test(util): add test for randomBits panic on error

util_test.go

@@ -0,0 +1,57 @@
+// Copyright 2016 Google Inc.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package uuid
+
+import (
+	"bytes"
+	"errors"
+	"testing"
+)
+
+// mockReader is a mock implementation of io.Reader for testing
+type mockReader struct {
+	data []byte // Data to be read
+	err  error  // Error to return during read
+}
+
+func (m *mockReader) Read(p []byte) (n int, err error) {
+	if m.err != nil {
+		return 0, m.err
+	}
+	n = copy(p, m.data)
+	return n, nil
+}
+
+// TestRandomBits tests the randomBits function
+func TestRandomBits(t *testing.T) {
+	originalRander := rander
+	defer func() { rander = originalRander }()
+
+	t.Run("fills slice with random data", func(t *testing.T) {
+		mockData := []byte{0x01, 0x02, 0x03, 0x04}
+		rander = &mockReader{data: mockData}
+
+		b := make([]byte, len(mockData))
+		randomBits(b)
+
+		if !bytes.Equal(b, mockData) {
+			t.Errorf("expected %v, got %v", mockData, b)
+		}
+	})
+
+	t.Run("panics on read error", func(t *testing.T) {
+		rander = &mockReader{err: errors.New("read error")}
+
+		defer func() {
+			if r := recover(); r == nil {
+				t.Error("expected panic, but did not occur")
+			} else if errMsg, ok := r.(string); !ok || errMsg != "read error" {
+				t.Errorf("expected panic with message 'read error', got %v", r)
+			}
+		}()
+
+		randomBits(make([]byte, 4))
+	})
+}