/// <summary>
/// Adds the intervals from the given rule set to the end of the zone
/// being built. The rule is deemed to take effect from the end of the previous
/// zone interval, or the start of time if this is the first rule set (which must
/// be a fixed one). Intervals are added until the rule set expires, or
/// until we determine that the rule set continues to the end of time,
/// possibly with a tail zone - a pair of standard/daylight rules which repeat
/// forever.
/// </summary>
private void AddIntervals(ZoneRuleSet ruleSet)
{
// We use the last zone interval computed so far (if there is one) to work out where to start.
var lastZoneInterval = zoneIntervals.LastOrDefault();
var start = lastZoneInterval?.End ?? Instant.BeforeMinValue;
// Simple case: a zone line with fixed savings (or - for 0)
// instead of a rule name. Just a single interval.
if (ruleSet.IsFixed)
{
zoneIntervals.Add(ruleSet.CreateFixedInterval(start));
return;
}
// Work on a copy of the rule set. We eliminate rules from it as they expire,
// so that we can tell when we're down to an infinite pair which can be represented
// as a tail zone.
var activeRules = new List <ZoneRecurrence>(ruleSet.Rules);
// Surprisingly tricky bit to work out: how to handle the transition from
// one rule set to another. We know the instant at which the new rule set
// come in, but not what offsets/name to use from that point onwards: which
// of the new rules is in force. We find out which rule would have taken
// effect most recently before or on the transition instant - but using
// the offsets from the final interval before the transition, instead
// of the offsets which would have been in force if the new rule set were
// actually extended backwards forever.
//
// It's possible that the most recent transition we find would actually
// have started before that final interval anyway - but this appears to
// match what zic produces.
//
// If we don't have a zone interval at all, we're starting at the start of
// time, so there definitely aren't any preceding rules.
var firstRule = lastZoneInterval is null ? null :
activeRules
.Select(rule => new { rule, prev = rule.PreviousOrSame(start, lastZoneInterval.StandardOffset, lastZoneInterval.Savings) })
.Where(pair => pair.prev != null)
.OrderBy(pair => pair.prev !.Value.Instant)
.Select(pair => pair.rule)
.LastOrDefault();
// Every transition in this rule set will use the same standard offset.
var standardOffset = ruleSet.StandardOffset;
// previousTransition here is ongoing as we loop through the transitions. It's not like
// lastZoneInterval, lastStandard and lastSavings, which refer to the last aspects of the
// previous rule set. When we set it up, this is effectively the *first* transition leading
// into the period in which the new rule set is
ZoneTransition previousTransition;
if (firstRule != null)
{
previousTransition = new ZoneTransition(start, firstRule.Name, standardOffset, firstRule.Savings);
}
else
{
// None of the rules in the current set have *any* transitions in the past, apparently.
// For an example of this, see Europe/Prague (in 2015e, anyway). A zone line with the
// Czech rule takes effect in 1944, but all the rules are from 1945 onwards.
// Use standard time until the first transition, regardless of the previous savings,
// and take the name for this first interval from the first standard time rule.
var name = activeRules.First(rule => rule.Savings == Offset.Zero).Name;
previousTransition = new ZoneTransition(start, name, standardOffset, Offset.Zero);
}
// Main loop - we keep going round until we run out of rules or hit infinity, each of which
// corresponds with a return statement in the loop.
while (true)
{
ZoneTransition?bestTransition = null;
for (int i = 0; i < activeRules.Count; i++)
{
var rule = activeRules[i];
var nextTransition = rule.Next(previousTransition.Instant, standardOffset, previousTransition.Savings);
// Once a rule is no longer active, remove it from the list. That way we can tell
// when we can create a tail zone.
if (nextTransition is null)
{
activeRules.RemoveAt(i);
i--;
continue;
}
var zoneTransition = new ZoneTransition(nextTransition.Value.Instant, rule.Name, standardOffset, rule.Savings);
if (!zoneTransition.IsTransitionFrom(previousTransition))
{
continue;
}
if (bestTransition is null || zoneTransition.Instant <= bestTransition.Instant)
{
bestTransition = zoneTransition;
}
}
Instant currentUpperBound = ruleSet.GetUpperLimit(previousTransition.Savings);
if (bestTransition is null || bestTransition.Instant >= currentUpperBound)
{
// No more transitions to find. (We may have run out of rules, or they may be beyond where this rule set expires.)
// Add a final interval leading up to the upper bound of the rule set, unless the previous transition took us up to
// this current bound anyway.
