units.em

/* units.em
 *
 * Variables to define an explicit system of units
 * 
 * These variables (in the module u) define a system of units to use in
 * Sirikata.
 *
 * Inherently, the choice of variables assigned to the value 1 defines the
 * a set of conventions for interacting with modules that do not adhere to
 * this unit system.  For example, if the documentation of a third-party
 * module says "ThirdPartyTime() returns the time in milliseconds since the
 * party started", the right way to store the return value using this units
 * module would be:
 *		var clock = ThirdPartyTime() * u.ms;
 *
 * I chose SI/standard scientific units as the basis for this module; most
 * units that I have defined to be numerically equal to 1 are from this
 * family.
 *
 * Note that there is no type-safety or other kind of safety associated with
 * using these units, only readability and convenience.  They are just
 * numbers, so if you don't pick the right units, you'll just get the wrong
 * numbers.  (Of course, if you're using mostly standard units, a lot of them
 * will be 1, so you might get lucky.  Still, watch your multiplication and
 * division!)
 */


if(typeof(u) === 'undefined')
	u = {};

/**
 * @function convert(quantity, targetUnits)
 * Use to pass a value in this unit system to a module expecting a different
 * system of units.
 * @return the value quantity in the units given by targetUnits
 */
u.convert = function(quantity, targetUnits) {
	return quantity / targetUnits;
};

/* ===Standard SI prefixes===
 *
 * Multiply these by any unit to derive another unit.  (Watch your order of
 * operations!)
 *
 * If you need prefixes outside of this range (exa, femto, etc.), you may want
 * to restructure your measurement system for numerical stability reasons.
 */
u.micro	= 1e-6;
u.milli	= 0.001;
u.centi	= 0.01;
u.deci	= 0.1;
u.deka	= 10;
u.hecto	= 100;
u.kilo	= 1000;
u.mega	= 1e6;

/**
 * @constant
 * Standard units of time.
 */
u.second			= (u.s 		= 1);

	u.millisecond	= (u.ms		= u.milli * u.second);	
	
	u.minute 		= (u.min	= 60 * u.second);
	u.hour 			= (u.hr		= 60 * u.minute);
	u.day						= 24 * u.hour;
	u.week						= 7 * u.day;
	u.year						= 365.25 * u.day;

/**
 * @constant
 * Standard units of length.
 */
u.meter 			= (u.m		= 1);

	u.millimeter	= (u.mm		= u.milli * u.meter);
	u.centimeter	= (u.cm		= u.centi * u.meter);
	u.kilometer		= (u.km		= u.kilo * u.meter);
	
	u.foot 			= (u.ft		= 0.3048 * u.meter);
	u.inch						= u.foot / 12;
	u.mile 						= 5280 * u.foot;

/**
 * @constant
 * Standard units of angle measure.
 */
u.radian			= (u.rad	= 1);

	u.turn						= 2 * Math.PI * u.radian;
	u.degree		= (u.deg	= u.turn / 360);

/**
 * @constant
 * Standard units of solid angle measure.
 */
u.steradian			= (u.sr		= 1);

	u.sphere					= 4 * Math.PI * u.steradian;
	u.squareDegree				= u.degree * u.degree;

/**
 * @constant
 * Standard units of mass.
 */
u.kilogram			= (u.kg		= 1);

	u.gram			= (u.g		= 0.001 * u.kilogram);
		// SI's a bit weird here.  Who am I to go against convention, though?
	
	u.pound			= (u.lb		= 0.45359237 * u.kilogram);
	u.ounce			= (u.oz		= u.pound / 16);
	u.ton						= 2000 * u.pound;

/**
 * @constant
 * Standard units of force.
 */
u.newton			= (u.N		= u.kilogram * u.meter / (u.second * u.second));
		// Scan the units above and you'll see that u.N == 1.  Better to define
		// as much as possible in terms of existing units, though.
	u.poundForce	= (u.lbf	= u.pound * 9.80665 * u.meter /
										  (u.second * u.second));