diff --git a/config/impl.go b/config/impl.go
index c7d0516..1d28de8 100644
--- a/config/impl.go
+++ b/config/impl.go
@@ -146,12 +146,40 @@ func (this *config) saveUser () error {
 	return nil
 }
 
-func (this *config) processUserDiff (changed []string) {
-	// TODO
+func (this *config) processUserDiff (changed map[string] struct { }) {
+	for key := range changed {
+		// this is the user file, and nothing has precedence over it, so
+		// the change always matters
+		this.broadcastChange(key)
+	}
 }
 
-func (this *config) processSystemDiff (index int, changed []string) {
-	// TODO
+func (this *config) processSystemDiff (index int, changed map[string] struct { }) {
+	for key := range changed {
+		// if specified in the user file, the change doesn't matter
+		if this.data.user != nil {
+			if has, _ := this.data.user.Has(key); has {
+				continue
+			}
+		}
+		
+		// if specified in any system files with precedence greater than
+		// this one, the change doesn't matter
+		for _, system := range this.data.system[:index] {
+			if _, has := system[key]; has {
+				continue
+			}
+		}
+
+		// the change does matter
+		this.broadcastChange(key)
+	}
+}
+
+func (this *config) broadcastChange (key string) {
+	for _, listener := range this.on.change.Listeners() {
+		listener(key)
+	}
 }
 
 func (this *config) get (key string, fallback Value) (Value, error) {
@@ -246,3 +274,29 @@ func (this *config) OnChange (callback func (string)) event.Cookie {
 	defer this.lock.Unlock()
 	return this.on.change.Connect(callback)
 }
+
+func diffValueMaps (first, second map[string] Value) map[string] struct { } {
+	diff := make(map[string] struct { })
+	
+	// - keys only first has
+	// - keys both have, but are different
+	for key, firstValue := range first {
+		secondValue, ok := second[key]
+		if !ok {
+			diff[key] = struct { } { }
+			continue
+		}
+		if !firstValue.Equals(secondValue) {
+			diff[key] = struct { } { }
+		}
+	}
+
+	// - keys only second has
+	for key := range second {
+		if _, has := first[key]; !has {
+			diff[key] = struct { } { }
+		}
+	}
+
+	return diff
+}
diff --git a/config/impl_test.go b/config/impl_test.go
new file mode 100644
index 0000000..a162fe1
--- /dev/null
+++ b/config/impl_test.go
@@ -0,0 +1,53 @@
+package config
+
+import "maps"
+import "testing"
+
+func TestDiffValueMapsNone (test *testing.T) {
+	str := `
+thing = something
+
+otherThing  = otherValue
+# comment
+otherThing = true
+otherThing = 234
+
+yetAnotherThing = 0.23498
+`
+	file1 := parseFileString(test, str)
+	file2 := parseFileString(test, str)
+	diff := diffValueMaps(file1.Map(), file2.Map())
+	if len(diff) != 0 {
+		test.Fatalf("diff not empty:\n%v", diff)
+	}
+}
+
+func TestDiffValueMaps (test *testing.T) {
+	file1 := parseFileString(test,
+`key4=0
+key1=value1
+# comment
+key2=34`)
+	file2 := parseFileString(test,
+`key1=value2
+key2=34
+# comment
+
+key3=0.2`)
+	diff := diffValueMaps(file1.Map(), file2.Map())
+	correct := map[string] struct { } {
+		"key1": struct { } { },
+		"key3": struct { } { },
+		"key4": struct { } { },
+	}
+	if !maps.Equal(diff, correct) {
+		test.Error("diffs do not match")
+		test.Errorf("EXPECTED:\n%v", correct)
+		test.Errorf("GOT:\n%v", diff)
+		test.Fail()
+	}
+}
+
+// TODO we need way more tests!
+// need to test watching files. maybe make a temp dir and do it there. remember
+// to defer cleaning up the dir and closing of the config.