units.js

const unitString =
`meter
kilometer = meter * 1000
megameter = kilometer * 1000
centimeter = meter / 100
millimeter = centimeter / 10
micrometer = millimeter / 1000
foot = meter * 0.3048
inch = foot / 12
yard = foot * 3
mile = foot * 5280
astronomical unit = meter * 149597870700
parsec = kilometer * 30856775814671.9

second = meter * 299792458
millisecond = second / 1000
microsecond = millisecond / 1000
nanosecond = microsecond / 1000
minute = second * 60
hour = minute * 60
day = hour * 24
week = day * 7
year = day * 365.25
millenium = year * 1000
galactic year = millenium * 230000`

/*
handle scaling in the interface
*/

var xFactor = 1;
var tFactor = 1;

//retrieve from the edit boxes, unscaling as we go
function getEdit(mode)
{
	if(mode === "ax")
		return document.getElementById("ax").checked;
	else
		return scaleOut(mode, document.getElementById(mode + "edit").value);
}

//set the edit boxes, scaling as we go
function setEdit(mode, value)
{
	if(mode === "ax")
		document.getElementById("ax").checked = value;
	else if(Math.abs(value) > TOL)
		document.getElementById(mode + "edit").value = expRound(scaleIn(mode, value));
	else
		document.getElementById(mode +"edit").value = "0e0";
}

//convert to exponential notation, with tolerance filter
function expRound(val)
{
	var pwr = Math.floor(Math.log10(Math.abs(val)));
	val /= Math.pow(10, pwr);
	val = TOL * Math.round(val / TOL);
	var d = Math.floor(Math.log10(Math.abs(val)));
	while(Math.abs(d) > TOL)
	{
		pwr += d;
		val /= Math.pow(10, d);
		val = TOL * Math.round(val / TOL);
		d = Math.floor(Math.log10(Math.abs(val)));
	}
	return val + "e" + pwr;
}

//apply scale factors
function scaleIn(mode, value)
{
	switch(mode)
	{
		case "x":
			return parseFloat(value * xFactor);
		case "t":
			return parseFloat(value * tFactor);
		case "v":
			return parseFloat(value * xFactor / tFactor);
	}
}

//remove scale factors
function scaleOut(mode, value)
{
	switch(mode)
	{
		case "x":
			return parseFloat(value) / xFactor;
		case "t":
			return parseFloat(value) / tFactor;
		case "v":
			return parseFloat(value) * tFactor / xFactor;
	}
}

/*
collect units
*/

var units = [];
var splitUnits = [[], []];

function unitLoad()
{
    packs = unitString.split("\n");

    //read distance units
    var block = 0;
    packs.forEach(function(pack)
    {
        if(pack.indexOf("=") == -1 && pack.length > 0)
        {
            units[pack] = 1;
            splitUnits[0].push(pack);
        }
        else if(pack.length > 0)
        {
            if(pack.indexOf("*") != -1)
                units[pack.substring(0, pack.indexOf(" = "))] =
                        units[pack.substring(pack.indexOf(" = ") + 3, pack.indexOf(" * "))] *
                        parseFloat(pack.substring(pack.indexOf(" * ") + 3));
            else
                units[pack.substring(0, pack.indexOf(" = "))] =
                        units[pack.substring(pack.indexOf(" = ") + 3, pack.indexOf(" / "))] /
                        parseFloat(pack.substring(pack.indexOf(" / ") + 3));
            splitUnits[block].push(pack.substring(0, pack.indexOf(" = ")));
        }
        else
        {
            block += 1;
        }
    });

    //build dropdown menus
    xUnitHTML = "";
    splitUnits[0].forEach(function(unit)
        {xUnitHTML += "<option value=\"" + units[unit] + "\">" + unit + "</option>";});
    splitUnits[1].forEach(function(unit)
        {xUnitHTML += "<option value=\"" + units[unit] + "\">light " + unit + "</option>";});
    tUnitHTML = "";
    splitUnits[1].forEach(function(unit)
        {tUnitHTML += "<option value=\"" + units[unit] + "\">" + unit + "</option>";});
    splitUnits[0].forEach(function(unit)
        {tUnitHTML += "<option value=\"" + units[unit] + "\">light " + unit + "</option>";});

    //store dropdown menus
    document.getElementById("xUnit").innerHTML = xUnitHTML;
    xFactor = 1 / document.getElementById("xUnit").value;
    document.getElementById("tUnit").innerHTML = tUnitHTML;
    tFactor = 1 / document.getElementById("tUnit").value;
}

//define update functions
document.getElementById("xUnit").addEventListener("change", function(){xFactor = 1 / this.value; update()});
document.getElementById("tUnit").addEventListener("change", function(){tFactor = 1 / this.value; update()});

//load approximate units from file
function approxUnits(xFact, tFact)
{
    bestX = "";
    bestT = "";
    for(var i = 0; i < splitUnits.length; i++)
    {
        splitUnits[i].forEach(function(unit)
        {
            if(bestX == "" || Math.abs(Math.log(units[unit] / xFact)) <
                    Math.abs(Math.log(units[bestX] / xFact)))
                bestX = unit;
            if(bestT == "" || Math.abs(Math.log(units[unit] / tFact)) <
                    Math.abs(Math.log(units[bestT] / tFact)))
                bestT = unit;
        });
    }
    document.getElementById("xUnit").value = units[bestX];
    document.getElementById("tUnit").value = units[bestT];
    xFactor = 1 / units[bestX];
    tFactor = 1 / units[bestT];
}