Modify rappor parameters.

components/rappor/rappor_metric.cc

 // Copyright 2014 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/rappor_metric.h"
 
 #include "base/logging.h"
 
 namespace rappor {
 
 RapporMetric::RapporMetric(const std::string& metric_name,
                            const RapporParameters& parameters,
-                           int32_t cohort)
+                           int32_t cohort_seed)
     : metric_name_(metric_name),
       parameters_(parameters),
       bloom_filter_(parameters.bloom_filter_size_bytes,
                     parameters.bloom_filter_hash_function_count,
-                    cohort * parameters.bloom_filter_hash_function_count) {
-  DCHECK_GE(cohort, 0);
+                    (cohort_seed % parameters.num_cohorts) *

[ulfar, 2014/05/27 17:52:14]

This is the only place where I see parameters.num_cohorts being used.  In
particular, I didn't see anyplace where the actual cohort number (mod
num_cohorts) is being reported to the service.  Is the cohort_seed now being
reported to the service, and the service is expected to do the modulus before
analysis?

[Steven Holte, 2014/05/27 17:58:42]

Yes, it's reporting the seed, and we'll just aggregate over the modulus in the
analysis.

+                        parameters.bloom_filter_hash_function_count) {
+  DCHECK_GE(cohort_seed, 0);
+  DCHECK_LT(cohort_seed, RapporParameters::kMaxCohorts);
 }
 
 RapporMetric::~RapporMetric() {}
 
 void RapporMetric::AddSample(const std::string& str) {
   bloom_filter_.AddString(str);
 }
 
 ByteVector RapporMetric::GetReport(const std::string& secret) const {
   // Generate a deterministically random mask of fake data using the
@@ skipping 21 matching lines @@
   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 rappor