// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

part of intl;

/// Given a month and day number, return the day of the year, all one-based.
///
/// For example,
///  * January 2nd (1, 2) -> 2.
///  * February 5th (2, 5) -> 36.
///  * March 1st of a non-leap year (3, 1) -> 60.
int _dayOfYear(int month, int day, bool leapYear) {
  if (month == 1) return day;
  if (month == 2) return day + 31;
  return ordinalDayFromMarchFirst(month, day) + 59 + (leapYear ? 1 : 0);
}

/// Return true if this is a leap year. Rely on [DateTime] to do the
/// underlying calculation, even though it doesn't expose the test to us.
bool _isLeapYear(DateTime date) {
  var feb29 = DateTime(date.year, 2, 29);
  return feb29.month == 2;
}

/// Return the day of the year counting March 1st as 1, after which the
/// number of days per month is constant, so it's easier to calculate.
/// Formula from http://en.wikipedia.org/wiki/Ordinal_date
int ordinalDayFromMarchFirst(int month, int day) =>
    ((30.6 * month) - 91.4).floor() + day;

/// A class for holding onto the data for a date so that it can be built
/// up incrementally.
class _DateBuilder {
  // Default the date values to the EPOCH so that there's a valid date
  // in case the format doesn't set them.
  int year = 1970,
      month = 1,
      day = 1,
      dayOfYear = 0,
      hour = 0,
      minute = 0,
      second = 0,
      fractionalSecond = 0;
  bool pm = false;
  bool utc = false;

  /// The locale, kept for logging purposes when there's an error.
  final String _locale;

  /// The date result produced from [asDate].
  ///
  /// Kept as a field to cache the result and to reduce the possibility of error
  /// after we've verified.
  DateTime _date;

  /// The number of times we've retried, for error reporting.
  int _retried = 0;

  /// Is this constructing a pure date.
  ///
  /// This is important because some locales change times at midnight,
  /// e.g. Brazil. So if we try to create a DateTime representing a date at
  /// midnight on the day of transition it will jump forward or back 1 hour.  If
  /// it jumps forward that's mostly harmless if we only care about the
  /// date. But if it jumps backwards that will change the date, which is
  /// bad. Compensate by adjusting the time portion forward. But only do that
  /// when we're explicitly trying to construct a date, which we can tell from
  /// the format.

  // We do set it, the analyzer just can't tell.
  // ignore: prefer_final_fields
  var _dateOnly = false;

  /// The function we will call to create a DateTime from its component pieces.
  ///
  /// This is normally only modified in tests that want to introduce errors.
  final _DateTimeConstructor _dateTimeConstructor;

  _DateBuilder(this._locale, this._dateTimeConstructor);

  // Functions that exist just to be closurized so we can pass them to a general
  // method.
  void setYear(x) {
    year = x;
  }

  void setMonth(x) {
    month = x;
  }

  void setDay(x) {
    day = x;
  }

  void setDayOfYear(x) {
    dayOfYear = x;
  }

  /// If [dayOfYear] has been set, return it, otherwise return [day], indicating
  /// the day of the month.
  int get dayOrDayOfYear => dayOfYear == 0 ? day : dayOfYear;

  void setHour(x) {
    hour = x;
  }

  void setMinute(x) {
    minute = x;
  }

  void setSecond(x) {
    second = x;
  }

  void setFractionalSecond(x) {
    fractionalSecond = x;
  }

  int get hour24 => pm ? hour + 12 : hour;

  /// Verify that we correspond to a valid date. This will reject out of
  /// range values, even if the DateTime constructor would accept them. An
  /// invalid message will result in throwing a [FormatException].
  void verify(String s) {
    _verify(month, 1, 12, 'month', s);
    _verify(hour24, 0, 23, 'hour', s);
    _verify(minute, 0, 59, 'minute', s);
    _verify(second, 0, 59, 'second', s);
    _verify(fractionalSecond, 0, 999, 'fractional second', s);
    // Verifying the day is tricky, because it depends on the month. Create
    // our resulting date and then verify that our values agree with it
    // as an additional verification. And since we're doing that, also
    // check the year, which we otherwise can't verify, and the hours,
    // which will catch cases like '14:00:00 PM'.
    var date = asDate();
    // On rare occasions, possibly related to DST boundaries, a parsed date will
    // come out as 1:00am. We compensate for the case of going backwards in
    // _correctForErrors, but we may not be able to compensate for a midnight
    // that doesn't exist. So tolerate an hour value of zero or one in these
    // cases.
    var minimumDate = _dateOnly && date.hour == 1 ? 0 : date.hour;
    _verify(hour24, minimumDate, date.hour, 'hour', s, date);
    if (dayOfYear > 0) {
      // We have an ordinal date, compute the corresponding date for the result
      // and compare to that.
      var leapYear = _isLeapYear(date);
      var correspondingDay = _dayOfYear(date.month, date.day, leapYear);
      _verify(
          dayOfYear, correspondingDay, correspondingDay, 'dayOfYear', s, date);
    } else {
      // We have the day of the month, compare directly.
      _verify(day, date.day, date.day, 'day', s, date);
    }
    _verify(year, date.year, date.year, 'year', s, date);
  }

