The Tor Browser: js/src/vm/DateTime.h@6474c204b198 (annotated)

js/src/vm/DateTime.h@6474c204b198 (annotated)

js/src/vm/DateTime.h

Wed, 31 Dec 2014 06:09:35 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Wed, 31 Dec 2014 06:09:35 +0100
changeset 0: 6474c204b198
permissions: -rw-r--r--

Cloned upstream origin tor-browser at tor-browser-31.3.0esr-4.5-1-build1
revision ID fc1c9ff7c1b2defdbc039f12214767608f46423f for hacking purpose.

 /* -*- 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 */

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js/src/vm/DateTime.h@6474c204b198 (annotated)

js/src/vm/DateTime.h