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Add support for Islamic calendar (#3)
* Add Islamic calendar support * Add isIslamicCalendarLeapYear() * Basic Islamic leap year tests * Preliminary Islamic calendar to JDN tests * Add round-trip testing for Islamic calendar * Improve documentation * Reword comment * Reword documentation comments * Add Islamic Calendar JD support * Add Islamic calendar to README
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| Original file line number | Diff line number | Diff line change |
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| // | ||
| // | ||
| // Copyright © 2021-2023 Stephen F. Booth <[email protected]> | ||
| // Part of https://github.com/sbooth/JulianDayNumber | ||
| // MIT license | ||
| // | ||
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| import Foundation | ||
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| /// Converts a date in the Islamic calendar to a Julian day number. | ||
| /// | ||
| /// The Julian day number (JDN) is the integer assigned to a whole solar day in the Julian day count starting from noon Universal Time, | ||
| /// with JDN 0 assigned to the day starting at noon on Monday, January 1, 4713 BC (-4712-01-01 12:00:00) in the proleptic Julian calendar. | ||
| /// This date is Shaabán 17, 5499 B.H. (-5498-08-16 12:00:00) in the proleptic Islamic calendar. | ||
| /// | ||
| /// - note: No validation checks are performed on the date values. | ||
| /// | ||
| /// - parameter Y: A year number between `-9999` and `99999`. | ||
| /// - parameter M: A month number between `1` (Muharram) and `12` (Dhu ́’l-Hijjab). | ||
| /// - parameter D: A day number between `1` and the maximum number of days in month `M` for year `Y`. | ||
| /// | ||
| /// - returns: The JDN corresponding to the requested date. | ||
| public func islamicCalendarDateToJulianDayNumber(year Y: Int, month M: Int, day D: Int) -> Int { | ||
| // Richards' algorithm is only valid for positive JDNs. | ||
| // JDN 0 is -5498-08-16 in the proleptic Islamic calendar. | ||
| // Adjust the year of earlier dates forward in time by a multiple of | ||
| // 30 (to account for leap years in the Islamic calendar) | ||
| // before calculating the JDN and then translate the result backward | ||
| // in time by the period of adjustment. | ||
| if Y < -5498 || (Y == -5498 && (M < 8 || (M == 8 && D < 16))) { | ||
| // 30 years = 10,631 days (19 years of 354 days and 11 leap years of 355 days) | ||
| let periods = (-5498 - Y) / 30 + 1 | ||
| let mappedY = Y + periods * 30 | ||
| let mappedJ = islamicCalendarDateToJulianDayNumber(year: mappedY, month: M, day: D) | ||
| return mappedJ - periods * 10631 | ||
| } | ||
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| let h = M - m | ||
| let g = Y + y - (n - h) / n | ||
| let f = (h - 1 + n) % n | ||
| let e = (p * g + q) / r + D - 1 - j | ||
| let J = e + (s * f + t) / u | ||
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| return J | ||
| } | ||
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| /// The earliest supported Julian day number using the Islamic calendar. | ||
| /// | ||
| /// This JDN corresponds to -9999-01-01 12:00:00 in the Islamic calendar | ||
| let earliestSupportedIslamicCalendarJDN = -1595227 | ||
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| /// The latest supported Julian day number using the Islamic calendar. | ||
| /// | ||
| /// This JDN corresponds to 99999-12-29 12:00:00 in the Islamic calendar | ||
| let latestSupportedIslamicCalendarJDN = 37384751 | ||
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| /// Converts the Julian day number `J` to a date in the Islamic calendar. | ||
| /// | ||
| /// - parameter J: A Julian day number between `-1595227` and `37384751`. | ||
| /// | ||
| /// - returns: The calendar date corresponding to `J`. | ||
| public func julianDayNumberToIslamicCalendarDate(_ J: Int) -> (year: Int, month: Int, day: Int) { | ||
| // Richards' algorithm is only valid for positive JDNs. | ||
| // Adjust negative JDNs forward in time by a multiple of | ||
| // 30 years (to account for leap years in the Islamic calendar) | ||
| // before calculating the proleptic Islamic date and then translate | ||
| // the result backward in time by the amount of forward adjustment. | ||
| if J < 0 { | ||
| // 30 years = 10,631 days (19 years of 354 days and 11 leap years of 355 days) | ||
| let periods = -J / 10631 + 1 | ||
| let mappedJ = J + periods * 10631 | ||
| let mappedYMD = julianDayNumberToIslamicCalendarDate(mappedJ) | ||
| return (mappedYMD.year - periods * 30, mappedYMD.month, mappedYMD.day) | ||
| } | ||
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| let f = J + j | ||
| let e = r * f + v | ||
| let g = (e % p) / r | ||
| let h = u * g + w | ||
| let D = (h % s) / u + 1 | ||
| let M = ((h / s + m) % n) + 1 | ||
| let Y = e / p - y + (n + m - M) / n | ||
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| return (Y, M, D) | ||
| } | ||
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| // Constants for Islamic calendar conversions | ||
| private let y = 5519 | ||
| private let j = 7664 | ||
| private let m = 0 | ||
| private let n = 12 | ||
| private let r = 30 | ||
| private let p = 10631 | ||
| private let q = 14 | ||
| private let v = 15 | ||
| private let u = 100 | ||
| private let s = 2951 | ||
| private let t = 51 | ||
| private let w = 10 |
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