1 #include <time.h>
2
3 /*
4 * BUG: Doesn't do leap years in full glory,
5 * or calendar changes. In 2038 the sign bit
6 * will be needed in time_t, but we say it
7 * can't be represented.
8 */
9 static int
dysize(int y)10 dysize(int y)
11 {
12 y += 1900; /* arg is a tm_year, number of years since 1900 */
13 if((y%4) == 0 && ((y%100) !=0 || (y%400) == 0))
14 return 366;
15 return 365;
16 }
17
18 static int
dmsize(int m,int y)19 dmsize(int m, int y)
20 {
21 static char sizes[12] =
22 { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
23
24 if(m == 1)
25 return (dysize(y)==366)? 29 : 28;
26 else
27 return sizes[m];
28 }
29
30 /* Reduce *v to [0, mult), adding 1 to *next for every mult
31 * subtracted from *v, and return 1 if reduction worked (no overflow)
32 */
33 static int
reduce(int * v,int * next,int mult)34 reduce(int *v, int *next, int mult)
35 {
36 int oldnext;
37
38 while(*v < 0){
39 *v += mult;
40 oldnext = *next;
41 --*next;
42 if(!(*next < oldnext))
43 return 0;
44 }
45 while(*v >= mult){
46 *v -= mult;
47 oldnext = *next;
48 ++*next;
49 if(!(*next > oldnext))
50 return 0;
51 }
52 return 1;
53 }
54
55 static int
jan1(int yr)56 jan1(int yr)
57 {
58 int y, d;
59
60 y = yr+1900;
61 d = (4+y+(y+3)/4-(y-1701)/100+(y-1601)/400+3)%7;
62 return d;
63 }
64
65 time_t
mktime(struct tm * t)66 mktime(struct tm *t)
67 {
68 time_t a;
69 int i, d;
70 struct tm *ptm;
71
72 if(!(reduce(&t->tm_sec, &t->tm_min, 60) &&
73 reduce(&t->tm_min, &t->tm_hour, 60) &&
74 reduce(&t->tm_hour, &t->tm_mday, 24) &&
75 reduce(&t->tm_mon, &t->tm_year, 12)))
76 return -1;
77 while(t->tm_mday < 1){
78 if(--t->tm_mon == -1){
79 t->tm_mon = 11;
80 t->tm_year--;
81 }
82 t->tm_mday += dmsize(t->tm_mon, t->tm_year);
83 }
84 while(t->tm_mday > dmsize(t->tm_mon, t->tm_year)){
85 t->tm_mday -= dmsize(t->tm_mon, t->tm_year);
86 if(++t->tm_mon == 12){
87 t->tm_mon = 0;
88 t->tm_year++;
89 }
90 }
91 a = t->tm_sec + 60*t->tm_min + 3600*t->tm_hour;
92 t->tm_yday = t->tm_mday-1;
93 for(i=0; i<t->tm_mon; i++)
94 t->tm_yday += dmsize(i, t->tm_year);
95 a += t->tm_yday*86400L;
96 if(t->tm_year < 70){
97 for(i=t->tm_year; i<70; i++)
98 if((a -= dysize(i)*86400L) < 0)
99 return -1;
100 }else if(t->tm_year > 70){
101 for(i=70; i<t->tm_year; i++)
102 if((a += dysize(i)*86400L) < 0)
103 return -1;
104 }
105 /*
106 * Now a is number of seconds past Jan 1 1970.
107 * Convert to GMT.
108 */
109 ptm = gmtime(&a);
110 d = ptm->tm_hour;
111 ptm = localtime(&a);
112 d -= ptm->tm_hour;
113 if(d < 0)
114 d += 24;
115 if(t->tm_isdst == 0 && ptm->tm_isdst)
116 d--;
117 if(t->tm_isdst > 0 && !ptm->tm_isdst)
118 d++;
119 a += d*3600;
120 t->tm_wday = (jan1(t->tm_year)+t->tm_yday)%7;
121 return a;
122 }
123