use crate::size_hint;

use crate::PeekingNext;

use alloc::collections::VecDeque;

use std::iter::Fuse;

/// See [`peek_nth()`] for more information.

#[derive(Clone, Debug)]

#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]

pub struct PeekNth<I>

where

    I: Iterator,

{

    iter: Fuse<I>,

    buf: VecDeque<I::Item>,

}

/// A drop-in replacement for [`std::iter::Peekable`] which adds a `peek_nth`

/// method allowing the user to `peek` at a value several iterations forward

/// without advancing the base iterator.

///

/// This differs from `multipeek` in that subsequent calls to `peek` or

/// `peek_nth` will always return the same value until `next` is called

/// (making `reset_peek` unnecessary).

pub fn peek_nth<I>(iterable: I) -> PeekNth<I::IntoIter>

where

    I: IntoIterator,

{

    PeekNth {

        iter: iterable.into_iter().fuse(),

        buf: VecDeque::new(),

    }

}

impl<I> PeekNth<I>

where

    I: Iterator,

{

    /// Works exactly like the `peek` method in `std::iter::Peekable`

    pub fn peek(&mut self) -> Option<&I::Item> {

        self.peek_nth(0)

    }

    /// Works exactly like the `peek_mut` method in `std::iter::Peekable`

    pub fn peek_mut(&mut self) -> Option<&mut I::Item> {

        self.peek_nth_mut(0)

    }

    /// Returns a reference to the `nth` value without advancing the iterator.

    ///

    /// # Examples

    ///

    /// Basic usage:

    ///

    /// ```

    /// use itertools::peek_nth;

    ///

    /// let xs = vec![1, 2, 3];

    /// let mut iter = peek_nth(xs.into_iter());

    ///

    /// assert_eq!(iter.peek_nth(0), Some(&1));

    /// assert_eq!(iter.next(), Some(1));

    ///

    /// // The iterator does not advance even if we call `peek_nth` multiple times

    /// assert_eq!(iter.peek_nth(0), Some(&2));

    /// assert_eq!(iter.peek_nth(1), Some(&3));

    /// assert_eq!(iter.next(), Some(2));

    ///

    /// // Calling `peek_nth` past the end of the iterator will return `None`

    /// assert_eq!(iter.peek_nth(1), None);

    /// ```

    pub fn peek_nth(&mut self, n: usize) -> Option<&I::Item> {

        let unbuffered_items = (n + 1).saturating_sub(self.buf.len());

        self.buf.extend(self.iter.by_ref().take(unbuffered_items));

        self.buf.get(n)

    }

    /// Returns a mutable reference to the `nth` value without advancing the iterator.

    ///

    /// # Examples

    ///

    /// Basic usage:

    ///

    /// ```

    /// use itertools::peek_nth;

    ///

    /// let xs = vec![1, 2, 3, 4, 5];

    /// let mut iter = peek_nth(xs.into_iter());

    ///

    /// assert_eq!(iter.peek_nth_mut(0), Some(&mut 1));

    /// assert_eq!(iter.next(), Some(1));

    ///

    /// // The iterator does not advance even if we call `peek_nth_mut` multiple times

    /// assert_eq!(iter.peek_nth_mut(0), Some(&mut 2));

    /// assert_eq!(iter.peek_nth_mut(1), Some(&mut 3));

    /// assert_eq!(iter.next(), Some(2));

    ///

    /// // Peek into the iterator and set the value behind the mutable reference.

    /// if let Some(p) = iter.peek_nth_mut(1) {

    ///     assert_eq!(*p, 4);

    ///     *p = 9;

    /// }

    ///

    /// // The value we put in reappears as the iterator continues.

    /// assert_eq!(iter.next(), Some(3));

    /// assert_eq!(iter.next(), Some(9));

    ///

    /// // Calling `peek_nth_mut` past the end of the iterator will return `None`

    /// assert_eq!(iter.peek_nth_mut(1), None);

    /// ```

    pub fn peek_nth_mut(&mut self, n: usize) -> Option<&mut I::Item> {

        let unbuffered_items = (n + 1).saturating_sub(self.buf.len());

        self.buf.extend(self.iter.by_ref().take(unbuffered_items));

        self.buf.get_mut(n)

    }

    /// Works exactly like the `next_if` method in `std::iter::Peekable`

    pub fn next_if(&mut self, func: impl FnOnce(&I::Item) -> bool) -> Option<I::Item> {

        match self.next() {

            Some(item) if func(&item) => Some(item),

            Some(item) => {

                self.buf.push_front(item);

                None

            }

            _ => None,

        }

    }

    /// Works exactly like the `next_if_eq` method in `std::iter::Peekable`

    pub fn next_if_eq<T>(&mut self, expected: &T) -> Option<I::Item>

    where

        T: ?Sized,

        I::Item: PartialEq<T>,

    {

        self.next_if(|next| next == expected)

    }

}

impl<I> Iterator for PeekNth<I>

where

    I: Iterator,

{

    type Item = I::Item;

    fn next(&mut self) -> Option<Self::Item> {

        self.buf.pop_front().or_else(|| self.iter.next())

    }

    fn size_hint(&self) -> (usize, Option<usize>) {

        size_hint::add_scalar(self.iter.size_hint(), self.buf.len())

    }

    fn fold<B, F>(self, mut init: B, mut f: F) -> B

    where

        F: FnMut(B, Self::Item) -> B,

    {

        init = self.buf.into_iter().fold(init, &mut f);

        self.iter.fold(init, f)

    }

}

impl<I> ExactSizeIterator for PeekNth<I> where I: ExactSizeIterator {}

impl<I> PeekingNext for PeekNth<I>

where

    I: Iterator,

{

    fn peeking_next<F>(&mut self, accept: F) -> Option<Self::Item>

    where

        F: FnOnce(&Self::Item) -> bool,

    {

        self.peek().filter(|item| accept(item))?;

        self.next()

    }

}