Joel Galenson | 6f79871 | 2021-04-01 17:03:06 -0700 | [diff] [blame] | 1 | use crate::size_hint; |
| 2 | use crate::PeekingNext; |
| 3 | use alloc::collections::VecDeque; |
| 4 | use std::iter::Fuse; |
| 5 | |
Joel Galenson | b593e25 | 2021-06-21 13:15:57 -0700 | [diff] [blame] | 6 | /// See [`peek_nth()`] for more information. |
Joel Galenson | 6f79871 | 2021-04-01 17:03:06 -0700 | [diff] [blame] | 7 | #[derive(Clone, Debug)] |
| 8 | pub struct PeekNth<I> |
| 9 | where |
| 10 | I: Iterator, |
| 11 | { |
| 12 | iter: Fuse<I>, |
| 13 | buf: VecDeque<I::Item>, |
| 14 | } |
| 15 | |
Joel Galenson | b593e25 | 2021-06-21 13:15:57 -0700 | [diff] [blame] | 16 | /// A drop-in replacement for [`std::iter::Peekable`] which adds a `peek_nth` |
Joel Galenson | 6f79871 | 2021-04-01 17:03:06 -0700 | [diff] [blame] | 17 | /// method allowing the user to `peek` at a value several iterations forward |
| 18 | /// without advancing the base iterator. |
| 19 | /// |
| 20 | /// This differs from `multipeek` in that subsequent calls to `peek` or |
| 21 | /// `peek_nth` will always return the same value until `next` is called |
| 22 | /// (making `reset_peek` unnecessary). |
| 23 | pub fn peek_nth<I>(iterable: I) -> PeekNth<I::IntoIter> |
| 24 | where |
| 25 | I: IntoIterator, |
| 26 | { |
| 27 | PeekNth { |
| 28 | iter: iterable.into_iter().fuse(), |
| 29 | buf: VecDeque::new(), |
| 30 | } |
| 31 | } |
| 32 | |
| 33 | impl<I> PeekNth<I> |
| 34 | where |
| 35 | I: Iterator, |
| 36 | { |
| 37 | /// Works exactly like the `peek` method in `std::iter::Peekable` |
| 38 | pub fn peek(&mut self) -> Option<&I::Item> { |
| 39 | self.peek_nth(0) |
| 40 | } |
| 41 | |
| 42 | /// Returns a reference to the `nth` value without advancing the iterator. |
| 43 | /// |
| 44 | /// # Examples |
| 45 | /// |
| 46 | /// Basic usage: |
| 47 | /// |
| 48 | /// ```rust |
| 49 | /// use itertools::peek_nth; |
| 50 | /// |
| 51 | /// let xs = vec![1,2,3]; |
| 52 | /// let mut iter = peek_nth(xs.iter()); |
| 53 | /// |
| 54 | /// assert_eq!(iter.peek_nth(0), Some(&&1)); |
| 55 | /// assert_eq!(iter.next(), Some(&1)); |
| 56 | /// |
| 57 | /// // The iterator does not advance even if we call `peek_nth` multiple times |
| 58 | /// assert_eq!(iter.peek_nth(0), Some(&&2)); |
| 59 | /// assert_eq!(iter.peek_nth(1), Some(&&3)); |
| 60 | /// assert_eq!(iter.next(), Some(&2)); |
| 61 | /// |
| 62 | /// // Calling `peek_nth` past the end of the iterator will return `None` |
| 63 | /// assert_eq!(iter.peek_nth(1), None); |
| 64 | /// ``` |
| 65 | pub fn peek_nth(&mut self, n: usize) -> Option<&I::Item> { |
| 66 | let unbuffered_items = (n + 1).saturating_sub(self.buf.len()); |
| 67 | |
| 68 | self.buf.extend(self.iter.by_ref().take(unbuffered_items)); |
| 69 | |
| 70 | self.buf.get(n) |
| 71 | } |
| 72 | } |
| 73 | |
| 74 | impl<I> Iterator for PeekNth<I> |
| 75 | where |
| 76 | I: Iterator, |
| 77 | { |
| 78 | type Item = I::Item; |
| 79 | |
| 80 | fn next(&mut self) -> Option<Self::Item> { |
| 81 | self.buf.pop_front().or_else(|| self.iter.next()) |
| 82 | } |
| 83 | |
| 84 | fn size_hint(&self) -> (usize, Option<usize>) { |
| 85 | size_hint::add_scalar(self.iter.size_hint(), self.buf.len()) |
| 86 | } |
| 87 | } |
| 88 | |
| 89 | impl<I> ExactSizeIterator for PeekNth<I> where I: ExactSizeIterator {} |
| 90 | |
| 91 | impl<I> PeekingNext for PeekNth<I> |
| 92 | where |
| 93 | I: Iterator, |
| 94 | { |
| 95 | fn peeking_next<F>(&mut self, accept: F) -> Option<Self::Item> |
| 96 | where |
| 97 | F: FnOnce(&Self::Item) -> bool, |
| 98 | { |
| 99 | self.peek().filter(|item| accept(item))?; |
| 100 | self.next() |
| 101 | } |
| 102 | } |