CoreFoundation/CFDate.c

/*
 * Copyright (c) 2008-2009 Brent Fulgham <bfulgham@gmail.org>.  All rights reserved.
 *
 * This source code is a modified version of the CoreFoundation sources released by Apple Inc. under
 * the terms of the APSL version 2.0 (see below).
 *
 * For information about changes from the original Apple source release can be found by reviewing the
 * source control system for the project at https://sourceforge.net/svn/?group_id=246198.
 *
 * The original license information is as follows:
 * 
 * Copyright (c) 2008 Apple Inc. All rights reserved.
 *
 * @APPLE_LICENSE_HEADER_START@
 * 
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this
 * file.
 * 
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 * 
 * @APPLE_LICENSE_HEADER_END@
 */
/*	CFDate.c
	Copyright 1998-2002, Apple, Inc. All rights reserved.
	Responsibility: Christopher Kane
*/

#include <CoreFoundation/CFDate.h>
#include <CoreFoundation/CFTimeZone.h>
#include <CoreFoundation/CFDictionary.h>
#include <CoreFoundation/CFArray.h>
#include <CoreFoundation/CFString.h>
#include <CoreFoundation/CFNumber.h>
#include "CFInternal.h"
#include <math.h>
#if DEPLOYMENT_TARGET_LINUX
#include <time.h>
#endif
#if DEPLOYMENT_TARGET_MACOSX
#include <sys/time.h>
#endif

#if defined(_MSC_VER)
extern void gettimeofday(struct timeval *tv, void *dummy);
#endif

const CFTimeInterval kCFAbsoluteTimeIntervalSince1970 = 978307200.0L;
const CFTimeInterval kCFAbsoluteTimeIntervalSince1904 = 3061152000.0L;

/* cjk: The Julian Date for the reference date is 2451910.5,
        I think, in case that's ever useful. */

__private_extern__ double __CFTSRRate = 0.0;
static double __CF1_TSRRate = 0.0;

#if DEPLOYMENT_TARGET_WINDOWS
// We should export this as SPI or API to clients - 3514284
CFAbsoluteTime _CFAbsoluteTimeFromFileTime(const FILETIME *ft) {
    CFAbsoluteTime ret = (CFTimeInterval)ft->dwHighDateTime * 429.49672960;
    ret += (CFTimeInterval)ft->dwLowDateTime / 10000000.0;
    ret -= (11644473600.0 + kCFAbsoluteTimeIntervalSince1970);
    /* seconds between 1601 and 1970, 1970 and 2001 */
    return ret;
}
#endif

__private_extern__ int64_t __CFTimeIntervalToTSR(CFTimeInterval ti) {
    if ((ti * __CFTSRRate) > INT64_MAX / 2) return (INT64_MAX / 2);
    return (int64_t)(ti * __CFTSRRate);
}

__private_extern__ CFTimeInterval __CFTSRToTimeInterval(int64_t tsr) {
    return (CFTimeInterval)((double)tsr * __CF1_TSRRate);
}

#ifdef DEPLOYMENT_TARGET_WINDOWS
__private_extern__ CFAbsoluteTime __CFDateWindowsSystemTimeToAbsoluteTime(SYSTEMTIME *the_time) {
   // this song and dance seems to be required to get enough precision
   // and date offset that the OSX version returns - steps were culled
   // from the remarks section of RtlTimeToSecondsSince1970 in the
   // msdn documentation

   SYSTEMTIME darwin_epoch;
   FILETIME also_the_time, also_epoch;
   ULARGE_INTEGER also_also_the_time, also_also_epoch;

   memset(&darwin_epoch, 0, sizeof(SYSTEMTIME));
   SystemTimeToFileTime(the_time, &also_the_time);

   // MSDN says casting a pointer from FILETIME to ULARGE_INTEGER* or __int64* can cause alignment faults on WIN64
   also_also_the_time.LowPart = also_the_time.dwLowDateTime;
   also_also_the_time.HighPart = also_the_time.dwHighDateTime;

   darwin_epoch.wYear = 2001;
   darwin_epoch.wMonth = 1;
   darwin_epoch.wDay = 1;
   SystemTimeToFileTime(&darwin_epoch, &also_epoch);
   also_also_epoch.LowPart = also_epoch.dwLowDateTime;
   also_also_epoch.HighPart = also_epoch.dwHighDateTime;

   return((double)(also_also_the_time.QuadPart - also_also_epoch.QuadPart) / 10000000.0);
}
#endif // DEPLOYMENT_TARGET_WINDOWS

