Hello
I am trying to use WritableBitmap.BackBuffer as used in this example, see the section examples.
What I am trying to do really is write an int[] into a space a nativeint points to.
To be able to write from int [] to some memory I started from this answer on SO.
Microsoft.FSharp.NativeInterop.NativePtr.write seems to be a good function to use to write.
After trying and reading a bit two questions arises.
WritableBitmap.BackBuffer has the type nativeint, how to convert to nativeptr that NativePter.write wants?
It seems that I can only write one int at a time but I want to write a whole int [].
I admit that I am in deep water but it is in the deep water you learn to swim :)
Thank in advance
Gorgen
I think that the NativePtr.write function can be only used to write single value at a time, so if you want to copy an array, you'll have to use a for loop.
An easier option may be to use the Marshal.Copy method (see MSDN) which takes a source array (there are overloads for arrays containing elements of various types) and intptr as the destination.
Something like this should work:
let imageData = [| ... |] // generate one dimensional array with image data
writeableBitmap.Lock()
let buffer = writeableBitmap.BackBuffer
Marshal.Copy(imageData, 0, buffer, imageData.Length)
I think Tomas's suggested approach makes sense. However, to answer your first question, you can convert a nativeint to a nativeptr using the NativeInterop.NativePtr.ofNativeInt function.
Related
Has anyone built a "universal" string class for C++Builder that manages all of the conversions to/from ASCII and Unicode?
I had a vision of a class that would accept AnsiString, UnicodeString, WideString, char*, wchar_t*, std::string, and variant values, and would provide any of those back out. AND the copy constructor has to do a deep copy, not just provide a pointer to the same buffer space (as AnsiString and UnicodeString do).
I figure someone else besides me must have to pass strings to both old interfaces that use char* and new ones that use (wide) strings. If you have built, or know of, something you're willing to share, please let me know. Most of the time it's not too big a deal, until I have to pass a map<std::string, std::string>, then it starts getting ugly.
We do not, and will not, support any internationalization whatsoever, so I don't need to worry about encoding. I just want a class that will return my little ASCII strings in whatever format makes the compiler happy... sanely.
UPDATE: to address the comments:
So, std::map<std::string, std::string> is ugly, because you can't do:
parammap[AnsiString(widekey).c_str()] = AnsiString(widevalue).c_str();
Oh no no no. You have to do this:
AnsiString akey = widekey;
AnsiString aval = widevalue;
parammap[akey.c_str()] = aval.c_str();
The person who originally wrote this code tried to keep it as port-friendly as possible, so he standardized on char* for all of the function calls he wrote (circa 2000, it wasn't a bad assumption). Sometimes I was trying to convert everything to char *s before I realized that the function was then immediately turning around and converting it back to wide. There are multiple interface layers, and it took me a while to figure out how it all went together.
Add in some creative compiler bugs, where it would get confused, especially when pulling string values out of Variants. In some places, I had to do:
String wstr = passedvariant.AsType(varString);
String astr = wstr;
std::string key = astr.c_str();
Then life happened, we ended up starting the port over (for the 3rd time. Don't ask), and I finally got smart and wrapped the low-level library in a layer that does all of the conversions, and retooled the middle layers to deal in Strings, so the application layer can just use String except for that map. For the map<string, string>, I created a function to do the converting, so it was one line in a bunch of places instead of six (the three line conversion above for both key and value).
Lastly, I wasn't actually asking for anyone to make suggestions on how to make my code better. I was asking if anyone had or knew of a universal string class. Our code is the way it is for reasons, and I'm not rewriting all of it to make it prettier. I just wanted not to have to touch so many lines... again. It would have been so much nicer to have the compiler keep track of which format is needed and convert it.
I'm trying to achieve a static cast like coercion that doesn't result in copying of any data.
A naive static cast does not work
let pkt = byte_buffer :> PktHeader
FS0193: Type constraint mismatch. The type byte[] is not compatible with type PktHeader The type 'byte[]' is not compatible with the type 'PktHeader' (FS0193) (program)
where the packet is initially held in a byte array because of the way System.Net.Sockets.Socket.Receive() is defined.
The low level packet struct is defined something like this
[<Struct; StructLayout(LayoutKind.Explicit)>]
type PktHeader =
[<FieldOffset(0)>] val mutable field1: uint16
[<FieldOffset(2)>] val mutable field2: uint16
[<FieldOffset(4)>] val mutable field3: uint32
.... many more fields follow ....
Efficiency is important in this real world scenario because wasteful copying of data could rule out F# as an implementation language.
How do you achieve zero copy efficiencies in this scenario?
EDIT on Nov 29
my question was predicated on the implicit belief that a C/C++/C# style unsafe static cast is a useful construct, as if this is self evident. However, on 2nd thought this kind of cast is not idiomatic in F# since it is inherently an imperative language technique fraught with peril. For this reason I've accepted the answer by V.B. where SBE/FlatBuffers data access is promulgated as best practice.
