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use std::collections::VecDeque; pub struct RestorableIter<T, I> where T: Iterator<Item = I>, I: Clone, { inner: T, buf: VecDeque<Option<I>>, saving: bool, } impl<T, I> RestorableIter<T, I> where T: Iterator<Item = I>, I: Clone, { pub fn new(inner: T) -> Self { RestorableIter { inner, buf: VecDeque::with_capacity(4), saving: false, } } #[inline] pub fn save(&mut self) { self.saving = true; } #[inline] pub fn restore(&mut self) { self.saving = false; } #[inline] pub fn clear(&mut self) { self.buf = VecDeque::with_capacity(4); self.saving = false; } } impl<T, I> Iterator for RestorableIter<T, I> where T: Iterator<Item = I>, I: Clone, { type Item = I; fn next(&mut self) -> Option<Self::Item> { if self.saving { let next_item = self.inner.next(); self.buf.push_back(next_item.clone()); next_item } else { match self.buf.pop_front() { Some(saved_item) => saved_item, None => self.inner.next(), } } } } pub trait Restorable<I>: Iterator where I: Clone, { fn iter_restorable(self) -> RestorableIter<Self, I> where Self: Sized + Iterator<Item = I>, { RestorableIter::new(self) } } impl<I, T: ?Sized> Restorable<I> for T where T: Iterator<Item = I>, I: Clone, { } #[cfg(test)] mod tests { use crate::*; #[test] fn test_restorable_new() { let mut iterator = RestorableIter::new(0..5); assert_eq!(iterator.next(), Some(0)); assert_eq!(iterator.next(), Some(1)); assert_eq!(iterator.next(), Some(2)); iterator.save(); assert_eq!(iterator.next(), Some(3)); assert_eq!(iterator.next(), Some(4)); iterator.restore(); assert_eq!(iterator.next(), Some(3)); assert_eq!(iterator.next(), Some(4)); assert_eq!(iterator.next(), None); } #[test] fn test_iter_restorable() { let mut iterator = (0..5).iter_restorable(); assert_eq!(iterator.next(), Some(0)); assert_eq!(iterator.next(), Some(1)); assert_eq!(iterator.next(), Some(2)); iterator.save(); assert_eq!(iterator.next(), Some(3)); assert_eq!(iterator.next(), Some(4)); iterator.restore(); assert_eq!(iterator.next(), Some(3)); assert_eq!(iterator.next(), Some(4)); assert_eq!(iterator.next(), None); } }