CFAbsoluteTime CFAbsoluteTimeGetCurrent(void) {
    CFAbsoluteTime ret;
#if DEPLOYMENT_TARGET_MACOSX || DEPLOYMENT_TARGET_LINUX || DEPLOYMENT_TARGET_FREEBSD
    struct timeval tv;
    gettimeofday(&tv, NULL);
    ret = (CFTimeInterval)tv.tv_sec - kCFAbsoluteTimeIntervalSince1970;
    ret += (1.0E-6 * (CFTimeInterval)tv.tv_usec);
#elif DEPLOYMENT_TARGET_WINDOWS
    FILETIME ft;
    GetSystemTimeAsFileTime(&ft);
    ret = _CFAbsoluteTimeFromFileTime(&ft);
#else
#error CFAbsoluteTimeGetCurrent unimplemented for this platform
#endif
   return ret;
}

__private_extern__ void __CFDateInitialize(void) {
#if DEPLOYMENT_TARGET_MACOSX
    struct mach_timebase_info info;
    mach_timebase_info(&info);
    __CFTSRRate = (1.0E9 / (double)info.numer) * (double)info.denom;
#elif DEPLOYMENT_TARGET_WINDOWS
    LARGE_INTEGER freq;
    if (!QueryPerformanceFrequency(&freq)) {
        HALT;
    }
    __CFTSRRate = (double)freq.QuadPart;
#elif DEPLOYMENT_TARGET_LINUX
	// On Linux, __CFReadTSR (see ForFoundation.h) is implemented on
	// the POSIX RT clock_* APIs which work at nanosecond granularity
	// (though not necessarily nanosecond resolution). Simply set the
	// TSRRate to 1 ns.
	__CFTSRRate = 1.0E9;
#else
#warning "__CFTSRRate not initialized for this platform!"
#endif
	__CF1_TSRRate = 1.0 / __CFTSRRate;
    CFDateGetTypeID(); // cause side-effects
}

#if 1
struct __CFDate {
    CFRuntimeBase _base;
    CFAbsoluteTime _time;       /* immutable */
};

static Boolean __CFDateEqual(CFTypeRef cf1, CFTypeRef cf2) {
    CFDateRef date1 = (CFDateRef)cf1;
    CFDateRef date2 = (CFDateRef)cf2;
    if (date1->_time != date2->_time) return false;
    return true;
}

static CFHashCode __CFDateHash(CFTypeRef cf) {
    CFDateRef date = (CFDateRef)cf;
    return (CFHashCode)(float)floor(date->_time);
}

static CFStringRef __CFDateCopyDescription(CFTypeRef cf) {
    CFDateRef date = (CFDateRef)cf;
    return CFStringCreateWithFormat(CFGetAllocator(date), NULL, CFSTR("<CFDate %p [%p]>{time = %0.09g}"), cf, CFGetAllocator(date), date->_time);
}

static CFTypeID __kCFDateTypeID = _kCFRuntimeNotATypeID;

static const CFRuntimeClass __CFDateClass = {
    0,
    "CFDate",
    NULL,       // init
    NULL,       // copy
    NULL,       // dealloc
    __CFDateEqual,
    __CFDateHash,
    NULL,       //
    __CFDateCopyDescription
};

CFTypeID CFDateGetTypeID(void) {
    if (_kCFRuntimeNotATypeID == __kCFDateTypeID) __kCFDateTypeID = _CFRuntimeRegisterClass(&__CFDateClass);
    return __kCFDateTypeID;
}

CFDateRef CFDateCreate(CFAllocatorRef allocator, CFAbsoluteTime at) {
    CFDateRef memory; 
    uint32_t size;
    size = sizeof(struct __CFDate) - sizeof(CFRuntimeBase);
    memory = (CFDateRef)_CFRuntimeCreateInstance(allocator, CFDateGetTypeID(), size, NULL);
    if (NULL == memory) {
        return NULL;
    }
    ((struct __CFDate *)memory)->_time = at;
    return memory;
}

