use borsh::{BorshDeserialize, BorshSerialize};
use core::{iter::FusedIterator, ops::Range};

use super::{Vector, ERR_INDEX_OUT_OF_BOUNDS};
use crate::env;

/// An iterator over references to each element in the stored vector.
#[derive(Debug)]
pub struct Iter<'a, T>
where
    T: BorshSerialize + BorshDeserialize,
{
    /// Underlying vector to iterate through
    vec: &'a Vector<T>,
    /// Range of indices to iterate.
    range: Range<u32>,
}

impl<'a, T> Iter<'a, T>
where
    T: BorshSerialize + BorshDeserialize,
{
    pub(super) fn new(vec: &'a Vector<T>) -> Self {
        Self { vec, range: Range { start: 0, end: vec.len() } }
    }

    /// Returns number of elements left to iterate.
    fn remaining(&self) -> usize {
        self.range.len()
    }
}

impl<'a, T> Iterator for Iter<'a, T>
where
    T: BorshSerialize + BorshDeserialize,
{
    type Item = &'a T;

    fn next(&mut self) -> Option<Self::Item> {
        <Self as Iterator>::nth(self, 0)
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        let remaining = self.remaining();
        (remaining, Some(remaining))
    }

    fn count(self) -> usize {
        self.remaining()
    }

    fn nth(&mut self, n: usize) -> Option<Self::Item> {
        let idx = self.range.nth(n)?;
        Some(self.vec.get(idx).unwrap_or_else(|| env::panic_str(ERR_INDEX_OUT_OF_BOUNDS)))
    }
}

impl<'a, T> ExactSizeIterator for Iter<'a, T> where T: BorshSerialize + BorshDeserialize {}
impl<'a, T> FusedIterator for Iter<'a, T> where T: BorshSerialize + BorshDeserialize {}

impl<'a, T> DoubleEndedIterator for Iter<'a, T>
where
    T: BorshSerialize + BorshDeserialize,
{
    fn next_back(&mut self) -> Option<Self::Item> {
        <Self as DoubleEndedIterator>::nth_back(self, 0)
    }

    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        let idx = self.range.nth_back(n)?;
        Some(self.vec.get(idx).unwrap_or_else(|| env::panic_str(ERR_INDEX_OUT_OF_BOUNDS)))
    }
}

/// An iterator over exclusive references to each element of a stored vector.
#[derive(Debug)]
pub struct IterMut<'a, T>
where
    T: BorshSerialize + BorshDeserialize,
{
    /// Mutable reference to vector used to iterate through.
    vec: &'a mut Vector<T>,
    /// Range of indices to iterate.
    range: Range<u32>,
}

impl<'a, T> IterMut<'a, T>
where
    T: BorshSerialize + BorshDeserialize,
{
    /// Creates a new iterator for the given storage vector.
    pub(crate) fn new(vec: &'a mut Vector<T>) -> Self {
        let end = vec.len();
        Self { vec, range: Range { start: 0, end } }
    }

    /// Returns the amount of remaining elements to yield by the iterator.
    fn remaining(&self) -> usize {
        self.range.len()
    }
}

impl<'a, T> IterMut<'a, T>
where
    T: BorshSerialize + BorshDeserialize,
{
    fn get_mut<'b>(&'b mut self, at: u32) -> Option<&'a mut T> {
        self.vec.get_mut(at).map(|value| {
            //* SAFETY: The lifetime can be swapped here because we can assert that the iterator
            //*         will only give out one mutable reference for every individual item
            //*         during the iteration, and there is no overlap. This must be checked
            //*         that no element in this iterator is ever revisited during iteration.
            unsafe { &mut *(value as *mut T) }
        })
    }
}

impl<'a, T> Iterator for IterMut<'a, T>
where
    T: BorshSerialize + BorshDeserialize,
{
    type Item = &'a mut T;

    fn next(&mut self) -> Option<Self::Item> {
        <Self as Iterator>::nth(self, 0)
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        let remaining = self.remaining();
        (remaining, Some(remaining))
    }

    fn count(self) -> usize {
        self.remaining()
    }

    fn nth(&mut self, n: usize) -> Option<Self::Item> {
        let idx = self.range.nth(n)?;
        Some(self.get_mut(idx).unwrap_or_else(|| env::panic_str(ERR_INDEX_OUT_OF_BOUNDS)))
    }
}

impl<'a, T> ExactSizeIterator for IterMut<'a, T> where T: BorshSerialize + BorshDeserialize {}
impl<'a, T> FusedIterator for IterMut<'a, T> where T: BorshSerialize + BorshDeserialize {}

impl<'a, T> DoubleEndedIterator for IterMut<'a, T>
where
    T: BorshSerialize + BorshDeserialize,
{
    fn next_back(&mut self) -> Option<Self::Item> {
        <Self as DoubleEndedIterator>::nth_back(self, 0)
    }

