dist/ntp/libntp/clocktime.c

   1 /*      $NetBSD$        */
   2
   3 /*
   4  * clocktime - compute the NTP date from a day of year, hour, minute
   5  *             and second.
   6  */
   7 #include "ntp_fp.h"
   8 #include "ntp_unixtime.h"
   9 #include "ntp_stdlib.h"
  10
  11 /*
  12  * Hacks to avoid excercising the multiplier.  I have no pride.
  13  */
  14 #define MULBY10(x)      (((x)<<3) + ((x)<<1))
  15 #define MULBY60(x)      (((x)<<6) - ((x)<<2))   /* watch overflow */
  16 #define MULBY24(x)      (((x)<<4) + ((x)<<3))
  17
  18 /*
  19  * Two days, in seconds.
  20  */
  21 #define TWODAYS         (2*24*60*60)
  22
  23 /*
  24  * We demand that the time be within CLOSETIME seconds of the receive
  25  * time stamp.  This is about 4 hours, which hopefully should be
  26  * wide enough to collect most data, while close enough to keep things
  27  * from getting confused.
  28  */
  29 #define CLOSETIME       (4*60*60)
  30
  31
  32 int
  33 clocktime(
  34         int yday,
  35         int hour,
  36         int minute,
  37         int second,
  38         int tzoff,
  39         u_long rec_ui,
  40         u_long *yearstart,
  41         u_int32 *ts_ui
  42         )
  43 {
  44         register long tmp;
  45         register u_long date;
  46         register u_long yst;
  47
  48         /*
  49          * Compute the offset into the year in seconds.  Note that
  50          * this could come out to be a negative number.
  51          */
  52         tmp = (long)(MULBY24((yday-1)) + hour + tzoff);
  53         tmp = MULBY60(tmp) + (long)minute;
  54         tmp = MULBY60(tmp) + (long)second;
  55
  56         /*
  57          * Initialize yearstart, if necessary.
  58          */
  59         yst = *yearstart;
  60         if (yst == 0) {
  61                 yst = calyearstart(rec_ui);
  62                 *yearstart = yst;
  63         }
  64
  65         /*
  66          * Now the fun begins.  We demand that the received clock time
  67          * be within CLOSETIME of the receive timestamp, but
  68          * there is uncertainty about the year the timestamp is in.
  69          * Use the current year start for the first check, this should
  70          * work most of the time.
  71          */
  72         date = (u_long)(tmp + (long)yst);
  73         if (date < (rec_ui + CLOSETIME) &&
  74             date > (rec_ui - CLOSETIME)) {
  75                 *ts_ui = date;
  76                 return 1;
  77         }
  78
  79         /*
  80          * Trouble.  Next check is to see if the year rolled over and, if
  81          * so, try again with the new year's start.
  82          */
  83         yst = calyearstart(rec_ui);
  84         if (yst != *yearstart) {
  85                 date = (u_long)((long)yst + tmp);
  86                 *ts_ui = date;
  87                 if (date < (rec_ui + CLOSETIME) &&
  88                     date > (rec_ui - CLOSETIME)) {
  89                         *yearstart = yst;
  90                         return 1;
  91                 }
  92         }
  93
  94         /*
  95          * Here we know the year start matches the current system
  96          * time.  One remaining possibility is that the time code
  97          * is in the year previous to that of the system time.  This
  98          * is only worth checking if the receive timestamp is less
  99          * than a couple of days into the new year.
 100          */
 101         if ((rec_ui - yst) < TWODAYS) {
 102                 yst = calyearstart(yst - TWODAYS);
 103                 if (yst != *yearstart) {
 104                         date = (u_long)(tmp + (long)yst);
 105                         if (date < (rec_ui + CLOSETIME) &&
 106                             date > (rec_ui - CLOSETIME)) {
 107                                 *yearstart = yst;
 108                                 *ts_ui = date;
 109                                 return 1;
 110                         }
 111                 }
 112         }
 113
 114         /*
 115          * One last possibility is that the time stamp is in the year
 116          * following the year the system is in.  Try this one before
 117          * giving up.
 118          */
 119         yst = calyearstart(rec_ui + TWODAYS);
 120         if (yst != *yearstart) {
 121                 date = (u_long)((long)yst + tmp);
 122                 if (date < (rec_ui + CLOSETIME) &&
 123                     date > (rec_ui - CLOSETIME)) {
 124                         *yearstart = yst;
 125                         *ts_ui = date;
 126                         return 1;
 127                 }
 128         }
 129
 130         /*
 131          * Give it up.
 132          */
 133         return 0;
 134 }