github.com/afumu/libc@v0.0.6/musl/src/time/__secs_to_tm.c (about)

     1  #include "time_impl.h"
     2  #include <limits.h>
     3  
     4  /* 2000-03-01 (mod 400 year, immediately after feb29 */
     5  #define LEAPOCH (946684800LL + 86400*(31+29))
     6  
     7  #define DAYS_PER_400Y (365*400 + 97)
     8  #define DAYS_PER_100Y (365*100 + 24)
     9  #define DAYS_PER_4Y   (365*4   + 1)
    10  
    11  int __secs_to_tm(long long t, struct tm *tm)
    12  {
    13  	long long days, secs, years;
    14  	int remdays, remsecs, remyears;
    15  	int qc_cycles, c_cycles, q_cycles;
    16  	int months;
    17  	int wday, yday, leap;
    18  	static const char days_in_month[] = {31,30,31,30,31,31,30,31,30,31,31,29};
    19  
    20  	/* Reject time_t values whose year would overflow int */
    21  	if (t < INT_MIN * 31622400LL || t > INT_MAX * 31622400LL)
    22  		return -1;
    23  
    24  	secs = t - LEAPOCH;
    25  	days = secs / 86400;
    26  	remsecs = secs % 86400;
    27  	if (remsecs < 0) {
    28  		remsecs += 86400;
    29  		days--;
    30  	}
    31  
    32  	wday = (3+days)%7;
    33  	if (wday < 0) wday += 7;
    34  
    35  	qc_cycles = days / DAYS_PER_400Y;
    36  	remdays = days % DAYS_PER_400Y;
    37  	if (remdays < 0) {
    38  		remdays += DAYS_PER_400Y;
    39  		qc_cycles--;
    40  	}
    41  
    42  	c_cycles = remdays / DAYS_PER_100Y;
    43  	if (c_cycles == 4) c_cycles--;
    44  	remdays -= c_cycles * DAYS_PER_100Y;
    45  
    46  	q_cycles = remdays / DAYS_PER_4Y;
    47  	if (q_cycles == 25) q_cycles--;
    48  	remdays -= q_cycles * DAYS_PER_4Y;
    49  
    50  	remyears = remdays / 365;
    51  	if (remyears == 4) remyears--;
    52  	remdays -= remyears * 365;
    53  
    54  	leap = !remyears && (q_cycles || !c_cycles);
    55  	yday = remdays + 31 + 28 + leap;
    56  	if (yday >= 365+leap) yday -= 365+leap;
    57  
    58  	years = remyears + 4*q_cycles + 100*c_cycles + 400LL*qc_cycles;
    59  
    60  	for (months=0; days_in_month[months] <= remdays; months++)
    61  		remdays -= days_in_month[months];
    62  
    63  	if (months >= 10) {
    64  		months -= 12;
    65  		years++;
    66  	}
    67  
    68  	if (years+100 > INT_MAX || years+100 < INT_MIN)
    69  		return -1;
    70  
    71  	tm->tm_year = years + 100;
    72  	tm->tm_mon = months + 2;
    73  	tm->tm_mday = remdays + 1;
    74  	tm->tm_wday = wday;
    75  	tm->tm_yday = yday;
    76  
    77  	tm->tm_hour = remsecs / 3600;
    78  	tm->tm_min = remsecs / 60 % 60;
    79  	tm->tm_sec = remsecs % 60;
    80  
    81  	return 0;
    82  }