use std::{fmt, iter::FromIterator, ops::AddAssign, sync::Arc};

use arrayvec::ArrayVec;
pub use cursor::Cursor;
pub use cursor::FilterCursor;

mod cursor;

#[cfg(feature = "test-util")]
const TREE_BASE: usize = 2;
#[cfg(not(feature = "test-util"))]
const TREE_BASE: usize = 6;

pub trait Item: Clone + fmt::Debug {
    type Summary: for<'a> AddAssign<&'a Self::Summary> + Default + Clone + fmt::Debug;

    fn summary(&self) -> Self::Summary;
}

pub trait KeyedItem: Item {
    type Key: for<'a> Dimension<'a, Self::Summary> + Ord;

    fn key(&self) -> Self::Key;
}

pub trait Dimension<'a, Summary: Default>: 'a + Clone + fmt::Debug + Default {
    fn add_summary(&mut self, summary: &'a Summary);
}

impl<'a, T: Default> Dimension<'a, T> for () {
    fn add_summary(&mut self, _: &'a T) {}
}

#[derive(Copy, Clone, Eq, PartialEq)]
pub enum SeekBias {
    Left,
    Right,
}

#[derive(Debug, Clone)]
pub struct SumTree<T: Item>(Arc<Node<T>>);

impl<T: Item> Default for SumTree<T> {
    fn default() -> Self {
        Self::new()
    }
}

impl<T: Item> SumTree<T> {
    pub fn new() -> Self {
        SumTree(Arc::new(Node::Leaf {
            summary: T::Summary::default(),
            items: ArrayVec::new(),
            item_summaries: ArrayVec::new(),
        }))
    }

    pub fn from_item(item: T) -> Self {
        let mut tree = Self::new();
        tree.push(item);
        tree
    }

    #[cfg(feature = "test-util")]
    pub fn items(&self) -> Vec<T> {
        let mut cursor = self.cursor::<(), ()>();
        cursor.descend_to_first_item(self, |_| true);
        cursor.cloned().collect()
    }

    pub fn cursor<'a, S, U>(&'a self) -> Cursor<'a, T, S, U>
    where
        S: Dimension<'a, T::Summary>,
        U: Dimension<'a, T::Summary>,
    {
        Cursor::new(self)
    }

    pub fn filter<'a, F, U>(&'a self, filter_node: F) -> FilterCursor<'a, F, T, U>
    where
        F: Fn(&T::Summary) -> bool,
        U: Dimension<'a, T::Summary>,
    {
        FilterCursor::new(self, filter_node)
    }

    #[allow(dead_code)]
    pub fn first(&self) -> Option<&T> {
        self.leftmost_leaf().0.items().first()
    }

    pub fn last(&self) -> Option<&T> {
        self.rightmost_leaf().0.items().last()
    }

    pub fn extent<'a, D: Dimension<'a, T::Summary>>(&'a self) -> D {
        let mut extent = D::default();
        match self.0.as_ref() {
            Node::Internal { summary, .. } | Node::Leaf { summary, .. } => {
                extent.add_summary(summary)
            }
        }
        extent
    }

    pub fn summary(&self) -> T::Summary {
        match self.0.as_ref() {
            Node::Internal { summary, .. } => summary.clone(),
            Node::Leaf { summary, .. } => summary.clone(),
        }
    }

    pub fn extend<I>(&mut self, iter: I)
    where
        I: IntoIterator<Item = T>,
    {
        let mut leaf: Option<Node<T>> = None;

        for item in iter {
            if leaf.is_some() && leaf.as_ref().unwrap().items().len() == 2 * TREE_BASE {
                self.push_tree(SumTree(Arc::new(leaf.take().unwrap())));
            }

            if leaf.is_none() {
                leaf = Some(Node::Leaf::<T> {
                    summary: T::Summary::default(),
                    items: ArrayVec::new(),
                    item_summaries: ArrayVec::new(),
                });
            }

