tests: add missing tests for stack implementation

internal/stack/stack_test.go

@@ -1 +1,196 @@
 package stack_test
+
+import (
+	"testing"
+
+	"github.com/StevanFreeborn/advent-of-code-2025/internal/stack"
+)
+
+func TestNew(t *testing.T) {
+	s := stack.New[int]()
+
+	if s == nil {
+		t.Errorf("Expected new stack to be non-nil")
+	}
+
+	if !s.IsEmpty() {
+		t.Errorf("Expected new stack to be empty")
+	}
+
+	if s.Size() != 0 {
+		t.Errorf("Expected new stack size to be 0, got %d", s.Size())
+	}
+}
+
+func TestPush(t *testing.T) {
+	s := stack.New[int]()
+
+	s.Push(1)
+
+	if s.IsEmpty() {
+		t.Errorf("Expected stack to be non-empty after push")
+	}
+
+	if s.Size() != 1 {
+		t.Errorf("Expected stack size to be 1 after one push, got %d", s.Size())
+	}
+}
+
+func TestPop(t *testing.T) {
+	s := stack.New[int]()
+
+	s.Push(1)
+	s.Push(2)
+
+	item, ok := s.Pop()
+
+	if !ok {
+		t.Errorf("Expected Pop to succeed")
+	}
+
+	if item != 2 {
+		t.Errorf("Expected popped item to be 2, got %d", item)
+	}
+
+	if s.Size() != 1 {
+		t.Errorf("Expected stack size to be 1 after one pop, got %d", s.Size())
+	}
+
+	item, ok = s.Pop()
+
+	if !ok {
+		t.Errorf("Expected Pop to succeed")
+	}
+
+	if item != 1 {
+		t.Errorf("Expected popped item to be 1, got %d", item)
+	}
+
+	if !s.IsEmpty() {
+		t.Errorf("Expected stack to be empty after popping all items")
+	}
+
+	item, ok = s.Pop()
+
+	if ok {
+		t.Errorf("Expected Pop to fail on empty stack")
+	}
+
+	var zero int
+
+	if item != zero {
+		t.Errorf("Expected popped item to be zero value, got %d", item)
+	}
+}
+
+func TestPeek(t *testing.T) {
+	s := stack.New[int]()
+
+	s.Push(1)
+	s.Push(2)
+
+	item, ok := s.Peek()
+
+	if !ok {
+		t.Errorf("Expected Peek to succeed")
+	}
+
+	if item != 2 {
+		t.Errorf("Expected peeked item to be 2, got %d", item)
+	}
+
+	if s.Size() != 2 {
+		t.Errorf("Expected stack size to remain 2 after Peek, got %d", s.Size())
+	}
+
+	s.Pop()
+
+	item, ok = s.Peek()
+
+	if !ok {
+		t.Errorf("Expected Peek to succeed")
+	}
+
+	if item != 1 {
+		t.Errorf("Expected peeked item to be 1, got %d", item)
+	}
+
+	s.Pop()
+
+	item, ok = s.Peek()
+
+	if ok {
+		t.Errorf("Expected Peek to fail on empty stack")
+	}
+
+	var zero int
+
+	if item != zero {
+		t.Errorf("Expected peeked item to be zero value, got %d", item)
+	}
+}
+
+func TestIsEmpty(t *testing.T) {
+	s := stack.New[int]()
+
+	if !s.IsEmpty() {
+		t.Errorf("Expected new stack to be empty")
+	}
+
+	s.Push(1)
+
+	if s.IsEmpty() {
+		t.Errorf("Expected stack to be non-empty after push")
+	}
+
+	s.Pop()
+
+	if !s.IsEmpty() {
+		t.Errorf("Expected stack to be empty after popping all items")
+	}
+}
+
+func TestSize(t *testing.T) {
+	s := stack.New[int]()
+
+	if s.Size() != 0 {
+		t.Errorf("Expected new stack size to be 0, got %d", s.Size())
+	}
+
+	s.Push(1)
+
+	if s.Size() != 1 {
+		t.Errorf("Expected stack size to be 1 after one push, got %d", s.Size())
+	}
+}
+
+func TestConcurrentAccess(t *testing.T) {
+	s := stack.New[int]()
+	done := make(chan bool)
+
+	go func() {
+		for i := range 1000 {
+			s.Push(i)
+		}
+		done <- true
+	}()
+
+	<-done
+
+	if s.Size() != 1000 {
+		t.Errorf("Expected stack size to be 1000 after concurrent pushes, got %d", s.Size())
+	}
+
+	go func() {
+		for range 1000 {
+			s.Pop()
+		}
+		done <- true
+	}()
+
+	<-done
+
+	if s.Size() != 0 {
+		t.Errorf("Expected stack size to be 0 after concurrent pops, got %d", s.Size())
+	}
+}