refactor: implement Order and Pattern types: slices returning default values for out of bound indices

order.go

@@ -0,0 +1,21 @@
+package sointu
+
+// Order is the pattern order for a track, in practice just a slice of integers,
+// but provides convenience functions that return -1 values for indices out of
+// bounds of the array, and functions to increase the size of the slice only by
+// necessary amount when a new item is added, filling the unused slots with -1s.
+type Order []int
+
+func (s Order) Get(index int) int {
+	if index < 0 || index >= len(s) {
+		return -1
+	}
+	return s[index]
+}
+
+func (s *Order) Set(index, value int) {
+	for len(*s) <= index {
+		*s = append(*s, -1)
+	}
+	(*s)[index] = value
+}