CFTimeInterval CFDateGetAbsoluteTime(CFDateRef date) {
    CF_OBJC_FUNCDISPATCH0(CFDateGetTypeID(), CFTimeInterval, date, "timeIntervalSinceReferenceDate");
    __CFGenericValidateType(date, CFDateGetTypeID());
    return date->_time;
}

CFTimeInterval CFDateGetTimeIntervalSinceDate(CFDateRef date, CFDateRef otherDate) {
    CF_OBJC_FUNCDISPATCH1(CFDateGetTypeID(), CFTimeInterval, date, "timeIntervalSinceDate:", otherDate);
    __CFGenericValidateType(date, CFDateGetTypeID());
    __CFGenericValidateType(otherDate, CFDateGetTypeID());
    return date->_time - otherDate->_time;
}   
    
CFComparisonResult CFDateCompare(CFDateRef date, CFDateRef otherDate, void *context) {
    CF_OBJC_FUNCDISPATCH1(CFDateGetTypeID(), CFComparisonResult, date, "compare:", otherDate);
    __CFGenericValidateType(date, CFDateGetTypeID());
    __CFGenericValidateType(otherDate, CFDateGetTypeID());
    if (date->_time < otherDate->_time) return kCFCompareLessThan;
    if (date->_time > otherDate->_time) return kCFCompareGreaterThan;
    return kCFCompareEqualTo;
}
#endif

CF_INLINE int32_t __CFDoubleModToInt(double d, int32_t modulus) {
    int32_t result = (int32_t)(float)floor(d - floor(d / modulus) * modulus);
    if (result < 0) result += modulus;
    return result;
}

CF_INLINE double __CFDoubleMod(double d, int32_t modulus) {
    double result = d - floor(d / modulus) * modulus;
    if (result < 0.0) result += (double)modulus;
    return result;
}

static const uint8_t daysInMonth[16] = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31, 0, 0, 0};
static const uint16_t daysBeforeMonth[16] = {0, 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365, 0, 0};
static const uint16_t daysAfterMonth[16] = {365, 334, 306, 275, 245, 214, 184, 153, 122, 92, 61, 31, 0, 0, 0, 0};

CF_INLINE bool isleap(int64_t year) {
    int64_t y = (year + 1) % 400;	/* correct to nearest multiple-of-400 year, then find the remainder */
    if (y < 0) y = -y;
    return (0 == (y & 3) && 100 != y && 200 != y && 300 != y);
}

/* year arg is absolute year; Gregorian 2001 == year 0; 2001/1/1 = absolute date 0 */
CF_INLINE uint8_t __CFDaysInMonth(int8_t month, int64_t year, bool leap) {
    return daysInMonth[month] + (2 == month && leap);
}

/* year arg is absolute year; Gregorian 2001 == year 0; 2001/1/1 = absolute date 0 */
CF_INLINE uint16_t __CFDaysBeforeMonth(int8_t month, int64_t year, bool leap) {
    return daysBeforeMonth[month] + (2 < month && leap);
}

/* year arg is absolute year; Gregorian 2001 == year 0; 2001/1/1 = absolute date 0 */
CF_INLINE uint16_t __CFDaysAfterMonth(int8_t month, int64_t year, bool leap) {
    return daysAfterMonth[month] + (month < 2 && leap);
}

/* year arg is absolute year; Gregorian 2001 == year 0; 2001/1/1 = absolute date 0 */
static void __CFYMDFromAbsolute(int64_t absolute, int64_t *year, int8_t *month, int8_t *day) {
    int64_t b = absolute / 146097; // take care of as many multiples of 400 years as possible
    int64_t y = b * 400;
    uint16_t ydays;
    absolute -= b * 146097;
    while (absolute < 0) {
	y -= 1;
	absolute += __CFDaysAfterMonth(0, y, isleap(y));
    }
    /* Now absolute is non-negative days to add to year */
    ydays = __CFDaysAfterMonth(0, y, isleap(y));
    while (ydays <= absolute) {
	y += 1;
	absolute -= ydays;
	ydays = __CFDaysAfterMonth(0, y, isleap(y));
    }
    /* Now we have year and days-into-year */
    if (year) *year = y;
    if (month || day) {
	int8_t m = absolute / 33 + 1; /* search from the approximation */
	bool leap = isleap(y);
	while (__CFDaysBeforeMonth(m + 1, y, leap) <= absolute) m++;
	if (month) *month = m;
	if (day) *day = absolute - __CFDaysBeforeMonth(m, y, leap) + 1;
    }
}