    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        let idx = self.range.nth_back(n)?;
        Some(self.get_mut(idx).unwrap_or_else(|| env::panic_str(ERR_INDEX_OUT_OF_BOUNDS)))
    }
}

/// A draining iterator for [`Vector<T>`].
#[derive(Debug)]
pub struct Drain<'a, T>
where
    T: BorshSerialize + BorshDeserialize,
{
    /// Mutable reference to vector used to iterate through.
    vec: &'a mut Vector<T>,
    /// Range of indices to iterate.
    range: Range<u32>,
    /// Range of elements to delete.
    delete_range: Range<u32>,
}

impl<'a, T> Drain<'a, T>
where
    T: BorshSerialize + BorshDeserialize,
{
    /// Creates a new iterator for the given storage vector.
    pub(crate) fn new(vec: &'a mut Vector<T>, range: Range<u32>) -> Self {
        Self { vec, delete_range: range.clone(), range }
    }

    /// Returns the amount of remaining elements to yield by the iterator.
    fn remaining(&self) -> usize {
        self.range.len()
    }
    fn remove(&mut self, index: u32) -> T {
        self.vec
            .values
            .get_mut_inner(index)
            .replace(None)
            // Element must exist within bounds of vector
            .unwrap_or_else(|| env::abort())
    }
}

impl<'a, T> Drop for Drain<'a, T>
where
    T: BorshSerialize + BorshDeserialize,
{
    fn drop(&mut self) {
        let delete_indices = (self.delete_range.start..self.range.start)
            .chain(self.range.end..self.delete_range.end);

        // Delete any non-deleted elements from iterator (not loading from storage)
        for i in delete_indices {
            self.vec.values.set(i, None);
        }

        // Shift values after delete into slots deleted.
        let shift_len = self.delete_range.len() as u32;
        for i in self.delete_range.end..self.vec.len() {
            self.vec.swap(i, i - shift_len);
        }

        // Adjust length of vector.
        self.vec.len -= self.delete_range.len() as u32;
    }
}

impl<'a, T> Iterator for Drain<'a, T>
where
    T: BorshSerialize + BorshDeserialize,
{
    type Item = T;

    fn next(&mut self) -> Option<Self::Item> {
        // Load and replace value at next index
        let delete_idx = self.range.next()?;
        let prev = self.remove(delete_idx);

        Some(prev)
    }

    fn nth(&mut self, n: usize) -> Option<Self::Item> {
        for _ in 0..n {
            let next = self.range.next()?;
            // Delete all values in advance, values will be shifted over on drop.
            // This avoids having to load and deserialize any elements skipped over.
            self.vec.values.set(next, None);
        }
        self.next()
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        let remaining = self.remaining();
        (remaining, Some(remaining))
    }

    fn count(self) -> usize {
        self.remaining()
    }
}

impl<'a, T> ExactSizeIterator for Drain<'a, T> where T: BorshSerialize + BorshDeserialize {}
impl<'a, T> FusedIterator for Drain<'a, T> where T: BorshSerialize + BorshDeserialize {}

impl<'a, T> DoubleEndedIterator for Drain<'a, T>
where
    T: BorshSerialize + BorshDeserialize,
{
    fn next_back(&mut self) -> Option<Self::Item> {
        let delete_idx = self.range.next_back()?;
        let prev = self.remove(delete_idx);

        Some(prev)
    }

    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        // Only delete and don't load any values before n
        for _ in 0..n {
            let next = self.range.next_back()?;
            // Delete all values in advance, values will be shifted over on drop.
            // This avoids having to load and deserialize any elements skipped over.
            self.vec.values.set(next, None);
        }
        self.next_back()
    }
}