| 1 | // pest. The Elegant Parser | 
| 2 | // Copyright (c) 2018 DragoČ™ Tiselice | 
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| 3 | // | 
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| 4 | // Licensed under the Apache License, Version 2.0 | 
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| 5 | // <LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0> or the MIT | 
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| 6 | // license <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your | 
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| 7 | // option. All files in the project carrying such notice may not be copied, | 
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| 8 | // modified, or distributed except according to those terms. | 
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| 9 |  | 
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| 10 | use alloc::vec; | 
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| 11 | use alloc::vec::Vec; | 
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| 12 | use core::ops::{Index, Range}; | 
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| 13 |  | 
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| 14 | /// Implementation of a `Stack` which maintains popped elements and length of previous states | 
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| 15 | /// in order to rewind the stack to a previous state. | 
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| 16 | #[ derive(Debug)] | 
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| 17 | pub struct Stack<T: Clone> { | 
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| 18 | /// All elements in the stack. | 
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| 19 | cache: Vec<T>, | 
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| 20 | /// All elements that are in previous snapshots but may not be in the next state. | 
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| 21 | /// They will be pushed back to `cache` if the snapshot is restored, | 
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| 22 | /// otherwise be dropped if the snapshot is cleared. | 
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| 23 | /// | 
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| 24 | /// Those elements from a sequence of snapshots are stacked in one [`Vec`], and | 
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| 25 | /// `popped.len() == lengths.iter().map(|(len, remained)| len - remained).sum()` | 
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| 26 | popped: Vec<T>, | 
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| 27 | /// Every element corresponds to a snapshot, and each element has two fields: | 
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| 28 | /// - Length of `cache` when corresponding snapshot is taken (AKA `len`). | 
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| 29 | /// - Count of elements that come from corresponding snapshot | 
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| 30 | ///   and are still in next snapshot or current state (AKA `remained`). | 
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| 31 | /// | 
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| 32 | /// And `len` is never less than `remained`. | 
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| 33 | /// | 
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| 34 | /// On restoring, the `cache` can be divided into two parts: | 
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| 35 | /// - `0..remained` are untouched since the snapshot is taken. | 
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| 36 | /// | 
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| 37 | ///   There's nothing to do with those elements. Just let them stay where they are. | 
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| 38 | /// | 
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| 39 | /// - `remained..cache.len()` are pushed after the snapshot is taken. | 
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| 40 | lengths: Vec<(usize, usize)>, | 
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| 41 | } | 
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| 42 |  | 
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| 43 | impl<T: Clone> Default for Stack<T> { | 
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| 44 | fn default() -> Self { | 
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| 45 | Self::new() | 
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| 46 | } | 
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| 47 | } | 
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| 48 |  | 
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| 49 | impl<T: Clone> Stack<T> { | 
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| 50 | /// Creates a new `Stack`. | 
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| 51 | pub fn new() -> Self { | 
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| 52 | Stack { | 
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| 53 | cache: vec![], | 
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| 54 | popped: vec![], | 
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| 55 | lengths: vec![], | 
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| 56 | } | 
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| 57 | } | 
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| 58 |  | 
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| 59 | /// Returns `true` if the stack is currently empty. | 
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| 60 | #[ allow(dead_code)] | 
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| 61 | pub fn is_empty(&self) -> bool { | 
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| 62 | self.cache.is_empty() | 
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| 63 | } | 
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| 64 |  | 
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| 65 | /// Returns the top-most `&T` in the `Stack`. | 
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| 66 | pub fn peek(&self) -> Option<&T> { | 
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| 67 | self.cache.last() | 
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| 68 | } | 
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| 69 |  | 
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| 70 | /// Pushes a `T` onto the `Stack`. | 
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| 71 | pub fn push(&mut self, elem: T) { | 
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| 72 | self.cache.push(elem); | 
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| 73 | } | 
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| 74 |  | 
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| 75 | /// Pops the top-most `T` from the `Stack`. | 
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| 76 | pub fn pop(&mut self) -> Option<T> { | 
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| 77 | let len = self.cache.len(); | 
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| 78 | let popped = self.cache.pop(); | 
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| 79 | if let Some(popped) = &popped { | 
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| 80 | if let Some((_, remained_count)) = self.lengths.last_mut() { | 
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| 81 | // `len >= *unpopped_count` | 
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| 82 | if len == *remained_count { | 
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| 83 | *remained_count -= 1; | 
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| 84 | self.popped.push(popped.clone()); | 
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| 85 | } | 
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| 86 | } | 
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| 87 | } | 
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| 88 | popped | 
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| 89 | } | 
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| 90 |  | 
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| 91 | /// Returns the size of the stack | 
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| 92 | pub fn len(&self) -> usize { | 
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| 93 | self.cache.len() | 
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| 94 | } | 
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| 95 |  | 
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| 96 | /// Takes a snapshot of the current `Stack`. | 
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| 97 | pub fn snapshot(&mut self) { | 
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| 98 | self.lengths.push((self.cache.len(), self.cache.len())) | 
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| 99 | } | 
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| 100 |  | 
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| 101 | /// The parsing after the last snapshot was successful so clearing it. | 
