returns the day of the week of any date between 01.01.1700 to 31.12.2199

Abstract - DayOfTheWeek.md

DayOfTheWeek

This function returns the weekday of any date between 1700 and 2199.

print(try dayOfTheWeek("28.09.1964")) // Prints "Monday"
print(try dayOfTheWeek("12.03.2014")) // Prints "Wednesday"
print(try dayOfTheWeek("31.12.1980")) // Prints "Wednesday"

DayOfTheWeek.swift

/// please insert date in DD.MM.YYYY pattern
func dayOfTheWeek(_ string: String) throws -> String {
	guard string.characters.count == 10                     else { throw DayOfTheWeekError.wrongFormat("length") }
	var index0 = string.startIndex
	var index1 = string.index(index0, offsetBy: 2)
	guard let dayCode = UInt16(string[index0..<index1])     else { throw DayOfTheWeekError.wrongFormat("day") }
	index0 = string.index(index1, offsetBy: 1)
	index1 = string.index(index0, offsetBy: 2)
	guard let monthCode = UInt16(string[index0..<index1])	else { throw DayOfTheWeekError.wrongFormat("month") }
	index0 = string.index(index1, offsetBy: 1)
	index1 = string.index(index0, offsetBy: 4)
	guard let yearCode = UInt16(string[index0..<index1])	else { throw DayOfTheWeekError.wrongFormat("year") }
	
	let leapYear = yearCode % 4 == 0 ? (yearCode % 100 == 0 ? (yearCode % 400 != 0) : true) : false
	
	var result : UInt16 = dayCode
	
	switch monthCode {
	case 1	: result += leapYear ? 5 : 6
	case 2	: result += leapYear ? 1 : 2
	case 3	: result += 2
	case 4	: result += 5
	case 5	: result += 0
	case 6	: result += 3
	case 7	: result += 5
	case 8	: result += 1
	case 9	: result += 4
	case 10	: result += 6
	case 11	: result += 2
	case 12	: result += 4
	default	: throw DayOfTheWeekError.wrongFormat("month")
	}
	
	let y = yearCode % 100
	
	result += (y / 4) + y
	
	switch yearCode / 100 {
	case 17	: result += 5
	case 18	: result += 3
	case 19	: result += 1
	case 20	: result += 0
	case 21	: result += 5
	default	: throw DayOfTheWeekError.wrongFormat("year")
	}
	
	switch result % 7 {
	case 1	: return "Monday"
	case 2	: return "Tuesday"
	case 3	: return "Wednesday"
	case 4	: return "Thursday"
	case 5	: return "Friday"
	case 6	: return "Saturday"
	case 0	: return "Sunday"
	default	: fatalError("\(result % 7) < 0 ?... because it's not in 0..<7.")
	}
}

enum DayOfTheWeekError : Error {
	case wrongFormat(String)
}