//! A variant of `SortedMap` that preserves insertion order.

use std::hash::{Hash, Hasher};

use crate::stable_hasher::{HashStable, StableHasher};
use rustc_index::vec::{Idx, IndexVec};

/// An indexed multi-map that preserves insertion order while permitting both *O*(log *n*) lookup of
/// an item by key and *O*(1) lookup by index.
///
/// This data structure is a hybrid of an [`IndexVec`] and a [`SortedMap`]. Like `IndexVec`,
/// `SortedIndexMultiMap` assigns a typed index to each item while preserving insertion order.
/// Like `SortedMap`, `SortedIndexMultiMap` has efficient lookup of items by key. However, this
/// is accomplished by sorting an array of item indices instead of the items themselves.
///
/// Unlike `SortedMap`, this data structure can hold multiple equivalent items at once, so the
/// `get_by_key` method and its variants return an iterator instead of an `Option`. Equivalent
/// items will be yielded in insertion order.
///
/// Unlike a general-purpose map like `BTreeSet` or `HashSet`, `SortedMap` and
/// `SortedIndexMultiMap` require *O*(*n*) time to insert a single item. This is because we may need
/// to insert into the middle of the sorted array. Users should avoid mutating this data structure
/// in-place.
///
/// [`SortedMap`]: super::SortedMap
#[derive(Clone, Debug)]
pub struct SortedIndexMultiMap<I: Idx, K, V> {
    /// The elements of the map in insertion order.
    items: IndexVec<I, (K, V)>,

    /// Indices of the items in the set, sorted by the item's key.
    idx_sorted_by_item_key: Vec<I>,
}

impl<I: Idx, K: Ord, V> SortedIndexMultiMap<I, K, V> {
    #[inline]
    pub fn new() -> Self {
        SortedIndexMultiMap { items: IndexVec::new(), idx_sorted_by_item_key: Vec::new() }
    }

    #[inline]
    pub fn len(&self) -> usize {
        self.items.len()
    }

    #[inline]
    pub fn is_empty(&self) -> bool {
        self.items.is_empty()
    }

    /// Returns an iterator over the items in the map in insertion order.
    #[inline]
    pub fn into_iter(self) -> impl DoubleEndedIterator<Item = (K, V)> {
        self.items.into_iter()
    }

    /// Returns an iterator over the items in the map in insertion order along with their indices.
    #[inline]
    pub fn into_iter_enumerated(self) -> impl DoubleEndedIterator<Item = (I, (K, V))> {
        self.items.into_iter_enumerated()
    }

    /// Returns an iterator over the items in the map in insertion order.
    #[inline]
    pub fn iter(&self) -> impl '_ + DoubleEndedIterator<Item = (&K, &V)> {
        self.items.iter().map(|(k, v)| (k, v))
    }

    /// Returns an iterator over the items in the map in insertion order along with their indices.
    #[inline]
    pub fn iter_enumerated(&self) -> impl '_ + DoubleEndedIterator<Item = (I, (&K, &V))> {
        self.items.iter_enumerated().map(|(i, (k, v))| (i, (k, v)))
    }

    /// Returns the item in the map with the given index.
    #[inline]
    pub fn get(&self, idx: I) -> Option<&(K, V)> {
        self.items.get(idx)
    }

    /// Returns an iterator over the items in the map that are equal to `key`.
    ///
    /// If there are multiple items that are equivalent to `key`, they will be yielded in
    /// insertion order.
    #[inline]
    pub fn get_by_key(&self, key: K) -> impl Iterator<Item = &V> + '_ {
        self.get_by_key_enumerated(key).map(|(_, v)| v)
    }

    /// Returns an iterator over the items in the map that are equal to `key` along with their
    /// indices.
    ///
    /// If there are multiple items that are equivalent to `key`, they will be yielded in
    /// insertion order.
    #[inline]
    pub fn get_by_key_enumerated(&self, key: K) -> impl Iterator<Item = (I, &V)> + '_ {
        let lower_bound = self.idx_sorted_by_item_key.partition_point(|&i| self.items[i].0 < key);
        self.idx_sorted_by_item_key[lower_bound..].iter().map_while(move |&i| {
            let (k, v) = &self.items[i];
            (k == &key).then_some((i, v))
        })
    }
}

impl<I: Idx, K: Eq, V: Eq> Eq for SortedIndexMultiMap<I, K, V> {}
impl<I: Idx, K: PartialEq, V: PartialEq> PartialEq for SortedIndexMultiMap<I, K, V> {
    fn eq(&self, other: &Self) -> bool {
        // No need to compare the sorted index. If the items are the same, the index will be too.
        self.items == other.items
    }
}

impl<I: Idx, K, V> Hash for SortedIndexMultiMap<I, K, V>
where
    K: Hash,
    V: Hash,
{
    fn hash<H: Hasher>(&self, hasher: &mut H) {
        self.items.hash(hasher)
    }
}

impl<I: Idx, K, V, C> HashStable<C> for SortedIndexMultiMap<I, K, V>
where
    K: HashStable<C>,
    V: HashStable<C>,
{
    fn hash_stable(&self, ctx: &mut C, hasher: &mut StableHasher) {
        let SortedIndexMultiMap {
            items,
            // We can ignore this field because it is not observable from the outside.
            idx_sorted_by_item_key: _,
        } = self;

        items.hash_stable(ctx, hasher)
    }
}

impl<I: Idx, K: Ord, V> FromIterator<(K, V)> for SortedIndexMultiMap<I, K, V> {
    fn from_iter<J>(iter: J) -> Self
    where
        J: IntoIterator<Item = (K, V)>,
    {
        let items = IndexVec::from_iter(iter);
        let mut idx_sorted_by_item_key: Vec<_> = items.indices().collect();

        // `sort_by_key` is stable, so insertion order is preserved for duplicate items.
        idx_sorted_by_item_key.sort_by_key(|&idx| &items[idx].0);

        SortedIndexMultiMap { items, idx_sorted_by_item_key }
    }
}

impl<I: Idx, K, V> std::ops::Index<I> for SortedIndexMultiMap<I, K, V> {
    type Output = V;

    fn index(&self, idx: I) -> &Self::Output {
        &self.items[idx].1
    }
}