diff --git a/cmd/joyful/main.go b/cmd/joyful/main.go
index 408fecd..271f922 100644
--- a/cmd/joyful/main.go
+++ b/cmd/joyful/main.go
@@ -2,9 +2,9 @@ package main
 
 import (
 	"fmt"
-	"os"
 	"time"
 
+	"git.annabunches.net/annabunches/joyful/internal/logger"
 	"git.annabunches.net/annabunches/joyful/internal/virtualdevice"
 	"github.com/holoplot/go-evdev"
 )
@@ -38,7 +38,7 @@ func main() {
 			},
 		},
 	)
-	fatalIfError(err, "Failed to create virtual device")
+	logger.FatalIfError(err, "Failed to create virtual device")
 
 	buffer := virtualdevice.NewEventBuffer(vDevice)
 
@@ -49,7 +49,7 @@ func main() {
 	fmt.Printf("Virtual device created as %s.\n", name)
 
 	pDevice, err := evdev.Open("/dev/input/event12")
-	fatalIfError(err, "Couldn't open physical device")
+	logger.FatalIfError(err, "Couldn't open physical device")
 
 	name, err = pDevice.Name()
 	if err != nil {
@@ -63,7 +63,7 @@ func main() {
 		last := combo
 
 		event, err := pDevice.ReadOne()
-		logIfError(err, "Error while reading event")
+		logger.LogIfError(err, "Error while reading event")
 
 		// FIXME: test code
 		for event.Code != evdev.SYN_REPORT {
@@ -103,7 +103,7 @@ func main() {
 			}
 
 			event, err = pDevice.ReadOne()
-			logIfError(err, "Error while reading event")
+			logger.LogIfError(err, "Error while reading event")
 		}
 
 		if combo > last && combo == 3 {
@@ -144,20 +144,3 @@ func jsButtons() []evdev.EvCode {
 
 	return buttons
 }
-
-func logIfError(err error, msg string) {
-	if err == nil {
-		return
-	}
-
-	fmt.Printf("%s: %s\n", msg, err.Error())
-}
-
-func fatalIfError(err error, msg string) {
-	if err == nil {
-		return
-	}
-
-	logIfError(err, msg)
-	os.Exit(1)
-}
diff --git a/internal/logger/logger.go b/internal/logger/logger.go
new file mode 100644
index 0000000..5b16c7b
--- /dev/null
+++ b/internal/logger/logger.go
@@ -0,0 +1,27 @@
+package logger
+
+import (
+	"fmt"
+	"os"
+)
+
+func Log(msg string) {
+	fmt.Println(msg)
+}
+
+func LogIfError(err error, msg string) {
+	if err == nil {
+		return
+	}
+
+	fmt.Printf("%s: %s\n", msg, err.Error())
+}
+
+func FatalIfError(err error, msg string) {
+	if err == nil {
+		return
+	}
+
+	LogIfError(err, msg)
+	os.Exit(1)
+}
diff --git a/internal/rules/keymapping.go b/internal/rules/keymapping.go
new file mode 100644
index 0000000..a69b2fa
--- /dev/null
+++ b/internal/rules/keymapping.go
@@ -0,0 +1,50 @@
+package rules
+
+import (
+	"git.annabunches.net/annabunches/joyful/internal/logger"
+	"github.com/holoplot/go-evdev"
+)
+
+type KeyMappingRule interface {
+	MatchEvent(*evdev.InputDevice, *evdev.InputEvent)
+}
+
+type SimpleMappingRule struct {
+	Input  RuleTarget
+	Output RuleTarget
+}
+
+type ComboMappingRule struct {
+	Input  []RuleTarget
+	Output RuleTarget
+}
+
+func (rule *SimpleMappingRule) MatchEvent(device *evdev.InputDevice, event *evdev.InputEvent) *evdev.InputEvent {
+	if event.Type != evdev.EV_KEY ||
+		device != rule.Input.Device ||
+		event.Code != rule.Input.Code {
+		return nil
+	}
+
+	// how we process inverted rules depends on the event type
+	value := event.Value
+	if rule.Input.Inverted {
+		switch rule.Input.Type {
+		case evdev.EV_KEY:
+			if value == 0 {
+				value = 1
+			} else {
+				value = 0
+			}
+		default:
+			logger.Log("Tried to handle inverted rule for unknown event type. Skipping rule.")
+			return nil
+		}
+	}
+
+	return &evdev.InputEvent{
+		Type:  rule.Output.Type,
+		Code:  rule.Output.Code,
+		Value: value,
+	}
+}
diff --git a/internal/rules/types.go b/internal/rules/types.go
new file mode 100644
index 0000000..a72643e
--- /dev/null
+++ b/internal/rules/types.go
@@ -0,0 +1,10 @@
+package rules
+
+import "github.com/holoplot/go-evdev"
+
+type RuleTarget struct {
+	Device   *evdev.InputDevice
+	Type     evdev.EvType
+	Code     evdev.EvCode
+	Inverted bool
+}