Add a utility function to calculate the next slice to download

content/browser/download/parallel_download_utils.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"
+
+namespace content {
+
+DownloadItem::ReceivedSlice FindNextSliceToDownload(
+    const std::vector<DownloadItem::ReceivedSlice>& received_slices) {
+  if (received_slices.empty())
+    return DownloadItem::ReceivedSlice(0, DownloadSaveInfo::kLengthFullContent);
+
+  std::vector<DownloadItem::ReceivedSlice>::const_iterator iter =
+      received_slices.begin();
+  DCHECK_GE(iter->offset, 0);
+  if (iter->offset != 0)
+    return DownloadItem::ReceivedSlice(0, iter->offset);
+
+  int64_t offset = 0;
+  int64_t remaining_bytes = DownloadSaveInfo::kLengthFullContent;
+  while (iter != received_slices.end()) {
+    offset = iter->offset + iter->received_bytes;
+    std::vector<DownloadItem::ReceivedSlice>::const_iterator next =
+        std::next(iter);
+    if (next == received_slices.end())
+      break;
+
+    DCHECK_GE(next->offset, offset);
+    if (next->offset > offset) {
+      remaining_bytes = next->offset - offset;
+      break;
+    }
+    iter = next;
+  }
+  return DownloadItem::ReceivedSlice(offset, remaining_bytes);
+}
+
+}  // namespace content