feat: input midi velocity into a separate track (includes many structural changes)

tracker/types/optional.go

@@ -0,0 +1,47 @@
+package types
+
+type (
+	// OptionalInteger is the simple struct, not to be confused with tracker.OptionalInt.
+	// It implements the tracker.optionalIntData interface, without needing to know so.
+	OptionalInteger struct {
+		value  int
+		exists bool
+	}
+)
+
+func NewOptionalInteger(value int, exists bool) OptionalInteger {
+	return OptionalInteger{value, exists}
+}
+
+func NewOptionalIntegerOf(value int) OptionalInteger {
+	return OptionalInteger{
+		value:  value,
+		exists: true,
+	}
+}
+
+func NewEmptyOptionalInteger() OptionalInteger {
+	// could also just use OptionalInteger{}
+	return OptionalInteger{
+		exists: false,
+	}
+}
+
+func (i OptionalInteger) Unpack() (int, bool) {
+	return i.value, i.exists
+}
+
+func (i OptionalInteger) Value() int {
+	if !i.exists {
+		panic("Access value of empty OptionalInteger")
+	}
+	return i.value
+}
+
+func (i OptionalInteger) Empty() bool {
+	return !i.exists
+}
+
+func (i OptionalInteger) Equals(value int) bool {
+	return i.exists && i.value == value
+}