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/* PD by Michelangelo Jones, 1:1/124. */
/*
** Returns 0 for new moon, 15 for full moon,
** 29 for the day before new, and so forth.
*/
/*
** This routine sometimes gets "off" by a few days,
** but is self-correcting.
*/
int moon_age(int month, int day, int year)
{
static short int ages[] =
{18, 0, 11, 22, 3, 14, 25, 6, 17,
28, 9, 20, 1, 12, 23, 4, 15, 26, 7};
static short int offsets[] =
{-1, 1, 0, 1, 2, 3, 4, 5, 7, 7, 9, 9};
if (day == 31)
day = 1;
return ((ages[(year + 1) % 19] + ((day + offsets[month-1]) % 30) +
(year < 1900)) % 30);
}
#ifdef TEST
#include <stdio.h>
#include <stdlib.h>
static char *description[] = {
"new", /* totally dark */
"waxing crescent", /* increasing to full & quarter light */
"in its first quarter", /* increasing to full & half light */
"waxing gibbous", /* increasing to full & > than half */
"full", /* fully lighted */
"waning gibbous", /* decreasing from full & > than half */
"in its last quarter", /* decreasing from full & half light */
"waning crescent" /* decreasing from full & quarter light */
};
static char *months[] = {"Jan", "Feb", "Mar", "Apr", "May", "Jun",
"Jul", "Aug", "Sep", "Oct", "Nov", "Dec"};
int main(int argc, char *argv[])
{
int month, day, year, phase;
if (4 > argc)
{
puts("Usage: MOON_AGE month day year");
return EXIT_FAILURE;
}
month = atoi(argv[1]);
day = atoi(argv[2]);
year = atoi(argv[3]);
if (100 > year)
year += 1900;
printf("moon_age(%d, %d, %d) returned %d\n", month, day, year,
phase = moon_age(month, day, year));
printf("Moon phase on %d %s %d is %s\n", day, months[month - 1], year,
description[(int)((phase + 2) * 16L / 59L)]);
}
#endif /* TEST */
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