  void _verify(int value, int min, int max, String desc, String originalInput,
      [DateTime parsed]) {
    if (value < min || value > max) {
      var parsedDescription = parsed == null ? '' : ' Date parsed as $parsed.';
      var errorDescription =
          'Error parsing $originalInput, invalid $desc value: $value'
          ' in $_locale'
          ' with time zone offset ${parsed?.timeZoneOffset ?? 'unknown'}.'
          ' Expected value between $min and $max.$parsedDescription.';
      if (_retried > 0) {
        errorDescription += ' Failed after $_retried retries.';
      }
      throw FormatException(errorDescription);
    }
  }

  /// Return a date built using our values. If no date portion is set,
  /// use the 'Epoch' of January 1, 1970.
  DateTime asDate({int retries = 3}) {
    // TODO(alanknight): Validate the date, especially for things which
    // can crash the VM, e.g. large month values.
    if (_date != null) return _date;

    if (utc) {
      _date = _dateTimeConstructor(year, month, dayOrDayOfYear, hour24, minute,
          second, fractionalSecond, utc);
    } else {
      var preliminaryResult = _dateTimeConstructor(year, month, dayOrDayOfYear,
          hour24, minute, second, fractionalSecond, utc);
      _date = _correctForErrors(preliminaryResult, retries);
    }
    return _date;
  }

  /// Given a local DateTime, check for errors and try to compensate for them if
  /// possible.
  DateTime _correctForErrors(DateTime result, int retries) {
    // There are 3 kinds of errors that we know of
    //
    // 1 - Issue 15560, sometimes we get UTC even when we asked for local, or
    // they get constructed as if in UTC and then have the offset subtracted.
    // Retry, possibly several times, until we get something that looks valid,
    // or we give up.
    //
    // 1a) - It appears that sometimes we get incorrect timezone offsets that
    // are not directly related to UTC. Also check for those and retry or
    // compensate.
    //
    // 2 - Timezone transitions. If we ask for the time during a timezone
    // transition then it will offset it by that transition. This is
    // particularly a problem if the timezone transition happens at midnight,
    // and we're looking for a date with no time component. This happens in
    // Brazil, and we can end up with 11:00pm the previous day. Add time to
    // compensate.
    //
    // 3 - Invalid input which the constructor nevertheless accepts. Just return
    // what it created, and verify will catch it if we're in strict mode.

    // If we've exhausted our retries, just return the input - it's not just a
    // flaky result.
    if (retries <= 0) {
      return result;
    }

    var leapYear = _isLeapYear(result);
    var resultDayOfYear = _dayOfYear(result.month, result.day, leapYear);

    // Check for the UTC failure. Are we expecting to produce a local time, but
    // the result is UTC. However, the local time might happen to be the same as
    // UTC. To be thorough, check if either the hour/day don't agree with what
    // we expect, or is a new DateTime in a non-UTC timezone.
    if (!utc &&
        result.isUtc &&
        (result.hour != hour24 ||
            result.day != resultDayOfYear ||
            !DateTime.now().isUtc)) {
      // This may be a UTC failure. Retry and if the result doesn't look
      // like it's in the UTC time zone, use that instead.
      _retried++;
      return asDate(retries: retries - 1);
    }

    if (_dateOnly && result.hour != 0) {
      // This could be a flake, try again.
      var tryAgain = asDate(retries: retries - 1);
      if (tryAgain != result) {
        // Trying again gave a different answer, so presumably it worked.
        return tryAgain;
      }

      // Trying again didn't work, try to force the offset.
      var expectedDayOfYear =
          dayOfYear == 0 ? _dayOfYear(month, day, leapYear) : dayOfYear;

