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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 an log of `StackOp`s in order to rewind the stack
/// to a previous state.
#[derive(Debug)]
pub struct Stack<T: Clone> {
ops: Vec<StackOp<T>>,
cache: Vec<T>,
snapshots: Vec<usize>,
}
impl<T: Clone> Stack<T> {
/// Creates a new `Stack`.
pub fn new() -> Self {
Stack {
ops: vec![],
cache: vec![],
snapshots: 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.ops.push(StackOp::Push(elem.clone()));
self.cache.push(elem);
}
/// Pops the top-most `T` from the `Stack`.
pub fn pop(&mut self) -> Option<T> {
let popped = self.cache.pop();
if let Some(ref val) = popped {
self.ops.push(StackOp::Pop(val.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.snapshots.push(self.ops.len());
}
/// The parsing after the last snapshot was successful so clearing it.
pub fn clear_snapshot(&mut self) {
self.snapshots.pop();
}
/// 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.snapshots.pop() {
Some(ops_index) => {
self.rewind_to(ops_index);
self.ops.truncate(ops_index);
}
None => {
self.cache.clear();
self.ops.clear();
}
}
}
// Rewind the stack to a particular index
fn rewind_to(&mut self, index: usize) {
let ops_to_rewind = &self.ops[index..];
for op in ops_to_rewind.iter().rev() {
match *op {
StackOp::Push(_) => {
self.cache.pop();
}
StackOp::Pop(ref elem) => {
self.cache.push(elem.clone());
}
}
}
}
}
impl<T: Clone> Index<Range<usize>> for Stack<T> {
type Output = [T];
fn index(&self, range: Range<usize>) -> &[T] {
self.cache.index(range)
}
}
#[derive(Debug)]
enum StackOp<T> {
Push(T),
Pop(T),
}
#[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 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);
}
}
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