diff options
Diffstat (limited to 'kernel/time/timeconv.c')
| -rw-r--r-- | kernel/time/timeconv.c | 128 |
1 files changed, 70 insertions, 58 deletions
diff --git a/kernel/time/timeconv.c b/kernel/time/timeconv.c index 62e3b46717a6..59b922c826e7 100644 --- a/kernel/time/timeconv.c +++ b/kernel/time/timeconv.c @@ -22,47 +22,16 @@ /* * Converts the calendar time to broken-down time representation - * Based on code from glibc-2.6 * * 2009-7-14: * Moved from glibc-2.6 to kernel by Zhaolei<zhaolei@cn.fujitsu.com> + * 2021-06-02: + * Reimplemented by Cassio Neri <cassio.neri@gmail.com> */ #include <linux/time.h> #include <linux/module.h> - -/* - * Nonzero if YEAR is a leap year (every 4 years, - * except every 100th isn't, and every 400th is). - */ -static int __isleap(long year) -{ - return (year) % 4 == 0 && ((year) % 100 != 0 || (year) % 400 == 0); -} - -/* do a mathdiv for long type */ -static long math_div(long a, long b) -{ - return a / b - (a % b < 0); -} - -/* How many leap years between y1 and y2, y1 must less or equal to y2 */ -static long leaps_between(long y1, long y2) -{ - long leaps1 = math_div(y1 - 1, 4) - math_div(y1 - 1, 100) - + math_div(y1 - 1, 400); - long leaps2 = math_div(y2 - 1, 4) - math_div(y2 - 1, 100) - + math_div(y2 - 1, 400); - return leaps2 - leaps1; -} - -/* How many days come before each month (0-12). */ -static const unsigned short __mon_yday[2][13] = { - /* Normal years. */ - {0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365}, - /* Leap years. */ - {0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366} -}; +#include <linux/kernel.h> #define SECS_PER_HOUR (60 * 60) #define SECS_PER_DAY (SECS_PER_HOUR * 24) @@ -77,9 +46,11 @@ static const unsigned short __mon_yday[2][13] = { */ void time64_to_tm(time64_t totalsecs, int offset, struct tm *result) { - long days, rem, y; + u32 u32tmp, day_of_century, year_of_century, day_of_year, month, day; + u64 u64tmp, udays, century, year; + bool is_Jan_or_Feb, is_leap_year; + long days, rem; int remainder; - const unsigned short *ip; days = div_s64_rem(totalsecs, SECS_PER_DAY, &remainder); rem = remainder; @@ -103,27 +74,68 @@ void time64_to_tm(time64_t totalsecs, int offset, struct tm *result) if (result->tm_wday < 0) result->tm_wday += 7; - y = 1970; - - while (days < 0 || days >= (__isleap(y) ? 366 : 365)) { - /* Guess a corrected year, assuming 365 days per year. */ - long yg = y + math_div(days, 365); - - /* Adjust DAYS and Y to match the guessed year. */ - days -= (yg - y) * 365 + leaps_between(y, yg); - y = yg; - } - - result->tm_year = y - 1900; - - result->tm_yday = days; - - ip = __mon_yday[__isleap(y)]; - for (y = 11; days < ip[y]; y--) - continue; - days -= ip[y]; - - result->tm_mon = y; - result->tm_mday = days + 1; + /* + * The following algorithm is, basically, Proposition 6.3 of Neri + * and Schneider [1]. In a few words: it works on the computational + * (fictitious) calendar where the year starts in March, month = 2 + * (*), and finishes in February, month = 13. This calendar is + * mathematically convenient because the day of the year does not + * depend on whether the year is leap or not. For instance: + * + * March 1st 0-th day of the year; + * ... + * April 1st 31-st day of the year; + * ... + * January 1st 306-th day of the year; (Important!) + * ... + * February 28th 364-th day of the year; + * February 29th 365-th day of the year (if it exists). + * + * After having worked out the date in the computational calendar + * (using just arithmetics) it's easy to convert it to the + * corresponding date in the Gregorian calendar. + * + * [1] "Euclidean Affine Functions and Applications to Calendar + * Algorithms". https://arxiv.org/abs/2102.06959 + * + * (*) The numbering of months follows tm more closely and thus, + * is slightly different from [1]. + */ + + udays = ((u64) days) + 2305843009213814918ULL; + + u64tmp = 4 * udays + 3; + century = div64_u64_rem(u64tmp, 146097, &u64tmp); + day_of_century = (u32) (u64tmp / 4); + + u32tmp = 4 * day_of_century + 3; + u64tmp = 2939745ULL * u32tmp; + year_of_century = upper_32_bits(u64tmp); + day_of_year = lower_32_bits(u64tmp) / 2939745 / 4; + + year = 100 * century + year_of_century; + is_leap_year = year_of_century ? !(year_of_century % 4) : !(century % 4); + + u32tmp = 2141 * day_of_year + 132377; + month = u32tmp >> 16; + day = ((u16) u32tmp) / 2141; + + /* + * Recall that January 1st is the 306-th day of the year in the + * computational (not Gregorian) calendar. + */ + is_Jan_or_Feb = day_of_year >= 306; + + /* Convert to the Gregorian calendar and adjust to Unix time. */ + year = year + is_Jan_or_Feb - 6313183731940000ULL; + month = is_Jan_or_Feb ? month - 12 : month; + day = day + 1; + day_of_year += is_Jan_or_Feb ? -306 : 31 + 28 + is_leap_year; + + /* Convert to tm's format. */ + result->tm_year = (long) (year - 1900); + result->tm_mon = (int) month; + result->tm_mday = (int) day; + result->tm_yday = (int) day_of_year; } EXPORT_SYMBOL(time64_to_tm); |