// Copyright 2022 The Pigweed Authors
//
// 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
//
//     https://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.

syntax = "proto3";

package pw.chrono;

option java_outer_classname = "Chrono";

message EpochType {
  enum Enum {
    UNKNOWN = 0;

    TIME_SINCE_BOOT = 1;

    // Time since 00:00:00 UTC, Thursday, 1 January 1970, including leap
    // seconds.
    UTC_WALL_CLOCK = 2;

    // Time since 00:00:00, 6 January 1980 UTC. Leap seconds are not inserted
    // into GPS. Thus, every time a leap second is inserted into UTC, UTC
    // falls another second behind GPS.
    GPS_WALL_CLOCK = 3;

    // Time since 00:00:00, 1 January 1958, and is offset 10 seconds ahead of
    // UTC at that date (i.e., its epoch, 1958-01-01 00:00:00 TAI, is
    // 1957-12-31 23:59:50 UTC). Leap seconds are not inserted into TAI. Thus,
    // every time a leap second is inserted into UTC, UTC falls another second
    // behind TAI.
    TAI_WALL_CLOCK = 4;
  };
}

// A representation of a clock's parameters.
//
// There are two major components to representing a steady, monotonic clock:
//
//   1. A representation of the clock's period.
//   2. A representation of the clock's epoch.
//
// To support a wide range of clock configurations, ClockParameters represents
// a clock's period as fractions of a second. Concretely:
//
//   Clock period (seconds) =
//       tick_period_seconds_numerator / tick_period_seconds_denominator
//
// So a simple 1KHz clock can be represented as:
//
//   tick_period_seconds_numerator = 1
//   tick_period_seconds_denominator = 1000
//   Clock period = 1 / 1000 = 0.001 seconds
//   Clock frequency = 1 / 0.001 = 1,000 Hz
//
// Failing to specify one or both of the period members of a ClockParameters
// message leaves the configuration specification incomplete and invalid.
//
// While clock period alone is enough to represent a duration if given a number
// of ticks, an epoch is required to make a clock represent a time point.
// EpochType optionally provides this information. Specifying an EpochType
// defines what a tick count of `0` represents. Some epoch types (e.g. UTC, GPS,
// TAI) allow the clock to resolve to real-world time points. If the EpochType
// is relative to boot or unknown, however, the clock is only sufficiently
// specified for relative time measurement without additional external
// information.
message ClockParameters {
  int32 tick_period_seconds_numerator = 1;    // Required
  int32 tick_period_seconds_denominator = 2;  // Required
  optional EpochType.Enum epoch_type = 3;
}

// A point in time relative to a clock's epoch.
message TimePoint {
  // The duration that has elapsed (number of clock ticks) since the epoch,
  // where the tick period and epoch are specified by the clock parameters.
  //
  // The meaning of `timestamp` is unspecified without an associated
  // ClockParameters.
  int64 timestamp = 1;                   // Required
  ClockParameters clock_parameters = 2;  // Required
}

// The time of a snapshot capture. Supports multiple timestamps to
// cover multiple time bases or clocks (e.g. time since boot, time
// from epoch, etc).
//
// This is an overlay proto for Snapshot, see more details here:
// https://pigweed.dev/pw_snapshot/proto_format.html#module-specific-data
message SnapshotTimestamps {
  repeated TimePoint timestamps = 22;
}