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| 102 | pub fn clear_snapshot(&mut self) { | 
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| 103 | if let Some((len, unpopped)) = self.lengths.pop() { | 
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| 104 | // Popped elements from previous state are no longer needed. | 
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| 105 | self.popped.truncate(self.popped.len() - (len - unpopped)); | 
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| 106 | } | 
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| 107 | } | 
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| 108 |  | 
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| 109 | /// Rewinds the `Stack` to the most recent `snapshot()`. If no `snapshot()` has been taken, this | 
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| 110 | /// function return the stack to its initial state. | 
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| 111 | pub fn restore(&mut self) { | 
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| 112 | match self.lengths.pop() { | 
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| 113 | Some((len_stack, remained)) => { | 
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| 114 | if remained < self.cache.len() { | 
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| 115 | // Remove those elements that are pushed after the snapshot. | 
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| 116 | self.cache.truncate(remained); | 
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| 117 | } | 
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| 118 | if len_stack > remained { | 
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| 119 | let rewind_count = len_stack - remained; | 
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| 120 | let new_len = self.popped.len() - rewind_count; | 
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| 121 | let recovered_elements = self.popped.drain(new_len..); | 
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| 122 | self.cache.extend(recovered_elements.rev()); | 
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| 123 | debug_assert_eq!(self.popped.len(), new_len); | 
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| 124 | } | 
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| 125 | } | 
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| 126 | None => { | 
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| 127 | self.cache.clear(); | 
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| 128 | // As `self.popped` and `self.lengths` should already be empty, | 
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| 129 | // there is no need to clear it. | 
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| 130 | debug_assert!(self.popped.is_empty()); | 
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| 131 | debug_assert!(self.lengths.is_empty()); | 
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| 132 | } | 
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| 133 | } | 
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| 134 | } | 
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| 135 | } | 
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| 136 |  | 
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| 137 | impl<T: Clone> Index<Range<usize>> for Stack<T> { | 
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| 138 | type Output = [T]; | 
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| 139 |  | 
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| 140 | fn index(&self, range: Range<usize>) -> &[T] { | 
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| 141 | self.cache.index(range) | 
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| 142 | } | 
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| 143 | } | 
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| 144 |  | 
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| 145 | #[ cfg(test)] | 
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| 146 | mod test { | 
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| 147 | use super::Stack; | 
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| 148 |  | 
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| 149 | #[ test] | 
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| 150 | fn snapshot_with_empty() { | 
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| 151 | let mut stack = Stack::new(); | 
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| 152 |  | 
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| 153 | stack.snapshot(); | 
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| 154 | // [] | 
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| 155 | assert!(stack.is_empty()); | 
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| 156 | // [0] | 
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| 157 | stack.push(0); | 
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| 158 | stack.restore(); | 
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| 159 | assert!(stack.is_empty()); | 
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| 160 | } | 
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| 161 |  | 
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| 162 | #[ test] | 
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| 163 | fn snapshot_twice() { | 
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| 164 | let mut stack = Stack::new(); | 
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| 165 |  | 
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| 166 | stack.push(0); | 
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| 167 |  | 
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| 168 | stack.snapshot(); | 
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| 169 | stack.snapshot(); | 
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| 170 | stack.restore(); | 
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| 171 | stack.restore(); | 
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| 172 |  | 
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| 173 | assert_eq!(stack[0..stack.len()], [0]); | 
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| 174 | } | 
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| 175 | #[ test] | 
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| 176 | fn restore_without_snapshot() { | 
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| 177 | let mut stack = Stack::new(); | 
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| 178 |  | 
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| 179 | stack.push(0); | 
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| 180 | stack.restore(); | 
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| 181 |  | 
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| 182 | assert_eq!(stack[0..stack.len()], [0; 0]); | 
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| 183 | } | 
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| 184 |  | 
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| 185 | #[ test] | 
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| 186 | fn snapshot_pop_restore() { | 
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| 187 | let mut stack = Stack::new(); | 
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| 188 |  | 
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| 189 | stack.push(0); | 
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| 190 | stack.snapshot(); | 
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| 191 | stack.pop(); | 
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| 192 | stack.restore(); | 
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| 193 |  | 
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| 194 | assert_eq!(stack[0..stack.len()], [0]); | 
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| 195 | } | 
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| 196 |  | 
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| 197 | #[ test] | 
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| 198 | fn snapshot_pop_push_restore() { | 
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| 199 | let mut stack = Stack::new(); | 
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| 200 |  | 
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| 201 | stack.push(0); | 
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| 202 | stack.snapshot(); | 
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| 203 | stack.pop(); | 
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| 204 | stack.push(1); | 
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| 205 | stack.restore(); | 
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| 206 |  | 
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| 207 | assert_eq!(stack[0..stack.len()], [0]); | 
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| 208 | } | 
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| 209 |  | 
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| 210 | #[ test] | 
