Don't copy StringSet if merging a subset

Make StringSet.Merge() work like StringLatestMap.Merge()

report/dns.go

@@ -27,10 +27,12 @@ func (r DNSRecords) Merge(other DNSRecords) DNSRecords {
 	cp := r.Copy()
 	for k, v := range other {
 		if v2, ok := cp[k]; ok {
-			cp[k] = DNSRecord{
-				Forward: v.Forward.Merge(v2.Forward),
-				Reverse: v.Reverse.Merge(v2.Reverse),
+			fMerged, fUnchanged := v.Forward.Merge(v2.Forward)
+			rMerged, rUnchanged := v.Reverse.Merge(v2.Reverse)
+			if fUnchanged && rUnchanged {
+				continue
 			}
+			cp[k] = DNSRecord{Forward: fMerged, Reverse: rMerged}
 		} else {
 			cp[k] = v
 		}

report/id_list.go

@@ -23,7 +23,8 @@ func (a IDList) Add(ids ...string) IDList {
 
 // Merge all elements from a and b into a new list
 func (a IDList) Merge(b IDList) IDList {
-	return IDList(StringSet(a).Merge(StringSet(b)))
+	merged, _ := StringSet(a).Merge(StringSet(b))
+	return IDList(merged)
 }
 
 // Contains returns true if id is in the list.

report/report.go

@@ -531,13 +531,12 @@ func (r Report) upgradeDNSRecords() Report {
 		if ok && (foundS || foundR) {
 			// Add address and names to report-level map
 			if existing, found := dns[addr]; found {
-				// Optimise the expected case that they are equal
-				if existing.Forward.Equal(snoopedNames) && existing.Reverse.Equal(reverseNames) {
+				var sUnchanged, rUnchanged bool
+				snoopedNames, sUnchanged = snoopedNames.Merge(existing.Forward)
+				reverseNames, rUnchanged = reverseNames.Merge(existing.Reverse)
+				if sUnchanged && rUnchanged {
 					continue
 				}
-				// Not equal - merge this node's data into existing data,
-				snoopedNames = snoopedNames.Merge(existing.Forward)
-				reverseNames = reverseNames.Merge(existing.Reverse)
 			}
 			dns[addr] = DNSRecord{Forward: snoopedNames, Reverse: reverseNames}
 		}

report/sets.go

@@ -35,7 +35,11 @@ func (s Sets) Add(key string, value StringSet) Sets {
 		s = emptySets
 	}
 	if existingValue, ok := s.psMap.Lookup(key); ok {
-		value = value.Merge(existingValue.(StringSet))
+		var unchanged bool
+		value, unchanged = existingValue.(StringSet).Merge(value)
+		if unchanged {
+			return s
+		}
 	}
 	return Sets{
 		psMap: s.psMap.Set(key, value),
@@ -94,7 +98,11 @@ func (s Sets) Merge(other Sets) Sets {
 	iter.ForEach(func(key string, value interface{}) {
 		set := value.(StringSet)
 		if existingSet, ok := result.Lookup(key); ok {
-			set = set.Merge(existingSet.(StringSet))
+			var unchanged bool
+			set, unchanged = existingSet.(StringSet).Merge(set)
+			if unchanged {
+				return
+			}
 		}
 		result = result.Set(key, set)
 	})

report/string_set.go

@@ -81,32 +81,54 @@ func (s StringSet) Add(strs ...string) StringSet {
 }
 
 // Merge combines the two StringSets and returns a new result.
-func (s StringSet) Merge(other StringSet) StringSet {
+// Second return value is true if the return value is s
+func (s StringSet) Merge(other StringSet) (StringSet, bool) {
 	switch {
 	case len(other) <= 0: // Optimise special case, to avoid allocating
-		return s // (note unit test DeepEquals breaks if we don't do this)
+		return s, true // (note unit test DeepEquals breaks if we don't do this)
 	case len(s) <= 0:
-		return other
+		return other, false
 	}
-	result := make(StringSet, len(s)+len(other))
-	for i, j, k := 0, 0, 0; ; k++ {
+
+	i, j := 0, 0
+loop:
+	for i < len(s) {
 		switch {
-		case i >= len(s):
-			copy(result[k:], other[j:])
-			return result[:k+len(other)-j]
 		case j >= len(other):
-			copy(result[k:], s[i:])
-			return result[:k+len(s)-i]
-		case s[i] < other[j]:
-			result[k] = s[i]
+			return s, true
+		case s[i] == other[j]:
 			i++
-		case s[i] > other[j]:
-			result[k] = other[j]
 			j++
-		default: // equal
-			result[k] = s[i]
+		case s[i] < other[j]:
 			i++
+		default:
+			break loop
+		}
+	}
+	if i >= len(s) && j >= len(other) {
+		return s, true
+	}
+
+	result := make(StringSet, i, len(s)+len(other))
+	copy(result, s[:i])
+
+	for i < len(s) {
+		switch {
+		case j >= len(other):
+			result = append(result, s[i:]...)
+			return result, false
+		case s[i] == other[j]:
+			result = append(result, s[i])
+			i++
+			j++
+		case s[i] < other[j]:
+			result = append(result, s[i])
+			i++
+		default:
+			result = append(result, other[j])
 			j++
 		}
 	}
+	result = append(result, other[j:]...)
+	return result, false
 }

report/topology_test.go

@@ -61,8 +61,8 @@ func TestStringSetMerge(t *testing.T) {
 		{input: report.MakeStringSet("a", "c"), other: report.MakeStringSet("a", "b"), want: report.MakeStringSet("a", "b", "c")},
 		{input: report.MakeStringSet("b"), other: report.MakeStringSet("a"), want: report.MakeStringSet("a", "b")},
 	} {
-		if want, have := testcase.want, testcase.input.Merge(testcase.other); !reflect.DeepEqual(want, have) {
-			t.Errorf("%v + %v: want %v, have %v", testcase.input, testcase.other, want, have)
+		if have, _ := testcase.input.Merge(testcase.other); !reflect.DeepEqual(testcase.want, have) {
+			t.Errorf("%v + %v: want %v, have %v", testcase.input, testcase.other, testcase.want, have)
 		}
 	}
 }