Avoid some string copies in RAPPOR code.

components/rappor/reports.cc

 // Copyright 2015 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 "components/rappor/reports.h"
 
 #include "base/logging.h"
 #include "base/rand_util.h"
+#include "base/strings/string_piece.h"
 #include "components/rappor/byte_vector_utils.h"
 #include "components/rappor/rappor_parameters.h"
 
 namespace rappor {
 
 namespace internal {
 
 ByteVector GenerateReport(const std::string& secret,
                           const NoiseParameters& parameters,
                           const ByteVector& value) {
   // Generate a deterministically random mask of fake data using the
   // client's secret key + real data as a seed.  The inclusion of the secret
   // in the seed avoids correlations between real and fake data.
   // The seed isn't a human-readable string.
-  const std::string personalization_string =
-      std::string(value.begin(), value.end());
+  const base::StringPiece personalization_string(
+      reinterpret_cast<const char*>(&value[0]), value.size());
   HmacByteVectorGenerator hmac_generator(value.size(), secret,
                                          personalization_string);
   const ByteVector fake_mask =
       hmac_generator.GetWeightedRandomByteVector(parameters.fake_prob);
   ByteVector fake_bits =
       hmac_generator.GetWeightedRandomByteVector(parameters.fake_one_prob);
 
   // Redact most of the real data by replacing it with the fake data, hiding
   // and limiting the amount of information an individual client reports on.
   const ByteVector* fake_and_redacted_bits =
       ByteVectorMerge(fake_mask, value, &fake_bits);
 
   // Generate biased coin flips for each bit.
   ByteVectorGenerator coin_generator(value.size());
   const ByteVector zero_coins =
       coin_generator.GetWeightedRandomByteVector(parameters.zero_coin_prob);
   ByteVector one_coins =
       coin_generator.GetWeightedRandomByteVector(parameters.one_coin_prob);
 
   // Create a randomized response report on the fake and redacted data, sending
   // the outcome of flipping a zero coin for the zero bits in that data, and of
   // flipping a one coin for the one bits in that data, as the final report.
   return *ByteVectorMerge(*fake_and_redacted_bits, zero_coins, &one_coins);
 }
 
 }  // namespace internal
 
 }  // namespace rappor