Add a utility function to calculate the next slice to download

content/browser/download/parallel_download_utils_unittest.cc

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "content/browser/download/parallel_download_utils.h"
+
+#include "content/public/browser/download_save_info.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace content {
+
+TEST(ParallelDownloadUtilsTest, FindNextSliceToDownload) {
+  std::vector<DownloadItem::ReceivedSlice> slices;
+  DownloadItem::ReceivedSlice slice = FindNextSliceToDownload(slices);
+  EXPECT_EQ(0, slice.offset);
+  EXPECT_EQ(DownloadSaveInfo::kLengthFullContent, slice.received_bytes);
+
+  slices.emplace_back(0, 500);
+  slice = FindNextSliceToDownload(slices);
+  EXPECT_EQ(500, slice.offset);
+  EXPECT_EQ(DownloadSaveInfo::kLengthFullContent, slice.received_bytes);
+
+  // Create a gap between slices.
+  slices.emplace_back(1000, 500);
+  slice = FindNextSliceToDownload(slices);
+  EXPECT_EQ(500, slice.offset);
+  EXPECT_EQ(500, slice.received_bytes);
+
+  // Fill the gap.
+  slices.emplace(slices.begin() + 1, slice);
+  slice = FindNextSliceToDownload(slices);
+  EXPECT_EQ(1500, slice.offset);
+  EXPECT_EQ(DownloadSaveInfo::kLengthFullContent, slice.received_bytes);
+
+  // Create a new gap at the beginning.
+  slices.erase(slices.begin());
+  slice = FindNextSliceToDownload(slices);
+  EXPECT_EQ(0, slice.offset);
+  EXPECT_EQ(500, slice.received_bytes);
+}
+
+}  // namespace content