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| 211 | fn snapshot_push_pop_restore() { | 
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| 212 | let mut stack = Stack::new(); | 
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| 213 |  | 
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| 214 | stack.push(0); | 
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| 215 | stack.snapshot(); | 
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| 216 | stack.push(1); | 
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| 217 | stack.push(2); | 
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| 218 | stack.pop(); | 
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| 219 | stack.restore(); | 
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| 220 |  | 
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| 221 | assert_eq!(stack[0..stack.len()], [0]); | 
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| 222 | } | 
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| 223 |  | 
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| 224 | #[ test] | 
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| 225 | fn snapshot_push_clear() { | 
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| 226 | let mut stack = Stack::new(); | 
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| 227 |  | 
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| 228 | stack.push(0); | 
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| 229 | stack.snapshot(); | 
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| 230 | stack.push(1); | 
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| 231 | stack.clear_snapshot(); | 
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| 232 |  | 
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| 233 | assert_eq!(stack[0..stack.len()], [0, 1]); | 
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| 234 | } | 
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| 235 |  | 
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| 236 | #[ test] | 
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| 237 | fn snapshot_pop_clear() { | 
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| 238 | let mut stack = Stack::new(); | 
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| 239 |  | 
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| 240 | stack.push(0); | 
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| 241 | stack.push(1); | 
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| 242 | stack.snapshot(); | 
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| 243 | stack.pop(); | 
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| 244 | stack.clear_snapshot(); | 
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| 245 |  | 
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| 246 | assert_eq!(stack[0..stack.len()], [0]); | 
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| 247 | } | 
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| 248 |  | 
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| 249 | #[ test] | 
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| 250 | fn stack_ops() { | 
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| 251 | let mut stack = Stack::new(); | 
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| 252 |  | 
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| 253 | // [] | 
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| 254 | assert!(stack.is_empty()); | 
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| 255 | assert_eq!(stack.peek(), None); | 
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| 256 | assert_eq!(stack.pop(), None); | 
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| 257 |  | 
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| 258 | // [0] | 
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| 259 | stack.push(0); | 
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| 260 | assert!(!stack.is_empty()); | 
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| 261 | assert_eq!(stack.peek(), Some(&0)); | 
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| 262 |  | 
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| 263 | // [0, 1] | 
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| 264 | stack.push(1); | 
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| 265 | assert!(!stack.is_empty()); | 
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| 266 | assert_eq!(stack.peek(), Some(&1)); | 
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| 267 |  | 
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| 268 | // [0] | 
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| 269 | assert_eq!(stack.pop(), Some(1)); | 
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| 270 | assert!(!stack.is_empty()); | 
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| 271 | assert_eq!(stack.peek(), Some(&0)); | 
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| 272 |  | 
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| 273 | // [0, 2] | 
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| 274 | stack.push(2); | 
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| 275 | assert!(!stack.is_empty()); | 
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| 276 | assert_eq!(stack.peek(), Some(&2)); | 
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| 277 |  | 
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| 278 | // [0, 2, 3] | 
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| 279 | stack.push(3); | 
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| 280 | assert!(!stack.is_empty()); | 
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| 281 | assert_eq!(stack.peek(), Some(&3)); | 
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| 282 |  | 
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| 283 | // Take a snapshot of the current stack | 
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| 284 | // [0, 2, 3] | 
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| 285 | stack.snapshot(); | 
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| 286 |  | 
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| 287 | // [0, 2] | 
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| 288 | assert_eq!(stack.pop(), Some(3)); | 
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| 289 | assert!(!stack.is_empty()); | 
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| 290 | assert_eq!(stack.peek(), Some(&2)); | 
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| 291 |  | 
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| 292 | // Take a snapshot of the current stack | 
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| 293 | // [0, 2] | 
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| 294 | stack.snapshot(); | 
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| 295 |  | 
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| 296 | // [0] | 
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| 297 | assert_eq!(stack.pop(), Some(2)); | 
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| 298 | assert!(!stack.is_empty()); | 
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| 299 | assert_eq!(stack.peek(), Some(&0)); | 
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| 300 |  | 
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| 301 | // [] | 
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| 302 | assert_eq!(stack.pop(), Some(0)); | 
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| 303 | assert!(stack.is_empty()); | 
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| 304 |  | 
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| 305 | // Test backtracking | 
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| 306 | // [0, 2] | 
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| 307 | stack.restore(); | 
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| 308 | assert_eq!(stack.pop(), Some(2)); | 
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| 309 | assert_eq!(stack.pop(), Some(0)); | 
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| 310 | assert_eq!(stack.pop(), None); | 
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| 311 |  | 
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| 312 | // Test backtracking | 
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| 313 | // [0, 2, 3] | 
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| 314 | stack.restore(); | 
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| 315 | assert_eq!(stack.pop(), Some(3)); | 
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| 316 | assert_eq!(stack.pop(), Some(2)); | 
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| 317 | assert_eq!(stack.pop(), Some(0)); | 
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| 318 | assert_eq!(stack.pop(), None); | 
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| 319 | } | 
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| 320 | } | 
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| 321 |  | 
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