Factorial approximation

flog.d

/// License: CC0

import std.math;

real flog10(long n) {
    real r = 0;
    foreach (i; 1..(n+1)) {
        r += log10(i);
    }
    return r;
}


private immutable int maxExp = cast(int) log10(ulong.max) - 1;
private void print(real e) {
    import std.stdio;
    import std.array;
    
    if (e < maxExp) {
        real r = pow(10.0, e);
        writeln(cast(ulong) r);
    } else {
        // Not sure, it's always correct.
        real shortPart = e % maxExp;
        real r = pow(10.0, shortPart);
        writeln(cast(ulong) r, "0".replicate(cast(int) (e - shortPart)));
    }
}

void main() {
    print(flog10(1));
    print(flog10(10));
    print(flog10(100));
    print(flog10(8000));
}

/*
$ time rdmd flog.d 

real    0m0.308s
user    0m0.260s
sys     0m0.040s
# on i5-4460

$ /usr/bin/time -v ./flog |& grep resident
        Maximum resident set size (kbytes): 2388
        Average resident set size (kbytes): 0
*/