10-21-2008
Quote:
Originally Posted by
neked
3) Even if you were a parameter substitution guru, and you used meaningful variable names and comments to make clearer what your parameter substitution tricks do, then future maintainers of the code might not be the same. This bit me a couple of days ago: I had to spend 20 minutes debugging a bash script riddled with those parameter substitution scripts. I estimate I would've spent closer to 5 minutes if the code was written using more obvious external tools (sed, basename, awk). This is about 15 minutes of human time wasted in order to save less than a few milliseconds of CPU time. Especially since the whole script runs only once a night, and does not exceed 0.030 seconds runtime on my modest 7 years old computer.
I've just substituted all occurrences of parameter substitutions with equivalent sed and basename commands in the script I refer to above, the execution time went up from an average of 0.030 seconds to 0.061 seconds. That means parameter substitution use shaved an average of 0.03 seconds per run. Considering that I've had to spend roughly 15 more minutes understanding the code to debug it, this means the code has to run 15*60/0.03 times to pay off the extra time I invested to debug it. That's 30,000 times. Considering that it runs once a night, this means we'll have to wait more than 80 years!
Even after 80 years, those 0.020 seconds shaved off CPU time per night are not as worthy to me as 15 minutes of my time which I could've spent
aternative activity redacted
Last edited by Perderabo; 10-21-2008 at 05:49 PM..
Reason: clean up language
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LEARN ABOUT SUSE
image::exiftool::shift
Image::ExifTool::Shift(3) User Contributed Perl Documentation Image::ExifTool::Shift(3)
NAME
Image::ExifTool::Shift.pl - ExifTool time shifting routines
DESCRIPTION
This module contains routines used by ExifTool to shift date and time values.
DETAILS
Time shifts are applied to standard EXIF-formatted date/time values (ie. "2005:03:14 18:55:00"). Date-only and time-only values may also
be shifted, and an optional timezone (ie. "-05:00") is also supported. Here are some general rules and examples to explain how shift
strings are interpreted:
Date-only values are shifted using the following formats:
'Y:M:D' - shift date by 'Y' years, 'M' months and 'D' days
'M:D' - shift months and days only
'D' - shift specified number of days
Time-only values are shifted using the following formats:
'h:m:s' - shift time by 'h' hours, 'm' minutes and 's' seconds
'h:m' - shift hours and minutes only
'h' - shift specified number of hours
Timezone shifts are specified in the following formats:
'+h:m' - shift timezone by 'h' hours and 'm' minutes
'-h:m' - negative shift of timezone hours and minutes
'+h' - shift timezone hours only
'-h' - negative shift of timezone hours only
A valid shift value consists of one or two arguments, separated by a space. If only one is provided, it is assumed to be a time shift when
applied to a time-only or a date/time value, or a date shift when applied to a date-only value. For example:
'7' - shift by 1 hour if applied to a time or date/time
value, or by one day if applied to a date value
'2:0' - shift 2 hours (time, date/time), or 2 months (date)
'5:0:0' - shift 5 hours (time, date/time), or 5 years (date)
'0:0:1' - shift 1 s (time, date/time), or 1 day (date)
If two arguments are given, the date shift is first, followed by the time shift:
'3:0:0 0' - shift date by 3 years
'0 15:30' - shift time by 15 hours and 30 minutes
'1:0:0 0:0:0+5:0' - shift date by 1 year and timezone by 5 hours
A date shift is simply ignored if applied to a time value or visa versa.
Numbers specified in shift fields may contain a decimal point:
'1.5' - 1 hour 30 minutes (time, date/time), or 1 day (date)
'2.5 0' - 2 days 12 hours (date/time), 12 hours (time) or
2 days (date)
And to save typing, a zero is assumed for any missing numbers:
'1::' - shift by 1 hour (time, date/time) or 1 year (date)
'26:: 0' - shift date by 26 years
'+:30 - shift timezone by 30 minutes
Below are some specific examples applied to real date and/or time values ('Dir' is the applied shift direction: '+' is positive, '-' is
negative):
Original Value Shift Dir Shifted Value
--------------------- ------- --- ---------------------
'20:30:00' '5' + '01:30:00'
'2005:01:27' '5' + '2005:02:01'
'11:54:00' '2.5 0' - '23:54:00'
'2005:11:02' '2.5 0' - '2005:10:31'
'2005:11:02 11:54:00' '2.5 0' - '2005:10:30 23:54:00'
'2004:02:28 08:00:00' '1 1.3' + '2004:02:29 09:18:00'
'07:00:00' '-5' + '07:00:00'
'07:00:00+01:00' '-5' + '07:00:00-04:00'
'07:00:00Z' '+2:30' - '07:00:00-02:30'
'1970:01:01' '35::' + '2005:01:01'
'2005:01:01' '400' + '2006:02:05'
'10:00:00.00' '::1.33' + '09:59:58.67'
NOTES
The format of the original date/time value is not changed when the time shift is applied. This means that the length of the date/time
string will not change, and only the numbers in the string will be modified. The only exception to this rule is that a 'Z' timezone is
changed to '+00:00' notation if a timezone shift is applied. A timezone will not be added to the date/time string.
TRICKY
This module is perhaps more complicated than it needs to be because it is designed to be very flexible in the way time shifts are specified
and applied...
The ability to shift dates by Y years, M months, etc, is somewhat contradictory to the goal of maintaining a constant shift for all time
values when applying a batch shift. This is because shifting by 1 month can be equivalent to anything from 28 to 31 days, and 1 year can
be 365 or 366 days, depending on the starting date.
The inconsistency is handled by shifting the first tag found with the actual specified shift, then calculating the equivalent time
difference in seconds for this shift and applying this difference to subsequent tags in a batch conversion. So if it works as designed,
the behaviour should be both intuitive and mathematically correct, and the user shouldn't have to worry about details such as this (in
keeping with Perl's "do the right thing" philosophy).
AUTHOR
Copyright 2003-2010, Phil Harvey (phil at owl.phy.queensu.ca)
This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
SEE ALSO
Image::ExifTool(3pm)
perl v5.12.1 2010-01-04 Image::ExifTool::Shift(3)