// (This is very rare, but can happen if changing rule brings the upper bound down to the time
// that the transition occurred. Example: 2008d, Europe/Sofia, April 1945.)
if (currentUpperBound > previousTransition.Instant)
{
zoneIntervals.Add(previousTransition.ToZoneInterval(currentUpperBound));
}
return;
}
// We have a non-final transition. so add an interval from the previous transition to
// this one.
zoneIntervals.Add(previousTransition.ToZoneInterval(bestTransition.Instant));
previousTransition = bestTransition;
// Tail zone handling.
// The final rule set must extend to infinity. There are potentially three ways
// this can happen:
// - All rules expire, leaving us with the final real transition, and an upper
// bound of infinity. This is handled above.
// - 1 rule is left, but it cannot create more than one transition in a row,
// so again we end up with no transitions to record, and we bail out with
// a final infinite interval.
// - 2 rules are left which would alternate infinitely. This is represented
// using a DaylightSavingZone as the tail zone.
//
// The code here caters for that last option, but needs to do it in stages.
// When we first realize we will have a tail zone (an infinite rule set,
// two rules left, both of which are themselves infinite) we can create the
// tail zone, but we don't yet know that we're into its regular tick/tock.
// It's possible that one rule only starts years after our current transition,
// so we need to hit the first transition of that rule before we can create a
// "seam" from the list of precomputed zone intervals to the calculated-on-demand
// part of history.
// For an example of why this is necessary, see Asia/Amman in 2013e: in late 2011
// we hit "two rules left" but the final rule only starts in 2013 - we don't want
// to see a bogus transition into that rule in 2012.
// We could potentially record fewer zone intervals by keeping track of which
// rules have created at least one transition, but this approach is simpler.
if (ruleSet.IsInfinite && activeRules.Count == 2)
{
if (tailZone != null)
{
// Phase two: both rules must now be active, so we're done.
return;
}
ZoneRecurrence startRule = activeRules[0];
ZoneRecurrence endRule = activeRules[1];
if (startRule.IsInfinite && endRule.IsInfinite)
{
// Phase one: build the zone, so we can go round once again and then return.
tailZone = new StandardDaylightAlternatingMap(standardOffset, startRule, endRule);
}
}
}
}
/// <summary>
/// Adds the intervals from the given rule set to the end of the zone
/// being built. The rule is deemed to take effect from the end of the previous
/// zone interval, or the start of time if this is the first rule set (which must
/// be a fixed one). Intervals are added until the rule set expires, or
/// until we determine that the rule set continues to the end of time,
/// possibly with a tail zone - a pair of standard/daylight rules which repeat
/// forever.
/// </summary>
private void AddIntervals(ZoneRuleSet ruleSet)
{
// We use the last zone interval computed so far (if there is one) to work out where to start.
var lastZoneInterval = zoneIntervals.LastOrDefault();
var start = lastZoneInterval?.End ?? Instant.BeforeMinValue;
// Simple case: a zone line with fixed savings (or - for 0)
// instead of a rule name. Just a single interval.
if (ruleSet.IsFixed)
{
zoneIntervals.Add(ruleSet.CreateFixedInterval(start));
return;
}
// Work on a copy of the rule set. We eliminate rules from it as they expire,
// so that we can tell when we're down to an infinite pair which can be represented
// as a tail zone.
var activeRules = new List<ZoneRecurrence>(ruleSet.Rules);
// Surprisingly tricky bit to work out: how to handle the transition from
// one rule set to another. We know the instant at which the new rule set
// come in, but not what offsets/name to use from that point onwards: which
// of the new rules is in force. We find out which rule would have taken
// effect most recently before or on the transition instant - but using
// the offsets from the final interval before the transition, instead
// of the offsets which would have been in force if the new rule set were
// actually extended backwards forever.
//
// It's possible that the most recent transition we find would actually
// have started before that final interval anyway - but this appears to
// match what zic produces.
//
// If we don't have a zone interval at all, we're starting at the start of
// time, so there definitely aren't any preceding rules.
var firstRule = lastZoneInterval == null ? null :
activeRules
.Select(rule => new { rule, prev = rule.PreviousOrSame(start, lastZoneInterval.StandardOffset, lastZoneInterval.Savings) })
.Where(pair => pair.prev != null)
.OrderBy(pair => pair.prev.Value.Instant)
.Select(pair => pair.rule)
.LastOrDefault();
// Every transition in this rule set will use the same standard offset.
var standardOffset = ruleSet.StandardOffset;
// previousTransition here is ongoing as we loop through the transitions. It's not like
// lastZoneInterval, lastStandard and lastSavings, which refer to the last aspects of the
// previous rule set. When we set it up, this is effectively the *first* transition leading
// into the period in which the new rule set is
ZoneTransition previousTransition;
if (firstRule != null)
{
previousTransition = new ZoneTransition(start, firstRule.Name, standardOffset, firstRule.Savings);
}
else
{
// None of the rules in the current set have *any* transitions in the past, apparently.