/* year arg is absolute year; Gregorian 2001 == year 0; 2001/1/1 = absolute date 0 */
static double __CFAbsoluteFromYMD(int64_t year, int8_t month, int8_t day) {
    double absolute = 0.0;
    int64_t idx;
    int64_t b = year / 400; // take care of as many multiples of 400 years as possible
    absolute += b * 146097.0;
    year -= b * 400;
    if (year < 0) {
	for (idx = year; idx < 0; idx++)
	    absolute -= __CFDaysAfterMonth(0, idx, isleap(idx));
    } else {
	for (idx = 0; idx < year; idx++)
	    absolute += __CFDaysAfterMonth(0, idx, isleap(idx));
    }
    /* Now add the days into the original year */
    absolute += __CFDaysBeforeMonth(month, year, isleap(year)) + day - 1;
    return absolute;
}

Boolean CFGregorianDateIsValid(CFGregorianDate gdate, CFOptionFlags unitFlags) {
    if ((unitFlags & kCFGregorianUnitsYears) && (gdate.year <= 0)) return false;
    if ((unitFlags & kCFGregorianUnitsMonths) && (gdate.month < 1 || 12 < gdate.month)) return false;
    if ((unitFlags & kCFGregorianUnitsDays) && (gdate.day < 1 || 31 < gdate.day)) return false;
    if ((unitFlags & kCFGregorianUnitsHours) && (gdate.hour < 0 || 23 < gdate.hour)) return false;
    if ((unitFlags & kCFGregorianUnitsMinutes) && (gdate.minute < 0 || 59 < gdate.minute)) return false;
    if ((unitFlags & kCFGregorianUnitsSeconds) && (gdate.second < 0.0 || 60.0 <= gdate.second)) return false;
    if ((unitFlags & kCFGregorianUnitsDays) && (unitFlags & kCFGregorianUnitsMonths) && (unitFlags & kCFGregorianUnitsYears) && (__CFDaysInMonth(gdate.month, gdate.year - 2001, isleap(gdate.year - 2001)) < gdate.day)) return false;
    return true;
}

CFAbsoluteTime CFGregorianDateGetAbsoluteTime(CFGregorianDate gdate, CFTimeZoneRef tz) {
    CFAbsoluteTime at;
    CFTimeInterval offset0, offset1;
    if (NULL != tz) {
	__CFGenericValidateType(tz, CFTimeZoneGetTypeID());
    }
    at = 86400.0 * __CFAbsoluteFromYMD(gdate.year - 2001, gdate.month, gdate.day);
    at += 3600.0 * gdate.hour + 60.0 * gdate.minute + gdate.second;
    if (NULL != tz) {
	offset0 = CFTimeZoneGetSecondsFromGMT(tz, at);
	offset1 = CFTimeZoneGetSecondsFromGMT(tz, at - offset0);
	at -= offset1;
    }
    return at;
}

CFGregorianDate CFAbsoluteTimeGetGregorianDate(CFAbsoluteTime at, CFTimeZoneRef tz) {
    CFGregorianDate gdate;
    int64_t absolute, year;
    int8_t month, day;
    CFAbsoluteTime fixedat;
    if (NULL != tz) {
	__CFGenericValidateType(tz, CFTimeZoneGetTypeID());
    }
    fixedat = at + (NULL != tz ? CFTimeZoneGetSecondsFromGMT(tz, at) : 0.0);
    absolute = (int64_t)floor(fixedat / 86400.0);
    __CFYMDFromAbsolute(absolute, &year, &month, &day);
    if (INT32_MAX - 2001 < year) year = INT32_MAX - 2001;
    gdate.year = year + 2001;
    gdate.month = month;
    gdate.day = day;
    gdate.hour = __CFDoubleModToInt(floor(fixedat / 3600.0), 24);
    gdate.minute = __CFDoubleModToInt(floor(fixedat / 60.0), 60);
    gdate.second = __CFDoubleMod(fixedat, 60);
    if (0.0 == gdate.second) gdate.second = 0.0;	// stomp out possible -0.0
    return gdate;
}

