organic/src/types/remove_trailing.rs

pub(crate) trait RemoveTrailing: Iterator + Sized {
    fn remove_trailing<R: Into<usize>>(self, amount_to_remove: R) -> RemoveTrailingIter<Self>;
}

impl<I> RemoveTrailing for I
where
    I: Iterator,
{
    fn remove_trailing<R: Into<usize>>(self, amount_to_remove: R) -> RemoveTrailingIter<Self> {
        RemoveTrailingIter {
            inner: self,
            buffer: Vec::new(),
            next_to_pop: 0,
            amount_to_remove: amount_to_remove.into(),
        }
    }
}

pub(crate) struct RemoveTrailingIter<I: Iterator> {
    inner: I,
    buffer: Vec<I::Item>,
    next_to_pop: usize,
    amount_to_remove: usize,
}

impl<I: Iterator> Iterator for RemoveTrailingIter<I> {
    type Item = I::Item;

    fn next(&mut self) -> Option<Self::Item> {
        if self.buffer.len() < self.amount_to_remove {
            self.buffer.reserve_exact(self.amount_to_remove);
        }
        while self.buffer.len() < self.amount_to_remove {
            if let Some(elem) = self.inner.next() {
                self.buffer.push(elem);
            } else {
                // The inner was smaller than amount_to_remove, so never return anything.
                return None;
            }
        }

        let new_value = self.inner.next();
        if self.amount_to_remove == 0 {
            return new_value;
        }

        if let Some(new_value) = new_value {
            let ret = std::mem::replace(&mut self.buffer[self.next_to_pop], new_value);
            self.next_to_pop = (self.next_to_pop + 1) % self.amount_to_remove;
            Some(ret)
        } else {
            // We have exactly the amount in the buffer than we wanted to cut off, so stop returning values.
            None
        }
    }
}