// For an example of this, see Europe/Prague (in 2015e, anyway). A zone line with the
// Czech rule takes effect in 1944, but all the rules are from 1945 onwards.
// Use standard time until the first transition, regardless of the previous savings,
// and take the name for this first interval from the first standard time rule.
var name = activeRules.First(rule => rule.Savings == Offset.Zero).Name;
previousTransition = new ZoneTransition(start, name, standardOffset, Offset.Zero);
}
// Main loop - we keep going round until we run out of rules or hit infinity, each of which
// corresponds with a return statement in the loop.
while (true)
{
ZoneTransition bestTransition = null;
for (int i = 0; i < activeRules.Count; i++)
{
var rule = activeRules[i];
var nextTransition = rule.Next(previousTransition.Instant, standardOffset, previousTransition.Savings);
// Once a rule is no longer active, remove it from the list. That way we can tell
// when we can create a tail zone.
if (nextTransition == null)
{
activeRules.RemoveAt(i);
i--;
continue;
}
var zoneTransition = new ZoneTransition(nextTransition.Value.Instant, rule.Name, standardOffset, rule.Savings);
if (!zoneTransition.IsTransitionFrom(previousTransition))
{
continue;
}
if (bestTransition == null || zoneTransition.Instant <= bestTransition.Instant)
{
bestTransition = zoneTransition;
}
}
Instant currentUpperBound = ruleSet.GetUpperLimit(previousTransition.Savings);
if (bestTransition == null || bestTransition.Instant >= currentUpperBound)
{
// No more transitions to find. (We may have run out of rules, or they may be beyond where this rule set expires.)
// Add a final interval leading up to the upper bound of the rule set, unless the previous transition took us up to
// this current bound anyway.
// (This is very rare, but can happen if changing rule brings the upper bound down to the time
// that the transition occurred. Example: 2008d, Europe/Sofia, April 1945.)
if (currentUpperBound > previousTransition.Instant)
{
zoneIntervals.Add(previousTransition.ToZoneInterval(currentUpperBound));
}
return;
}
// We have a non-final transition. so add an interval from the previous transition to
// this one.
zoneIntervals.Add(previousTransition.ToZoneInterval(bestTransition.Instant));
previousTransition = bestTransition;
// Tail zone handling.
// The final rule set must extend to infinity. There are potentially three ways
// this can happen:
// - All rules expire, leaving us with the final real transition, and an upper
// bound of infinity. This is handled above.
// - 1 rule is left, but it cannot create more than one transition in a row,
// so again we end up with no transitions to record, and we bail out with
// a final infinite interval.
// - 2 rules are left which would alternate infinitely. This is represented
// using a DaylightSavingZone as the tail zone.
//
// The code here caters for that last option, but needs to do it in stages.
// When we first realize we will have a tail zone (an infinite rule set,
// two rules left, both of which are themselves infinite) we can create the
// tail zone, but we don't yet know that we're into its regular tick/tock.
// It's possible that one rule only starts years after our current transition,
// so we need to hit the first transition of that rule before we can create a
// "seam" from the list of precomputed zone intervals to the calculated-on-demand
// part of history.
// For an example of why this is necessary, see Asia/Amman in 2013e: in late 2011
// we hit "two rules left" but the final rule only starts in 2013 - we don't want
// to see a bogus transition into that rule in 2012.
// We could potentially record fewer zone intervals by keeping track of which
// rules have created at least one transition, but this approach is simpler.
if (ruleSet.IsInfinite && activeRules.Count == 2)
{
if (tailZone != null)
{
// Phase two: both rules must now be active, so we're done.
return;
}
ZoneRecurrence startRule = activeRules[0];
ZoneRecurrence endRule = activeRules[1];
if (startRule.IsInfinite && endRule.IsInfinite)
{
// Phase one: build the zone, so we can go round once again and then return.
tailZone = new StandardDaylightAlternatingMap(standardOffset, startRule, endRule);
}
}
}
}
