I need to convert NSDate objects to (ICU) UDate objects. I have read that they are similar, but I was hoping that someone might have already investigated doing this and can help me shortcut the process. So has anyone got some sample code where they have done this, or can point me in the direction of where I can determine how to do this?
I haven't tested this, but probably (UDate)([someNsDate timeIntervalSince1970]/1000.0)
UDate = milliseconds since 1970 (double)
timeIntervalSince1970 = seconds since 1970 (double)
Related
I want to write a function that takes 'yyyymmdd' as its input and generates the next day (same 'yyyymmdd' format output). This function can be applied to solve problems like next-week or next-t-days.
I know that this task can be easily done using the time.delta approach. However, what is inside the time library? I mean, what sort of computation did it do to solve this "next-day" problem?
In Python, I wrote my own next-day function using a long if-elif-..-else statement on the day-level when int(dd) >= 28. It worked, but looks extremely ugly and inefficient... So I start to be curious about what's written inside the time library...
In particular, I want to know what is the most efficient way of performing such next-day task, and can we prove its optimality of efficiency?
Thank you very much for your help!
I do appreciate why differences between two LocalDateTime instance are expressed as Periods and not Durations, but I could not find a reason why Period is a class and not a struct.
I am helping to port a codebase that did lots of this:
DateTime t1;
DateTime t2;
TimeSpan diff = t2-t1;
// After port, with a surprising allocation
LocalDateTime t1;
LocalDateTime t2;
Period diff = t2-t1;
It seems like a bit of a perf/GC pitfall, and I'm just curious why Period is a class and not a struct?
The main reason for Period to be a class is that it would be huge - it has 6 long fields and 4 int fields. That would be 64 bytes - an awful lot to pass as a method argument etc. While some other structs in Noda Time are "pretty big" they're not that big.
But it's worth noting that the two pieces of code do radically different things. The Noda Time equivalent of TimeSpan isn't Period; it's Duration, which is a struct. If you don't need a calendrical calculation, you might want to consider converting your two LocalDateTime values to Instant values in UTC (or avoid using LocalDateTime to start with), and then take the difference between those two instants.
Internally, there are non-allocating ways of getting "the number of days" between dates, for example... we could potentially expose something like that publicly, but I think it would be worth doing benchmarking first to prove this is really important. (The GC is pretty good at collecting very-temporary objects. Sure, it's not free - but I think code would have to be doing very little other than this for it to become a major factor.)
I've seen several questions/answers here that suggest the best way to get a string representation of an integer in Objective-C is to use [NSString stringWithFormat:#"%d", x]. I'm afraid the C/C++ programmer in me is having a hard time believing I want to bring all the formatting code into play for such a simple task. That is, I assume stringWithFormat needs to parse through the format string looking for all the different type specifiers, field widths, and options that I could possibly use, then it has to be able to interpret that variable length list of parameters and use the format specifier to coerce x to the appropriate type, then go through a lengthy process of conversion, accounting for signed/unsigned values and negation along the way.
Needless to say in C/C++ I could simply use itoa(x) which does exactly one thing and does it extremely efficiently.
I'm not interested in arguing the relative merits of one language over another, but rather just asking the question: is the incredibly powerful [NSString stringWithFormat:#"%d", x] really the most efficient way to do this very, very simple task in Objective-C? Seems like I'm cracking a peanut with a sledge hammer.
You could use itoa() followed by any of +[NSString stringWithUTF8String:] -[NSString initWithBytes:length:encoding: or +[NSString stringWithCString:encoding:] if it makes you feel better, but I wouldn't worry about it unless you're sure this is a performance problem.
You could also use description method. It boxes it as NSNumber and converts to a NSString.
int intVariable = 1;
NSString* stringRepresentation = [#(intVariable) description];
How do you convert epoch time (unix timestamp) to standard time in D? Is there a way to customize the format?
You really should separate questions out, not ask two completely different questions at the same time.
How do you convert epoch time (unix timestamp) to standard time in D?
If you need to convert from unix time to SysTime's "std time," then you use unixTimeToStdTime:
assert(unixTimeToStdTime(0) == (Date(1970, 1, 1) - Date.init).total!"hnsecs");
So, if you do SysTime(unixTimeToStdTime(0)), you'll get a SysTime in your local time zone at the point in time when it was midnight, January 1st 1970 in UTC (i.e. the epoch time). Unlike the other SysTime constructors, it does not treat the time it's given as being in the timezone that it's given. Rather, it just sets its stdTime to the given value, and it's timezone to the given value. So, to construct an identical SysTime with the other constructors, you'd do something like
auto epochInLocalTime = SysTime(Date(1970, 1, 1), UTC()).toLocalTime();
If you want to convert in the opposite direction, stdTimeToUnixTime will convert a std time to a unix time. However, SysTime has toUnixTime on it, making it so that you generally don't need stdTimeToUnixTime.
time_t epoch = epochInLocalTime.toUnixTime();
However, one thing to be aware of is the fact that std.datetime truly deals with unix time - time_t is always considered to be in UTC. The reason that this matters is that for some inexplicable reason, Windows applies the local time's DST to time_t so that the UTC offset never changes, which means that it's wrong for a good chunk of the year. It works with all of the Microsoft's functions which use time_t, because they expect that nonsense, but you'll have interoperability problems if you try and use a Windows time_t with another box or with a library like std.datetime which actually uses time_t correctly.
Is there a way to customize the format?
You mean that you're looking for a way to provide a user-defined format for a string representing the time (like C's strftime)? std.datetime doesn't have that yet. The API still needs to be designed for it. Some discussion has taken place, but it hasn't been settled on yet. So, it'll happen eventually, but it could be a while.
In the interim, you have toISOString, toISOExtString, and toSimpleString, which use the ISO format, the ISO extended format, and Boost's simple string format respectively. In general, I'd suggest using toISOExtString, because it's both easily read by humans and standard. It's also generally best to put it in UTC (e.g. sysTime.toUTC()) when communicating with other computers (as opposed to printing it out for humans), because then the time zone is part of it, unlike with LocalTime, which doesn't append the time zone.
If you haven't read this article on std.datetime yet, then I suggest that you do, since it should give you a good overview of the module and how to use it.
I'm not familiar with D, but according to std.datetime, you could use these steps
long unixTimeToStdTime(time_t)
struct SysTime(long stdTime)
SysTime.dayOfGregorianCal()
struct Date(int day)
Date.toISOExtString()
Is there a way to convert a string value into time_t format?
Thanks in advance.
One approach could be to parse the string into the various components and use them to fill a tm structure. Once you have the structure filled with data you can use the C function mktime to convert the structure into a time_t type.
There might be more ideal ways to do this in visual C++, but in normal C/C++ this is probably the way i'd do it untill I'd find a better algorithm.