// SPDX-License-Identifier: MPL-2.0

use alloc::vec::Vec;

/// SlotVec is the variant of Vector.
/// It guarantees that the index of one item remains unchanged during adding
/// or deleting other items of the vector.
#[derive(Debug, Clone)]
pub struct SlotVec<T> {
    // The slots to store items.
    slots: Vec<Option<T>>,
    // The number of occupied slots.
    // The i-th slot is occupied if `self.slots[i].is_some()`.
    num_occupied: usize,
}

impl<T> SlotVec<T> {
    /// New an empty vector.
    pub const fn new() -> Self {
        Self {
            slots: Vec::new(),
            num_occupied: 0,
        }
    }

    pub fn with_capacity(capacity: usize) -> Self {
        Self {
            slots: Vec::with_capacity(capacity),
            num_occupied: 0,
        }
    }

    /// Return `true` if the vector contains no items.
    pub fn is_empty(&self) -> bool {
        self.num_occupied == 0
    }

    /// Return the number of items.
    pub fn len(&self) -> usize {
        self.num_occupied
    }

    /// Return the number of slots.
    pub fn slots_len(&self) -> usize {
        self.slots.len()
    }

    /// Get the item at position `idx`.
    ///
    /// Return `None` if `idx` is out of bounds or the item is not exist.
    pub fn get(&self, idx: usize) -> Option<&T> {
        if idx >= self.slots.len() {
            return None;
        }
        self.slots[idx].as_ref()
    }

    /// Put an item into the vector.
    /// It may be put into any existing empty slots or the back of the vector.
    ///
    /// Return the index of the inserted item.
    pub fn put(&mut self, entry: T) -> usize {
        let idx = if self.num_occupied == self.slots.len() {
            self.slots.push(Some(entry));
            self.slots.len() - 1
        } else {
            let idx = self.slots.iter().position(|x| x.is_none()).unwrap();
            self.slots[idx] = Some(entry);
            idx
        };
        self.num_occupied += 1;
        idx
    }

    /// Put and return the item at position `idx`.
    ///
    /// Return `None` if the item is not exist.
    pub fn put_at(&mut self, idx: usize, item: T) -> Option<T> {
        if idx >= self.slots.len() {
            self.slots.resize_with(idx + 1, Default::default);
        }
        let mut sub_item = Some(item);
        core::mem::swap(&mut sub_item, &mut self.slots[idx]);
        if sub_item.is_none() {
            self.num_occupied += 1;
        }
        sub_item
    }

    /// Remove and return the item at position `idx`.
    ///
    /// Return `None` if `idx` is out of bounds or the item has been removed.
    pub fn remove(&mut self, idx: usize) -> Option<T> {
        if idx >= self.slots.len() {
            return None;
        }
        let mut del_item = None;
        core::mem::swap(&mut del_item, &mut self.slots[idx]);
        if del_item.is_some() {
            debug_assert!(self.num_occupied > 0);
            self.num_occupied -= 1;
        }
        del_item
    }

    /// Create an iterator which gives both of the index and the item.
    /// The index may not be continuous.
    pub fn idxes_and_items(&self) -> impl Iterator<Item = (usize, &'_ T)> {
        self.slots
            .iter()
            .enumerate()
            .filter(|(_, x)| x.is_some())
            .map(|(idx, x)| (idx, x.as_ref().unwrap()))
    }

    /// Create an iterator which just gives the item.
    pub fn iter(&self) -> impl Iterator<Item = &'_ T> {
        self.slots.iter().filter_map(|x| x.as_ref())
    }
}

impl Default for SlotVec<()> {
    fn default() -> Self {
        Self::new()
    }
}
