import Foundation
// Zeller's Congruence implementation
// Returns the day of the week for a given date.
func dayOfWeek(_ d: Int, _ mInput: Int, _ yInput: Int) -> String {
// Zeller's output mapping:
// 0 = Saturday, 1 = Sunday, 2 = Monday, ... 6 = Friday
let names = [
"Saturday", "Sunday", "Monday", "Tuesday",
"Wednesday", "Thursday", "Friday"
]
var m = mInput
var y = yInput
// In Zeller's formula, January and February are counted
// as months 13 and 14 of the previous year.
if m < 3 {
m += 12 // Convert Jan → 13, Feb → 14
y -= 1 // Move to previous year
}
let K = y % 100 // Year of the century (last two digits)
let J = y / 100 // Zero-based century (e.g., 2024 → 20)
// Zeller's formula (clearer step-by-step version):
let term1 = d // day of month
let term2 = (13 * (m + 1)) / 5 // month adjustment
let term3 = K // year of century
let term4 = K / 4 // leap years in century
let term5 = J / 4 // leap centuries
let term6 = 5 * J // century correction
// Combine all terms and take modulo 7
let h = (term1 + term2 + term3 + term4 + term5 + term6) % 7
// Return the corresponding weekday name
return names[h]
}
let d = 30
let m = 5
let y = 2024
print(dayOfWeek(d, m, y))
/*
run:
Thursday
*/
