Class Stack

pest/src/stack.rs:17–41

/// 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 {