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author | Pieter Wuille <pieter.wuille@gmail.com> | 2019-11-04 10:54:38 -0800 |
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committer | Pieter Wuille <pieter.wuille@gmail.com> | 2019-11-07 09:12:26 -0800 |
commit | e6e622e5a0e22c2ac1b50b96af818e412d67ac54 (patch) | |
tree | 63f9df6728f3d29a26281ae08d083356048ce771 /src | |
parent | d0e8f4d5d8ddaccb37f98b7989fb944081e41ab8 (diff) | |
download | bitcoin-e6e622e5a0e22c2ac1b50b96af818e412d67ac54.tar.xz |
Implement O(1) OP_IF/NOTIF/ELSE/ENDIF logic
This optimization was first suggested by Sergio Demian Lerner in
https://bitslog.wordpress.com/2017/04/17/new-quadratic-delays-in-bitcoin-scripts/.
The implementation follows the suggested approach there, but with a slightly
simpler representation.
Diffstat (limited to 'src')
-rw-r--r-- | src/script/interpreter.cpp | 52 |
1 files changed, 46 insertions, 6 deletions
diff --git a/src/script/interpreter.cpp b/src/script/interpreter.cpp index 7a34c9c48d..cd6ab0ee1b 100644 --- a/src/script/interpreter.cpp +++ b/src/script/interpreter.cpp @@ -289,16 +289,56 @@ namespace { * expose whether the stack is empty and whether or not any false values are * present at all. To implement OP_ELSE, a toggle_top modifier is added, which * flips the last value without returning it. + * + * This uses an optimized implementation that does not materialize the + * actual stack. Instead, it just stores the size of the would-be stack, + * and the position of the first false value in it. */ class ConditionStack { private: - std::vector<bool> m_flags; + //! A constant for m_first_false_pos to indicate there are no falses. + static constexpr uint32_t NO_FALSE = std::numeric_limits<uint32_t>::max(); + + //! The size of the implied stack. + uint32_t m_stack_size = 0; + //! The position of the first false value on the implied stack, or NO_FALSE if all true. + uint32_t m_first_false_pos = NO_FALSE; + public: - bool empty() { return m_flags.empty(); } - bool all_true() { return !std::count(m_flags.begin(), m_flags.end(), false); } - void push_back(bool f) { m_flags.push_back(f); } - void pop_back() { m_flags.pop_back(); } - void toggle_top() { m_flags.back() = !m_flags.back(); } + bool empty() { return m_stack_size == 0; } + bool all_true() { 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. + } + } }; } |