Apply further optimisation to Sets.Merge()

Avoid memory allocation if the merged Sets will be the same as the input.

report/sets.go

@@ -105,7 +105,29 @@ func (m Sets) Merge(n Sets) Sets {
 	}
 
 	i, j := 0, 0
+loop:
+	for i < len(m) {
+		switch {
+		case j >= len(n):
+			return m
+		case m[i].key == n[j].key:
+			if !m[i].Value.ContainsSet(n[j].Value) {
+				break loop
+			}
+			i++
+			j++
+		case m[i].key < n[j].key:
+			i++
+		default:
+			break loop
+		}
+	}
+	if i >= len(m) && j >= len(n) {
+		return m
+	}
+
 	out := make([]stringSetEntry, i, len(m))
+	copy(out, m[:i])
 
 	for i < len(m) {
 		switch {

report/string_set.go

@@ -36,6 +36,22 @@ func (s StringSet) Contains(str string) bool {
 	return i < len(s) && s[i] == str
 }
 
+// ContainsSet returns true if the string set includes all strings in the other set
+func (s StringSet) ContainsSet(b StringSet) bool {
+	i, j := 0, 0
+	for i < len(s) && j < len(b) {
+		if s[i] == b[j] {
+			i++
+			j++
+		} else if s[i] < b[j] {
+			i++
+		} else {
+			return false
+		}
+	}
+	return j == len(b)
+}
+
 // Intersection returns the intersections of a and b
 func (s StringSet) Intersection(b StringSet) StringSet {
 	result, i, j := emptyStringSet, 0, 0