/* Note that the units of years and months are not equal length, but are treated as such. */
CFAbsoluteTime CFAbsoluteTimeAddGregorianUnits(CFAbsoluteTime at, CFTimeZoneRef tz, CFGregorianUnits units) {
    CFGregorianDate gdate;
    CFGregorianUnits working;
    CFAbsoluteTime candidate_at0, candidate_at1;
    uint8_t monthdays;

    if (NULL != tz) {
	__CFGenericValidateType(tz, CFTimeZoneGetTypeID());
    }

    /* Most people seem to expect years, then months, then days, etc.
	to be added in that order.  Thus, 27 April + (4 days, 1 month)
	= 31 May, and not 1 June. This is also relatively predictable.

	On another issue, months not being equal length, people also
	seem to expect late day-of-month clamping (don't clamp as you
	go through months), but clamp before adding in the days. Late
	clamping is also more predictable given random starting points
	and random numbers of months added (ie Jan 31 + 2 months could
	be March 28 or March 29 in different years with aggressive
	clamping). Proportionality (28 Feb + 1 month = 31 March) is
	also not expected.

	Also, people don't expect time zone transitions to have any
	effect when adding years and/or months and/or days, only.
	Hours, minutes, and seconds, though, are added in as humans
	would experience the passing of that time. What this means
	is that if the date, after adding years, months, and days
	lands on some date, and then adding hours, minutes, and
	seconds crosses a time zone transition, the time zone
	transition is accounted for. If adding years, months, and
	days gets the date into a different time zone offset period,
	that transition is not taken into account.
    */
    gdate = CFAbsoluteTimeGetGregorianDate(at, tz);
    /* We must work in a CFGregorianUnits, because the fields in the CFGregorianDate can easily overflow */
    working.years = gdate.year;
    working.months = gdate.month;
    working.days = gdate.day;
    working.years += units.years;
    working.months += units.months;
    while (12 < working.months) {
	working.months -= 12;
	working.years += 1;
    }
    while (working.months < 1) {
	working.months += 12;
	working.years -= 1;
    }
    monthdays = __CFDaysInMonth(working.months, working.years - 2001, isleap(working.years - 2001));
    if (monthdays < working.days) {	/* Clamp day to new month */
	working.days = monthdays;
    }
    working.days += units.days;
    while (monthdays < working.days) {
	working.months += 1;
	if (12 < working.months) {
	    working.months -= 12;
	    working.years += 1;
	}
	working.days -= monthdays;
	monthdays = __CFDaysInMonth(working.months, working.years - 2001, isleap(working.years - 2001));
    }
    while (working.days < 1) {
	working.months -= 1;
	if (working.months < 1) {
	    working.months += 12;
	    working.years -= 1;
	}
	monthdays = __CFDaysInMonth(working.months, working.years - 2001, isleap(working.years - 2001));
	working.days += monthdays;
    }
    gdate.year = working.years;
    gdate.month = working.months;
    gdate.day = working.days;
    /* Roll in hours, minutes, and seconds */
    candidate_at0 = CFGregorianDateGetAbsoluteTime(gdate, tz);
    candidate_at1 = candidate_at0 + 3600.0 * units.hours + 60.0 * units.minutes + units.seconds;
    /* If summing in the hours, minutes, and seconds delta pushes us
     * into a new time zone offset, that will automatically be taken
     * care of by the fact that we just add the raw time above. To
     * undo that effect, we'd have to get the time zone offsets for
     * candidate_at0 and candidate_at1 here, and subtract the
     * difference (offset1 - offset0) from candidate_at1. */
    return candidate_at1;
}