            if let Some(Node::Leaf {
                summary,
                items,
                item_summaries,
            }) = leaf.as_mut()
            {
                let item_summary = item.summary();
                *summary += &item_summary;
                items.push(item);
                item_summaries.push(item_summary);
            } else {
                unreachable!()
            }
        }

        if leaf.is_some() {
            self.push_tree(SumTree(Arc::new(leaf.take().unwrap())));
        }
    }

    pub fn push(&mut self, item: T) {
        let summary = item.summary();
        self.push_tree(SumTree::from_child_trees(vec![SumTree(Arc::new(
            Node::Leaf {
                summary: summary.clone(),
                items: ArrayVec::from_iter(Some(item)),
                item_summaries: ArrayVec::from_iter(Some(summary)),
            },
        ))]))
    }

    pub fn push_tree(&mut self, other: Self) {
        let other_node = other.0.clone();
        if !other_node.is_leaf() || !other_node.items().is_empty() {
            if self.0.height() < other_node.height() {
                for tree in other_node.child_trees() {
                    self.push_tree(tree.clone());
                }
            } else if let Some(split_tree) = self.push_tree_recursive(other) {
                *self = Self::from_child_trees(vec![self.clone(), split_tree]);
            }
        }
    }

    fn push_tree_recursive(&mut self, other: SumTree<T>) -> Option<SumTree<T>> {
        match Arc::make_mut(&mut self.0) {
            Node::Internal {
                height,
                summary,
                child_summaries,
                child_trees,
                ..
            } => {
                let other_node = other.0.clone();
                *summary += other_node.summary();

                let height_delta = *height - other_node.height();
                let mut summaries_to_append = ArrayVec::<T::Summary, { 2 * TREE_BASE }>::new();
                let mut trees_to_append = ArrayVec::<SumTree<T>, { 2 * TREE_BASE }>::new();
                if height_delta == 0 {
                    summaries_to_append.extend(other_node.child_summaries().iter().cloned());
                    trees_to_append.extend(other_node.child_trees().iter().cloned());
                } else if height_delta == 1 && !other_node.is_underflowing() {
                    summaries_to_append.push(other_node.summary().clone());
                    trees_to_append.push(other)
                } else {
                    let tree_to_append = child_trees.last_mut().unwrap().push_tree_recursive(other);
                    *child_summaries.last_mut().unwrap() =
                        child_trees.last().unwrap().0.summary().clone();

                    if let Some(split_tree) = tree_to_append {
                        summaries_to_append.push(split_tree.0.summary().clone());
                        trees_to_append.push(split_tree);
                    }
                }

                let child_count = child_trees.len() + trees_to_append.len();
                if child_count > 2 * TREE_BASE {
                    let left_summaries: ArrayVec<_, { 2 * TREE_BASE }>;
                    let right_summaries: ArrayVec<_, { 2 * TREE_BASE }>;
                    let left_trees;
                    let right_trees;

                    let midpoint = (child_count + child_count % 2) / 2;
                    {
                        let mut all_summaries = child_summaries
                            .iter()
                            .chain(summaries_to_append.iter())
                            .cloned();
                        left_summaries = all_summaries.by_ref().take(midpoint).collect();
                        right_summaries = all_summaries.collect();
                        let mut all_trees =
                            child_trees.iter().chain(trees_to_append.iter()).cloned();
                        left_trees = all_trees.by_ref().take(midpoint).collect();
                        right_trees = all_trees.collect();
                    }
                    *summary = sum(left_summaries.iter());
                    *child_summaries = left_summaries;
                    *child_trees = left_trees;

                    Some(SumTree(Arc::new(Node::Internal {
                        height: *height,
                        summary: sum(right_summaries.iter()),
                        child_summaries: right_summaries,
                        child_trees: right_trees,
                    })))
                } else {
                    child_summaries.extend(summaries_to_append);
                    child_trees.extend(trees_to_append);
                    None
                }
            }
            Node::Leaf {
                summary,
                items,
                item_summaries,
            } => {
                let other_node = other.0;

                let child_count = items.len() + other_node.items().len();
                if child_count > 2 * TREE_BASE {
                    let left_items;
                    let right_items;
                    let left_summaries;
                    let right_summaries: ArrayVec<T::Summary, { 2 * TREE_BASE }>;

                    let midpoint = (child_count + child_count % 2) / 2;
                    {
                        let mut all_items = items.iter().chain(other_node.items().iter()).cloned();
                        left_items = all_items.by_ref().take(midpoint).collect();
                        right_items = all_items.collect();

