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 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-

  * vim: set ts=8 sts=4 et sw=4 tw=99:

  * This Source Code Form is subject to the terms of the Mozilla Public

  * License, v. 2.0. If a copy of the MPL was not distributed with this

  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

 #ifndef vm_DateTime_h

 #define vm_DateTime_h

 #include "mozilla/FloatingPoint.h"

 #include "mozilla/MathAlgorithms.h"

 #include <stdint.h>

 #include "js/Value.h"

 #include "vm/NumericConversions.h"

 namespace js {

 /* Constants defined by ES5 15.9.1.10. */

 const double HoursPerDay = 24;

 const double MinutesPerHour = 60;

 const double SecondsPerMinute = 60;

 const double msPerSecond = 1000;

 const double msPerMinute = msPerSecond * SecondsPerMinute;

 const double msPerHour = msPerMinute * MinutesPerHour;

 /* ES5 15.9.1.2. */

 const double msPerDay = msPerHour * HoursPerDay;

 /*

  * Additional quantities not mentioned in the spec.  Be careful using these!

  * They aren't doubles (and aren't defined in terms of all the other constants

  * so that they can be used in constexpr scenarios; if you need constants that

  * trigger floating point semantics, you'll have to manually cast to get it.

  */

 const unsigned SecondsPerHour = 60 * 60;

 const unsigned SecondsPerDay = SecondsPerHour * 24;

 const double StartOfTime = -8.64e15;

 const double EndOfTime = 8.64e15;

 const double MaxTimeMagnitude = 8.64e15;

 /* ES5 15.9.1.14. */

 inline double

 TimeClip(double time)

 {

     /* Steps 1-2. */

     if (!mozilla::IsFinite(time) || mozilla::Abs(time) > MaxTimeMagnitude)

         return JS::GenericNaN();

     /* Step 3. */

     return ToInteger(time + (+0.0));

 }

 /*

  * Stores date/time information, particularly concerning the current local

  * time zone, and implements a small cache for daylight saving time offset

  * computation.

  *

  * The basic idea is premised upon this fact: the DST offset never changes more

  * than once in any thirty-day period.  If we know the offset at t_0 is o_0,

  * the offset at [t_1, t_2] is also o_0, where t_1 + 3_0 days == t_2,

  * t_1 <= t_0, and t0 <= t2.  (In other words, t_0 is always somewhere within a

  * thirty-day range where the DST offset is constant: DST changes never occur

  * more than once in any thirty-day period.)  Therefore, if we intelligently

  * retain knowledge of the offset for a range of dates (which may vary over

  * time), and if requests are usually for dates within that range, we can often

  * provide a response without repeated offset calculation.

  *

  * Our caching strategy is as follows: on the first request at date t_0 compute

  * the requested offset o_0.  Save { start: t_0, end: t_0, offset: o_0 } as the

  * cache's state.  Subsequent requests within that range are straightforwardly

  * handled.  If a request for t_i is far outside the range (more than thirty

  * days), compute o_i = dstOffset(t_i) and save { start: t_i, end: t_i,

  * offset: t_i }.  Otherwise attempt to *overextend* the range to either

  * [start - 30d, end] or [start, end + 30d] as appropriate to encompass

  * t_i.  If the offset o_i30 is the same as the cached offset, extend the

  * range.  Otherwise the over-guess crossed a DST change -- compute

  * o_i = dstOffset(t_i) and either extend the original range (if o_i == offset)

  * or start a new one beneath/above the current one with o_i30 as the offset.

  *

  * This cache strategy results in 0 to 2 DST offset computations.  The naive

  * always-compute strategy is 1 computation, and since cache maintenance is a

  * handful of integer arithmetic instructions the speed difference between

  * always-1 and 1-with-cache is negligible.  Caching loses if two computations

  * happen: when the date is within 30 days of the cached range and when that

  * 30-day range crosses a DST change.  This is relatively uncommon.  Further,

  * instances of such are often dominated by in-range hits, so caching is an

  * overall slight win.

  *

  * Why 30 days?  For correctness the duration must be smaller than any possible

  * duration between DST changes.  Past that, note that 1) a large duration

  * increases the likelihood of crossing a DST change while reducing the number

  * of cache misses, and 2) a small duration decreases the size of the cached

  * range while producing more misses.  Using a month as the interval change is

  * a balance between these two that tries to optimize for the calendar month at

  * a time that a site might display.  (One could imagine an adaptive duration

  * that accommodates near-DST-change dates better; we don't believe the

  * potential win from better caching offsets the loss from extra complexity.)

  */

 class DateTimeInfo

 {

   public:

     DateTimeInfo();

     /*

      * Get the DST offset in milliseconds at a UTC time.  This is usually

      * either 0 or |msPerSecond * SecondsPerHour|, but at least one exotic time

      * zone (Lord Howe Island, Australia) has a fractional-hour offset, just to

      * keep things interesting.

      */

     int64_t getDSTOffsetMilliseconds(int64_t utcMilliseconds);

     void updateTimeZoneAdjustment();

     /* ES5 15.9.1.7. */

     double localTZA() { return localTZA_; }

   private:

     /*

      * The current local time zone adjustment, cached because retrieving this

      * dynamically is Slow, and a certain venerable benchmark which shall not

      * be named depends on it being fast.

      *

      * SpiderMonkey occasionally and arbitrarily updates this value from the

      * system time zone to attempt to keep this reasonably up-to-date.  If

      * temporary inaccuracy can't be tolerated, JSAPI clients may call

      * JS_ClearDateCaches to forcibly sync this with the system time zone.

      */

     double localTZA_;

     /*

      * Compute the DST offset at the given UTC time in seconds from the epoch.

      * (getDSTOffsetMilliseconds attempts to return a cached value, but in case

      * of a cache miss it calls this method.  The cache is represented through

      * the offset* and *{Start,End}Seconds fields below.)

      */

     int64_t computeDSTOffsetMilliseconds(int64_t utcSeconds);

     int64_t offsetMilliseconds;

     int64_t rangeStartSeconds, rangeEndSeconds; // UTC-based

     int64_t oldOffsetMilliseconds;

     int64_t oldRangeStartSeconds, oldRangeEndSeconds; // UTC-based

     /*

      * Cached offset in seconds from the current UTC time to the current

      * local standard time (i.e. not including any offset due to DST).

      */

     int32_t utcToLocalStandardOffsetSeconds;

     static const int64_t MaxUnixTimeT = 2145859200; /* time_t 12/31/2037 */

     static const int64_t RangeExpansionAmount = 30 * SecondsPerDay;

     void sanityCheck();

 };

 }  /* namespace js */

 #endif /* vm_DateTime_h */