antecedent-graph 0.5.2

//! Shared graph algorithms (reachability, Kahn topo).
//!
//! SPDX-License-Identifier: MIT OR Apache-2.0

use crate::types::DenseNodeId;
use crate::workspace::GraphWorkspace;

/// Directed BFS reachability from `from` to `to` over `children`.
#[must_use]
pub fn bfs_reaches(
    children: &[Vec<DenseNodeId>],
    from: DenseNodeId,
    to: DenseNodeId,
    ws: &mut GraphWorkspace,
) -> bool {
    if from == to {
        return true;
    }
    if from.as_usize() >= children.len() || to.as_usize() >= children.len() {
        return false;
    }
    ws.prepare(children.len());
    ws.frontier.push(from);
    ws.visited.insert(from);
    while let Some(n) = ws.frontier.pop() {
        for &c in &children[n.as_usize()] {
            if c == to {
                return true;
            }
            if !ws.visited.contains(c) {
                ws.visited.insert(c);
                ws.frontier.push(c);
            }
        }
    }
    false
}

/// Kahn topological order. Returns `None` if a directed cycle exists.
#[must_use]
pub fn kahn_order(
    parents: &[Vec<DenseNodeId>],
    children: &[Vec<DenseNodeId>],
) -> Option<Vec<DenseNodeId>> {
    let n = parents.len();
    debug_assert_eq!(n, children.len());
    let mut indeg = vec![0u32; n];
    for (i, p) in parents.iter().enumerate() {
        indeg[i] = u32::try_from(p.len()).ok()?;
    }
    let mut q: Vec<DenseNodeId> = indeg
        .iter()
        .enumerate()
        .filter(|&(_, &d)| d == 0)
        .map(|(i, _)| DenseNodeId::from_raw(u32::try_from(i).expect("node fit")))
        .collect();
    let mut order = Vec::with_capacity(n);
    while let Some(u) = q.pop() {
        order.push(u);
        for &v in &children[u.as_usize()] {
            indeg[v.as_usize()] -= 1;
            if indeg[v.as_usize()] == 0 {
                q.push(v);
            }
        }
    }
    (order.len() == n).then_some(order)
}

/// Whether the directed graph is acyclic (Kahn count).
#[must_use]
pub fn is_dag(parents: &[Vec<DenseNodeId>], children: &[Vec<DenseNodeId>]) -> bool {
    kahn_order(parents, children).is_some()
}