Short-circuiting "and" and "or" operators

doc/script/operators.txt

@@ -56,6 +56,11 @@ In a table:
 | @true@	@false@		@true@		@false@		@true@
 | @true@	@true@		@true@		@true@		@false@
 
+The @and@ and @or@ operators use [[http://en.wikipedia.org/wiki/Short-circuit_evaluation|short-circuit evaluation]], which means that the second argument is only evaluated if the first argument does not suffice to determine the value of the expression.
+For example
+> true or card.field_that_does_not_exist
+evaluates to @true@ instead of giving an error.
+
 --Grouping and order--
 
 Operators are ordered as usual, so

src/script/context.cpp

@@ -74,6 +74,25 @@ ScriptValueP Context::eval(const Script& script, bool useScope) {
 					}
 					break;
 				}
+				// Short-circuiting and/or = conditional jump without pop
+				case I_JUMP_SC_AND: {
+					bool condition = *stack.back();
+					if (!condition) {
+						instr = &script.instructions[0] + i.data;
+					} else {
+						stack.pop_back();
+					}
+					break;
+				}
+				case I_JUMP_SC_OR: {
+					bool condition = *stack.back();
+					if (condition) {
+						instr = &script.instructions[0] + i.data;
+					} else {
+						stack.pop_back();
+					}
+					break;
+				}
 				
 				// Get a variable
 				case I_GET_VAR: {

src/script/dependency.cpp

@@ -207,8 +207,10 @@ ScriptValueP Context::dependencies(const Dependency& dep, const Script& script)
 					break;
 				}
 				// Conditional jump
-				case I_JUMP_IF_NOT: {
-					stack.pop_back(); // condition
+				case I_JUMP_IF_NOT: case I_JUMP_SC_AND: case I_JUMP_SC_OR: {
+					if (i.instr == I_JUMP_IF_NOT) {
+						stack.pop_back(); // pop condition
+					}
 					// create jump record
 					Jump* jump = new Jump;
 					jump->target = &script.instructions[i.data];
@@ -217,6 +219,9 @@ ScriptValueP Context::dependencies(const Dependency& dep, const Script& script)
 					getBindings(scope, jump->bindings);
 					jumps.push(jump);
 					// just fall through for the case that the condition holds
+					if (i.instr != I_JUMP_IF_NOT) {
+						stack.pop_back(); // pop condition afterwards, so it is not poped when jump is taken
+					}
 					break;
 				}
 				
@@ -231,9 +236,11 @@ ScriptValueP Context::dependencies(const Dependency& dep, const Script& script)
 					ScriptValueP& it = stack[stack.size() - 2]; // second element of stack
 					ScriptValueP val = it->next();
 					if (val) {
+						// we have not been through the body
 						it = dependency_dummy; // invalidate iterator, so we loop only once
 						stack.push_back(val);
 					} else {
+						// we have been through the body once already
 						stack.erase(stack.end() - 2); // remove iterator
 						instr = &script.instructions[i.data];
 					}

src/script/parser.cpp

@@ -663,15 +663,29 @@ void parseOper(TokenIterator& input, Script& script, Precedence minPrec, Instruc
 			parseOper(input, script, PREC_SET,  I_SET_VAR, instr.data);
 		}
 		else if (minPrec <= PREC_AND    && token==_("orelse"))parseOper(input, script, PREC_ADD,   I_BINARY, I_OR_ELSE);
-		else if (minPrec <= PREC_AND    && token==_("and"))   parseOper(input, script, PREC_CMP,   I_BINARY, I_AND);
+		else if (minPrec <= PREC_AND    && token==_("and")) {
+			// short-circuiting and:
+			//   "XXX and YYY"
+			// becomes
+			//   XXX
+			//   I_JUMP_SC_AND after     # if top==false then goto after else pop
+			//   YYY
+			//   after:
+			unsigned jmpSC = script.addInstruction(I_JUMP_SC_AND);
+			parseOper(input, script, PREC_CMP);
+			script.comeFrom(jmpSC);
+		}
 		else if (minPrec <= PREC_AND    && token==_("or" )) {
 			Token t = input.peek();
 			if (t == _("else")) {// or else
 				input.read(); // skip else
 				// TODO: deprecate "or else" in favor of "orelse"
-				parseOper(input, script, PREC_ADD,   I_BINARY, I_OR_ELSE);
+				parseOper(input, script, PREC_ADD, I_BINARY, I_OR_ELSE);
 			} else {
-				parseOper(input, script, PREC_CMP,   I_BINARY, I_OR);
+				// short-circuiting or
+				unsigned jmpSC = script.addInstruction(I_JUMP_SC_OR);
+				parseOper(input, script, PREC_CMP);
+				script.comeFrom(jmpSC);
 			}
 		}
 		else if (minPrec <= PREC_AND    && token==_("xor"))   parseOper(input, script, PREC_CMP,   I_BINARY, I_XOR);

