contrib/ntp/libntp/clocktime.c

   1 /*
   2  * clocktime - compute the NTP date from a day of year, hour, minute
   3  *             and second.
   4  */
   5 #include "ntp_fp.h"
   6 #include "ntp_unixtime.h"
   7 #include "ntp_stdlib.h"
   8
   9 /*
  10  * Hacks to avoid excercising the multiplier.  I have no pride.
  11  */
  12 #define MULBY10(x)      (((x)<<3) + ((x)<<1))
  13 #define MULBY60(x)      (((x)<<6) - ((x)<<2))   /* watch overflow */
  14 #define MULBY24(x)      (((x)<<4) + ((x)<<3))
  15
  16 /*
  17  * Two days, in seconds.
  18  */
  19 #define TWODAYS         (2*24*60*60)
  20
  21 /*
  22  * We demand that the time be within CLOSETIME seconds of the receive
  23  * time stamp.  This is about 4 hours, which hopefully should be
  24  * wide enough to collect most data, while close enough to keep things
  25  * from getting confused.
  26  */
  27 #define CLOSETIME       (4*60*60)
  28
  29
  30 int
  31 clocktime(
  32         int yday,
  33         int hour,
  34         int minute,
  35         int second,
  36         int tzoff,
  37         u_long rec_ui,
  38         u_long *yearstart,
  39         u_int32 *ts_ui
  40         )
  41 {
  42         register long tmp;
  43         register u_long date;
  44         register u_long yst;
  45
  46         /*
  47          * Compute the offset into the year in seconds.  Note that
  48          * this could come out to be a negative number.
  49          */
  50         tmp = (long)(MULBY24((yday-1)) + hour + tzoff);
  51         tmp = MULBY60(tmp) + (long)minute;
  52         tmp = MULBY60(tmp) + (long)second;
  53
  54         /*
  55          * Initialize yearstart, if necessary.
  56          */
  57         yst = *yearstart;
  58         if (yst == 0) {
  59                 yst = calyearstart(rec_ui);
  60                 *yearstart = yst;
  61         }
  62
  63         /*
  64          * Now the fun begins.  We demand that the received clock time
  65          * be within CLOSETIME of the receive timestamp, but
  66          * there is uncertainty about the year the timestamp is in.
  67          * Use the current year start for the first check, this should
  68          * work most of the time.
  69          */
  70         date = (u_long)(tmp + (long)yst);
  71         if (date < (rec_ui + CLOSETIME) &&
  72             date > (rec_ui - CLOSETIME)) {
  73                 *ts_ui = date;
  74                 return 1;
  75         }
  76
  77         /*
  78          * Trouble.  Next check is to see if the year rolled over and, if
  79          * so, try again with the new year's start.
  80          */
  81         yst = calyearstart(rec_ui);
  82         if (yst != *yearstart) {
  83                 date = (u_long)((long)yst + tmp);
  84                 *ts_ui = date;
  85                 if (date < (rec_ui + CLOSETIME) &&
  86                     date > (rec_ui - CLOSETIME)) {
  87                         *yearstart = yst;
  88                         return 1;
  89                 }
  90         }
  91
  92         /*
  93          * Here we know the year start matches the current system
  94          * time.  One remaining possibility is that the time code
  95          * is in the year previous to that of the system time.  This
  96          * is only worth checking if the receive timestamp is less
  97          * than a couple of days into the new year.
  98          */
  99         if ((rec_ui - yst) < TWODAYS) {
 100                 yst = calyearstart(yst - TWODAYS);
 101                 if (yst != *yearstart) {
 102                         date = (u_long)(tmp + (long)yst);
 103                         if (date < (rec_ui + CLOSETIME) &&
 104                             date > (rec_ui - CLOSETIME)) {
 105                                 *yearstart = yst;
 106                                 *ts_ui = date;
 107                                 return 1;
 108                         }
 109                 }
 110         }
 111
 112         /*
 113          * One last possibility is that the time stamp is in the year
 114          * following the year the system is in.  Try this one before
 115          * giving up.
 116          */
 117         yst = calyearstart(rec_ui + TWODAYS);
 118         if (yst != *yearstart) {
 119                 date = (u_long)((long)yst + tmp);
 120                 if (date < (rec_ui + CLOSETIME) &&
 121                     date > (rec_ui - CLOSETIME)) {
 122                         *yearstart = yst;
 123                         *ts_ui = date;
 124                         return 1;
 125                 }
 126         }
 127
 128         /*
 129          * Give it up.
 130          */
 131         return 0;
 132 }