//! This file implements the suggested solution from the professor,
//! incrementally updating the existing graph, instead of creating new
//! graphs.

use super::*;

/// We keep a list of graphs, one for each round.  Since each node in
/// each graph has exactly one edge out of it, we can simply represent
/// the graph as a list of integers.
type Graph = Vec<usize>;

/// Returns an empty graph, where every node is associated with an
/// index that is out of bounds.
fn default_graph(n: usize) -> Graph {
    std::iter::repeat(n).take(n).collect()
}

fn add_edge(graph: &mut Graph, source: usize, target: usize) {
    let len = graph.len();

    if source >= len {
        panic!("source = {source} out of bound: {len}");
    }

    if target >= len {
        panic!("target = {target} out of bound: {len}");
    }

    *graph.get_mut(source).unwrap() = target;
    *graph.get_mut(target).unwrap() = source;
}

// NOTE: This seems to be unnecessary now.
/// Returns the associated number.
///
/// To be more precise, if `node` is compared with a number `n` at
/// round `round`, then return `Some(round)`, otherwise return `None`.
///
/// If either `round` or `node` is out of bounds, this function
/// panics.
#[allow(dead_code)]
fn assoc(graphs: &[Graph], round: usize, node: usize) -> Option<usize> {
    if let Some(graph) = graphs.get(round) {
        if let Some(n) = graph.get(node).copied() {
            if n >= graph.len() {
                None
            } else {
                Some(n)
            }
        } else {
            panic!("node = {node} out of bound = {}", graph.len());
        }
    } else {
        panic!("round = {round} out of bound = {}", graphs.len());
    }
}

/// Initial **MIN** algorithm.
fn smi_init<T: PartialOrd>(a: &[T], graphs: &mut Vec<Graph>, count: &mut usize) -> usize {
    if a.is_empty() {
        panic!("invalid empty input");
    }

    let n = a.len();
    let logn_ceil = (n as f32).log2().ceil() as usize;

    let mut indices: Vec<_> = (0..n).collect();

    let mut upper = n;

    graphs.clear();
    graphs
        .try_reserve(logn_ceil)
        .unwrap_or_else(|e| panic!("reserving memory fails: {e}"));

    while upper > 1 {
        let mut graph = default_graph(n);

        let div = upper.div_euclid(2);

        for i in 0..div {
            *count += 1;

            let x = indices.get(2 * i).unwrap();
            let y = indices.get(2 * i + 1).unwrap();

            let ax = a.get(*x).unwrap();
            let ay = a.get(*y).unwrap();

            add_edge(graph.borrow_mut(), *x, *y);

            match ax.partial_cmp(ay) {
                Some(Ordering::Less) | Some(Ordering::Equal) => {
                    *indices.get_mut(i).unwrap() = *x;
                }
                Some(_) => {
                    *indices.get_mut(i).unwrap() = *y;
                }
                None => {
                    *count += 1;

                    if ax.partial_cmp(ax).is_some() {
                        *indices.get_mut(i).unwrap() = *x;
                    } else {
                        *indices.get_mut(i).unwrap() = *y;
                    }
                }
            }
        }

        let offset = upper.rem_euclid(2);

        if offset == 1 {
            *indices.get_mut(div).unwrap() = *indices.get(upper - 1).unwrap();
        }

        upper = div + offset;

        graphs.push(graph);
    }

    assert_eq!(graphs.len(), logn_ceil);

