Add "differentiate" node type

Can be used to transform absolute movement to relative movement

Makefile

@@ -12,7 +12,7 @@ CPPFLAGS += $(shell pkg-config --cflags $(DEPS))
 LDLIBS += $(shell pkg-config --libs $(DEPS))
 INTERP ?=
 MAIN = main
-OBJS = main.o events.o processing.o graph.o config.o event_code_names.o hash_table.o module_registry.o event_predicate.o nodes/getchar.o nodes/print.o nodes/evdev.o nodes/tee.o nodes/router.o nodes/modifiers.o nodes/modify_predicate.o nodes/uinput.o nodes/assign.o
+OBJS = main.o events.o processing.o graph.o config.o event_code_names.o hash_table.o module_registry.o event_predicate.o nodes/getchar.o nodes/print.o nodes/evdev.o nodes/tee.o nodes/router.o nodes/modifiers.o nodes/modify_predicate.o nodes/uinput.o nodes/assign.o nodes/differentiate.o
 
 all: $(MAIN)
 

nodes/differentiate.c

@@ -0,0 +1,84 @@
+#include "../graph.h"
+#include "../module_registry.h"
+
+typedef struct {
+	GraphNode as_GraphNode;
+	int64_t previous;
+} DifferentiateGraphNode;
+
+static bool
+handle_event(EventPositionBase * self, EventNode * event)
+{
+	DifferentiateGraphNode *node = DOWNCAST(DifferentiateGraphNode, GraphNode, DOWNCAST(GraphNode, EventPositionBase, self));
+	size_t count = node->as_GraphNode.outputs.length;
+	if (!count) {
+		event_destroy(event);
+		return true;
+	}
+
+	int64_t current = event->data.payload;
+	event->data.payload = current - node->previous;
+	node->previous = current;
+
+	if (count > 1) {
+		count = event_replicate(event, count - 1) + 1;
+	}
+	for (size_t i = 0; i < count; ++i) {
+		event->position = &node->as_GraphNode.outputs.elements[i]->as_EventPositionBase;
+		event = event->next;
+	}
+	return true;
+}
+
+static GraphNode *
+create(GraphNodeSpecification * spec, GraphNodeConfig * config, InitializationEnvironment * env)
+{
+	(void) config;
+	(void) env;
+	DifferentiateGraphNode * node = T_ALLOC(1, DifferentiateGraphNode);
+	if (!node) {
+		return NULL;
+	}
+
+	int64_t initial = 0;
+	if (config->options) {
+		// FIXME check config->options for NULL in all node types
+		initial = env_resolve_constant(env, config_setting_get_member(config->options, "initial"));
+	}
+
+	*node = (DifferentiateGraphNode) {
+		.as_GraphNode = {
+			.as_EventPositionBase = {
+				.handle_event = &handle_event,
+				.waiting_new_event = false,
+			},
+			.specification = spec,
+			.inputs = EMPTY_GRAPH_CHANNEL_LIST,
+			.outputs = EMPTY_GRAPH_CHANNEL_LIST,
+		},
+		.previous = initial,
+	};
+	return &node->as_GraphNode;
+}
+
+static void destroy
+(GraphNodeSpecification * self, GraphNode * target)
+{
+	(void) self;
+	free(target);
+}
+
+GraphNodeSpecification nodespec_differentiate = (GraphNodeSpecification) {
+	.create = &create,
+	.destroy = &destroy,
+	.register_io = NULL,
+	.name = "differentiate",
+	.documentation = "Subtracts the previous event payload from the current one\nAccepts events on any connector\nSends events on all connectors"
+	                 "\nOption 'initial' (optional): the value to subtract from the first event payload"
+	,
+};
+
+MODULE_CONSTRUCTOR(init)
+{
+	register_graph_node_specification(&nodespec_differentiate);
+}