Start rulemapping refactor to be more explicit about typing intentions.

internal/mappingrules/mapping_rule_button.go

@@ -0,0 +1,23 @@
+package mappingrules
+
+import "github.com/holoplot/go-evdev"
+
+// A Simple Mapping Rule can map a button to a button or an axis to an axis.
+type MappingRuleButton struct {
+	MappingRuleBase
+	Input  *RuleTargetButton
+	Output *RuleTargetButton
+}
+
+func (rule *MappingRuleButton) MatchEvent(device *evdev.InputDevice, event *evdev.InputEvent, mode *string) (*evdev.InputDevice, *evdev.InputEvent) {
+	if !rule.MappingRuleBase.modeCheck(mode) {
+		return nil, nil
+	}
+
+	if device != rule.Input.GetDevice() ||
+		event.Code != rule.Input.GetCode() {
+		return nil, nil
+	}
+
+	return rule.Output.Device, rule.Output.CreateEvent(rule.Input.NormalizeValue(event.Value), mode)
+}