Glue parallel download job and download file together.

content/browser/download/parallel_download_utils.cc

Index: content/browser/download/parallel_download_utils.cc
diff --git a/content/browser/download/parallel_download_utils.cc b/content/browser/download/parallel_download_utils.cc
index d21ea49f20c061e15a062e6fec4758ea0e905705..e49c4a9af65ba893ae85a9e9d362e07d5154295a 100644
--- a/content/browser/download/parallel_download_utils.cc
+++ b/content/browser/download/parallel_download_utils.cc
@@ -30,6 +30,50 @@ const int kParallelRequestCount = 2;
 
 }  // namespace
 
+bool ShouldUseParallelDownload(const DownloadCreateInfo& create_info) {
+  // 1. Accept-Ranges, Content-Length and strong validators response headers.
+  // 2. Feature |kParallelDownloading| enabled.
+  // 3. Content-Length is no less than the minimum slice size configuration.
+  // 3. (Undetermined) Http/1.1 protocol.
+  // 4. (Undetermined) Not under http proxy, e.g. data saver.
+
+  // Etag and last modified are stored into DownloadCreateInfo in
+  // DownloadRequestCore only if the response header complies to the strong
+  // validator rule.
+  bool has_strong_validator =
+      !create_info.etag.empty() || !create_info.last_modified.empty();
+
+  return has_strong_validator && create_info.accept_range &&
+         create_info.total_bytes >= GetMinSliceSizeConfig() &&
+         base::FeatureList::IsEnabled(features::kParallelDownloading);
+}
+
+std::vector<DownloadItem::ReceivedSlice> FindSlicesForRemainingContent(
+    int64_t bytes_received,
+    int64_t total_bytes,
+    int request_count) {
+  std::vector<DownloadItem::ReceivedSlice> slices_to_download;
+  if (bytes_received >= total_bytes || request_count <= 0)
+    return slices_to_download;
+
+  // TODO(xingliu): Consider to use minimum size of a slice.
+  int64_t bytes_left = total_bytes - bytes_received;
+  int64_t slice_size = bytes_left / request_count;
+  slice_size = slice_size > 0 ? slice_size : 1;
+  int64_t current_offset = bytes_received;
+  for (int i = 0, num_requests = bytes_left / slice_size; i < num_requests - 1;
+       ++i) {
+    slices_to_download.emplace_back(current_offset, slice_size);
+    current_offset += slice_size;
+  }
+
+  // Last slice is a half open slice, which results in sending range request
+  // like "Range:50-" to fetch from 50 bytes to the end of the file.
+  slices_to_download.emplace_back(current_offset,
+                                  DownloadSaveInfo::kLengthFullContent);
+  return slices_to_download;
+}
+
 std::vector<DownloadItem::ReceivedSlice> FindSlicesToDownload(
     const std::vector<DownloadItem::ReceivedSlice>& received_slices) {
   std::vector<DownloadItem::ReceivedSlice> result;