https://github.com/mozilla/gecko-dev
Tip revision: 68bba3abd53ee78d9996e00a47b9d024604d30bf authored by Ryan VanderMeulen on 29 July 2015, 14:12:35 UTC
Added tag B2G_2_0_END for changeset 2e6f1d4deff9 on a CLOSED TREE
Added tag B2G_2_0_END for changeset 2e6f1d4deff9 on a CLOSED TREE
Tip revision: 68bba3a
DateTime.h
/* -*- 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 */