/*-
 * Copyright (c) 1989, 1993
 *	The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software posted to USENET.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 * @(#) Copyright (c) 1989, 1993 The Regents of the University of California.  All rights reserved.
 * @(#)pom.c       8.1 (Berkeley) 5/31/93
 * $FreeBSD: src/games/pom/pom.c,v 1.9 1999/11/30 03:49:09 billf Exp $
 * $DragonFly: src/games/pom/pom.c,v 1.4 2006/08/08 17:08:49 pavalos Exp $
 */

/*
 * Phase of the Moon.  Calculates the current phase of the moon.
 * Based on routines from `Practical Astronomy with Your Calculator',
 * by Duffett-Smith.  Comments give the section from the book that
 * particular piece of code was adapted from.
 *
 * -- Keith E. Brandt  VIII 1984
 *
 */

#include <time.h>
#include <stdio.h>
#include <math.h>

#define	EPOCH	  85
#define	EPSILONg  279.611371	/* solar ecliptic long at EPOCH */
#define	RHOg	  282.680403	/* solar ecliptic long of perigee at EPOCH */
#define	ECCEN	  0.01671542	/* solar orbit eccentricity */
#define	lzero	  18.251907	/* lunar mean long at EPOCH */
#define	Pzero	  192.917585	/* lunar mean long of perigee at EPOCH */
#define	Nzero	  55.204723	/* lunar mean long of node at EPOCH */
#define isleap(y) ((((y) % 4) == 0 && ((y) % 100) != 0) || ((y) % 400) == 0)

static void	adj360 (double *);
static double	dtor (double);
static double	potm (double);

int
main(void)
{
	time_t tt;
	struct tm *GMT;
	double days, today, tomorrow;
	int cnt;

	(void) time(&tt);
	GMT = gmtime(&tt);
	days = (GMT->tm_yday + 1) + ((GMT->tm_hour +
	    (GMT->tm_min / 60.0) + (GMT->tm_sec / 3600.0)) / 24.0);
	for (cnt = EPOCH; cnt < GMT->tm_year; ++cnt)
		days += isleap(1900 + cnt) ? 366 : 365;
	today = potm(days) + .5;
	(void)printf("The Moon is ");
	if ((int)today == 100)
		(void)printf("Full\n");
	else if (!(int)today)
		(void)printf("New\n");
	else {
		tomorrow = potm(days + 1);
		if ((int)today == 50)
			(void)printf("%s\n", tomorrow > today ?
			    "at the First Quarter" : "at the Last Quarter");
		else {
			(void)printf("%s ", tomorrow > today ?
			    "Waxing" : "Waning");
			if (today > 50)
				(void)printf("Gibbous (%1.0f%% of Full)\n",
				    today);
			else if (today < 50)
				(void)printf("Crescent (%1.0f%% of Full)\n",
				    today);
		}
	}

	return 0;
}

/*
 * potm --
 *	return phase of the moon
 */
static double
potm(double days)
{
	double N, Msol, Ec, LambdaSol, l, Mm, Ev, Ac, A3, Mmprime;
	double A4, lprime, V, ldprime, D, Nm;

	N = 360 * days / 365.2422;				/* sec 42 #3 */
	adj360(&N);
	Msol = N + EPSILONg - RHOg;				/* sec 42 #4 */
	adj360(&Msol);
	Ec = 360 / M_PI * ECCEN * sin(dtor(Msol));		/* sec 42 #5 */
	LambdaSol = N + Ec + EPSILONg;				/* sec 42 #6 */
	adj360(&LambdaSol);
	l = 13.1763966 * days + lzero;				/* sec 61 #4 */
	adj360(&l);
	Mm = l - (0.1114041 * days) - Pzero;			/* sec 61 #5 */
	adj360(&Mm);
	Nm = Nzero - (0.0529539 * days);			/* sec 61 #6 */
	adj360(&Nm);
	Ev = 1.2739 * sin(dtor(2*(l - LambdaSol) - Mm));	/* sec 61 #7 */
	Ac = 0.1858 * sin(dtor(Msol));				/* sec 61 #8 */
	A3 = 0.37 * sin(dtor(Msol));
	Mmprime = Mm + Ev - Ac - A3;				/* sec 61 #9 */
	Ec = 6.2886 * sin(dtor(Mmprime));			/* sec 61 #10 */
	A4 = 0.214 * sin(dtor(2 * Mmprime));			/* sec 61 #11 */
	lprime = l + Ev + Ec - Ac + A4;				/* sec 61 #12 */
	V = 0.6583 * sin(dtor(2 * (lprime - LambdaSol)));	/* sec 61 #13 */
	ldprime = lprime + V;					/* sec 61 #14 */
	D = ldprime - LambdaSol;				/* sec 63 #2 */
	return(50 * (1 - cos(dtor(D))));			/* sec 63 #3 */
}

/*
 * dtor --
 *	convert degrees to radians
 */
static double
dtor(double deg)
{
	return(deg * M_PI / 180);
}

/*
 * adj360 --
 *	adjust value so 0 <= deg <= 360
 */
static void
adj360(double *deg)
{
	for (;;)
		if (*deg < 0)
			*deg += 360;
		else if (*deg > 360)
			*deg -= 360;
		else
			break;
}