Issue #64: Add additional runtime checks.

* Check for unsupported updates in ClaspFacade::update() and
  ClaspFacade::prepare(). The former requires real multi-shot mode,
  while the latter requires at least multi-solve mode. Throw
  std::logic_error if these requirements are not met.

src/clasp_facade.cpp

@@ -972,6 +972,7 @@ bool ClaspFacade::read() {
 
 void ClaspFacade::prepare(EnumMode enumMode) {
 	POTASSCO_REQUIRE(solve_.get() && !solving());
+	POTASSCO_REQUIRE(!solved() || ctx.solveMode() == SharedContext::solve_multi);
 	EnumOptions& en = config_->solve;
 	if (solved()) {
 		doUpdate(0, false, SIG_DFL);
@@ -1020,7 +1021,8 @@ ClaspFacade::Result ClaspFacade::solve(const LitVec& a, EventHandler* handler) {
 }
 
 ProgramBuilder& ClaspFacade::update(bool updateConfig, void (*sigAct)(int)) {
-	POTASSCO_REQUIRE(config_ && program() && !solving());
+	POTASSCO_REQUIRE(config_ && program() && !solving(), "Program updates not supported!");
+	POTASSCO_REQUIRE(!program()->frozen() || incremental(), "Program updates not supported!");
 	doUpdate(program(), updateConfig, sigAct);
 	return *program();
 }

tests/facade_test.cpp

@@ -203,6 +203,34 @@ TEST_CASE("Facade", "[facade]") {
 		REQUIRE(!asp.frozen());
 	}
 
+	SECTION("testUpdateChecks") {
+		Clasp::Asp::LogicProgram& asp = libclasp.startAsp(config);
+		lpAdd(asp, "a :- not b. b :- not a.");
+
+		SECTION("cannotSolveAgainInSingleSolveMode") {
+			REQUIRE(libclasp.solve().sat());
+			REQUIRE_THROWS_AS(libclasp.prepare(), std::logic_error);
+			REQUIRE_THROWS_AS(libclasp.solve(), std::logic_error);
+		}
+
+		SECTION("maySolveAgainInMultiSolveMode") {
+			libclasp.ctx.setSolveMode(Clasp::SharedContext::solve_multi);
+			REQUIRE(libclasp.solve().sat());
+			REQUIRE_NOTHROW(libclasp.prepare());
+			REQUIRE_FALSE(libclasp.solved());
+			REQUIRE(libclasp.solve().sat());
+		}
+
+		SECTION("cannotUpdateInSingleShotMode") {
+			Clasp::Asp::LogicProgram& asp = libclasp.startAsp(config);
+			libclasp.keepProgram();
+			lpAdd(asp, "a :- not b. b :- not a.");
+			REQUIRE(libclasp.solve().sat());
+			REQUIRE_THROWS_AS(libclasp.update(), std::logic_error);
+			REQUIRE_THROWS_AS(libclasp.prepare(), std::logic_error);
+		}
+	}
+
 	SECTION("testPrepareTooStrongBound") {
 		config.solve.numModels = 0;
 		config.solve.optBound.assign(1, 0);