Modhul:Easter

Module to calculate Easter dates and dates relative to Easter.

Calculate

Function to calculate Easter date or date of the event related to Easter.

arguments

Field	What is for?	Example argument
1	Year	i.e. 2024 or 1882.
method	Calculation method of the Easter date: Julian original calculation for Julian calendar Eastern or Orthodox original calculation for Julian calendar converted to Gregorian calendar Western or Roman revised calculation for Gregorian calendar. This is default method if not given.	i.e. Eastern to calculate Easter date in the Orthodox rite.
day	Name of the event related to Easter date or integer number of days before (negative) or after (positive) the Easter. The following days are available by name: Septuagesima 63 days before Easter - this is the earliest date returned by the funtion Sexagesima 52 days before Easter Fat Thursday 52 days before Easter Quinquagesima 49 days before Easter Ash Wednesday 46 days before Easter Palm Sunday 7 days before Easter Maundy Thursday 3 days before Easter Good Friday 2 days before Easter Holy Saturday 1 day before Easter Easter 0 - no before nor after - this is default value if not given Easter Monday 1 day after Easter Ascension Thursday 39 days after Easter Pentecost 49 day after Easter Corpus Christi 60 day after Easter	i.e. 49 or interchangeably Pentecost
format	Describes date format in the final output, which is the same as used in #time parser function. Default format is Y-m-d. There is a special value none that prevents from final date formatting.	i.e. j xd.

examples

{{#invoke:Easter|Calculate|{{CURRENTYEAR}}}}

2024-03-31

{{#invoke:Easter|Calculate|{{CURRENTYEAR}}|method=Eastern}}

2024-05-05

{{#invoke:Easter|Calculate|{{CURRENTYEAR}}|format=[[j xg]]}}

31 Maret

{{#invoke:Easter|Calculate|{{CURRENTYEAR}}|method=Eastern|format=[[j xg]]}}

5 Mèi

{{#invoke:Easter|Calculate|{{CURRENTYEAR}}|day=Ash Wednesday}}

2024-02-14

{{#invoke:Easter|Calculate|{{CURRENTYEAR}}|method=Eastern|day=Ash Wednesday}}

2024-03-20

{{#invoke:Easter|Calculate|{{CURRENTYEAR}}|format=[[j xg]] (l)|day=Ash Wednesday}}

14 Fèbruari (Rebo)

{{#invoke:Easter|Calculate|{{CURRENTYEAR}}|method=Eastern|format=[[j xg]] (l)|day=Ash Wednesday}}

20 Maret (Rebo)

---

local m = {}

local EasterData = {
    defaultMethod = 3,        -- default method of Easter date calculation when Easter type is not given
    defaultFormat = "Y-m-d",  -- default date output format
    noFormat      = "none",   -- prevent from final date formatting
    defaultOffset = 0,        -- the Easter date
    minimumOffset = -63,      -- Septuagesima
    maximumOffset = 68,       -- Feast of the Sacred Heart
 
    -- API
    apiEaster            = "Calculate", -- public function name
    argEasterYear        = 1,           -- index or name of the argument with year
    argEasterMethod      = "method",    -- index or name of the argument with calculation method
    argEasterOffset      = "day",       -- index or name of the argument with offset in days relative to the calculated Easter Sunday
    argEasterFormat      = "format",    -- index or name of the argument with date output format (#time style)
 
