        assert_eq!(upper_bound, 1);
        assert_eq!(to_scan, vec![0, 1, 2]);
    }

    #[test]
    fn test_augmentation_backtrack_updates_assignment_chain() {
        let predecessors = vec![1u8, 2u8, 2u8];
        let mut assigned_rows = vec![
            AssignmentState::Unassigned,
            AssignmentState::Unassigned,
            AssignmentState::Unassigned,
        ];
        let mut assigned_columns = vec![
            AssignmentState::Unassigned,
            AssignmentState::Assigned(1u8),
            AssignmentState::Unassigned,
        ];

        augmentation_backtrack(0u8, &predecessors, &mut assigned_rows, &mut assigned_columns, 2u8);

        assert_eq!(assigned_rows[0], AssignmentState::Assigned(1u8));
        assert_eq!(assigned_rows[1], AssignmentState::Assigned(2u8));
        assert_eq!(assigned_columns[1], AssignmentState::Assigned(0u8));
        assert_eq!(assigned_columns[2], AssignmentState::Assigned(1u8));
    }

    #[test]
    #[should_panic(expected = "augmentation_backtrack detected a predecessor cycle")]
    fn test_augmentation_backtrack_panics_on_predecessor_cycle() {
        let predecessors = vec![1u8];
        let mut assigned_rows = vec![AssignmentState::Unassigned];
        let mut assigned_columns = vec![
            AssignmentState::Unassigned,
            AssignmentState::Assigned(0u8),
            AssignmentState::Unassigned,
        ];

        augmentation_backtrack(0u8, &predecessors, &mut assigned_rows, &mut assigned_columns, 2u8);
    }

    #[test]
    fn test_augmenting_row_reduction_reduces_column_cost_on_strict_second_best() {
        let mut unassigned_rows = vec![0u8];
        let mut assigned_rows = vec![AssignmentState::Unassigned, AssignmentState::Unassigned];
        let mut assigned_columns = vec![AssignmentState::Unassigned, AssignmentState::Unassigned];
        let mut column_costs = vec![10.0_f64, 10.0_f64];

        augmenting_row_reduction_impl(
            &mut unassigned_rows,
            &mut assigned_rows,
            &mut assigned_columns,
            &mut column_costs,
            2,
            |_row, _col_costs| ((0u8, 1.0_f64), (Some(1u8), 3.0_f64)),
        );

        assert!((column_costs[0] - 8.0).abs() < f64::EPSILON);
        assert_eq!(assigned_rows[0], AssignmentState::Assigned(0u8));
        assert_eq!(assigned_columns[0], AssignmentState::Assigned(0u8));
        assert_eq!(unassigned_rows, [] as [u8; 0]);
    }

    #[test]
    fn test_augmenting_row_reduction_switches_to_second_minimum_on_tie() {
        let mut unassigned_rows = vec![0u8];
        let mut assigned_rows = vec![AssignmentState::Assigned(1u8), AssignmentState::Unassigned];
        let mut assigned_columns =
            vec![AssignmentState::Unassigned, AssignmentState::Assigned(0u8)];
        let mut column_costs = vec![10.0_f64, 10.0_f64];

        augmenting_row_reduction_impl(
            &mut unassigned_rows,
            &mut assigned_rows,
            &mut assigned_columns,
            &mut column_costs,
            3,
            |_row, _col_costs| ((0u8, 5.0_f64), (Some(1u8), 5.0_f64)),
        );

        assert_eq!(assigned_rows[1], AssignmentState::Assigned(0u8));
        assert_eq!(assigned_columns[0], AssignmentState::Assigned(1u8));
        assert_eq!(unassigned_rows, [] as [u8; 0]);
    }

    #[test]
    fn test_augmenting_row_reduction_iteration_guard_fallback_preserves_progress() {
        let mut unassigned_rows = vec![0u8];
        let mut assigned_rows = vec![AssignmentState::Assigned(1u8)];
        let mut assigned_columns =
            vec![AssignmentState::Unassigned, AssignmentState::Assigned(0u8)];
        let mut column_costs = vec![10.0_f64];

        augmenting_row_reduction_impl(
            &mut unassigned_rows,
            &mut assigned_rows,
            &mut assigned_columns,
            &mut column_costs,
            1,
            |_row, _col_costs| ((0u8, 1.0_f64), (None, 9.0_f64)),
        );

        assert_eq!(unassigned_rows, vec![1u8]);
        assert_eq!(assigned_rows[0], AssignmentState::Assigned(0u8));
        assert_eq!(assigned_columns[0], AssignmentState::Assigned(0u8));
    }
}