103 lines
2.6 KiB
Rust
103 lines
2.6 KiB
Rust
use crate::size_hint;
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use crate::PeekingNext;
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use alloc::collections::VecDeque;
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use std::iter::Fuse;
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/// See [`peek_nth()`] for more information.
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#[derive(Clone, Debug)]
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pub struct PeekNth<I>
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where
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I: Iterator,
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{
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iter: Fuse<I>,
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buf: VecDeque<I::Item>,
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}
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/// A drop-in replacement for [`std::iter::Peekable`] which adds a `peek_nth`
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/// method allowing the user to `peek` at a value several iterations forward
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/// without advancing the base iterator.
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///
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/// This differs from `multipeek` in that subsequent calls to `peek` or
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/// `peek_nth` will always return the same value until `next` is called
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/// (making `reset_peek` unnecessary).
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pub fn peek_nth<I>(iterable: I) -> PeekNth<I::IntoIter>
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where
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I: IntoIterator,
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{
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PeekNth {
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iter: iterable.into_iter().fuse(),
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buf: VecDeque::new(),
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}
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}
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impl<I> PeekNth<I>
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where
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I: Iterator,
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{
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/// Works exactly like the `peek` method in `std::iter::Peekable`
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pub fn peek(&mut self) -> Option<&I::Item> {
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self.peek_nth(0)
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}
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/// Returns a reference to the `nth` value without advancing the iterator.
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///
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/// # Examples
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///
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/// Basic usage:
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///
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/// ```rust
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/// use itertools::peek_nth;
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///
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/// let xs = vec![1,2,3];
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/// let mut iter = peek_nth(xs.iter());
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///
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/// assert_eq!(iter.peek_nth(0), Some(&&1));
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/// assert_eq!(iter.next(), Some(&1));
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///
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/// // The iterator does not advance even if we call `peek_nth` multiple times
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/// assert_eq!(iter.peek_nth(0), Some(&&2));
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/// assert_eq!(iter.peek_nth(1), Some(&&3));
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/// assert_eq!(iter.next(), Some(&2));
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///
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/// // Calling `peek_nth` past the end of the iterator will return `None`
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/// assert_eq!(iter.peek_nth(1), None);
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/// ```
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pub fn peek_nth(&mut self, n: usize) -> Option<&I::Item> {
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let unbuffered_items = (n + 1).saturating_sub(self.buf.len());
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self.buf.extend(self.iter.by_ref().take(unbuffered_items));
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self.buf.get(n)
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}
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}
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impl<I> Iterator for PeekNth<I>
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where
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I: Iterator,
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{
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type Item = I::Item;
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fn next(&mut self) -> Option<Self::Item> {
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self.buf.pop_front().or_else(|| self.iter.next())
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}
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fn size_hint(&self) -> (usize, Option<usize>) {
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size_hint::add_scalar(self.iter.size_hint(), self.buf.len())
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}
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}
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impl<I> ExactSizeIterator for PeekNth<I> where I: ExactSizeIterator {}
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impl<I> PeekingNext for PeekNth<I>
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where
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I: Iterator,
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{
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fn peeking_next<F>(&mut self, accept: F) -> Option<Self::Item>
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where
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F: FnOnce(&Self::Item) -> bool,
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{
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self.peek().filter(|item| accept(item))?;
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self.next()
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}
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}
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