                        let mut all_summaries = item_summaries
                            .iter()
                            .chain(other_node.child_summaries())
                            .cloned();
                        left_summaries = all_summaries.by_ref().take(midpoint).collect();
                        right_summaries = all_summaries.collect();
                    }
                    *items = left_items;
                    *item_summaries = left_summaries;
                    *summary = sum(item_summaries.iter());
                    Some(SumTree(Arc::new(Node::Leaf {
                        items: right_items,
                        summary: sum(right_summaries.iter()),
                        item_summaries: right_summaries,
                    })))
                } else {
                    *summary += other_node.summary();
                    items.extend(other_node.items().iter().cloned());
                    item_summaries.extend(other_node.child_summaries().iter().cloned());
                    None
                }
            }
        }
    }

    pub fn is_empty(&self) -> bool {
        match self.0.as_ref() {
            Node::Leaf { items, .. } => items.is_empty(),
            Node::Internal { .. } => false,
        }
    }

    /// Update the last element in the Sumtree with the provided f.
    pub fn update_last(&mut self, f: impl FnOnce(&mut T)) {
        if self.is_empty() {
            log::warn!("Trying to update the last item in an empty sumtree");
            return;
        }
        self.update_last_leaf_recursive(f);
    }

    fn update_last_leaf_recursive(&mut self, f: impl FnOnce(&mut T)) -> T::Summary {
        match Arc::make_mut(&mut self.0) {
            Node::Internal {
                summary,
                child_summaries,
                child_trees,
                ..
            } => {
                let last_summary = child_summaries
                    .last_mut()
                    .expect("Internal node should hold at least one child");
                let last_child = child_trees
                    .last_mut()
                    .expect("Internal node should hold at least one child");
                // If the current node is an internal node, recursively update its last child.
                *last_summary = last_child.update_last_leaf_recursive(f);
                // Get the new summary after the child has been updated.
                *summary = sum(child_summaries.iter());
                summary.clone()
            }
            Node::Leaf {
                summary,
                items,
                item_summaries,
            } => {
                let (item, item_summary) = (
                    items
                        .last_mut()
                        .expect("Leaf node should have at least one item"),
                    item_summaries
                        .last_mut()
                        .expect("Leaf node should have at least one item"),
                );
                (f)(item);
                *item_summary = item.summary();
                *summary = sum(item_summaries.iter());
                summary.clone()
            }
        }
    }

    fn from_child_trees(child_trees: Vec<SumTree<T>>) -> Self {
        let height = child_trees[0].0.height() + 1;
        let mut child_summaries = ArrayVec::new();
        for child in &child_trees {
            child_summaries.push(child.0.summary().clone());
        }
        let summary = sum(child_summaries.iter());
        SumTree(Arc::new(Node::Internal {
            height,
            summary,
            child_summaries,
            child_trees: ArrayVec::from_iter(child_trees),
        }))
    }

    fn leftmost_leaf(&self) -> &Self {
        match *self.0 {
            Node::Leaf { .. } => self,
            Node::Internal {
                ref child_trees, ..
            } => child_trees.first().unwrap().leftmost_leaf(),
        }
    }

    fn rightmost_leaf(&self) -> &Self {
        match *self.0 {
            Node::Leaf { .. } => self,
            Node::Internal {
                ref child_trees, ..
            } => child_trees.last().unwrap().rightmost_leaf(),
        }
    }
}

impl<T: KeyedItem> SumTree<T> {
    #[allow(dead_code)]
    pub fn insert(&mut self, item: T) {
        *self = {
            let mut cursor = self.cursor::<T::Key, ()>();
            let mut new_tree = cursor.slice(&item.key(), SeekBias::Left);
            new_tree.push(item);
            new_tree.push_tree(cursor.suffix());
            new_tree
        };
    }

    pub fn edit(&mut self, edits: &mut [Edit<T>]) {
        if edits.is_empty() {
            return;
        }

        edits.sort_unstable_by_key(|item| item.key());

