//! Compute a shortest path using the [Dijkstra search
//! algorithm](https://en.wikipedia.org/wiki/Dijkstra's_algorithm).

use indexmap::map::Entry::{Occupied, Vacant};
use indexmap::IndexMap;
use num_traits::Zero;
use std::cmp::Ordering;
use std::collections::{BinaryHeap, HashMap};
use std::hash::Hash;
use std::usize;

use super::reverse_path;

/// Compute a shortest path using the [Dijkstra search
/// algorithm](https://en.wikipedia.org/wiki/Dijkstra's_algorithm).
///
/// The shortest path starting from `start` up to a node for which `success` returns `true` is
/// computed and returned along with its total cost, in a `Some`. If no path can be found, `None`
/// is returned instead.
///
/// - `start` is the starting node.
/// - `successors` returns a list of successors for a given node, along with the cost for moving
/// from the node to the successor.
/// - `success` checks whether the goal has been reached. It is not a node as some problems require
/// a dynamic solution instead of a fixed node.
///
/// A node will never be included twice in the path as determined by the `Eq` relationship.
///
/// The returned path comprises both the start and end node.
///
/// # Example
///
/// We will search the shortest path on a chess board to go from (1, 1) to (4, 6) doing only knight
/// moves.
///
/// The first version uses an explicit type `Pos` on which the required traits are derived.
///
/// ```
/// use pathfinding::prelude::dijkstra;
///
/// #[derive(Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
/// struct Pos(i32, i32);
///
/// impl Pos {
///   fn successors(&self) -> Vec<(Pos, usize)> {
///     let &Pos(x, y) = self;
///     vec![Pos(x+1,y+2), Pos(x+1,y-2), Pos(x-1,y+2), Pos(x-1,y-2),
///          Pos(x+2,y+1), Pos(x+2,y-1), Pos(x-2,y+1), Pos(x-2,y-1)]
///          .into_iter().map(|p| (p, 1)).collect()
///   }
/// }
///
/// static GOAL: Pos = Pos(4, 6);
/// let result = dijkstra(&Pos(1, 1), |p| p.successors(), |p| *p == GOAL);
/// assert_eq!(result.expect("no path found").1, 4);
/// ```
///
/// The second version does not declare a `Pos` type, makes use of more closures,
/// and is thus shorter.
///
/// ```
/// use pathfinding::prelude::dijkstra;
///
/// static GOAL: (i32, i32) = (4, 6);
/// let result = dijkstra(&(1, 1),
///                       |&(x, y)| vec![(x+1,y+2), (x+1,y-2), (x-1,y+2), (x-1,y-2),
///                                      (x+2,y+1), (x+2,y-1), (x-2,y+1), (x-2,y-1)]
///                                  .into_iter().map(|p| (p, 1)),
///                       |&p| p == GOAL);
/// assert_eq!(result.expect("no path found").1, 4);
/// ```
pub fn dijkstra<N, C, FN, IN, FS>(start: &N, successors: FN, success: FS) -> Option<(Vec<N>, C)>
where
    N: Eq + Hash + Clone,
    C: Zero + Ord + Copy,
    FN: FnMut(&N) -> IN,
    IN: IntoIterator<Item = (N, C)>,
    FS: FnMut(&N) -> bool,
{
    let (parents, reached) = run_dijkstra(start, successors, success);
    reached.map(|target| {
        (
            reverse_path(&parents, |&(p, _)| p, target),
            parents.get_index(target).unwrap().1 .1,
        )
    })
}

/// Determine all reachable nodes from a starting point as well as the minimum cost to
/// reach them and a possible optimal parent node
/// using the [Dijkstra search algorithm](https://en.wikipedia.org/wiki/Dijkstra's_algorithm).
///
/// - `start` is the starting node.
/// - `successors` returns a list of successors for a given node, along with the cost for moving
/// from the node to the successor.
///
/// The result is a map where every reachable node (not including `start`) is associated with
/// an optimal parent node and a cost.
///
/// The [`build_path`] function can be used to build a full path from the starting point to one
/// of the reachable targets.
pub fn dijkstra_all<N, C, FN, IN>(start: &N, successors: FN) -> HashMap<N, (N, C)>
where
    N: Eq + Hash + Clone,
    C: Zero + Ord + Copy,
    FN: FnMut(&N) -> IN,
    IN: IntoIterator<Item = (N, C)>,
{
    dijkstra_partial(start, successors, |_| false).0
}

