Three lists comparator

threelist.go

package threelist

// Compare two lists old vs new using fullset
func Compare(fullset, old, new []string) ([]string, []string, []string) {
	// Initialize empty result lists
	keep := []string{}
	removed := []string{}
	added := []string{}

	// For each element of fullset
	for _, el := range fullset {
		// If known by old
		if contains(old, el) {
			if contains(new, el) {
				keep = append(keep, el)
			} else {
				removed = append(removed, el)
			}
		} else {
			// Not known by old
			if contains(new, el) {
				added = append(added, el)
			}
		}
	}

	return keep, removed, added
}

func contains(list []string, item string) bool {
	set := make(map[string]struct{}, len(list))
	for _, s := range list {
		set[s] = struct{}{}
	}

	_, ok := set[item]
	return ok
}