js/src/prmjtime.h

   1 /* -*- Mode: C; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
   2  *
   3  * ***** BEGIN LICENSE BLOCK *****
   4  * Version: MPL 1.1/GPL 2.0/LGPL 2.1
   5  *
   6  * The contents of this file are subject to the Mozilla Public License Version
   7  * 1.1 (the "License"); you may not use this file except in compliance with
   8  * the License. You may obtain a copy of the License at
   9  * http://www.mozilla.org/MPL/
  10  *
  11  * Software distributed under the License is distributed on an "AS IS" basis,
  12  * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
  13  * for the specific language governing rights and limitations under the
  14  * License.
  15  *
  16  * The Original Code is Mozilla Communicator client code, released
  17  * March 31, 1998.
  18  *
  19  * The Initial Developer of the Original Code is
  20  * Netscape Communications Corporation.
  21  * Portions created by the Initial Developer are Copyright (C) 1998
  22  * the Initial Developer. All Rights Reserved.
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  24  * Contributor(s):
  25  *
  26  * Alternatively, the contents of this file may be used under the terms of
  27  * either of the GNU General Public License Version 2 or later (the "GPL"),
  28  * or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
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  37  *
  38  * ***** END LICENSE BLOCK ***** */
  39
  40 #ifndef prmjtime_h___
  41 #define prmjtime_h___
  42 /*
  43  * PR date stuff for mocha and java. Placed here temporarily not to break
  44  * Navigator and localize changes to mocha.
  45  */
  46 #include <time.h>
  47
  48 #include "jslong.h"
  49 #ifdef MOZILLA_CLIENT
  50 #include "jscompat.h"
  51 #endif
  52
  53 struct JSContext;
  54
  55 /*
  56  * Implements a small cache for daylight saving time offset computation.
  57  *
  58  * The basic idea is premised upon this fact: the DST offset never changes more
  59  * than once in any thirty-day period.  If we know the offset at t_0 is o_0,
  60  * the offset at [t_1, t_2] is also o_0, where t_1 + 3_0 days == t_2,
  61  * t_1 <= t_0, and t0 <= t2.  (In other words, t_0 is always somewhere within a
  62  * thirty-day range where the DST offset is constant: DST changes never occur
  63  * more than once in any thirty-day period.)  Therefore, if we intelligently
  64  * retain knowledge of the offset for a range of dates (which may vary over
  65  * time), and if requests are usually for dates within that range, we can often
  66  * provide a response without repeated offset calculation.
  67  *
  68  * Our caching strategy is as follows: on the first request at date t_0 compute
  69  * the requested offset o_0.  Save { start: t_0, end: t_0, offset: o_0 } as the
  70  * cache's state.  Subsequent requests within that range are straightforwardly
  71  * handled.  If a request for t_i is far outside the range (more than thirty
  72  * days), compute o_i = dstOffset(t_i) and save { start: t_i, end: t_i,
  73  * offset: t_i }.  Otherwise attempt to *overextend* the range to either
  74  * [start - 30d, end] or [start, end + 30d] as appropriate to encompass
  75  * t_i.  If the offset o_i30 is the same as the cached offset, extend the
  76  * range.  Otherwise the over-guess crossed a DST change -- compute
  77  * o_i = dstOffset(t_i) and either extend the original range (if o_i == offset)
  78  * or start a new one beneath/above the current one with o_i30 as the offset.
  79  *
  80  * This cache strategy results in 0 to 2 DST offset computations.  The naive
  81  * always-compute strategy is 1 computation, and since cache maintenance is a
  82  * handful of integer arithmetic instructions the speed difference between
  83  * always-1 and 1-with-cache is negligible.  Caching loses if two computations
  84  * happen: when the date is within 30 days of the cached range and when that
  85  * 30-day range crosses a DST change.  This is relatively uncommon.  Further,
  86  * instances of such are often dominated by in-range hits, so caching is an
  87  * overall slight win.
  88  *
  89  * Why 30 days?  For correctness the duration must be smaller than any possible
  90  * duration between DST changes.  Past that, note that 1) a large duration
  91  * increases the likelihood of crossing a DST change while reducing the number
  92  * of cache misses, and 2) a small duration decreases the size of the cached
  93  * range while producing more misses.  Using a month as the interval change is
  94  * a balance between these two that tries to optimize for the calendar month at
  95  * a time that a site might display.  (One could imagine an adaptive duration
  96  * that accommodates near-DST-change dates better; we don't believe the
  97  * potential win from better caching offsets the loss from extra complexity.)
