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/*
* Copyright (c) 2018 Isaac van Bakel
*
* Permission is hereby granted, free of charge, to any person obtaining a copy of
* this software and associated documentation files (the "Software"), to deal in
* the Software without restriction, including without limitation the rights to
* use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
* the Software, and to permit persons to whom the Software is furnished to do so,
* subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in all
* copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
* FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
* COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
* IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/
/// A wrapper around an existing iterator to extend it with backtracking functionality
pub struct BacktrackingIterator<I> where I: Iterator {
iterator: I,
backtracking_vec: Vec<I::Item>,
state: BacktrackingState,
}
use crate::BacktrackingState::*;
enum BacktrackingState {
/// There may be some values in the history, but we're taking values off the iterator
Progressing,
/// We've been asked to backtrack, so we've started taking values from the history instead
/// The `position` field tracks where we are in the history, with 0 being at the start.
///
/// A `BacktrackingIterator` may be in this state with `position` as an invalid index into
/// the history - in this case, the next call to `next()` will restore it to the `Progressing`
/// state and yield a value from the internal `Iterator`.
Backtracking { position: usize },
}
impl<I> BacktrackingIterator<I> where I:Iterator {
/// Create a `BacktrackingIterator` from an existing iterator.
pub fn new(iterator: I) -> Self {
BacktrackingIterator {
iterator,
backtracking_vec: vec![],
state: Progressing,
}
}
/// Give the current position in the history, as a usize.
pub fn history_position(&self) -> usize {
match self.state {
Progressing => self.backtracking_vec.len(),
Backtracking { position } => position,
}
}
/// Start the iterator again from all the elements in the current history
/// The iterator will repeat every element which was emitted since the last
/// call to `forget()`.
///
/// ```
/// extern crate backtracking_iterator;
/// use backtracking_iterator::BacktrackingIterator;
///
/// let v = vec![1_u8, 2_u8];
/// let mut bt = BacktrackingIterator::new(v.into_iter());
/// bt.next();
/// bt.backtrack();
/// assert!(bt.next().unwrap() == 1_u8);
/// ```
pub fn backtrack(&mut self) {
self.state = Backtracking { position: 0_usize };
}
/// Clear the current history.
/// ```
/// extern crate backtracking_iterator;
/// use backtracking_iterator::BacktrackingIterator;
///
/// let v = vec![1_u8, 2_u8];
/// let mut bt = BacktrackingIterator::new(v.into_iter());
/// bt.next();
///
/// //Before we call this, 1_u8 is in the history
/// bt.forget();
///
/// bt.backtrack();
/// assert!(bt.next().unwrap() == 2_u8);
/// ```
pub fn forget(&mut self) {
self.backtracking_vec.clear();
}
/// Produce an iterator which goes back over the current history in reverse,
/// and yields references to items in the history.
/// ```
/// extern crate backtracking_iterator;
/// use backtracking_iterator::BacktrackingIterator;
///
/// let v = vec![1_u8, 2_u8];
/// let mut bt = BacktrackingIterator::new(v.into_iter());
/// bt.next();
///
/// let mut wb = bt.walk_back();
///
/// assert!(wb.next().unwrap() == &1_u8);
/// ```
pub fn walk_back(&self) -> Walkback<I> {
Walkback::new(self)
}
/// Restart this iterator, backtracking from the given position in the backwalk.
/// Has no expected behaviour if you don't do the sensible thing i.e. get this `usize`
/// from a `Walkback`.
/// ```
/// extern crate backtracking_iterator;
/// use backtracking_iterator::BacktrackingIterator;
///
/// let v = vec![1_u8, 2_u8, 3_u8];
/// let mut bt = BacktrackingIterator::new(v.into_iter());
/// bt.next(); // 1_u8
/// bt.next(); // 2_u8
/// let wb_pos = {
/// let mut wb = bt.walk_back();
/// assert!(wb.next().unwrap() == &2_u8);
/// wb.history_position()
/// };
///
/// bt.go_from(wb_pos);
/// assert!(bt.next().unwrap() == 2_u8);
pub fn go_from(&mut self, start_from: usize) {
self.state = Backtracking { position: start_from };
}
}
/// In order to be able to backtrack, the iterator values must be `Clone`able
/// The reason for this is simple - the value will both be owned by the caller,
/// and stored in the history to be repeated later.
impl<I> Iterator for BacktrackingIterator<I> where I: Iterator, I::Item: Clone {
type Item = I::Item;
fn next(&mut self) -> Option<Self::Item> {
use crate::{Backtracking, Progressing};
match self.state {
Progressing => {
if let Some(val) = self.iterator.next() {
self.backtracking_vec.push(val.clone());
Some(val)
} else {
None
}
},
Backtracking { position } => {
if position >= self.backtracking_vec.len() {
self.state = Progressing;
self.next()
} else {
let backtracked_value = self.backtracking_vec[position].clone();
let new_position = position + 1;
self.state = Backtracking { position: new_position };
Some(backtracked_value)
}
},
}
}
}
/// A backwalk through a `BacktrackingIterator`'s history. Yields references
/// to items in the history, and can be used to walk back to a desired point.
/// The current position is before the most-recently-yielded element. To restart
/// a `BacktrackingIterator` at the current position of the backwalk, use the
/// `history_position()` method.
pub struct Walkback<'history, I> where I: Iterator {
backtracker: &'history BacktrackingIterator<I>,
reverse_position: usize,
}
impl<'history, I> Walkback<'history, I> where I: Iterator {
fn new(backtracker: &'history BacktrackingIterator<I>) -> Self {
let history_len = backtracker.backtracking_vec.len();
Walkback {
backtracker: &backtracker,
reverse_position: history_len,
}
}
pub fn history_position(&self) -> usize {
self.reverse_position
}
}
impl<'history, I> Iterator for Walkback<'history, I> where I: Iterator {
type Item = &'history I::Item;
fn next(&mut self) -> Option<Self::Item> {
if self.reverse_position == 0 {
None
} else {
let new_position = self.reverse_position - 1_usize;
let val = &self.backtracker.backtracking_vec[new_position];
self.reverse_position = new_position;
Some(val)
}
}
}
#[cfg(test)]
mod tests {
#[test]
fn basic_test() {
use crate::{BacktrackingIterator};
let num_vec = vec![1_u8, 2, 3, 4, 5, 6];
let vec_iter = num_vec.into_iter();
let mut bt_iter = BacktrackingIterator::new(vec_iter);
assert!(bt_iter.next().unwrap() == 1_u8);
assert!(bt_iter.next().unwrap() == 2_u8);
bt_iter.backtrack();
assert!(bt_iter.next().unwrap() == 1_u8);
assert!(bt_iter.next().unwrap() == 2_u8);
bt_iter.forget();
bt_iter.backtrack();
assert!(bt_iter.next().unwrap() == 3_u8);
assert!(bt_iter.next().unwrap() == 4_u8);
assert!(bt_iter.next().unwrap() == 5_u8);
assert!(bt_iter.next().unwrap() == 6_u8);
assert!(!bt_iter.next().is_some());
bt_iter.backtrack();
assert!(bt_iter.next().unwrap() == 3_u8);
}
#[test]
fn backwalk_test() {
use crate::{BacktrackingIterator};
let num_vec = vec![1_u8, 2, 3, 4, 5, 6];
let vec_iter = num_vec.into_iter();
let mut bt_iter = BacktrackingIterator::new(vec_iter);
for _ in 1..=6 {
bt_iter.next();
}
let mut wb = bt_iter.walk_back();
for i in 1_u8..=6 {
assert!(wb.next().unwrap() == &(7 - i));
}
}
}