/// Determine some reachable nodes from a starting point as well as the minimum cost to
/// reach them and a possible optimal parent node
/// using the [Dijkstra search algorithm](https://en.wikipedia.org/wiki/Dijkstra's_algorithm).
///
/// - `start` is the starting node.
/// - `successors` returns a list of successors for a given node, along with the cost for moving
/// from the node to the successor.
/// - `stop` is a function which is called every time a node is examined (including `start`).
///   A `true` return value will stop the algorithm.
///
/// The result is a map where every node examined before the algorithm stopped (not including
/// `start`) is associated with an optimal parent node and a cost, as well as the node which
/// caused the algorithm to stop if any.
///
/// The [`build_path`] function can be used to build a full path from the starting point to one
/// of the reachable targets.
pub fn dijkstra_partial<N, C, FN, IN, FS>(
    start: &N,
    successors: FN,
    stop: FS,
) -> (HashMap<N, (N, C)>, Option<N>)
where
    N: Eq + Hash + Clone,
    C: Zero + Ord + Copy,
    FN: FnMut(&N) -> IN,
    IN: IntoIterator<Item = (N, C)>,
    FS: FnMut(&N) -> bool,
{
    let (parents, reached) = run_dijkstra(start, successors, stop);
    (
        parents
            .iter()
            .skip(1)
            .map(|(n, (p, c))| (n.clone(), (parents.get_index(*p).unwrap().0.clone(), *c)))
            .collect(),
        reached.map(|i| parents.get_index(i).unwrap().0.clone()),
    )
}

fn run_dijkstra<N, C, FN, IN, FS>(
    start: &N,
    mut successors: FN,
    mut stop: FS,
) -> (IndexMap<N, (usize, C)>, Option<usize>)
where
    N: Eq + Hash + Clone,
    C: Zero + Ord + Copy,
    FN: FnMut(&N) -> IN,
    IN: IntoIterator<Item = (N, C)>,
    FS: FnMut(&N) -> bool,
{
    let mut to_see = BinaryHeap::new();
    to_see.push(SmallestHolder {
        cost: Zero::zero(),
        index: 0,
    });
    let mut parents: IndexMap<N, (usize, C)> = IndexMap::new();
    parents.insert(start.clone(), (usize::max_value(), Zero::zero()));
    let mut target_reached = None;
    while let Some(SmallestHolder { cost, index }) = to_see.pop() {
        let successors = {
            let (node, &(_, c)) = parents.get_index(index).unwrap();
            if stop(node) {
                target_reached = Some(index);
                break;
            }
            // We may have inserted a node several time into the binary heap if we found
            // a better way to access it. Ensure that we are currently dealing with the
            // best path and discard the others.
            if cost > c {
                continue;
            }
            successors(node)
        };
        for (successor, move_cost) in successors {
            let new_cost = cost + move_cost;
            let n;
            match parents.entry(successor) {
                Vacant(e) => {
                    n = e.index();
                    e.insert((index, new_cost));
                }
                Occupied(mut e) => {
                    if e.get().1 > new_cost {
                        n = e.index();
                        e.insert((index, new_cost));
                    } else {
                        continue;
                    }
                }
            }

            to_see.push(SmallestHolder {
                cost: new_cost,
                index: n,
            });
        }
    }
    (parents, target_reached)
}

/// Build a path leading to a target according to a parents map, which must
/// contain no loop. This function can be used after [`dijkstra_all`] or
/// [`dijkstra_partial`] to build a path from a starting point to a reachable target.
///
/// - `target` is reachable target.
/// - `parents` is a map containing an optimal parent (and an associated
///    cost which is ignored here) for every reachable node.
///
/// This function returns a vector with a path from the farthest parent up to
/// `target`, including `target` itself.
///
/// # Panics
///
/// If the `parents` map contains a loop, this function will attempt to build
/// a path of infinite length and panic when memory is exhausted.
#[allow(clippy::implicit_hasher)]
pub fn build_path<N, C>(target: &N, parents: &HashMap<N, (N, C)>) -> Vec<N>
where
    N: Eq + Hash + Clone,
{
    let mut rev = vec![target.clone()];
    let mut next = target.clone();
    while let Some((parent, _)) = parents.get(&next) {
        rev.push(parent.clone());
        next = parent.clone();
    }
    rev.reverse();
    rev
}

struct SmallestHolder<K> {
    cost: K,
    index: usize,
}

impl<K: PartialEq> PartialEq for SmallestHolder<K> {
    fn eq(&self, other: &Self) -> bool {
        self.cost == other.cost
    }
}

impl<K: PartialEq> Eq for SmallestHolder<K> {}

impl<K: Ord> PartialOrd for SmallestHolder<K> {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        Some(self.cmp(other))
    }
}

impl<K: Ord> Ord for SmallestHolder<K> {
    fn cmp(&self, other: &Self) -> Ordering {
        other.cost.cmp(&self.cost)
    }
}