# How can I calculate the exponential integral?

(I'm not sure this is the right forum.)

I'm writing a program that uses the prime-counting function. Right now, I'm using x/log(x), but I want to switch to something more accurate. A better approximation is the logarithmic integral function (actually, its Eulerian variant), which can be computed from the exponential integral. Now how can I compute the exponential integral? I'm on a macOS Intel system using Swift, so I can use the various advanced floating-point functions provided by Apple's system libraries if needed to help.