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.
This commit is contained in:
parent
d0e8f4d5d8
commit
e6e622e5a0
1 changed files with 46 additions and 6 deletions
|
@ -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.
|
||||
}
|
||||
}
|
||||
};
|
||||
}
|
||||
|
||||
|
|
Loading…
Reference in a new issue