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<p class=MsoNormal><span style='font-family:"Calibri","sans-serif";color:windowtext'>The
implementation of ‘timesub’ uses quite a bit of complex logic,
approximations, and looping to do what can be done with a single
expression. The part of the function that digests the number of days into
y/m/d fields has been known since before computers as “Julian Day Number”
and that can be harnessed directly simply by subtracting a constant for the
Epoch base.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Calibri","sans-serif";color:windowtext'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:"Calibri","sans-serif";color:windowtext'>Here
is my version, which is not so digested into my own library that you can’t
follow the logic:<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Calibri","sans-serif";color:windowtext'><o:p> </o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'>void timezone_t::state::timesub (<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> const __time64_t t, // input time number<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> const long offset, // added to time, but figures
date from original (e.g. like a denormal value)<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> ymd_hms& tmp // output<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> ) const<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> {<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> // more complex than it needs to be!<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> static const int JDayEpoch= Julian_day_number
(ymd(EPOCH_YEAR, 1, 1));<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> int days_in_epoch = t / SECSPERDAY;<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> int rem = t - days_in_epoch * SECSPERDAY;<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> int corr= count_leap_seconds (JDayEpoch,
JDayEpoch+days_in_epoch);<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> tmp= Julian_day_number (JDayEpoch+days_in_epoch);<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> tmp.set_normalized (hms(0,0,rem+offset - corr));<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> }<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Calibri","sans-serif";color:windowtext'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:"Calibri","sans-serif";color:windowtext'>In
your code, the part I’m referring to is the middle. First it finds ‘corr’
for leap seconds, and afterwards it adds the seconds into the h/m/s possibly
overflowing into the d/m/y. I have those in callable functions, but the
approach is the same. It’s just that the middle part, converting a
number of seconds into y/m/d, is not as hard as you make it out to be. <o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Calibri","sans-serif";color:windowtext'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:"Calibri","sans-serif";color:windowtext'>This
is a direct implementation of the published formula, which involves no looping or
successive approximations or peeling off each month individually:<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Calibri","sans-serif";color:windowtext'><o:p> </o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'>ymd Julian_day_number (int J)<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> {<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> const int j= J+32044;<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> const int g= j / 146097;<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> const int dg= j % 146097;<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> const int c= (dg / 36524 + 1) * 3 / 4;<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> const int dc= dg - c * 36524;<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> const int b= dc / 1461;<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> const int db= dc % 1461;<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> const int a= (db/365+1) * 3 / 4;<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> const int da= db - a * 365;<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> const int y= g*400 + c*100 + b*4 + a;<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> const int m= (da*5+308) / 153 - 2;<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> const int d= da - (m+4) *153 / 5 + 122;<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> return ymd (y-4800+(m+2)/12, (m+2)%12+1, d+1);<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:.5in'><span style='font-family:"Calibri","sans-serif";
color:windowtext'> }<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Calibri","sans-serif";color:windowtext'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:"Calibri","sans-serif";color:windowtext'>(The
comment that it is (still) more complex than it needs to be is because the
normalization code in my library can handle that directly, so I can just call
existing code.)<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Calibri","sans-serif";color:windowtext'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:"Calibri","sans-serif";color:windowtext'>--John<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Calibri","sans-serif";color:windowtext'><o:p> </o:p></span></p>
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