//
// This program computes the factorial of numbers greater than 20.
// Swift's built‑in Int cannot store extremely large values,
// so we implement a simple arbitrary‑precision integer using base‑10 digits.
//
import Foundation
//
// A minimal BigInt type using base‑10 digits.
// Digits are stored least‑significant first for easy multiplication.
//
struct BigInt {
var digits: [Int] = [1] // represents the number 1
// Multiply BigInt by a small Int
mutating func multiply(by n: Int) {
var carry = 0
for i in 0..<digits.count {
let product = digits[i] * n + carry
digits[i] = product % 10
carry = product / 10
}
while carry > 0 {
digits.append(carry % 10)
carry /= 10
}
}
// Convert BigInt to a human‑readable string
func toString() -> String {
digits.reversed().map(String.init).joined()
}
}
//
// Compute factorial using BigInt.
// The algorithm multiplies numbers from 2 to n.
//
func factorialBig(_ n: Int) -> BigInt {
var result = BigInt()
for i in 2...n {
result.multiply(by: i)
}
return result
}
//
// Main program: read input, compute factorial, print result.
//
print("Enter a number greater than 20: ", terminator: "")
let input = readLine() ?? "0"
let n = Int(input) ?? 0
let result = factorialBig(n)
print("\nFactorial of \(n) is:\n")
print(result.toString())
/*
run:
Enter a number greater than 20: 25
Factorial of 25 is:
15511210043330985984000000
*/
