// Copyright 2017 Google Inc. All Rights Reserved.
//
// This file contains the declarations for the EventMetric class and related
// types.
#ifndef WVCDM_METRICS_EVENT_METRIC_H_
#define WVCDM_METRICS_EVENT_METRIC_H_

#include <cstdarg>
#include <functional>
#include <map>
#include <memory>
#include <ostream>
#include <sstream>
#include <stdint.h>
#include <string>
#include <vector>

#include "lock.h"

#include "distribution.h"
#include "metric_publisher.h"

namespace wvcdm {
namespace metrics {

class EventMetricTest;

// This base class provides the common defintion used by all templated
// instances of EventMetric.
class BaseEventMetric : public MetricPublisher {
 public:
  // Publish metric values to the MetricNotification. |subscriber| must
  // not be null and is owned by the caller.
  virtual void Publish(MetricNotification* subscriber);

 protected:
  // Instantiates a BaseEventMetric.
  BaseEventMetric(const std::string& metric_name)
      : metric_name_(metric_name) {}

  virtual ~BaseEventMetric();

  // Record will look for an existing instance of the Distribution identified
  // by the field_names_values string and update it. If the instance does
  // not exist, this will create it.
  void Record(const std::string& field_names_values, double value);

 private:
  friend class EventMetricTest;
  const std::string metric_name_;
  // value_map_ contains a mapping from the field name/value pairs to the
  // distribution instance which holds the metric information (min, max, sum,
  // etc.).
  std::map<std::string, Distribution*> value_map_;

  /*
   * This locks the internal state of the event metric to ensure safety across
   * multiple threads preventing the caller from worrying about locking.
   */
  Lock internal_lock_;
};

// This is a placeholder type for unused type parameters.
struct Unused {
  // Required for compilation. Should never be used.
  inline friend std::ostream& operator<< (std::ostream& out, const Unused&)
      { return out; }
};


// This class converts the size_t value into the highest power of two
// below the value.  E.g. for 7, the value is 4.  For 11, the value is 8.
// This class is intended to simplify the use of EventMetric Fields that may
// have many possible values, but we want to bucket them into a small set of
// numbers (32 or 64).
class Pow2Bucket {
 public:
  explicit Pow2Bucket(size_t value) : value_(GetLowerBucket(value)) {}

  Pow2Bucket(const Pow2Bucket& value) : value_(value.value_) {}

  // Support for converting to string.
  friend std::ostream& operator<<(std::ostream& os, const Pow2Bucket& log)
      { return os << log.value_; }

 private:
  inline size_t GetLowerBucket(size_t value) {
    if (!value) {
      return 0;
    }

    size_t log = 0;
    while (value) {
      log++;
      value >>= 1;
    }

    return 1 << (log - 1);
  }

  size_t value_;
};

// The EventMetric class is used to capture statistics about an event such as
// a method call. EventMetric keeps track of the count, mean, min, max and
// variance for the value being measured. For example, we may want to track the
// latency of a particular operation. Each time the operation is run, the time
// is calculated and provided to the EventMetric by using the
// EventMetric::Record method to capture and maintain statistics about the
// latency (max, min, count, mean, variance).
//
// The EventMetric class supports the ability to breakdown the statistics based
// on certain "field" values. For example, if a particular operation can run in
// one of two modes, it's useful to track the latency of the operation in each
// mode separately. You can use Fields to define how to breakdown the
// statistics. Each Field is a separate dimention. The statistics for each
// combination of field values are tracked independently.
//
// Example usage:
//
// EventMetric<int, int> my_metric("multiple/fields/metric",
//                                 "request_type", "error_code",  // Field names);
//
// my_metric.Record(1, 7, 23);  // (latency value, request type, error code).
//
// A MetricNotification may be used to allow the EventMetric instance to
// notify that an update occurred and provide the updated value.
//
// class MyMetricNotification : public MetricNotification {
//   // Add implementation here.
// }
//
// MyMetricNotification notification;
// my_metric.Publish(notification);
template<typename F1=Unused,
         typename F2=Unused,
         typename F3=Unused,
         typename F4=Unused>
class EventMetric : public BaseEventMetric {
 public:
  // Create an EventMetric instance with the name |metric_name|.
  EventMetric(const std::string& metric_name);

  // Overloaded constructors with variable field name arguments. The number
  // of |field_name| arguments must match the number of used Field type
  // arguments.
  EventMetric(const std::string& metric_name,
              const char* field_name);
  EventMetric(const std::string& metric_name,
              const char* field_name1,
              const char* field_name2);
  EventMetric(const std::string& metric_name,
              const char* field_name1,
              const char* field_name2,
              const char* field_name3);
  EventMetric(const std::string& metric_name,
              const char* field_name1,
              const char* field_name2,
              const char* field_name3,
              const char* field_name4);

