Report JSON that is not an Ink story instead of aborting

inkcpp_compiler/json_compiler.cpp

@@ -30,8 +30,25 @@ void json_compiler::compile(
     const nlohmann::json& input, emitter* output, compilation_results* results
 )
 {
+	/* Look the top-level keys up rather than reaching for them with operator[].
+	   On a const json, operator[] with a missing key is an assert(), which
+	   aborts the process instead of reporting an error, and any JSON at all can
+	   be handed to a compiler that accepts "a .json file". Every other failure
+	   here is an ink_exception the caller can catch. */
+	if (! input.is_object()) {
+		throw ink_exception("this JSON is not an Ink story: its top level is not an object");
+	}
+	const auto version_itr = input.find("inkVersion");
+	if (version_itr == input.end()) {
+		throw ink_exception("this JSON is not an Ink story: it has no \"inkVersion\"");
+	}
+	const auto root_itr = input.find("root");
+	if (root_itr == input.end()) {
+		throw ink_exception("this JSON is not an Ink story: it has no \"root\"");
+	}
+
 	// Get the runtime version
-	_ink_version = input["inkVersion"];
+	_ink_version = *version_itr;
 
 	// Start the output
 	set_results(results);
@@ -45,7 +62,7 @@ void json_compiler::compile(
 		_emitter->set_list_meta(_list_meta);
 	}
 	// Compile the root container
-	compile_container(input["root"], 0, 0);
+	compile_container(*root_itr, 0, 0);
 
 	// finalize
 	_emitter->finish(_next_container_index);
@@ -133,6 +150,14 @@ void json_compiler::compile_container(
     const std::string& name_override
 )
 {
+	/* Every container is an array whose last element is its metadata object, so
+	   rbegin() and the end() - 1 below are only meaningful for a non-empty
+	   array. Reaching them with anything else dereferences an end iterator.
+	   This guard covers the root and every nested container in one place. */
+	if (! container.is_array() || container.empty()) {
+		throw ink_exception("malformed Ink story: a container must be a non-empty array");
+	}
+
 	// Grab metadata from the last object in this container
 	container_meta meta;
 	bool           is_knot = name_override != "" && index_in_parent == -1;