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use crate::size_hint; | ||
use std::collections::VecDeque; | ||
use std::iter::Fuse; | ||
use crate::PeekingNext; | ||
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/// See [`peek_nth()`](../fn.peek_nth.html) for more information. | ||
#[derive(Clone, Debug)] | ||
pub struct PeekNth<I> | ||
where | ||
I: Iterator, | ||
{ | ||
iter: Fuse<I>, | ||
buf: VecDeque<I::Item>, | ||
} | ||
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/// 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(), | ||
} | ||
} | ||
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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) | ||
} | ||
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/// 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> { | ||
if n < self.buf.len() { | ||
return Some(&self.buf[n]); | ||
} | ||
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for _ in self.buf.len()..=n { | ||
match self.iter.next() { | ||
Some(next_item) => { | ||
self.buf.push_back(next_item); | ||
} | ||
None => return None, | ||
} | ||
} | ||
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Some(&self.buf[n]) | ||
} | ||
} | ||
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impl<I> Iterator for PeekNth<I> | ||
where | ||
I: Iterator, | ||
{ | ||
type Item = I::Item; | ||
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fn next(&mut self) -> Option<I::Item> { | ||
if self.buf.is_empty() { | ||
self.iter.next() | ||
} else { | ||
self.buf.pop_front() | ||
} | ||
} | ||
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fn size_hint(&self) -> (usize, Option<usize>) { | ||
size_hint::add_scalar(self.iter.size_hint(), self.buf.len()) | ||
} | ||
} | ||
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impl<I> ExactSizeIterator for PeekNth<I> where I: ExactSizeIterator {} | ||
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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, | ||
{ | ||
if self.buf.is_empty() { | ||
if let Some(r) = self.peek() { | ||
if !accept(r) { | ||
return None; | ||
} | ||
} | ||
} else if let Some(r) = self.buf.get(0) { | ||
if !accept(r) { | ||
return None; | ||
} | ||
} | ||
self.next() | ||
} | ||
} |
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