pest/stack.rs
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// pest. The Elegant Parser
// Copyright (c) 2018 Dragoș Tiselice
//
// Licensed under the Apache License, Version 2.0
// <LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0> or the MIT
// license <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. All files in the project carrying such notice may not be copied,
// modified, or distributed except according to those terms.
use alloc::vec;
use alloc::vec::Vec;
use core::ops::{Index, Range};
/// Implementation of a `Stack` which maintains popped elements and length of previous states
/// in order to rewind the stack to a previous state.
#[derive(Debug)]
pub struct Stack<T: Clone> {
/// All elements in the stack.
cache: Vec<T>,
/// All elements that are in previous snapshots but may not be in the next state.
/// They will be pushed back to `cache` if the snapshot is restored,
/// otherwise be dropped if the snapshot is cleared.
///
/// Those elements from a sequence of snapshots are stacked in one [`Vec`], and
/// `popped.len() == lengths.iter().map(|(len, remained)| len - remained).sum()`
popped: Vec<T>,
/// Every element corresponds to a snapshot, and each element has two fields:
/// - Length of `cache` when corresponding snapshot is taken (AKA `len`).
/// - Count of elements that come from corresponding snapshot
/// and are still in next snapshot or current state (AKA `remained`).
///
/// And `len` is never less than `remained`.
///
/// On restoring, the `cache` can be divided into two parts:
/// - `0..remained` are untouched since the snapshot is taken.
///
/// There's nothing to do with those elements. Just let them stay where they are.
///
/// - `remained..cache.len()` are pushed after the snapshot is taken.
lengths: Vec<(usize, usize)>,
}
impl<T: Clone> Default for Stack<T> {
fn default() -> Self {
Self::new()
}
}
impl<T: Clone> Stack<T> {
/// Creates a new `Stack`.
pub fn new() -> Self {
Stack {
cache: vec![],
popped: vec![],
lengths: vec![],
}
}
/// Returns `true` if the stack is currently empty.
#[allow(dead_code)]
pub fn is_empty(&self) -> bool {
self.cache.is_empty()
}
/// Returns the top-most `&T` in the `Stack`.
pub fn peek(&self) -> Option<&T> {
self.cache.last()
}
/// Pushes a `T` onto the `Stack`.
pub fn push(&mut self, elem: T) {
self.cache.push(elem);
}
/// Pops the top-most `T` from the `Stack`.
pub fn pop(&mut self) -> Option<T> {
let len = self.cache.len();
let popped = self.cache.pop();
if let Some(popped) = &popped {
if let Some((_, remained_count)) = self.lengths.last_mut() {
// `len >= *unpopped_count`
if len == *remained_count {
*remained_count -= 1;
self.popped.push(popped.clone());
}
}
}
popped
}
/// Returns the size of the stack
pub fn len(&self) -> usize {
self.cache.len()
}
/// Takes a snapshot of the current `Stack`.
pub fn snapshot(&mut self) {
self.lengths.push((self.cache.len(), self.cache.len()))
}
/// The parsing after the last snapshot was successful so clearing it.
pub fn clear_snapshot(&mut self) {
if let Some((len, unpopped)) = self.lengths.pop() {
// Popped elements from previous state are no longer needed.
self.popped.truncate(self.popped.len() - (len - unpopped));
}
}
/// Rewinds the `Stack` to the most recent `snapshot()`. If no `snapshot()` has been taken, this
/// function return the stack to its initial state.
pub fn restore(&mut self) {
match self.lengths.pop() {
Some((len_stack, remained)) => {
if remained < self.cache.len() {
// Remove those elements that are pushed after the snapshot.
self.cache.truncate(remained);
}
if len_stack > remained {
let rewind_count = len_stack - remained;
let new_len = self.popped.len() - rewind_count;
let recovered_elements = self.popped.drain(new_len..);
self.cache.extend(recovered_elements.rev());
debug_assert_eq!(self.popped.len(), new_len);
}
}
None => {
self.cache.clear();
// As `self.popped` and `self.lengths` should already be empty,
// there is no need to clear it.
