sets: add Add() method

Part of PCI-3790

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [v0.12.2] - UNRELEASED
+
+### Added
+
+- pkg/sets: add Intersect() and Add() methods.
+
 ## [v0.12.1] - 2024-04-03
 
 ### Minor breaking change

sets/sets.go

@@ -60,6 +60,15 @@ func (s *Set[T]) Contains(item T) bool {
 	return found
 }
 
+// Add inserts item into s. Returns true if the item was present.
+func (s *Set[T]) Add(item T) bool {
+	if s.Contains(item) {
+		return true
+	}
+	s.items[item] = struct{}{}
+	return false
+}
+
 // Remove deletes item from s. Returns true if the item was present.
 func (s *Set[T]) Remove(item T) bool {
 	if !s.Contains(item) {

sets/sets_test.go

@@ -271,3 +271,41 @@ func TestRemoveNotFound(t *testing.T) {
 		t.Run(tc.name, func(t *testing.T) { test(t, tc) })
 	}
 }
+
+func TestAdd(t *testing.T) {
+	type testCase struct {
+		name     string
+		items    []int
+		wantList []int
+	}
+
+	test := func(t *testing.T, tc testCase) {
+		s := sets.New[int](5)
+		for _, item := range tc.items {
+			s.Add(item)
+		}
+		qt.Assert(t, qt.DeepEquals(s.OrderedList(), tc.wantList))
+	}
+
+	testCases := []testCase{
+		{
+			name:     "one item",
+			items:    []int{3},
+			wantList: []int{3},
+		},
+		{
+			name:     "multiple items",
+			items:    []int{3, 0, 42},
+			wantList: []int{0, 3, 42},
+		},
+		{
+			name:     "duplicates",
+			items:    []int{10, 5, 5, 10, 1},
+			wantList: []int{1, 5, 10},
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) { test(t, tc) })
+	}
+}