[roll20 Easter Calculator] A script to calculate the date of easter sunday, and also the day of the week for any given year, month, and day. The output isnt polished, it's simply presented as a proof of concept for now. #roll20 #roll20script

easter.js

/*
Easter date Calculation
best easter calculator: https://www.staff.science.uu.nl/~gent0113/easter/easter_text2a.htm
script: https://www.staff.science.uu.nl/~gent0113/easter/addfiles/easter.js
root: https://www.staff.science.uu.nl/~gent0113/easter/eastercalculator.htm
historical easters: http://www.kevinlaughery.com/east4099.html

USAGE
!easter [year] [optional: true for gregorian calendar, false for julian]
    gives the date of easter, and the date of paschal full moon, the full moon that immediately precedes easter sunday.
    Paschal full moon can vary from the actual full moon date: full moon can sometimes occur in different days in different tiem zones
    so back in 326 AD, the church came up with a way of calculating a standardised full moon date, which would be universal across the globe.
!easter 1805  
!easter 1805 true

!weekday [year] [month] [day] [optional: true for gregorian calendar, false for julian]
    given a year, month, and day, !weekday tells what day of the week it is (Sunday, Monday, etc.)
!weekday 1805 11 29
!weekday 1805 11 29 true
    I set julian calendar as default for these, because most people (me included) will usually be using this 
    in historical or fantasy campaigns where julian is most appropriate.
    The output isnt polished, it's simply presented as a proof of concept for now.
*/

var easterMoon = easterMoon || (function () {
    'use strict';

    const version = '0.1',
        scriptName = 'Easter Moon',
        lastUpdate = 1539862639541, //enter Date.now() in console to get the date 
        months = ['January','February','March','April','May','June','July','August','September','October','November','December'],
        days = ['Monday','Tuesday','Wednesday','Thursday','Friday','Saturday','Sunday'],
    
        checkInstall = () => {
            log(`-=> ${scriptName} v${version} <=-   [${new Date(lastUpdate)}]`);
        },

        handleInput = (msg) => {
            // copy the body of your on(chat:message) event to this function
            if (msg.type === 'api' && msg.content.split(' ')[0] === '!easter' ) {
                let args = msg.content.split(' ');
                // args = !easter year gregorian/true/false
                let year = parseInt(args[1],10) || 2018;
                let easterDate = {};
                let moonday = '';
                if(args.length > 2 && args[2].toString().toLowerCase() == 'true') {
                    log('gregorian');
                    easterDate = easterG(year);
                    moonday = weekday(year,easterDate.moon_month,easterDate.moon_day,true);
                } else {
                    easterDate = easterJ(year);
                    moonday = weekday(year,easterDate.moon_month,easterDate.moon_day);
                }
                
                sendChat('',`Easter in ${year} falls on the ${easterDate.day}${ordinal(easterDate.day)} of ${months[easterDate.month -1]}.`);
                sendChat('',`Paschal Full Moon in ${year} fell on ${moonday}, the ${easterDate.moon_day}${ordinal(easterDate.moon_day)} of ${months[easterDate.moon_month -1]}.`);
            } else if (msg.type === 'api' && msg.content.split(' ')[0] === '!weekday' ) {
                let args = msg.content.split(' ');
                if (args.length <4) {
                    sendChat('', 'Too Few Arguments');
                    return;
                }
                let year = parseInt(args[1],10) || 2018;
                let month = parseInt(args[2],10) || 2018;
                let day = parseInt(args[3],10) || 2018;
                let calendar = false;
                if(args.length > 4) {
                    if(args[4].toString().toLowerCase() === 'true') calendar = true;
                }
                let thisday = weekday(year,month,day,calendar);
                sendChat('',`The ${day}${ordinal(day)} of ${months[month -1]} in ${year} is a ${thisday}.`);
            }
        },
        
