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Annotation of embedaddon/ntp/libntp/caltontp.c, revision 1.1.1.1

1.1       misho       1: /*
                      2:  * caltontp - convert a date to an NTP time
                      3:  */
                      4: #include <sys/types.h>
                      5: 
                      6: #include "ntp_types.h"
                      7: #include "ntp_calendar.h"
                      8: #include "ntp_stdlib.h"
                      9: #include "ntp_assert.h"
                     10: 
                     11: /*
                     12:  * Juergen Perlinger, 2008-11-12
                     13:  * Add support for full calendar calculatios. If the day-of-year is provided
                     14:  * (that is, not zero) it will be used instead of month and day-of-month;
                     15:  * otherwise a full turn through the calendar calculations will be taken.
                     16:  *
                     17:  * I know that Harlan Stenn likes to see assertions in production code, and I
                     18:  * agree there, but it would be a tricky thing here. The algorithm is quite
                     19:  * capable of producing sensible answers even to seemingly weird inputs: the
                     20:  * date <any year here>-03-00, the 0.th March of the year, will be automtically
                     21:  * treated as the last day of February, no matter whether the year is a leap
                     22:  * year or not. So adding constraints is merely for the benefit of the callers,
                     23:  * because the only thing we can check for consistency is our input, produced
                     24:  * by somebody else.
                     25:  *
                     26:  * BTW: A total roundtrip using 'caljulian' would be a quite shaky thing:
                     27:  * Because of the truncation of the NTP time stamp to 32 bits and the epoch
                     28:  * unfolding around the current time done by 'caljulian' the roundtrip does
                     29:  * *not* necessarily reproduce the input, especially if the time spec is more
                     30:  * than 68 years off from the current time...
                     31:  */
                     32: u_long
                     33: caltontp(
                     34:        const struct calendar *jt
                     35:        )
                     36: {
                     37:        ntp_u_int32_t days;     /* full days in NTP epoch */
                     38:        ntp_u_int32_t years;    /* complete ACE years before date */
                     39:        ntp_u_int32_t month;    /* adjusted month for calendar */
                     40:        
                     41:        NTP_INSIST(jt != NULL);
                     42: 
                     43:        NTP_REQUIRE(jt->month <= 13);   /* permit month 0..13! */
                     44:        NTP_REQUIRE(jt->monthday <= 32);
                     45:        NTP_REQUIRE(jt->yearday <= 366);
                     46:        NTP_REQUIRE(jt->hour <= 24);
                     47:        NTP_REQUIRE(jt->minute <= MINSPERHR);
                     48:        NTP_REQUIRE(jt->second <= SECSPERMIN);
                     49: 
                     50:        /*
                     51:         * First convert the date to fully elapsed days since NTP epoch. The
                     52:         * expressions used here give us initially days since 0001-01-01, the
                     53:         * beginning of the christian era in the proleptic gregorian calendar;
                     54:         * they are rebased on-the-fly into days since beginning of the NTP
                     55:         * epoch, 1900-01-01.
                     56:         */
                     57:        if (jt->yearday) {
                     58:                /*
                     59:                 * Assume that the day-of-year contains a useable value and
                     60:                 * avoid all calculations involving month and day-of-month.
                     61:                 */
                     62:                years = jt->year - 1;
                     63:                days  = years * DAYSPERYEAR     /* days in previous years */
                     64:                      + years / 4               /* plus prior years's leap days */
                     65:                      - years / 100             /* minus leapless century years */
                     66:                      + years / 400             /* plus leapful Gregorian yrs */
                     67:                      + jt->yearday             /* days this year */
                     68:                      - DAY_NTP_STARTS;         /* rebase to NTP epoch */
                     69:        } else {
                     70:                /*
                     71:                 * The following code is according to the excellent book
                     72:                 * 'Calendrical Calculations' by Nachum Dershowitz and Edward
                     73:                 * Reingold. It does a full calendar evaluation, using one of
                     74:                 * the alternate algorithms: Shift to a hypothetical year
                     75:                 * starting on the previous march,1st; merge years, month and
                     76:                 * days; undo the the 9 month shift (which is 306 days). The
                     77:                 * advantage is that we do NOT need to now whether a year is a
                     78:                 * leap year or not, because the leap day is the LAST day of
                     79:                 * the year.
                     80:                 */
                     81:                month  = (ntp_u_int32_t)jt->month + 9;
                     82:                years  = jt->year - 1 + month / 12;
                     83:                month %= 12;
                     84:                days   = years * DAYSPERYEAR    /* days in previous years */
                     85:                       + years / 4              /* plus prior years's leap days */
                     86:                       - years / 100            /* minus leapless century years */
                     87:                       + years / 400            /* plus leapful Gregorian yrs */
                     88:                       + (month * 153 + 2) / 5  /* plus days before month */
                     89:                       + jt->monthday           /* plus day-of-month */
                     90:                       - 306                    /* minus 9 months */
                     91:                       - DAY_NTP_STARTS;        /* rebase to NTP epoch */
                     92:        }
                     93: 
                     94:        /*
                     95:         * Do the obvious: Merge everything together, making sure integer
                     96:         * promotion doesn't play dirty tricks on us; there is probably some
                     97:         * redundancy in the casts, but this drives it home with force. All
                     98:         * arithmetic is done modulo 2**32, because the result is truncated
                     99:         * anyway.
                    100:         */
                    101:        return               days       * SECSPERDAY
                    102:            + (ntp_u_int32_t)jt->hour   * MINSPERHR*SECSPERMIN
                    103:            + (ntp_u_int32_t)jt->minute * SECSPERMIN
                    104:            + (ntp_u_int32_t)jt->second;
                    105: }