    -- errors
    errorMissingYear     = "Missing mandatory argument with year",
    errorInvalidYear     = "The argument with year value is incorrect: '%s'",
    errorInvalidOffset   = "Incorrect argument 'day': '%s'",
    errorInvalidMethod   = "Incorrect argument 'method': '%s'",
    errorYearOutOfRange  = "Easter dates are available between years 326 and 4099; year: %d",
    errorIncorrectMethod = "Western or Orthodox Easter exists since 1583; year: %d",
    errorUnknownMethod   = "Unknown method: %d",
 
    methods = {
        ["Julian"]      = 1,
        ["Eastern"]     = 2,
        ["Orthodox"]    = 2, -- alias for Eastern
        ["Western"]     = 3,
        ["Roman"]       = 3, -- alias for Roman
    },
 
    relativeDates = {
        ["Septuagesima"]             = -63,
        ["Sexagesima"]               = -56,
        ["Fat Thursday"]             = -52,
        ["Quinquagesima"]            = -49,
        ["Ash Wednesday"]            = -46,
        ["Palm Sunday"]              =  -7,
        ["Maundy Thursday"]          =  -3,
        ["Good Friday"]              =  -2,
        ["Holy Saturday"]            =  -1, 
        ["Easter"]                   =   0,
        ["Easter Monday"]            =   1,
        ["Ascension Thursday"]       =  39,
        ["Pentecost"]                =  49,
        ["Corpus Christi"]           =  60,
    },
}
 
local function formatEasterError(message, ...)
    if select('#', ... ) > 0 then
        message = string.format(message, ...)
    end
    return "<span class=\"error\">" .. message .. "</span>"
end
 
local function loadEasterYear(year)
    if not year then
        return false, formatEasterError(EasterData.errorMissingYear)
    end
    local result = tonumber(year)
    if not result or math.floor(result) ~= result then
        return false, formatEasterError(EasterData.errorInvalidYear, year)
    end
 
    return true, result
end
 
local function loadEasterMethod(method, year)
    local result = EasterData.defaultMethod
    if method then
        result = EasterData.methods[method]
        if not result then
            return false, formatEasterError(EasterData.errorInvalidMethod, method)
        end
    end
 
    if year < 1583 then
        result = 1
    end
 
    return true, result
end
 
local function loadEasterOffset(day)
    if not day then
        return true, ""
    end
 
    local data = EasterData.relativeDates
    local offset = tonumber(day)
    if not offset then
        offset = data[day]
    end
    if not offset or offset ~= math.floor(offset) or offset < EasterData.minimumOffset or offset > EasterData.maximumOffset then
        return false, formatEasterError(EasterData.errorInvalidOffset, day)
    end
 
    if offset < -1 then
        return true, string.format(" %d days", offset)
    elseif offset == -1 then
        return true, " -1 day"
    elseif offset == 0 then
        return true, ""
    elseif offset == 1 then
        return true, " +1 day"
    else -- if offset > 1 then
        return true, string.format(" +%d days", offset)
    end
end
 
local function loadEasterFormat(fmt)
    if fmt == EasterData.noFormat then
        return true, nil
    elseif not fmt then
        return true, EasterData.defaultFormat
    else
        return true, fmt
    end
end
 
--[[
 PURPOSE:     This function returns Easter Sunday day and month
              for a specified year and method.
 
 INPUTS:      Year   - Any year between 326 and 4099.
              Method - 1 = the original calculation based on the
                           Julian calendar
                       2 = the original calculation, with the
                           Julian date converted to the
                           equivalent Gregorian calendar
                       3 = the revised calculation based on the
                           Gregorian calendar
 
 OUTPUTS:     None.
 
 RETURNS:     0, error message - Error; invalid arguments
              month, day       - month and day of the Sunday
 
 NOTES:
              The code is translated from DN OSP 6.4.0 sources.
              The roots of the code might be found in
              http://www.gmarts.org/index.php?go=415
 
 ORIGINAL NOTES:
 
              This algorithm is an arithmetic interpretation
              of the 3 step Easter Dating Method developed
              by Ron Mallen 1985, as a vast improvement on
              the method described in the Common Prayer Book
 
              Published Australian Almanac 1988
              Refer to this publication, or the Canberra Library
              for a clear understanding of the method used
 
              Because this algorithm is a direct translation of
              the official tables, it can be easily proved to be
              100% correct
 