      // If we're _dateOnly, then hours should be zero, but might have been
      // offset to e.g. 11:00pm the previous day. Add that time back in. This
      // might be because of an erratic error, but it might also be because of a
      // time zone (Brazil) where there is no midnight at a daylight savings
      // time transition. In that case we will retry, but eventually give up and
      // return 1:00am on the correct date.
      var daysPrevious = expectedDayOfYear - resultDayOfYear;
      // For example, if it's the day before at 11:00pm, we offset by (24 - 23),
      // so +1. If it's the same day at 1:00am, we offset by (0 - 1), so -1.
      var offset = (daysPrevious * 24) - result.hour;
      var adjusted = result.add(Duration(hours: offset));
      // Check if the adjustment worked. This can fail on a time zone transition
      // where midnight doesn't exist.
      if (adjusted.hour == 0) {
        return adjusted;
      }
      // Adjusting did not work. Just check if the adjusted date is right. And
      // if it's not, just give up and return [result]. The scenario where this
      // might correctly happen is if we're in a Brazil time zone, jump forward
      // to 1:00 am because of a DST transition, and trying to go backwards 1
      // hour takes us back to 11:00pm the day before. In that case the 1:00am
      // answer on the correct date is preferable.
      var adjustedDayOfYear =
          _dayOfYear(adjusted.month, adjusted.day, leapYear);
      if (adjustedDayOfYear != expectedDayOfYear) {
        return result;
      }
      return adjusted;
    }
    // None of our corrections applied, just return the uncorrected date.
    return result;
  }
}

/// A simple and not particularly general stream class to make parsing
/// dates from strings simpler. It is general enough to operate on either
/// lists or strings.
// TODO(alanknight): With the improvements to the collection libraries
// since this was written we might be able to get rid of it entirely
// in favor of e.g. aString.split('') giving us an iterable of one-character
// strings, or else make the implementation trivial. And consider renaming,
// as _Stream is now just confusing with the system Streams.
class _Stream {
  dynamic contents;
  int index = 0;

  _Stream(this.contents);

  bool atEnd() => index >= contents.length;

  dynamic next() => contents[index++];

  /// Return the next [howMany] items, or as many as there are remaining.
  /// Advance the stream by that many positions.
  dynamic read([int howMany = 1]) {
    var result = peek(howMany);
    index += howMany;
    return result;
  }

  /// Does the input start with the given string, if we start from the
  /// current position.
  bool startsWith(String pattern) {
    if (contents is String) return contents.startsWith(pattern, index);
    return pattern == peek(pattern.length);
  }

  /// Return the next [howMany] items, or as many as there are remaining.
  /// Does not modify the stream position.
  dynamic peek([int howMany = 1]) {
    dynamic result;
    if (contents is String) {
      String stringContents = contents;
      result = stringContents.substring(
          index, min(index + howMany, stringContents.length));
    } else {
      // Assume List
      result = contents.sublist(index, index + howMany);
    }
    return result;
  }

  /// Return the remaining contents of the stream
  dynamic rest() => peek(contents.length - index);

  /// Find the index of the first element for which [f] returns true.
  /// Advances the stream to that position.
  int findIndex(Function f) {
    while (!atEnd()) {
      if (f(next())) return index - 1;
    }
    return null;
  }

  /// Find the indexes of all the elements for which [f] returns true.
  /// Leaves the stream positioned at the end.
  List<dynamic> findIndexes(Function f) {
    var results = [];
    while (!atEnd()) {
      if (f(next())) results.add(index - 1);
    }
    return results;
  }

  /// Assuming that the contents are characters, read as many digits as we
  /// can see and then return the corresponding integer, advancing the receiver.
  ///
  /// For non-ascii digits, the optional arguments are a regular expression
  /// [digitMatcher] to find the next integer, and the codeUnit of the local
  /// zero [zeroDigit].
  int nextInteger({RegExp digitMatcher, int zeroDigit}) {
    var string =
        (digitMatcher ?? DateFormat._asciiDigitMatcher).stringMatch(rest());
    if (string == null || string.isEmpty) return null;
    read(string.length);
    if (zeroDigit != null && zeroDigit != DateFormat._asciiZeroCodeUnit) {
      // Trying to optimize this, as it might get called a lot.
      var oldDigits = string.codeUnits;
      var newDigits = List<int>(string.length);
      for (var i = 0; i < string.length; i++) {
        newDigits[i] = oldDigits[i] - zeroDigit + DateFormat._asciiZeroCodeUnit;
      }
      string = String.fromCharCodes(newDigits);
    }
    return int.parse(string);
  }
}