    *indices.first().unwrap()
}

fn smi_step<T: PartialOrd>(
    a: &[T],
    previous_min_index: usize,
    added_indices: &HashSet<usize>,
    graphs: &mut Vec<Graph>,
    count: &mut usize,
) -> usize {
    let n = a.len();

    if previous_min_index >= n {
        panic!("invalid min index: {previous_min_index} with n = {n}");
    }

    if graphs.is_empty() {
        panic!("invalid empty round");
    }

    let mut to_compare: Option<usize> = None;

    for graph in graphs.iter_mut() {
        let assoc = {
            if let Some(assoc) = graph.get(previous_min_index).copied() {
                if assoc >= graph.len() {
                    continue;
                } else {
                    assoc
                }
            } else {
                panic!(
                    "prev = {previous_min_index}, graph length = {}",
                    graph.len()
                );
            }
        };

        if let Some(cand) = to_compare {
            *count += 1;

            let acand = a.get(cand).unwrap();
            let a_assoc = a.get(assoc).unwrap();

            let cand_already_added = added_indices.contains(&cand);
            let assoc_already_added = added_indices.contains(&assoc);

            if cand_already_added && assoc_already_added {
                to_compare = None;
                continue;
            } else if cand_already_added {
                to_compare = Some(assoc);
                continue;
            } else if assoc_already_added {
                to_compare = Some(cand);
                continue;
            }

            add_edge(graph.borrow_mut(), cand, assoc);

            match acand.partial_cmp(a_assoc) {
                Some(Ordering::Less) | Some(Ordering::Equal) => {}
                _ => {
                    to_compare = Some(assoc);
                }
            }
        } else {
            to_compare = Some(assoc);
        }
    }

    if to_compare.is_none() {
        let mut unadded: Vec<_> = Vec::new();

        for i in 0..n {
            if !added_indices.contains(&i) {
                unadded.push(i);
            }
        }

        dbg!(previous_min_index, &unadded);

        return *unadded.first().unwrap();
    }

    to_compare.unwrap()
}

pub fn smi<T: PartialOrd>(a: &[T], count: &mut usize) -> Vec<usize> {
    if a.is_empty() {
        return Vec::new();
    }

    let n = a.len();

    let mut graphs: Vec<Graph> = Vec::with_capacity((n.ilog2() as usize) + 1);

    let mut result = Vec::with_capacity(n);
    let mut added_indices = HashSet::with_capacity(n);

    let min = smi_init(a, &mut graphs, count);

    result.push(min);
    added_indices.insert(min);

    let mut step_min = min;

    for _ in 1..n {
        step_min = smi_step(a, step_min, &added_indices, &mut graphs, count);

        result.push(step_min);
        added_indices.insert(step_min);
    }

    result
}

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

    use rand::{
        distributions::{Distribution, Uniform},
        thread_rng,
    };

    #[test]
    fn test_smi_init_and_one_step() {
        let input = vec![1, 2, 3, 0, 9, 2, 3];

        let mut count = 0;

        let mut graphs = Vec::with_capacity((input.len().ilog2() as usize) + 1);

        let min = smi_init(&input, &mut graphs, &mut count);

        let mut added_indices = HashSet::with_capacity(7);

        added_indices.insert(min);

        assert_eq!(min, 3);

        let second_min = smi_step(&input, min, &added_indices, &mut graphs, &mut count);

        assert_eq!(second_min, 0);
    }

    #[test]
    fn test_smi_itself() {
        for i in 1..=5 {
            let two_i = 1 << i;

            let mut rng = thread_rng();

            let input = {
                let uniform = Uniform::new(-100i32, 100i32);
                uniform
                    .sample_iter(&mut rng)
                    .take(10 * two_i)
                    .collect::<Vec<_>>()
            };

            let n = input.len();

            // println!("input = {input:?}");
            println!("input length = {n}");

            let nlogn = n * ((n as f32).log2().ceil() as usize);

            let mut count = 0;

            let sort_result = smi(&input, &mut count);

            println!("count = {count}, nlog(n) = {nlogn}");
            // println!("sort_result = {sort_result:?}");

            let sort_result_numbers: Vec<_> = sort_result
                .iter()
                .copied()
                .map(|x| *input.get(x).unwrap())
                .collect();

            // println!("sort_result_numbers = {sort_result_numbers:?}");

            let mut answer = input.clone();

            answer.sort_unstable();

            assert_eq!(answer, sort_result_numbers);

            println!("correctly sorted");
            println!();
        }
    }
}