conditionstack.h

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// Copyright (c) 2024 The Bitcoin developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
#ifndef BITCOIN_SCRIPT_CONDITIONSTACK_H
#define BITCOIN_SCRIPT_CONDITIONSTACK_H
 
#include <cassert>
#include <cstdint>
#include <limits>
 
class ConditionStack {
private:
    static constexpr uint32_t NO_FALSE = std::numeric_limits<uint32_t>::max();
 
    uint32_t m_stack_size = 0;
    uint32_t m_first_false_pos = NO_FALSE;
 
public:
    bool empty() const { return m_stack_size == 0; }
    bool all_true() const { return m_first_false_pos == NO_FALSE; }
    void push_back(bool f) {
        if (m_first_false_pos == NO_FALSE && !f) {
            // The stack consists of all true values, and a false is added.
            // The first false value will appear at the current size.
            m_first_false_pos = m_stack_size;
        }
        ++m_stack_size;
    }
    void pop_back() {
        assert(m_stack_size > 0);
        --m_stack_size;
        if (m_first_false_pos == m_stack_size) {
            // When popping off the first false value, everything becomes true.
            m_first_false_pos = NO_FALSE;
        }
    }
    void toggle_top() {
        assert(m_stack_size > 0);
        if (m_first_false_pos == NO_FALSE) {
            // The current stack is all true values; the first false will be the
            // top.
            m_first_false_pos = m_stack_size - 1;
        } else if (m_first_false_pos == m_stack_size - 1) {
            // The top is the first false value; toggling it will make
            // everything true.
            m_first_false_pos = NO_FALSE;
        } else {
            // There is a false value, but not on top. No action is needed as
            // toggling anything but the first false value is unobservable.
        }
    }
};
 
#endif // BITCOIN_SCRIPT_CONDITIONSTACK_H