1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320
// pest. The Elegant Parser
// Copyright (c) 2018 DragoČ™ 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);
}
}