1 files changed, 67 insertions, 3 deletions

diff --git a/src/script/interpreter.cpp b/src/script/interpreter.cpp
index e4e86031e6..5bf418472a 100644
--- a/src/script/interpreter.cpp
+++ b/src/script/interpreter.cpp
@@ -278,6 +278,70 @@ int FindAndDelete(CScript& script, const CScript& b)
     return nFound;
 }
 
+namespace {
+/** A data type to abstract out the condition stack during script execution.
+ *
+ * Conceptually it acts like a vector of booleans, one for each level of nested
+ * IF/THEN/ELSE, indicating whether we're in the active or inactive branch of
+ * each.
+ *
+ * The elements on the stack cannot be observed individually; we only need to
+ * 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:
+    //! 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_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.
+        }
+    }
+};
+}
+
 bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript& script, unsigned int flags, const BaseSignatureChecker& checker, SigVersion sigversion, ScriptError* serror)
 {
     static const CScriptNum bnZero(0);
@@ -293,7 +357,7 @@ bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript&
     CScript::const_iterator pbegincodehash = script.begin();
     opcodetype opcode;
     valtype vchPushValue;
-    std::vector<bool> vfExec;
+    ConditionStack vfExec;
     std::vector<valtype> altstack;
     set_error(serror, SCRIPT_ERR_UNKNOWN_ERROR);
     if (script.size() > MAX_SCRIPT_SIZE)
@@ -305,7 +369,7 @@ bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript&
     {
         while (pc < pend)
         {
-            bool fExec = !count(vfExec.begin(), vfExec.end(), false);
+            bool fExec = vfExec.all_true();
 
             //
             // Read instruction
@@ -494,7 +558,7 @@ bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript&
                 {
                     if (vfExec.empty())
                         return set_error(serror, SCRIPT_ERR_UNBALANCED_CONDITIONAL);
-                    vfExec.back() = !vfExec.back();
+                    vfExec.toggle_top();
                 }
                 break;