Return to README CVS log

Up to [CSRG BSD Unix] / 43BSDReno / usr.bin / cal

Annotation of 43BSDReno/usr.bin/cal/README, revision 1.1

1.1     ! root        1: Program Design
        !             2: 
        !             3: This program exactly duplicates the operation of the original unix "cal"
        !             4: program.  It was designed with that intent, so no "improvements" were made
        !             5: to either the command line syntax or to the error reporting.  The main
        !             6: goal was to allow replacement of the existing binary with a freely
        !             7: redistibutable version without breaking any existing applications that
        !             8: might be built on top of the original.
        !             9: 
        !            10: The date routines were written from scratch, basically from first
        !            11: principles.  The algorithm for calculating the day of week from any
        !            12: gregorian date was "reverse engineered". This was necessary as most of
        !            13: the documented algorithms have to do with date calculations for other
        !            14: calendars (e.g. julian) and are only accurate when converted to gregorian
        !            15: within a narrow range of dates.
        !            16: 
        !            17: I take 1 jan 1 to be a Saturday because that's what cal says and I couldn't
        !            18: change that even if I was dumb enough to try. From this we can easily
        !            19: calculate the day of week for any date. The algorithm for a zero based
        !            20: day of week:
        !            21: 
        !            22:        calculate the number of days in all prior years (year-1)*365
        !            23:        add the number of leap years (days?) since year 1 
        !            24:                (not including this year as that is covered later)
        !            25:        add the day number within the year
        !            26:                this compensates for the non-inclusive leap year
        !            27:                calculation
        !            28:        if the day in question occurs before the gregorian reformation
        !            29:                (3 sep 1752 for our purposes), then simply return 
        !            30:                (value so far - 1 + SATURDAY's value of 6) modulo 7.
        !            31:        if the day in question occurs during the reformation (3 sep 1752
        !            32:                to 13 sep 1752 inclusive) return THURSDAY. This is my
        !            33:                idea of what happened then. It does not matter much as
        !            34:                this program never tries to find day of week for any day
        !            35:                that is not the first of a month.
        !            36:        otherwise, after the reformation, use the same formula as the
        !            37:                days before with the additional step of subtracting the
        !            38:                number of days (11) that were adjusted out of the calendar
        !            39:                just before taking the modulo.
        !            40: 
        !            41: It must be noted that the number of leap years calculation is sensitive
        !            42: to the date for which the leap year is being calculated. A year that occurs
        !            43: before the reformation is determined to be a leap year if its modulo of
        !            44: 4 equals zero. But after the reformation, a year is only a leap year if
        !            45: its modulo of 4 equals zero and its modulo of 100 does not. Of course,
        !            46: there is an exception for these century years. If the modulo of 400 equals
        !            47: zero, then the year is a leap year anyway. This is, in fact, what the
        !            48: gregorian reformation was all about (a bit of error in the old algorithm
        !            49: that caused the calendar to be inaccurate.)
        !            50: 
        !            51: Once we have the day in year for the first of the month in question, the
        !            52: rest is trivial. Running diff on any output of this program and the
        !            53: equivalent output from the original cal reports no difference.  This was
        !            54: confirmed by a script that ran them for all possible inputs (and took
        !            55: approximately 36 hours to complete on a sun-3.)

This archive runs on limited infrastructure. Preserving old code on modern bandwidth. Automated agents are requested to crawl responsibly.