  // Record will update the statistics of the EventMetric broken down by the
  // given field values.
  void Record(double value,
              F1 field1 = Unused(),
              F2 field2 = Unused(),
              F3 field3 = Unused(),
              F4 field4 = Unused());

 private:
  friend class EventMetricTest;
  std::vector<std::string> field_names_;
};

// This is an internal namespace for helper functions only.
namespace impl {

// This method formats the collection of field name/value pairs.
// The format of the string is:
//
//   [{field:value[&field:value]*}]
//
// If there are no pairs, returns a blank string.
//
// TODO(blueeyes): Add a check for the count of field_names and the count
// of field values (check that the fields are not type Unused).
template<typename F1, typename F2, typename F3, typename F4>
std::string MakeFieldNameString(const std::vector<std::string>& field_names,
                                const F1 field1, const F2 field2,
                                const F3 field3, const F4 field4) {
  std::stringstream field_name_and_values;
  std::vector<std::string>::const_iterator field_name_iterator =
      field_names.begin();
  if (field_name_iterator == field_names.end()) {
    return field_name_and_values.str();
  }
  // There is at least one name/value pair. prepend open brace.
  field_name_and_values << "{";
  field_name_and_values << *field_name_iterator << ':' << field1;
  if (++field_name_iterator == field_names.end()) {
    field_name_and_values << "}";
    return field_name_and_values.str();
  }
  field_name_and_values << '&' << *field_name_iterator << ':' << field2;
  if (++field_name_iterator == field_names.end()) {
    field_name_and_values << "}";
    return field_name_and_values.str();
  }
  field_name_and_values << '&' << *field_name_iterator << ':' << field3;
  if (++field_name_iterator == field_names.end()) {
    field_name_and_values << "}";
    return field_name_and_values.str();
  }
  field_name_and_values << '&' << *field_name_iterator << ':' << field4;
  field_name_and_values << "}";
  return field_name_and_values.str();
}

// This specialization of the helper method is a shortcut for EventMetric
// instances with no fields.
template<>
inline std::string MakeFieldNameString<Unused, Unused, Unused, Unused>(
    const std::vector<std::string>& field_names,
    const Unused unused1, const Unused unused2,
    const Unused unused3, const Unused unused4) {
  return "";
}

// This helper function appends the field names to a vector of strings.
inline void AppendFieldNames(std::vector<std::string>* field_name_vector,
                             int field_count, ...) {
  va_list field_names;

  va_start(field_names, field_count);
  for (int x = 0; x < field_count; x++) {
    field_name_vector->push_back(va_arg(field_names, const char*));
  }
  va_end(field_names);
}

}  // namespace impl

// Overloaded template constructor implementations for EventMetric.
template<typename F1, typename F2, typename F3, typename F4>
EventMetric<F1, F2, F3, F4>::EventMetric(
    const std::string& metric_name)
        : BaseEventMetric(metric_name) {}

template<typename F1, typename F2, typename F3, typename F4>
EventMetric<F1, F2, F3, F4>::EventMetric(
    const std::string& metric_name,
    const char* field_name)
        : BaseEventMetric(metric_name) {
  impl::AppendFieldNames(&field_names_, 1, field_name);
}
template<typename F1, typename F2, typename F3, typename F4>
EventMetric<F1, F2, F3, F4>::EventMetric(
    const std::string& metric_name,
    const char* field_name1,
    const char* field_name2)
        : BaseEventMetric(metric_name) {
  impl::AppendFieldNames(&field_names_, 2, field_name1, field_name2);
}
template<typename F1, typename F2, typename F3, typename F4>
EventMetric<F1, F2, F3, F4>::EventMetric(
    const std::string& metric_name,
    const char* field_name1,
    const char* field_name2,
    const char* field_name3)
        : BaseEventMetric(metric_name) {
  impl::AppendFieldNames(&field_names_,
                         3, field_name1, field_name2, field_name3);
}
template<typename F1, typename F2, typename F3, typename F4>
EventMetric<F1, F2, F3, F4>::EventMetric(
    const std::string& metric_name,
    const char* field_name1,
    const char* field_name2,
    const char* field_name3,
    const char* field_name4)
        : BaseEventMetric(metric_name) {
  impl::AppendFieldNames(&field_names_,
                         4, field_name1, field_name2,
                         field_name3, field_name4);
}

template<typename F1, typename F2, typename F3, typename F4>
void EventMetric<F1, F2, F3, F4>::Record(
    double value, F1 field1, F2 field2, F3 field3, F4 field4) {
  std::string field_name_values =
      impl::MakeFieldNameString(field_names_, field1, field2, field3, field4);
  BaseEventMetric::Record(field_name_values, value);
}

}  // namespace metrics
}  // namespace wvcdm

#endif  // WVCDM_METRICS_EVENT_METRIC_H_