Easter calculation (#2)

* Add Easter month and day calculation

* Add comment on algorithm source

* Documentation cleanup

Sources/JulianDayNumber/Easter.swift

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+//
+// Copyright © 2021-2023 Stephen F. Booth <[email protected]>
+// Part of https://github.com/sbooth/JulianDayNumber
+// MIT license
+//
+
+import Foundation
+
+// From the Explanatory Supplement to the Astronomical Almanac, 3rd edition, S.E Urban and P.K. Seidelmann eds., (Mill Valley, CA: University Science Books), Chapter 15, pp. 585-624.
+
+/// Returns the month `M` and day `D` of Easter in year `Y`.
+///
+/// Years before 1582 are interpreted in the Julian calendar and years after
+/// 1582 are interpreted in the Gregorian calendar.
+///
+/// - parameter year: A year number.
+///
+/// - returns: The month and day of Easter in the requested year.
+public func easter(year Y: Int) -> (month: Int, day: Int) {
+	return Y < 1582 ? easterInJulianCalendar(year: Y) : easterInGregorianCalendar(year: Y)
+}
+
+/// Returns the month `M` and day `D` of Easter in year `Y` in the Julian calendar.
+///
+/// - parameter year: A year number.
+///
+/// - returns: The month and day of Easter in the requested year.
+public func easterInJulianCalendar(year Y: Int) -> (month: Int, day: Int) {
+	let a = 22 + ((225 - 11 * (Y % 19)) % 30)
+	let g = a + ((56 + 6 * Y - Y / 4 - a) % 7)
+	let M = 3 + g / 32
+	let D = 1 + ((g - 1) % 31)
+	return (M, D)
+}
+
+/// Returns the month `M` and day `D` of Easter in year `Y` in the Gregorian calendar.
+///
+/// - parameter year: A year number.
+///
+/// - returns: The month and day of Easter in the requested year.
+public func easterInGregorianCalendar(year Y: Int) -> (month: Int, day: Int) {
+	let a = Y / 100
+	let b = a - a / 4
+	let c = (Y % 19)
+	let e = ((15 + 19 * c + b - (a - (a - 17) / 25) / 3) % 30)
+	let f = e - (c + 11 * e) / 319
+	let g = 22 + f + ((140004 - Y - Y / 4 + b - f) % 7)
+	let M = 3 + g / 32
+	let D = 1 + ((g - 1) % 31)
+	return (M, D)
+}

Tests/JulianDayNumberTests/EasterTests.swift

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+//
+// Copyright © 2021-2023 Stephen F. Booth <[email protected]>
+// Part of https://github.com/sbooth/JulianDayNumber
+// MIT license
+//
+
+import XCTest
+@testable import JulianDayNumber
+
+final class EasterTests: XCTestCase {
+	func testEaster() {
+		// Dates from Meeus (1998)
+		XCTAssertTrue(easter(year: 1991) == (3, 31))
+		XCTAssertTrue(easter(year: 1992) == (4, 19))
+		XCTAssertTrue(easter(year: 1993) == (4, 11))
+		XCTAssertTrue(easter(year: 1954) == (4, 18))
+		XCTAssertTrue(easter(year: 2000) == (4, 23))
+		XCTAssertTrue(easter(year: 1818) == (3, 22))
+		XCTAssertTrue(easter(year: 179) == (4, 12))
+		XCTAssertTrue(easter(year: 711) == (4, 12))
+		XCTAssertTrue(easter(year: 1243) == (4, 12))
+	}
+}