queue: add fixed size circular buffer

This commit introduces a fixed size circular buffer  which stores
elements in a fixed size underlying array, wrapping to overwrite items
when the buffer gets full.

queue/circular_buf_test.go

@@ -0,0 +1,198 @@
+package queue
+
+import (
+	"reflect"
+	"testing"
+)
+
+// TestNewCircularBuffer tests the size parameter check when creating a circular
+// buffer.
+func TestNewCircularBuffer(t *testing.T) {
+	tests := []struct {
+		name          string
+		size          int
+		expectedError error
+	}{
+		{
+			name:          "zero size",
+			size:          0,
+			expectedError: errInvalidSize,
+		},
+		{
+			name:          "negative size",
+			size:          -1,
+			expectedError: errInvalidSize,
+		},
+		{
+			name:          "ok size",
+			size:          1,
+			expectedError: nil,
+		},
+	}
+
+	for _, test := range tests {
+		test := test
+
+		t.Run(test.name, func(t *testing.T) {
+			_, err := NewCircularBuffer(test.size)
+			if err != test.expectedError {
+				t.Fatalf("expected: %v, got: %v",
+					test.expectedError, err)
+			}
+		})
+	}
+}
+
+// TestCircularBuffer tests the adding and listing of items in a circular
+// buffer.
+func TestCircularBuffer(t *testing.T) {
+	tests := []struct {
+		name          string
+		size          int
+		itemCount     int
+		expectedItems []interface{}
+	}{
+		{
+			name:          "no elements",
+			size:          5,
+			itemCount:     0,
+			expectedItems: nil,
+		},
+		{
+			name:      "single element",
+			size:      5,
+			itemCount: 1,
+			expectedItems: []interface{}{
+				0,
+			},
+		},
+		{
+			name:      "no wrap, not full",
+			size:      5,
+			itemCount: 4,
+			expectedItems: []interface{}{
+				0, 1, 2, 3,
+			},
+		},
+		{
+			name:      "no wrap, exactly full",
+			size:      5,
+			itemCount: 5,
+			expectedItems: []interface{}{
+				0, 1, 2, 3, 4,
+			},
+		},
+		{
+			// The underlying array should contain {5, 1, 2, 3, 4}.
+			name:      "wrap, one over",
+			size:      5,
+			itemCount: 6,
+			expectedItems: []interface{}{
+				1, 2, 3, 4, 5,
+			},
+		},
+		{
+			// The underlying array should contain {5, 6, 2, 3, 4}.
+			name:      "wrap, two over",
+			size:      5,
+			itemCount: 7,
+			expectedItems: []interface{}{
+				2, 3, 4, 5, 6,
+			},
+		},
+	}
+
+	for _, test := range tests {
+		test := test
+
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+
+			buffer, err := NewCircularBuffer(test.size)
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+
+			for i := 0; i < test.itemCount; i++ {
+				buffer.Add(i)
+			}
+
+			// List the items in the buffer and check that the list
+			// is as expected.
+			list := buffer.List()
+			if !reflect.DeepEqual(test.expectedItems, list) {
+				t.Fatalf("expected %v, got: %v",
+					test.expectedItems, list)
+			}
+		})
+	}
+}
+
+// TestLatest tests fetching of the last item added to a circular buffer.
+func TestLatest(t *testing.T) {
+	tests := []struct {
+		name string
+		size int
+
+		// items is the number of items to add to the buffer.
+		items int
+
+		// expectedItem is the value we expect from Latest().
+		expectedItem interface{}
+	}{
+		{
+			name:         "no items",
+			size:         3,
+			items:        0,
+			expectedItem: nil,
+		},
+		{
+			name:         "one item",
+			size:         3,
+			items:        1,
+			expectedItem: 0,
+		},
+		{
+			name:         "exactly full",
+			size:         3,
+			items:        3,
+			expectedItem: 2,
+		},
+		{
+			name:         "overflow to index 0",
+			size:         3,
+			items:        4,
+			expectedItem: 3,
+		},
+		{
+			name:         "overflow twice to index 0",
+			size:         3,
+			items:        7,
+			expectedItem: 6,
+		},
+	}
+
+	for _, test := range tests {
+		test := test
+
+		t.Run(test.name, func(t *testing.T) {
+			//t.Parallel()
+
+			buffer, err := NewCircularBuffer(test.size)
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+
+			for i := 0; i < test.items; i++ {
+				buffer.Add(i)
+			}
+
+			latest := buffer.Latest()
+
+			if !reflect.DeepEqual(latest, test.expectedItem) {
+				t.Fatalf("expected: %v, got: %v",
+					test.expectedItem, latest)
+			}
+		})
+	}
+}