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15649301
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2013-03-26T23:28:27.757
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2017-05-23T11:50:37.993
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The <code>getDateFromJulianDay</code> function originally proposed is too computationally intensive for effective use on an embedded device, containing many multiplication and division operations on large <code>long</code> variables or, as originally written in C++, <a href="http://qt.gitorious.org/qt/qt/blobs/4.7/src/corelib/tools/qdatetime.cpp#line140" rel="nofollow noreferrer"><code>longlong</code> variables</a>. I think I hunted down an efficient epoch to date algorithm for an embedded device. After fruitless Googling, I found myself back on Stack Overflow, and found the question <a href="https://stackoverflow.com/questions/1692184/converting-epoch-time-to-real-date-time">Converting epoch time to “real” date/time</a>, asking about self-written epoch time to date implementation and provides a suitable algorithm. This <a href="https://stackoverflow.com/a/1692210/2209295">answer</a> to the question references the <a href="http://www.raspberryginger.com/jbailey/minix/html/gmtime_8c-source.html" rel="nofollow noreferrer">gmtime.c source code</a>, and provided the source in C I needed to write a Python conversion algorithm: <pre><code>/* * gmtime - convert the calendar time into broken down time */ /* $Header: /opt/proj/minix/cvsroot/src/lib/ansi/gmtime.c,v 1.1.1.1 2005/04/21 14:56:05 beng Exp $ */ #include <time.h> #include <limits.h> #include "loc_time.h" struct tm * gmtime(register const time_t *timer) { static struct tm br_time; register struct tm *timep = &br_time; time_t time = *timer; register unsigned long dayclock, dayno; int year = EPOCH_YR; dayclock = (unsigned long)time % SECS_DAY; dayno = (unsigned long)time / SECS_DAY; timep->tm_sec = dayclock % 60; timep->tm_min = (dayclock % 3600) / 60; timep->tm_hour = dayclock / 3600; timep->tm_wday = (dayno + 4) % 7; /* day 0 was a thursday */ while (dayno >= YEARSIZE(year)) { dayno -= YEARSIZE(year); year++; } timep->tm_year = year - YEAR0; timep->tm_yday = dayno; timep->tm_mon = 0; while (dayno >= _ytab[LEAPYEAR(year)][timep->tm_mon]) { dayno -= _ytab[LEAPYEAR(year)][timep->tm_mon]; timep->tm_mon++; } timep->tm_mday = dayno + 1; timep->tm_isdst = 0; return timep; } </code></pre> Additionally, the <a href="https://stackoverflow.com/a/3136466/2209295">analysis</a> of the question <a href="https://stackoverflow.com/questions/3136341/why-is-gmtime-implemented-this-way">Why is gmtime implemented this way?</a> helped affirm that the <code>gmtime</code> function is fairly efficient. Using the <a href="http://www.raspberryginger.com/jbailey/minix/html/" rel="nofollow noreferrer">raspberryginger.com minix Doxygen documentation site</a>, I was able to find the C macros and constants that were included in <a href="http://www.raspberryginger.com/jbailey/minix/html/gmtime_8c-source.html" rel="nofollow noreferrer">gmtime.c</a> from <a href="http://www.raspberryginger.com/jbailey/minix/html/loc__time_8h-source.html" rel="nofollow noreferrer">loc_time.h</a>. The relevant code snippet: <pre><code>#define YEAR0 1900 /* the first year */ #define EPOCH_YR 1970 /* EPOCH = Jan 1 1970 00:00:00 */ #define SECS_DAY (24L * 60L * 60L) #define LEAPYEAR(year) (!((year) % 4) && (((year) % 100) || !((year) % 400))) #define YEARSIZE(year) (LEAPYEAR(year) ? 