Use testify, write a couple more tests, and start a major refactor.

internal/mappingrules/interfaces.go

@@ -0,0 +1,28 @@
+package mappingrules
+
+import "github.com/holoplot/go-evdev"
+
+type MappingRule interface {
+	MatchEvent(*evdev.InputDevice, *evdev.InputEvent, *string) *evdev.InputEvent
+	OutputName() string
+}
+
+// RuleTargets represent either a device input to match on, or an output to produce.
+// Some RuleTarget types may work via side effects, such as RuleTargetModeSelect.
+type RuleTarget interface {
+	// NormalizeValue takes the raw input value and possibly modifies it based on the Target settings.
+	// (e.g., inverting the value if Inverted == true)
+	NormalizeValue(int32) int32
+
+	// CreateEvent typically takes the (probably normalized) value and returns an event that can be emitted
+	// on a virtual device.
+	//
+	// For RuleTargetModeSelect, this method modifies the active mode and returns nil.
+	//
+	// TODO: should we normalize inside this function to simplify the interface?
+	CreateEvent(int32, *string) *evdev.InputEvent
+
+	GetCode() evdev.EvCode
+	GetDeviceName() string
+	GetDevice() *evdev.InputDevice
+}