src/script/script.cpp

@@ -101,6 +101,8 @@ static const unsigned int INVALID_ADDRESS = 0x03FFFFFF;
 unsigned int Script::addInstruction(InstructionType t) {
 	assert( t == I_JUMP
 	     || t == I_JUMP_IF_NOT
+	     || t == I_JUMP_SC_AND
+	     || t == I_JUMP_SC_OR
 	     || t == I_LOOP
 	     || t == I_LOOP_WITH_KEY
 	     || t == I_POP);
@@ -134,6 +136,8 @@ void Script::addInstruction(InstructionType t, const String& s) {
 void Script::comeFrom(unsigned int pos) {
 	assert( instructions.at(pos).instr == I_JUMP
 	     || instructions.at(pos).instr == I_JUMP_IF_NOT
+	     || instructions.at(pos).instr == I_JUMP_SC_AND
+	     || instructions.at(pos).instr == I_JUMP_SC_OR
 	     || instructions.at(pos).instr == I_LOOP
 	     || instructions.at(pos).instr == I_LOOP_WITH_KEY);
 	assert( instructions.at(pos).data == INVALID_ADDRESS );
@@ -166,6 +170,8 @@ String Script::dumpInstr(unsigned int pos, Instruction i) const {
 		case I_PUSH_CONST:	ret += _("push");		break;
 		case I_JUMP:		ret += _("jump");		break;
 		case I_JUMP_IF_NOT:	ret += _("jnz");		break;
+		case I_JUMP_SC_AND:	ret += _("jump sc and");break;
+		case I_JUMP_SC_OR:	ret += _("jump sc or");	break;
 		case I_GET_VAR:		ret += _("get");		break;
 		case I_SET_VAR:		ret += _("set");		break;
 		case I_MEMBER_C:	ret += _("member_c");	break;
@@ -221,7 +227,10 @@ String Script::dumpInstr(unsigned int pos, Instruction i) const {
 		case I_PUSH_CONST: case I_MEMBER_C:							// const
 			ret += _("\t") + constants[i.data]->typeName();
 			break;
-		case I_JUMP: case I_JUMP_IF_NOT: case I_LOOP: case I_LOOP_WITH_KEY: case I_MAKE_OBJECT: case I_CALL: case I_CLOSURE: case I_DUP:	// int
+		case I_JUMP: case I_JUMP_IF_NOT: case I_JUMP_SC_AND: case I_JUMP_SC_OR:
+		case I_LOOP: case I_LOOP_WITH_KEY:
+		case I_MAKE_OBJECT:
+		case I_CALL: case I_CLOSURE: case I_DUP:	// int
 			ret += String::Format(_("\t%d"), i.data);
 			break;
 		case I_GET_VAR: case I_SET_VAR: case I_NOP:					// variable
@@ -310,6 +319,9 @@ const Instruction* Script::backtraceSkip(const Instruction* instr, int to_skip)
 			}
 			case I_JUMP_IF_NOT: case I_LOOP: case I_LOOP_WITH_KEY:
 				return nullptr; // give up
+			case I_JUMP_SC_AND: case I_JUMP_SC_OR:
+				// assume the fallthrough case, in which case we compared and poped the top of the stack
+				to_skip += 1; break;
 			default:
 				break; // nett stack effect 0
 		}

src/script/script.hpp

@@ -26,6 +26,8 @@ enum InstructionType
 ,	I_PUSH_CONST	= 1  ///< arg = const val  : push a constant onto the stack
 ,	I_JUMP			= 2  ///< arg = address    : move the instruction pointer to the given position
 ,	I_JUMP_IF_NOT	= 3  ///< arg = address    : move the instruction pointer if the top of the stack is false
+,	I_JUMP_SC_AND	= 19 ///< arg = address    : (short-circuiting and) jump and don't pop if the top of the stack is false
+,	I_JUMP_SC_OR	= 20 ///< arg = address    : (short-circuiting or)  jump and don't pop if the top of the stack is true
 	// Variables
 ,	I_GET_VAR		= 4  ///< arg = var        : find a variable, push its value onto the stack, it is an error if the variable is not found
 ,	I_SET_VAR		= 5  ///< arg = var        : assign the top value from the stack to a variable (doesn't pop)