debug_assert!(self.popped.is_empty());
debug_assert!(self.lengths.is_empty());
}
}
}
}
impl<T: Clone> Index<Range<usize>> for Stack<T> {
type Output = [T];
fn index(&self, range: Range<usize>) -> &[T] {
self.cache.index(range)
}
}
#[cfg(test)]
mod test {
use super::Stack;
#[test]
fn snapshot_with_empty() {
let mut stack = Stack::new();
stack.snapshot();
// []
assert!(stack.is_empty());
// [0]
stack.push(0);
stack.restore();
assert!(stack.is_empty());
}
#[test]
fn snapshot_twice() {
let mut stack = Stack::new();
stack.push(0);
stack.snapshot();
stack.snapshot();
stack.restore();
stack.restore();
assert_eq!(stack[0..stack.len()], [0]);
}
#[test]
fn restore_without_snapshot() {
let mut stack = Stack::new();
stack.push(0);
stack.restore();
assert_eq!(stack[0..stack.len()], [0; 0]);
}
#[test]
fn snapshot_pop_restore() {
let mut stack = Stack::new();
stack.push(0);
stack.snapshot();
stack.pop();
stack.restore();
assert_eq!(stack[0..stack.len()], [0]);
}
#[test]
fn snapshot_pop_push_restore() {
let mut stack = Stack::new();
stack.push(0);
stack.snapshot();
stack.pop();
stack.push(1);
stack.restore();
assert_eq!(stack[0..stack.len()], [0]);
}
#[test]
fn snapshot_push_pop_restore() {
let mut stack = Stack::new();
stack.push(0);
stack.snapshot();
stack.push(1);
stack.push(2);
stack.pop();
stack.restore();
assert_eq!(stack[0..stack.len()], [0]);
}
#[test]
fn snapshot_push_clear() {
let mut stack = Stack::new();
stack.push(0);
stack.snapshot();
stack.push(1);
stack.clear_snapshot();
assert_eq!(stack[0..stack.len()], [0, 1]);
}
#[test]
fn snapshot_pop_clear() {
let mut stack = Stack::new();
stack.push(0);
stack.push(1);
stack.snapshot();
stack.pop();
stack.clear_snapshot();
assert_eq!(stack[0..stack.len()], [0]);
}
#[test]
fn stack_ops() {
let mut stack = Stack::new();
// []
assert!(stack.is_empty());
assert_eq!(stack.peek(), None);
assert_eq!(stack.pop(), None);
// [0]
stack.push(0);
assert!(!stack.is_empty());
assert_eq!(stack.peek(), Some(&0));
// [0, 1]
stack.push(1);
assert!(!stack.is_empty());
assert_eq!(stack.peek(), Some(&1));
// [0]
assert_eq!(stack.pop(), Some(1));
assert!(!stack.is_empty());
assert_eq!(stack.peek(), Some(&0));
// [0, 2]
stack.push(2);
assert!(!stack.is_empty());
assert_eq!(stack.peek(), Some(&2));
// [0, 2, 3]
stack.push(3);
assert!(!stack.is_empty());
assert_eq!(stack.peek(), Some(&3));
// Take a snapshot of the current stack
// [0, 2, 3]
stack.snapshot();
// [0, 2]
assert_eq!(stack.pop(), Some(3));
assert!(!stack.is_empty());
assert_eq!(stack.peek(), Some(&2));
// Take a snapshot of the current stack
// [0, 2]
stack.snapshot();
// [0]
assert_eq!(stack.pop(), Some(2));
assert!(!stack.is_empty());
assert_eq!(stack.peek(), Some(&0));
// []
assert_eq!(stack.pop(), Some(0));
assert!(stack.is_empty());
// Test backtracking
// [0, 2]
stack.restore();
assert_eq!(stack.pop(), Some(2));
assert_eq!(stack.pop(), Some(0));
assert_eq!(stack.pop(), None);
// Test backtracking
// [0, 2, 3]
stack.restore();
assert_eq!(stack.pop(), Some(3));
assert_eq!(stack.pop(), Some(2));
assert_eq!(stack.pop(), Some(0));
assert_eq!(stack.pop(), None);
}
}