  98  */
  99 class DSTOffsetCache {
 100   public:
 101     inline DSTOffsetCache();
 102     JSInt64 getDSTOffsetMilliseconds(int64 localTimeMilliseconds, JSContext *cx);
 103
 104     inline void purge();
 105 #ifdef JS_METER_DST_OFFSET_CACHING
 106     void dumpStats();
 107 #endif
 108
 109   private:
 110     JSInt64 computeDSTOffsetMilliseconds(int64 localTimeSeconds);
 111
 112     JSInt64 offsetMilliseconds;
 113     JSInt64 rangeStartSeconds, rangeEndSeconds;
 114
 115     JSInt64 oldOffsetMilliseconds;
 116     JSInt64 oldRangeStartSeconds, oldRangeEndSeconds;
 117
 118 #ifdef JS_METER_DST_OFFSET_CACHING
 119     size_t totalCalculations;
 120     size_t hit;
 121     size_t missIncreasing;
 122     size_t missDecreasing;
 123     size_t missIncreasingOffsetChangeUpper;
 124     size_t missIncreasingOffsetChangeExpand;
 125     size_t missLargeIncrease;
 126     size_t missDecreasingOffsetChangeLower;
 127     size_t missDecreasingOffsetChangeExpand;
 128     size_t missLargeDecrease;
 129 #endif
 130
 131     static const JSInt64 MAX_UNIX_TIMET = 2145859200; /* time_t 12/31/2037 */
 132     static const JSInt64 MILLISECONDS_PER_SECOND = 1000;
 133     static const JSInt64 SECONDS_PER_MINUTE = 60;
 134     static const JSInt64 SECONDS_PER_HOUR = 60 * SECONDS_PER_MINUTE;
 135     static const JSInt64 SECONDS_PER_DAY = 24 * SECONDS_PER_HOUR;
 136
 137     static const JSInt64 RANGE_EXPANSION_AMOUNT = 30 * SECONDS_PER_DAY;
 138
 139   private:
 140     void sanityCheck();
 141
 142 #ifdef JS_METER_DST_OFFSET_CACHING
 143 #define NOTE_GENERIC(member) this->member++
 144 #else
 145 #define NOTE_GENERIC(member) ((void)0)
 146 #endif
 147     void noteOffsetCalculation() {
 148         NOTE_GENERIC(totalCalculations);
 149     }
 150     void noteCacheHit() {
 151         NOTE_GENERIC(hit);
 152     }
 153     void noteCacheMissIncrease() {
 154         NOTE_GENERIC(missIncreasing);
 155     }
 156     void noteCacheMissDecrease() {
 157         NOTE_GENERIC(missDecreasing);
 158     }
 159     void noteCacheMissIncreasingOffsetChangeUpper() {
 160         NOTE_GENERIC(missIncreasingOffsetChangeUpper);
 161     }
 162     void noteCacheMissIncreasingOffsetChangeExpand() {
 163         NOTE_GENERIC(missIncreasingOffsetChangeExpand);
 164     }
 165     void noteCacheMissLargeIncrease() {
 166         NOTE_GENERIC(missLargeIncrease);
 167     }
 168     void noteCacheMissDecreasingOffsetChangeLower() {
 169         NOTE_GENERIC(missDecreasingOffsetChangeLower);
 170     }
 171     void noteCacheMissDecreasingOffsetChangeExpand() {
 172         NOTE_GENERIC(missDecreasingOffsetChangeExpand);
 173     }
 174     void noteCacheMissLargeDecrease() {
 175         NOTE_GENERIC(missLargeDecrease);
 176     }
 177 #undef NOTE_GENERIC
 178 };
 179
 180 JS_BEGIN_EXTERN_C
 181
 182 typedef struct PRMJTime       PRMJTime;
 183
 184 /*
 185  * Broken down form of 64 bit time value.
 186  */
 187 struct PRMJTime {
 188     JSInt32 tm_usec;            /* microseconds of second (0-999999) */
 189     JSInt8 tm_sec;              /* seconds of minute (0-59) */
 190     JSInt8 tm_min;              /* minutes of hour (0-59) */
 191     JSInt8 tm_hour;             /* hour of day (0-23) */
 192     JSInt8 tm_mday;             /* day of month (1-31) */
 193     JSInt8 tm_mon;              /* month of year (0-11) */
 194     JSInt8 tm_wday;             /* 0=sunday, 1=monday, ... */
 195     JSInt32 tm_year;            /* absolute year, AD */
 196     JSInt16 tm_yday;            /* day of year (0 to 365) */
 197     JSInt8 tm_isdst;            /* non-zero if DST in effect */
 198 };
 199
 200 /* Some handy constants */
 201 #define PRMJ_USEC_PER_SEC       1000000L
 202 #define PRMJ_USEC_PER_MSEC      1000L
 203
 204 /* Return the current local time in micro-seconds */
 205 extern JSInt64
 206 PRMJ_Now(void);
 207
 208 /* Release the resources associated with PRMJ_Now; don't call PRMJ_Now again */
 209 #if defined(JS_THREADSAFE) && (defined(HAVE_GETSYSTEMTIMEASFILETIME) || defined(HAVE_SYSTEMTIMETOFILETIME))
 210 extern void
 211 PRMJ_NowShutdown(void);
 212 #else
 213 #define PRMJ_NowShutdown()
 214 #endif
 215
 216 /* get the difference between this time zone and  gmt timezone in seconds */
 217 extern JSInt32
 218 PRMJ_LocalGMTDifference(void);
 219
 220 /* Format a time value into a buffer. Same semantics as strftime() */
 221 extern size_t
 222 PRMJ_FormatTime(char *buf, int buflen, const char *fmt, PRMJTime *tm);
 223
 224 JS_END_EXTERN_C
 225
 226 #endif /* prmjtime_h___ */
 227