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+// Copyright 2015 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// RAPPOR encoder.
+//
+// See README.md and encoder_demo.cc for an example.
+
+#ifndef RAPPOR_H_
+#define RAPPOR_H_
+
+#include <string>
+
+#include "rappor_deps.h"  // for dependency injection
+
+namespace rappor {
+
+// For debug logging
+void log(const char* fmt, ...);
+
+// RAPPOR encoding parameters.
+class Params {
+ public:
+  Params(int num_bits, int num_hashes, int num_cohorts,
+         float prob_f, float prob_p, float prob_q)
+      : num_bits_(num_bits),
+        num_hashes_(num_hashes),
+        num_cohorts_(num_cohorts),
+        prob_f_(prob_f),
+        prob_p_(prob_p),
+        prob_q_(prob_q) {
+  }
+
+  // Accessors
+  int num_bits() { return num_bits_; }
+  int num_hashes() { return num_hashes_; }
+  int num_cohorts() { return num_cohorts_; }
+  float prob_f() { return prob_f_; }
+  float prob_p() { return prob_p_; }
+  float prob_q() { return prob_q_; }
+
+ private:
+  friend class Encoder;
+
+  // k: size of bloom filter, PRR, and IRR.  0 < k <= 32.
+  int num_bits_;
+
+  // number of bits set in the Bloom filter ("h")
+  int num_hashes_;
+
+  // Total number of cohorts ("m").  Note that the cohort assignment is what
+  // is used in the client, not m.  We include it here for documentation (it
+  // can be unset, unlike the other params.)
+  int num_cohorts_;
+
+  float prob_f_;  // noise probability for PRR, quantized to 1/128
+
+  float prob_p_;  // noise probability for IRR, quantized to 1/128
+  float prob_q_;  // noise probability for IRR, quantized to 1/128
+};
+
+// Encoder: take client values and transform them with the RAPPOR privacy
+// algorithm.
+class Encoder {
+ public:
+  // Note that invalid parameters cause runtime assertions in the constructor.
+  // Encoders are intended to be created at application startup with constant
+  // arguments, so errors should be caught early.
+
+  // encoder_id: A unique ID for this encoder -- typically the name of the
+  //   metric being encoded, so that different metrics have different PRR
+  //   mappings.
+  // params: RAPPOR encoding parameters, which affect privacy and decoding.
+  //   (held by reference; it must outlive the Encoder)
+  // deps: application-supplied dependencies.
+  //   (held by reference; it must outlive the Encoder)
+  Encoder(const std::string& encoder_id, const Params& params,
+          const Deps& deps);
+
+  // Encode raw bits (represented as an integer), setting output parameter
+  // irr_out.  Only valid when the return value is 'true' (success).
+  bool EncodeBits(const Bits bits, Bits* irr_out) const;
+
+  // Encode a string, setting output parameter irr_out.  Only valid when the
+  // return value is 'true' (success).
+  bool EncodeString(const std::string& value, Bits* irr_out) const;
+  // For use with HmacDrbg hash function and any num_bits divisible by 8.
+  bool EncodeString(const std::string& value,
+                    std::vector<uint8_t>* irr_out) const;
+
+  // For testing/simulation use only.
+  bool _EncodeBitsInternal(const Bits bits, Bits* prr_out, Bits* irr_out)
+    const;
+  bool _EncodeStringInternal(const std::string& value, Bits* bloom_out,
+                             Bits* prr_out, Bits* irr_out) const;
+
+  // Accessor for the assigned cohort.
+  uint32_t cohort() { return cohort_; }
+  // Set a cohort manually, if previously generated.
+  void set_cohort(uint32_t cohort);
+
+ private:
+  bool MakeBloomFilter(const std::string& value, Bits* bloom_out) const;
+  bool MakeBloomFilter(const std::string& value,
+                       std::vector<uint8_t>* bloom_out) const;
+  bool GetPrrMasks(const Bits bits, Bits* uniform, Bits* f_mask) const;
+
+  // static helper function for initialization
+  static uint32_t AssignCohort(const Deps& deps, int num_cohorts);
+
+  const std::string encoder_id_;
+  const Params& params_;
+  const Deps& deps_;
+  uint32_t cohort_;
+  std::string cohort_str_;
+};
+
+}  // namespace rappor
+
+#endif  // RAPPOR_H_