[refactor] interpreter: define interface for vfExec

Includes comments added by Pieter Wuille.

src/script/interpreter.cpp

@@ -278,6 +278,30 @@ 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.
+ */
+class ConditionStack {
+private:
+    std::vector<bool> m_flags;
+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 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 +317,7 @@ 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 +329,7 @@ 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 +518,7 @@ 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;