// Copyright 2020 The Fuchsia 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 "src/local_aggregation_1_1/backfill_manager.h"

#include <algorithm>
#include <chrono>

#include "src/lib/util/datetime_util.h"
#include "src/local_aggregation_1_1/local_aggregation.pb.h"
#include "src/public/lib/statusor/status_macros.h"
#include "src/public/lib/statusor/statusor.h"
#include "src/registry/metric_definition.pb.h"

namespace cobalt::local_aggregation {

using TimeInfo = util::TimeInfo;
using time_point = std::chrono::system_clock::time_point;

BackfillManager::BackfillManager(uint32_t total_backfill_window)
    : backstop_days_(total_backfill_window) {}

lib::statusor::StatusOr<std::vector<TimeInfo>> BackfillManager::CalculateBackfill(
    TimeInfo last_time_info, time_point end_time,
    const util::CivilTimeConverterInterface& civil_time_converter, const MetricDefinition& metric,
    bool is_daily) const {
  // Calculate the end day index and hour ID with respect to the time zone specified in `metric`.
  CB_ASSIGN_OR_RETURN(TimeInfo end_info,
                      TimeInfo::FromTimePoint(end_time, civil_time_converter, metric));

  // Calculate the backstop day index with respect to the current UTC time. It is not useful to send
  // observations for earlier day indices, since the shuffler's lookback period is calculated with
  // respect to UTC. Observations with smaller day indices will be dropped.
  uint32_t backstop_day_index = util::TimePointToDayIndexUtc(end_time) - backstop_days_;

  std::vector<TimeInfo> backfill;
  // If calculating backfill for a daily report, select every day index since the last one (or the
  // backstop day index, whichever is larger), up to the day index of `end_time` in `metric`'s time
  // zone.
  //
  // If calculating for an hourly report, we need to be more careful, since not all hour IDs are
  // valid for a given time zone. For each hour within the backfill period, we need to compute the
  // hour ID with respect to `metric`'s time zone as of the beginning of that hour. This accounts
  // for the possibility that the UTC offset or DST status may change within the backfill period.
  if (is_daily) {
    uint32_t start_day_index = std::max(last_time_info.day_index + 1, backstop_day_index);
    for (uint32_t i = start_day_index; i < end_info.day_index; ++i) {
      backfill.push_back(TimeInfo::FromDayIndex(i));
    }
  } else {
    time_point time = end_time - util::kOneHour;
    while (time >= time_point::min()) {
      CB_ASSIGN_OR_RETURN(TimeInfo time_info,
                          TimeInfo::FromTimePoint(time, civil_time_converter, metric));
      if (time_info.hour_id <= last_time_info.hour_id || time_info.day_index < backstop_day_index) {
        break;
      }
      backfill.push_back(time_info);
      time -= util::kOneHour;
    }
    std::reverse(backfill.begin(), backfill.end());
  }

  return backfill;
}

}  // namespace cobalt::local_aggregation