diff -r 000000000000 -r 6474c204b198 js/src/vm/DateTime.h --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/js/src/vm/DateTime.h Wed Dec 31 06:09:35 2014 +0100 @@ -0,0 +1,162 @@ +/* -*- 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 + +#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 */