french.c

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/* $selId: french.c,v 2.0 1995/10/24 01:13:06 lees Exp $
 * Copyright 1993-1995, Scott E. Lee, all rights reserved.
 * Permission granted to use, copy, modify, distribute and sell so long as
 * the above copyright and this permission statement are retained in all
 * copies.  THERE IS NO WARRANTY - USE AT YOUR OWN RISK.
 *
 * $Id: french.c,v 1.2 1998/12/28 09:34:47 sas Exp $
 */

/**************************************************************************
 *
 * These are the externally visible components of this file:
 *
 *     void
 *     SdnToFrench(
 *         long int  sdn,
 *         int      *pYear,
 *         int      *pMonth,
 *         int      *pDay);
 *
 * Convert a SDN to a French republican calendar date.  If the input SDN is
 * before the first day of year 1 or after the last day of year 14, the
 * three output values will all be set to zero, otherwise *pYear will be in
 * the range 1 to 14 inclusive; *pMonth will be in the range 1 to 13
 * inclusive; *pDay will be in the range 1 to 30 inclusive.  If *pMonth is
 * 13, the SDN represents one of the holidays at the end of the year and
 * *pDay will be in the range 1 to 6 inclusive.
 *
 *     long int
 *     FrenchToSdn(
 *         int year,
 *         int month,
 *         int day);
 *
 * Convert a French republican calendar date to a SDN.  Zero is returned
 * when the input date is detected as invalid or out of the supported
 * range.  The return value will be > 0 for all valid, supported dates, but
 * there are some invalid dates that will return a positive value.  To
 * verify that a date is valid, convert it to SDN and then back and compare
 * with the original.
 *
 *     char *FrenchMonthName[14];
 *
 * Convert a French republican month number (1 to 13) to the name of the
 * French republican month (null terminated).  An index of 13 (for the
 * "extra" days at the end of the year) will return the string "Extra".  An
 * index of zero will return a zero length string.
 *
 * VALID RANGE
 *
 *     These routines only convert dates in years 1 through 14 (Gregorian
 *     dates 22 September 1792 through 22 September 1806).  This more than
 *     covers the period when the calendar was in use.
 *
 *     I would support a wider range of dates, but I have not been able to
 *     find an authoritative definition of when leap years were to have
 *     occurred.  There are suggestions that it was to skip a leap year ever
 *     100 years like the Gregorian calendar.
 *
 * CALENDAR OVERVIEW
 *
 *     The French republican calendar was adopted in October 1793 during
 *     the French Revolution and was abandoned in January 1806.  The intent
 *     was to create a new calendar system that was based on scientific
 *     principals, not religious traditions.
 *
 *     The year is divided into 12 months of 30 days each.  The remaining 5
 *     to 6 days in the year are grouped at the end and are holidays.  Each
 *     month is divided into three decades (instead of weeks) of 10 days
 *     each.
 *
 *     The epoch (first day of the first year) is 22 September 1792 in the
 *     Gregorian calendar.  Leap years are every fourth year (year 3, 7,
 *     11, etc.)
 *
 * TESTING
 *
 *     This algorithm has been tested from the year 1 to 14.  The source
 *     code of the verification program is included in this package.
 *
 * REFERENCES
 *
 *     I have found no detailed, authoritative reference on this calendar.
 *     The algorithms are based on a preponderance of less authoritative
 *     sources.
 *
 **************************************************************************/

#include "sdncal.h"

#define SDN_OFFSET         2375474
#define DAYS_PER_4_YEARS   1461
#define DAYS_PER_MONTH     30
#define FIRST_VALID        2375840
#define LAST_VALID         2380952

void SdnToFrench(
					long int sdn,
					int *pYear,
					int *pMonth,
					int *pDay)
{
	long int temp;
	int dayOfYear;

	if (sdn < FIRST_VALID || sdn > LAST_VALID) {
		*pYear = 0;
		*pMonth = 0;
		*pDay = 0;
		return;
	}
	temp = (sdn - SDN_OFFSET) * 4 - 1;
	*pYear = temp / DAYS_PER_4_YEARS;
	dayOfYear = (temp % DAYS_PER_4_YEARS) / 4;
	*pMonth = dayOfYear / DAYS_PER_MONTH + 1;
	*pDay = dayOfYear % DAYS_PER_MONTH + 1;
}

long int FrenchToSdn(
						int year,
						int month,
						int day)
{
	/* check for invalid dates */
	if (year < 1 || year > 14 ||
		month < 1 || month > 13 ||
		day < 1 || day > 30) {
		return (0);
	}
	return ((year * DAYS_PER_4_YEARS) / 4
			+ (month - 1) * DAYS_PER_MONTH
			+ day
			+ SDN_OFFSET);
}

char *FrenchMonthName[14] =
{
	"",
	"Vendemiaire",
	"Brumaire",
	"Frimaire",
	"Nivose",
	"Pluviose",
	"Ventose",
	"Germinal",
	"Floreal",
	"Prairial",
	"Messidor",
	"Thermidor",
	"Fructidor",
	"Extra"
};