        *self = {
            let mut cursor = self.cursor::<T::Key, ()>();
            let mut new_tree = SumTree::new();
            let mut buffered_items = Vec::new();

            cursor.seek(&T::Key::default(), SeekBias::Left);
            for edit in edits {
                let new_key = edit.key();
                let mut old_item = cursor.item();

                if old_item
                    .as_ref()
                    .is_some_and(|old_item| old_item.key() < new_key)
                {
                    new_tree.extend(buffered_items.drain(..));
                    let slice = cursor.slice(&new_key, SeekBias::Left);
                    new_tree.push_tree(slice);
                    old_item = cursor.item();
                }
                if old_item.is_some_and(|old_item| old_item.key() == new_key) {
                    cursor.next();
                }
                match edit {
                    Edit::Insert(item) => {
                        buffered_items.push(item.clone());
                    }
                    Edit::Remove(_) => {}
                }
            }

            new_tree.extend(buffered_items);
            new_tree.push_tree(cursor.suffix());
            new_tree
        };
    }
}

#[derive(Clone, Debug)]
pub enum Node<T: Item> {
    Internal {
        height: u8,
        summary: T::Summary,
        child_summaries: ArrayVec<T::Summary, { 2 * TREE_BASE }>,
        child_trees: ArrayVec<SumTree<T>, { 2 * TREE_BASE }>,
    },
    Leaf {
        summary: T::Summary,
        items: ArrayVec<T, { 2 * TREE_BASE }>,
        item_summaries: ArrayVec<T::Summary, { 2 * TREE_BASE }>,
    },
}

impl<T: Item> Node<T> {
    fn is_leaf(&self) -> bool {
        matches!(self, Node::Leaf { .. })
    }

    fn height(&self) -> u8 {
        match self {
            Node::Internal { height, .. } => *height,
            Node::Leaf { .. } => 0,
        }
    }

    fn summary(&self) -> &T::Summary {
        match self {
            Node::Internal { summary, .. } => summary,
            Node::Leaf { summary, .. } => summary,
        }
    }

    fn child_summaries(&self) -> &[T::Summary] {
        match self {
            Node::Internal {
                child_summaries, ..
            } => child_summaries.as_slice(),
            Node::Leaf { item_summaries, .. } => item_summaries.as_slice(),
        }
    }

    fn child_trees(&self) -> &ArrayVec<SumTree<T>, { 2 * TREE_BASE }> {
        match self {
            Node::Internal { child_trees, .. } => child_trees,
            Node::Leaf { .. } => panic!("Leaf nodes have no child trees"),
        }
    }

    fn items(&self) -> &ArrayVec<T, { 2 * TREE_BASE }> {
        match self {
            Node::Leaf { items, .. } => items,
            Node::Internal { .. } => panic!("Internal nodes have no items"),
        }
    }

    fn is_underflowing(&self) -> bool {
        match self {
            Node::Internal { child_trees, .. } => child_trees.len() < TREE_BASE,
            Node::Leaf { items, .. } => items.len() < TREE_BASE,
        }
    }
}

#[derive(Debug)]
#[allow(dead_code)]
pub enum Edit<T: KeyedItem> {
    Insert(T),
    Remove(T),
}

impl<T: KeyedItem> Edit<T> {
    fn key(&self) -> T::Key {
        match self {
            Edit::Insert(item) | Edit::Remove(item) => item.key(),
        }
    }
}

fn sum<'a, T, I>(iter: I) -> T
where
    T: 'a + Default + AddAssign<&'a T>,
    I: Iterator<Item = &'a T>,
{
    let mut sum = T::default();
    for value in iter {
        sum += value;
    }
    sum
}

#[cfg(test)]
#[path = "lib_test.rs"]
mod tests;