366 : 365) #define FIRSTSUNDAY(timp) (((timp)->tm_yday - (timp)->tm_wday + 420) % 7) #define FIRSTDAYOF(timp) (((timp)->tm_wday - (timp)->tm_yday + 420) % 7) #define TIME_MAX ULONG_MAX #define ABB_LEN 3 extern const int _ytab[2][10]; </code></pre> And the <code>extern const int _ytab</code> was defined in <a href="http://www.raspberryginger.com/jbailey/minix/html/lib_2ansi_2misc_8c-source.html#l00082" rel="nofollow noreferrer">misc.c</a>: <pre><code>const int _ytab[2][12] = { { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 }, { 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 } }; </code></pre> Some other things I found: <ul> <li>The <a href="http://www.raspberryginger.com/jbailey/minix/html/gmtime_8c.html" rel="nofollow noreferrer">gmtime.c File Reference</a> was very helpful for finding dependencies.</li> <li>The <code>gmtime</code> function starts indexing the Month, Day of Week, and Day of Year at the number zero, (maximal ranges of 0-11, 0-6, 0-365, respectively), whereas the Day of Month starts at the number 1, (1-31), see the <a href="http://publib.boulder.ibm.com/infocenter/zvm/v6r1/index.jsp?topic=/com.ibm.zvm.v610.edclv/gmtime.htm" rel="nofollow noreferrer">IBM <code>gmtime()</code> reference</a>.</li> </ul> <hr> I re-wrote the <code>gmtime</code> function for Python 1.5.2+: <pre class="lang-py prettyprint-override"><code>def is_leap_year(year): return ( not ((year) % 4) and ( ((year) % 100) or (not((year) % 400)) ) ) def year_size(year): if is_leap_year(year): return 366 else: return 365 def ntp_time_to_date(ntp_time): year = 1900 # EPOCH_YR for NTP ytab = [ [ 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31], [ 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31] ] (dayno,dayclock) = divmod(ntp_time, 86400L) dayno = int(dayno) # Calculate time of day from seconds on the day's clock. (hour, sec_past_hour) = divmod(dayclock,3600) hour = int(hour) (min, sec) = divmod(int(sec_past_hour),60) while (dayno >= year_size(year)): dayno = dayno - year_size(year) year = year + 1 month = 1 # NOTE: month range is (1-12) while (dayno >= ytab[is_leap_year(year)][month]): dayno = dayno - ytab[is_leap_year(year)][month] month = month + 1 day = dayno + 1 return (year, month, day, hour, min, sec) </code></pre> Modifications I made re-factoring the C++ <code>gmtime</code> function to my Python function <code>ntp_time_to_date(ntp_time)</code>: <ul> <li>Changed epoch from UNIX epoch of 1970 to NTP epoch of 1900 (<a href="http://www.eecis.udel.edu/~mills/y2k.html#era" rel="nofollow noreferrer">the prime epoch for NTP</a>).</li> <li>Slightly streamlined time of day calculation. <ul> <li>Comparing time of day calculation of <code>gmtime</code> to <code>ntp_time_to_date</code>: <ul> <li>Both <code>(dayclock % 3600) / 60</code> and <code>dayclock / 3600</code> occur behind the scenes in <code>divmod(dayclock,3600)</code> and <code>divmod(sec_past_hour,60)</code>.</li> <li>Only real difference is that <code>divmod(sec_past_hour,60)</code> avoids modulo of <code>dayclock</code> (0-86399) by 60 via <code>dayclock % 60</code>, and instead does modulo of <code>sec_past_hour</code> (0-3599) by 60 within <code>divmod(sec_past_hour,60)</code>.</li> </ul></li> </ul></li> <li>Removed variables and code I did not need, for example, day of week.</li> <li>Changed indexing of Month to start at 1, so Month range is (1-12) instead of (0-11)</li> <li>Type cast variables away from <code>long</code> as soon as values were less than 65535 to greatly decrease code execution time. <ul> <li>The requires long variables are: <ul> <li><code>ntp_time</code>, seconds since 1900 (0-4294967295)</li> <li><code>dayclock</code>, seconds into day (0-86399)</li> </ul></li> <li>The largest of the rest of the variables is the calculated year within the date.</li> </ul></li> </ul> <hr> The Python <code>ntp_time_to_date</code> function (with its dependencies) runs successfully on the Telit GC-864 on an embedded version of Python 1.5.2+, as well as on Python 2.7.3, but of course use the datetime library if you can.
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