UniqueString tweak

Previously UniqueString used MPI_Wtime, which could have a very low
resolution (1e-5s) on some machines. In this PR, we switch to use
amrex::second(), which uses the highest resolution steady clock (usually
1.e-9s). We have also changed a hardwired number according to resolution of
the clock so that the digits are not wasted on trailing zeros.

Src/Base/AMReX_Utility.cpp

@@ -183,10 +183,17 @@ amrex::FileExists(const std::string &filename)
 std::string
 amrex::UniqueString()
 {
- std::stringstream tempstring;
- tempstring << std::setprecision(11) << std::fixed << ParallelDescriptor::second();
- auto const tsl = tempstring.str().length();
- return(tempstring.str().substr(tsl/2, tsl));
+ constexpr int len = 7;
+ static const auto n = std::max
+ (len,
+ static_cast<int>(
+ std::round(std::log10(double(MaxResSteadyClock::period::den)
+ /double(MaxResSteadyClock::period::num)))));
+ std::stringstream tempstring;
+ tempstring << std::setprecision(n) << std::fixed << amrex::second()+100.0;
+ auto const ts = tempstring.str();
+ auto const tsl = ts.length();
+ return ts.substr(tsl-len,tsl); // tsl-len >= 0
 }
 
 void