/// <summary>
/// Returns the given instant adjusted one year backward taking into account leap years and other
/// adjustments like day of week.
/// </summary>
/// <param name="instant">The <see cref="Instant"/> lower bound for the next trasnition.</param>
/// <param name="standardOffset">The <see cref="Offset"/> standard offset.</param>
/// <param name="previousSavings">The <see cref="Offset"/> savings adjustment at the given Instant.</param>
/// <returns>The previous transition, or null if there is no previous transition.</returns>
internal Transition?Previous(Instant instant, Offset standardOffset, Offset previousSavings)
{
CalendarSystem calendar = CalendarSystem.Iso;
Offset wallOffset = standardOffset + previousSavings;
int year = instant == Instant.MaxValue ? Int32.MaxValue : calendar.GetYear(instant.Plus(wallOffset));
if (year > toYear)
{
// First pull instant back to the start of the year after toYear
instant = calendar.GetLocalInstant(toYear + 1, 1, 1, 0, 0).Minus(wallOffset);
}
Instant previous = yearOffset.Previous(instant, standardOffset, previousSavings);
if (previous <= instant)
{
year = calendar.GetYear(previous.Plus(wallOffset));
if (year < fromYear)
{
return(null);
}
}
return(new Transition(previous, wallOffset, standardOffset + Savings));
}
/// <summary>
/// Returns the given instant adjusted one year forward taking into account leap years and other
/// adjustments like day of week.
/// </summary>
/// <remarks>
/// If the given instant is before the starting year, the year of the given instant is
/// adjusted to the beginning of the starting year. The first transition after the
/// adjusted instant is determined. If the next adjustment is after the ending year, this
/// method returns null; otherwise the next transition is returned.
/// </remarks>
/// <param name="instant">The <see cref="Instant"/> lower bound for the next transition.</param>
/// <param name="standardOffset">The <see cref="Offset"/> standard offset.</param>
/// <param name="previousSavings">The <see cref="Offset"/> savings adjustment at the given Instant.</param>
/// <returns>The next transition, or null if there is no next transition.</returns>
internal Transition?Next(Instant instant, Offset standardOffset, Offset previousSavings)
{
CalendarSystem calendar = CalendarSystem.Iso;
Offset wallOffset = standardOffset + previousSavings;
int year = instant == Instant.MinValue ? Int32.MinValue : calendar.GetYear(instant.Plus(wallOffset));
if (year < fromYear)
{
// First advance instant to start of from year.
instant = calendar.GetLocalInstant(fromYear, 1, 1, 0, 0).Minus(wallOffset);
// Back off one tick to account for next recurrence being exactly at the beginning
// of the year.
instant = instant - Duration.Epsilon;
}
Instant next = yearOffset.Next(instant, standardOffset, previousSavings);
if (next >= instant)
{
year = calendar.GetYear(next.Plus(wallOffset));
if (year > toYear)
{
return(null);
}
}
return(new Transition(next, wallOffset, standardOffset + Savings));
}
