use crate::size_hint;

use crate::PeekingNext;

use alloc::collections::VecDeque;

use std::iter::Fuse;

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

#[derive(Clone, Debug)]

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)

    }

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

    ///

    /// # Examples

    ///

    /// Basic usage:

    ///

    /// ```rust

    /// use itertools::peek_nth;

    ///

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

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

    }

}

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())

    }

}

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()

    }

}