/*
 * Copyright 2023, gRPC Authors 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.
 */
import Dispatch

/// A timeout for a gRPC call.
///
/// It's a combination of an amount (expressed as an integer of at maximum 8 digits), and a unit, which is
/// one of ``Timeout/Unit`` (hours, minutes, seconds, milliseconds, microseconds or nanoseconds).
///
/// Timeouts must be positive and at most 8-digits long.
@available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
@usableFromInline
struct Timeout: CustomStringConvertible, Hashable, Sendable {
  /// Possible units for a ``Timeout``.
  internal enum Unit: Character {
    case hours = "H"
    case minutes = "M"
    case seconds = "S"
    case milliseconds = "m"
    case microseconds = "u"
    case nanoseconds = "n"
  }

  /// The largest amount of any unit of time which may be represented by a gRPC timeout.
  static let maxAmount: Int64 = 99_999_999

  private let amount: Int64
  private let unit: Unit

  @usableFromInline
  var duration: Duration {
    Duration(amount: amount, unit: unit)
  }

  /// The wire encoding of this timeout as described in the gRPC protocol.
  /// See "Timeout" in https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md#requests
  var wireEncoding: String {
    "\(amount)\(unit.rawValue)"
  }

  @usableFromInline
  var description: String {
    return self.wireEncoding
  }

  @usableFromInline
  init?(decoding value: String) {
    guard (2 ... 8).contains(value.count) else {
      return nil
    }

    if let amount = Int64(value.dropLast()),
      let unit = Unit(rawValue: value.last!)
    {
      self = Self.init(amount: amount, unit: unit)
    } else {
      return nil
    }
  }

  /// Create a ``Timeout`` from a ``Duration``.
  ///
  /// - Important: It's not possible to know with what precision the duration was created: that is,
  /// it's not possible to know whether `Duration.seconds(value)` or `Duration.milliseconds(value)`
  /// was used. For this reason, the unit chosen for the ``Timeout`` (and thus the wire encoding) may be
  /// different from the one originally used to create the ``Duration``. Despite this, we guarantee that
  /// both durations will be equivalent if there was no loss in precision during the transformation.
  /// For example, `Duration.hours(123)` will yield a ``Timeout`` with `wireEncoding` equal to
  /// `"442800S"`, which is in seconds. However, 442800 seconds and 123 hours are equivalent.
  /// However, you must note that there may be some loss of precision when dealing with transforming
  /// between units. For example, for very low precisions, such as a duration of only a few attoseconds,
  /// given the smallest unit we have is whole nanoseconds, we cannot represent it. Same when converting
  /// for instance, milliseconds to seconds. In these scenarios, we'll round to the closest whole number in
  /// the target unit.
  @usableFromInline
  init(duration: Duration) {
    let (seconds, attoseconds) = duration.components

    if seconds == 0 {
      // There is no seconds component, so only pay attention to the attoseconds.
      // Try converting to nanoseconds first, and continue rounding up if the
      // max amount of digits is exceeded.
      let nanoseconds = Int64(Double(attoseconds) / 1e+9)
      self.init(rounding: nanoseconds, unit: .nanoseconds)
    } else if Self.exceedsDigitLimit(seconds) {
      // We don't have enough digits to represent this amount in seconds, so
      // we will have to use minutes or hours.
      // We can also ignore attoseconds, since we won't have enough precision
      // anyways to represent the (at most) one second that the attoseconds
      // component can express.
      self.init(rounding: seconds, unit: .seconds)
    } else {
      // We can't convert seconds to nanoseconds because that would take us
      // over the 8 digit limit (1 second = 1e+9 nanoseconds).
      // We can however, try converting to microseconds or milliseconds.
      let nanoseconds = Int64(Double(attoseconds) / 1e+9)
      let microseconds = nanoseconds / 1000
      if microseconds == 0 {
        self.init(amount: seconds, unit: .seconds)
      } else {
        let secondsInMicroseconds = seconds * 1000 * 1000
        let totalMicroseconds = microseconds + secondsInMicroseconds
        self.init(rounding: totalMicroseconds, unit: .microseconds)
      }
    }
  }

  /// Create a timeout by rounding up the timeout so that it may be represented in the gRPC
  /// wire format.
  private init(rounding amount: Int64, unit: Unit) {
    var roundedAmount = amount
    var roundedUnit = unit

    if roundedAmount <= 0 {
      roundedAmount = 0
    } else {
      while roundedAmount > Timeout.maxAmount {
        switch roundedUnit {
        case .nanoseconds:
          roundedAmount = roundedAmount.quotientRoundedUp(dividingBy: 1000)
          roundedUnit = .microseconds
        case .microseconds:
          roundedAmount = roundedAmount.quotientRoundedUp(dividingBy: 1000)
          roundedUnit = .milliseconds
        case .milliseconds:
          roundedAmount = roundedAmount.quotientRoundedUp(dividingBy: 1000)
          roundedUnit = .seconds
        case .seconds:
          roundedAmount = roundedAmount.quotientRoundedUp(dividingBy: 60)
          roundedUnit = .minutes
        case .minutes:
          roundedAmount = roundedAmount.quotientRoundedUp(dividingBy: 60)
          roundedUnit = .hours
        case .hours:
          roundedAmount = Timeout.maxAmount
          roundedUnit = .hours
        }
      }
    }

    self.init(amount: roundedAmount, unit: roundedUnit)
  }

  private static func exceedsDigitLimit(_ value: Int64) -> Bool {
    value > Timeout.maxAmount
  }

  /// Creates a `GRPCTimeout`.
  ///
  /// - Precondition: The amount should be greater than or equal to zero and less than or equal
  ///   to `GRPCTimeout.maxAmount`.
  internal init(amount: Int64, unit: Unit) {
    precondition((0 ... Timeout.maxAmount).contains(amount))

    self.amount = amount
    self.unit = unit
  }
}

extension Int64 {
  /// Returns the quotient of this value when divided by `divisor` rounded up to the nearest
  /// multiple of `divisor` if the remainder is non-zero.
  ///
  /// - Parameter divisor: The value to divide this value by.
  fileprivate func quotientRoundedUp(dividingBy divisor: Int64) -> Int64 {
    let (quotient, remainder) = self.quotientAndRemainder(dividingBy: divisor)
    return quotient + (remainder != 0 ? 1 : 0)
  }
}

@available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
extension Duration {
  /// Construct a `Duration` given a number of minutes represented as an `Int64`.
  ///
  ///       let d: Duration = .minutes(5)
  ///
  /// - Returns: A `Duration` representing a given number of minutes.
  internal static func minutes(_ minutes: Int64) -> Duration {
    return Self.init(secondsComponent: 60 * minutes, attosecondsComponent: 0)
  }

  /// Construct a `Duration` given a number of hours represented as an `Int64`.
  ///
  ///       let d: Duration = .hours(3)
  ///
  /// - Returns: A `Duration` representing a given number of hours.
  internal static func hours(_ hours: Int64) -> Duration {
    return Self.init(secondsComponent: 60 * 60 * hours, attosecondsComponent: 0)
  }

  internal init(amount: Int64, unit: Timeout.Unit) {
    switch unit {
    case .hours:
      self = Self.hours(amount)
    case .minutes:
      self = Self.minutes(amount)
    case .seconds:
      self = Self.seconds(amount)
    case .milliseconds:
      self = Self.milliseconds(amount)
    case .microseconds:
      self = Self.microseconds(amount)
    case .nanoseconds:
      self = Self.nanoseconds(amount)
    }
  }
}