Factorial

Factorial.swift

postfix operator !! { }

public postfix func !! <I: UnsignedIntegerType>(x: I) -> UInt {
    #if arch(x86_64) || arch(arm64)
        precondition(
            x.toUIntMax() < 21,
            "Invalid input to compute a factorial(< 21 on 64-bit arch)."
        )
    #else
        precondition(
            x.toUIntMax() < 13,
            "Invalid input to compute a factorial.(< 13 on 32-bit arch)"
        )
    #endif
    
    var x: UInt = numericCast(x)
    var b: UInt = 0 , c: UInt = 0 , fractorial: UInt = 0
    
    c = x - 1;
    fractorial = 1;
    while(c>0) {
        fractorial = 0;
        b = c;
        while b > 0 {
            if b & 1 != 0 {
                fractorial += x; // p = p + N;
            }
            // if you would like to use double choose the alternative forms instead shifts
            // the code is fast even!
            // you can use the same tips on double or 64 bit int etc.... but you must... ;-)
            b = b>>1; // b/=2; (b = b / 2;) ( b >> 1; a.s.r. is more efficent for int or long..!)
            x = x<<1; // N += N; N = N + N; N = N * 2; (N <<=1; a.s.l. is more efficent for int or long..!)
        } // end of: while(b>0)
        x = fractorial;
        c = c - 1; // c = c - 1;
    } // end of: while(c > 0)
    
    return fractorial
}

public postfix func !! <I: SignedIntegerType>(x: I) -> Int {
    #if arch(x86_64) || arch(arm64)
        precondition(
            x.toIntMax() < 21,
            "Invalid input to compute a factorial(< 21 on 64-bit arch)."
        )
    #else
        precondition(
            x.toIntMax() < 13,
            "Invalid input to compute a factorial.(< 13 on 32-bit arch)"
        )
    #endif
    
    var x: Int = numericCast(x)
    var b: Int = 0 , c: Int = 0 , fractorial: Int = 0
    
    c = x - 1;
    fractorial = 1;
    while(c>0) {
        fractorial = 0;
        b = c;
        while b > 0 {
            if b & 1 != 0 {
                fractorial += x; // p = p + N;
            }
            // if you would like to use double choose the alternative forms instead shifts
            // the code is fast even!
            // you can use the same tips on double or 64 bit int etc.... but you must... ;-)
            b = b>>1; // b/=2; (b = b / 2;) ( b >> 1; a.s.r. is more efficent for int or long..!)
            x = x<<1; // N += N; N = N + N; N = N * 2; (N <<=1; a.s.l. is more efficent for int or long..!)
        } // end of: while(b>0)
        x = fractorial;
        c = c - 1; // c = c - 1;
    } // end of: while(c > 0)
    
    return fractorial
}