        ordinal = (day) => {
            switch(day) {
                case 1:
                case 21:
                case 31:
                    return 'st';
                case 2:
                case 22:
                    return 'nd';
                case 3:
                case 23:
                    return 'rd';
                default:
                    return 'th';
            }
        },
        gmod = (m,n) => {
            // generalized modulo function (m mod n) - also valid for negative values of m
            return ((m%n)+n)%n;
        },
        weekday = (year,month,day,gregorian) => {
            if(gregorian === undefined) gregorian = false;
            let a = gmod(year,100);
            let b = Math.floor(a/4);
            const monthKey = [1,4,4,0,2,5,0,3,6,1,4,6];
            let c= b + day + monthKey[month-1];
            if(1 === yearType(year,(gregorian ? 'g': 'j')) && month < 3) {
                c -= 1;
            }
            let d = c;
            if(gregorian) {
                d += [6,4,2,0][gmod(Math.floor(year/100),4)];
            } else {
                d += 18 - Math.floor(year/100);
            }
            let e = d+ a;
            let f = gmod(e,7);
            log(`a ${a}; b ${b}; c ${c}; d ${d}; e ${e}; f ${f}.`);
            return days[gmod(f -2,7)];
            

        },
        yearType = (year,cal) => {
            // determine year type (yt=0 for common; yt=1 for bissextile) for Julian calendar (cal='j') or Gregorian calendar (cal='g')
            let yt=0;
            if(gmod(year,4) == 0) yt=1;
            if(cal == 'g'){
                if(gmod(year,400) == 0) yt=1;
                else if(gmod(year,100) == 0) yt=0;
            }
            return yt;
        },

        easterJ = (year, emod) => {
            //emod = 0 or 1, dionysian or armenian
            if (emod === undefined) emod = 1;
            let a = gmod(year,19);
            let gn = a + 1;
            let b = gmod (year, 4);
            let c = gmod(year, 7);
            let d = gmod((19 * a + 15), 30);
            if (emod === 1 && gn === 1) d += 1;
            let e = gmod((2 * b + 4 * c - d +6), 7);
            let moon = d+= 113;
            let sunday = moon + e +1;
            let fm_month = Math.floor(moon/31);
            let fm_day = gmod(moon,31) +1;
            let month = Math.floor(sunday / 31);
            let day = gmod(sunday,31) +1;
            let output = {year: year, sunday: sunday, month: month, day: day, moon: moon, moon_month: fm_month, moon_day: fm_day};
            return output;
        },

        easterG = (year) => {
            // Gregorian Easter Sunday date
            let a=gmod(year,19);
            let gn=a+1;                                           // Golden Number
            let b=Math.floor(year/100);
            let c=gmod(year,100);
            let d=Math.floor(b/4);
            let e=gmod(b,4);
            let f=Math.floor((b+8)/25);
            let g=Math.floor((b-f+1)/3);
            let h=gmod((19*a+b-d-g+15),30);
            let i=Math.floor(c/4);
            let k=gmod(c,4);
            let l=gmod((2*e+2*i-h-k+4),7);
            let m=Math.floor((a+11*h+22*l)/451);
            let ep=gmod((11*a+8-b+d+Math.floor((8*b+13)/25)),30);
            let moon = 0;
            if(ep <= 23) moon=136-ep;                          // Easter full moon [days after -92 March]
            else if((ep == 25) && (gn > 11)) moon=140;
            else if((ep == 24) || (ep == 25)) moon=141;
            else if(ep >= 26) moon=166-ep;
            let sunday=114+h-7*m+l;                                  // Easter Sunday [days after -92 March]
            let fm_month=Math.floor(moon/31);                          // month Easter full moon [March = 3; April = 4]
            let fm_day=gmod(moon,31)+1;                              // day Easter full moon
            let month=Math.floor(sunday/31);                          // month Easter Sunday [March = 3; April = 4]
            let day=gmod(sunday,31)+1;                              // day Easter Sunday
            let output = {year: year, sunday: sunday, month: month, day: day, moon: moon, moon_month: fm_month, moon_day: fm_day};
            return output;
        },

        registerEventHandlers = function () {
            on('chat:message', handleInput);
        };

    return {
        CheckInstall: checkInstall,
        RegisterEventHandlers: registerEventHandlers
    };

}());

on('ready', function () {
    'use strict';
    easterMoon.CheckInstall();
    easterMoon.RegisterEventHandlers();
});