/* at1 - at2.  The only constraint here is that this needs to be the inverse
of CFAbsoluteTimeByAddingGregorianUnits(), but that's a very rigid constraint.
Unfortunately, due to the nonuniformity of the year and month units, this
inversion essentially has to approximate until it finds the answer. */
CFGregorianUnits CFAbsoluteTimeGetDifferenceAsGregorianUnits(CFAbsoluteTime at1, CFAbsoluteTime at2, CFTimeZoneRef tz, CFOptionFlags unitFlags) {
    const int32_t seconds[5] = {366 * 24 * 3600, 31 * 24 * 3600, 24 * 3600, 3600, 60};
    CFGregorianUnits units = {0, 0, 0, 0, 0, 0.0};
    CFAbsoluteTime atold, atnew = at2;
    int32_t idx, incr;
    incr = (at2 < at1) ? 1 : -1;
    /* Successive approximation: years, then months, then days, then hours, then minutes. */
    for (idx = 0; idx < 5; idx++) {
	if (unitFlags & (1 << idx)) {
	    ((int32_t *)&units)[idx] = -3 * incr + (int32_t)((at1 - atnew) / seconds[idx]);
	    do {
		atold = atnew;
		((int32_t *)&units)[idx] += incr;
		atnew = CFAbsoluteTimeAddGregorianUnits(at2, tz, units);
	    } while ((1 == incr && atnew <= at1) || (-1 == incr && at1 <= atnew));
	    ((int32_t *)&units)[idx] -= incr;
	    atnew = atold;
	}
    }
    if (unitFlags & kCFGregorianUnitsSeconds) {
	units.seconds = at1 - atnew;
    }
    if (0.0 == units.seconds) units.seconds = 0.0;	// stomp out possible -0.0
    return units;
}

SInt32 CFAbsoluteTimeGetDayOfWeek(CFAbsoluteTime at, CFTimeZoneRef tz) {
    int64_t absolute;
    CFAbsoluteTime fixedat;
    if (NULL != tz) {
	__CFGenericValidateType(tz, CFTimeZoneGetTypeID());
    }
    fixedat = at + (NULL != tz ? CFTimeZoneGetSecondsFromGMT(tz, at) : 0.0);
    absolute = (int64_t)floor(fixedat / 86400.0);
    return (absolute < 0) ? ((absolute + 1) % 7 + 7) : (absolute % 7 + 1); /* Monday = 1, etc. */
}

SInt32 CFAbsoluteTimeGetDayOfYear(CFAbsoluteTime at, CFTimeZoneRef tz) {
    CFAbsoluteTime fixedat;
    int64_t absolute, year;
    int8_t month, day;
    if (NULL != tz) {
	__CFGenericValidateType(tz, CFTimeZoneGetTypeID());
    }
    fixedat = at + (NULL != tz ? CFTimeZoneGetSecondsFromGMT(tz, at) : 0.0);
    absolute = (int64_t)floor(fixedat / 86400.0);
    __CFYMDFromAbsolute(absolute, &year, &month, &day);
    return __CFDaysBeforeMonth(month, year, isleap(year)) + day;
}

/* "the first week of a year is the one which includes the first Thursday" (ISO 8601) */
SInt32 CFAbsoluteTimeGetWeekOfYear(CFAbsoluteTime at, CFTimeZoneRef tz) {
    int64_t absolute, year;
    int8_t month, day;
    CFAbsoluteTime fixedat;
    if (NULL != tz) {
	__CFGenericValidateType(tz, CFTimeZoneGetTypeID());
    }
    fixedat = at + (NULL != tz ? CFTimeZoneGetSecondsFromGMT(tz, at) : 0.0);
    absolute = (int64_t)floor(fixedat / 86400.0);
    __CFYMDFromAbsolute(absolute, &year, &month, &day);
    double absolute0101 = __CFAbsoluteFromYMD(year, 1, 1);
    int64_t dow0101 = __CFDoubleModToInt(absolute0101, 7) + 1;
    /* First three and last three days of a year can end up in a week of a different year */
    if (1 == month && day < 4) {
	if ((day < 4 && 5 == dow0101) || (day < 3 && 6 == dow0101) || (day < 2 && 7 == dow0101)) {
	    return 53;
	}
    }
    if (12 == month && 28 < day) {
	double absolute20101 = __CFAbsoluteFromYMD(year + 1, 1, 1);
	int64_t dow20101 = __CFDoubleModToInt(absolute20101, 7) + 1;
	if ((28 < day && 4 == dow20101) || (29 < day && 3 == dow20101) || (30 < day && 2 == dow20101)) {
	    return 1;
	}
    }
    /* Days into year, plus a week-shifting correction, divided by 7. First week is 1. */
    return (__CFDaysBeforeMonth(month, year, isleap(year)) + day + (dow0101 - 11) % 7 + 2) / 7 + 1;
}