              It's free! Please do not modify code or comments!
]]
local function calculateEasterDate(year, method)
    if year < 326 or year > 4099 then
        -- Easter dates are valid for years between 326 and 4099
        return 0, formatEasterError(EasterData.errorYearOutOfRange, year)
    end
    if year < 1583 and method ~= 1 then
        -- Western or Orthodox Easter is valid since 1583
        return 0, formatEasterError(EasterData.errorIncorrectMethod, year)
    end
 
    -- intermediate result
    local firstDig = math.floor(year / 100)
    local remain19 = year % 19
    local temp = 0
    -- table A to E results
    local tA = 0
    local tB = 0
    local tC = 0
    local tD = 0
    local tE = 0
     -- Easter Sunday day
    local d = 0
 
    if method == 1 or method == 2 then
        -- calculate PFM date
        tA   = ((225 - 11 * remain19) % 30) + 21
        -- find the next Sunday
        tB   = (tA - 19) % 7
        tC   = (40 - firstDig) % 7
        temp = year % 100
        tD   = (temp + math.floor(temp / 4)) % 7
        tE   = ((20 - tB - tC - tD) % 7) + 1
        d    = tA + tE
        if method == 2 then
            -- convert Julian to Gregorian date
            -- 10 days were skipped in the Gregorian calendar from 5-14 Oct 1582
            temp = 10
            -- only 1 in every 4 century years are leap years in the Gregorian
            -- calendar (every century is a leap year in the Julian calendar)
            if year > 1600 then
                temp = temp + firstDig - 16 - math.floor((firstDig - 16) / 4)
            end
            d = d + temp
        end
    elseif method == 3 then
        -- calculate PFM date
        temp = math.floor((firstDig - 15) / 2)  + 202 - 11 * remain19
        if firstDig > 26 then
            temp = temp - 1
        end
        if firstDig > 38 then
            temp = temp - 1
        end
        if firstDig == 21 or firstDig == 24 or firstDig == 25 or firstDig == 33 or firstDig == 36 or firstDig == 37 then
            temp = temp - 1
        end
        temp = temp % 30
        tA   = temp + 21
        if temp == 29 then
            tA = tA - 1
        end
        if temp == 28 and remain19 > 10 then
            tA = tA - 1
        end
        -- find the next Sunday
        tB   = (tA - 19) % 7
        tC   = (40 - firstDig) % 4
        if tC == 3 then
            tC = tC + 1
        end
        if tC > 1 then
            tC = tC + 1
        end
        temp = year % 100
        tD   = (temp + math.floor(temp / 4)) % 7
        tE   = ((20 - tB - tC - tD) % 7) + 1
        d    = tA + tE
    else
        -- Unknown method
        return 0, formatEeasteError(EasterData.errorUnknownMethod, method)
    end
    if d > 61 then
        -- when the oryginal calculation os converted to the Gregorian
        -- calendar, Easter Sunday can occur in May
        return 5, d - 61
    elseif d > 31 then
        return 4, d - 31
    else
        return 3, d
    end
end
 
local function Easter(args)
    local ok
    local year
    ok, year = loadEasterYear(args[EasterData.argEasterYear])
    if not ok then
        return year
    end
 
    local method
    ok, method = loadEasterMethod(args[EasterData.argEasterMethod], year)
    if not ok then
        return method
    end
 
    local offset
    ok, offset = loadEasterOffset(args[EasterData.argEasterOffset])
    if not ok then
        return offset
    end
 
    local format
    ok, format = loadEasterFormat(args[EasterData.argEasterFormat])
    if not ok then
        return format
    end
 
    local month, day = calculateEasterDate(year, method)
    if month == 0 then
        return day
    end
 
    local result = string.format("%04d-%02d-%02d%s", year, month, day, offset)
    if format then
        result = mw.language.getContentLanguage():formatDate(format, result)
    end
 
    return result
end
 
m[EasterData.apiEaster] = function (frame)
    return Easter(frame.args)
end
 
return m