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use crate::grind::Grinder;
use std::collections::VecDeque;
pub struct Lookahead<T: Grinder> {
inner: T,
buffer: VecDeque<T::Item>,
}
impl<T> Lookahead<T>
where
T: Grinder,
{
pub fn new(inner: T) -> Lookahead<T> {
Lookahead {
inner: inner,
buffer: VecDeque::new(),
}
}
pub fn lookahead(&mut self, offset: usize) -> &T::Item {
for _ in self.buffer.len()..offset + 1 {
self.buffer.push_back(self.inner.next());
}
&self.buffer[offset]
}
pub fn undo(&mut self, item: T::Item) {
self.buffer.push_front(item);
}
}
impl<T> Grinder for Lookahead<T>
where
T: Grinder,
{
type Item = T::Item;
type Error = T::Error;
fn emit(&mut self, err: Self::Error) {
self.inner.emit(err)
}
fn next(&mut self) -> Self::Item {
match self.buffer.pop_front() {
Some(v) => v,
None => self.inner.next(),
}
}
}
impl<T> From<T> for Lookahead<T>
where
T: Grinder,
{
fn from(inner: T) -> Lookahead<T> {
Lookahead::new(inner)
}
}