use std::cmp::Reverse;
use std::collections::BinaryHeap;

/// Computes shortest distances from `source` in a directed, weighted graph.
///
/// `graph[u]` contains `(v, weight)` edges from `u` to `v`.
/// Unreachable vertices have distance `None`.
pub fn dijkstra(graph: &[Vec<(usize, u64)>], source: usize) -> Vec<Option<u64>> {
    let mut distances = vec![None; graph.len()];

    if source >= graph.len() {
        return distances;
    }

    // The heap stores the smallest tentative distance first.
    let mut heap = BinaryHeap::new();
    distances[source] = Some(0);
    heap.push(Reverse((0_u64, source)));

    while let Some(Reverse((distance, vertex))) = heap.pop() {
        // Ignore entries made obsolete by a shorter path.
        if distances[vertex] != Some(distance) {
            continue;
        }

        for &(neighbor, weight) in &graph[vertex] {
            assert!(neighbor < graph.len(), "edge points outside the graph");

            // Ignore paths whose total weight cannot fit in u64.
            let Some(candidate) = distance.checked_add(weight) else {
                continue;
            };

            if distances[neighbor].is_none_or(|current| candidate < current) {
                distances[neighbor] = Some(candidate);
                heap.push(Reverse((candidate, neighbor)));
            }
        }
    }

    distances
}

#[cfg(test)]
mod tests {
    use super::dijkstra;

    #[test]
    fn finds_shortest_paths() {
        let graph = vec![
            vec![(1, 4), (2, 1)],
            vec![(3, 1)],
            vec![(1, 2), (3, 5)],
            vec![],
        ];

        assert_eq!(
            dijkstra(&graph, 0),
            vec![Some(0), Some(3), Some(1), Some(4)]
        );
    }

    #[test]
    fn leaves_unreachable_vertices_unset() {
        let graph = vec![vec![(1, 7)], vec![], vec![]];

        assert_eq!(dijkstra(&graph, 0), vec![Some(0), Some(7), None]);
    }
}