// Zeller's Congruence implementation
// Returns the day of the week for a given date.
fn day_of_week(d: i32, m: i32, y: i32) -> &'static str {
// Zeller's output mapping:
// 0 = Saturday, 1 = Sunday, 2 = Monday, ... 6 = Friday
let names = [
"Saturday", "Sunday", "Monday", "Tuesday",
"Wednesday", "Thursday", "Friday",
];
let mut m = m;
let mut y = y;
// 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
names[h as usize]
}
fn main() {
let d = 30;
let m = 5;
let y = 2024;
println!("{}", day_of_week(d, m, y));
}
/*
run:
Thursday
*/