A pure F# approach for conversion
let convertByteArrayToStruct<'a when 'a : struct> (byteArr : byte[]) =
let handle = GCHandle.Alloc(byteArr, GCHandleType.Pinned)
let structure = Marshal.PtrToStructure (handle.AddrOfPinnedObject(), typeof<'a>)
handle.Free()
structure :?> 'a
This is a minimum example but I'd recommend introducing some checks on the length of the byte array because, as it's written there, it will produce undefined results if you give it a byte array which is too short. You could check against Marshall.SizeOf(typeof<'a>).
There is no pure F# solution to do a less safe conversion than this (and this is already an approach prone to runtime failure). Alternative options could include interop with C# to use unsafe and fixed to do the conversion.
Ultimately though, you are asking for a way to subvert the F# type system which is not really what the language is designed for. One of the principle advantages of F# is the power of the type system and it's ability to help you produce statically verifiable code.
F# and very low-level performance optimizations are not best friends, but then... some smart people do magic even with Java, which doesn't have value types and real generic collections for them.
1) I am a big fan of a flyweight pattern lately. If you architecture allows for it, you could wrap a byte array and access struct members via offsets. A C# example here. SBE/FlatBuffers even have tools to generate a wrapper automatically from a definition.
2) If you could stay within unsafe context in C# to do the work, pointer casting is very easy and efficient. However, that requires pinning the byte array and keeping its handle for later release, or staying within fixed keyword. If you have many small ones without a pool, you could have problems with GC.
3) The third option is abusing .NET type system and cast a byte array with IL like this (this could be coded in F#, if you insist :) ):
static T UnsafeCast(object value) {
ldarg.1 //load type object
ret //return type T
}
I tried this option and even have a snippet somewhere if you need, but this approach makes me uncomfortable because I do not understand its consequences to GC. We have two objects backed by the same memory, what would happen when one of them is GCed? I was going to ask a new question on SO about this detail, will post it soon.
The last approach could be good for arrays of structs, but for a single struct it will box it or copy it anyway. Since structs are on the stack and passed by value, you will probably get better results just by casting a pointer to byte[] in unsafe C# or using Marshal.PtrToStructure as in another answer here, and then copy by value. Copying is not the worst thing, especially on the stack, but allocation of new objects and GC is the enemy, so you need byte arrays pooled and this will add much more to the overall performance than you struct casting issue.
But if your struct is very big, option 1 could still be better.
How can I increase the length of a tuple in Erlang? For example, suppose Tup={1,2,3}, and now I want to add another element to it. Is there any way to do this?
Tuple is not suppose to be a flexible data structure. If you are resizing it often, then you should consider using other erlang data structures like lists, maps or sets - depends on your expectation. Here is nice introduction to key-value stores.
But if you really have to extend that tuple, then you can use erlang:append_element/2:
{1,2,3,4} = erlang:append_element({1,2,3}, 4).
Tuples aren't mutable so you can't, strictly speaking, increase the length.
Generally, if you want a variable-number-of-things datatype, a tuple will be very inconvenient. For example, iterating over all elements of a list is highly idiomatic, while iterating over all elements of a tuple whose size is unknown at compile-time is a pain.
However, a common pattern is to get a tuple as a result from some function and return elements of that tuple plus additions.
country_coords(Name) ->
{Lat, Lng} = find_address(Name),
{_Street, _City, _Zip, Country} = geocode(Lat, Lng),
{ok, Lat, Lng, Country}.
erlang:append_element(tuple_whose_length_to_increase, element_to_be).This is the inbuilt function but tuples,lists are not meant to be flexible.So avoid using this function unless there is no other way
I'm trying to learn how to think in a functional programming way, for this, I'm trying to learn Erlang and solving easy problems from codingbat. I came with the common problem of comparing elements inside a list. For example, compare a value of the i-th position element with the value of the i+1-th position of the list. So, I have been thinking and searching how to do this in a functional way in Erlang (or any functional language).
Please, be gentle with me, I'm very newb in this functional world, but I want to learn
Thanks in advance
Define a list:
L = [1,2,3,4,4,5,6]
Define a function f, which takes a list
If it matches a list of one element or an empty list, return the empty list
If it matches the first element and the second element then take the first element and construct a new list by calling the rest of the list recursivly
Otherwise skip the first element of the list.
In Erlang code
f ([]) -> [];
f ([_]) -> [];
f ([X, X|Rest]) -> [X | f(Rest)];
f ([_|Rest]) -> f(Rest).
Apply function
f(L)
This should work... haven't compiled and run it but it should get you started. Also in case you need to do modifications to it to behave differently.
Welcome to Erlang ;)
I try to be gentle ;-) So main thing in functional approach is thinking in terms: What is input? What should be output? There is nothing like comparing the i-th element with the i+1-th element alone. There have to be always purpose of it which will lead to data transformation. Even Mazen Harake's example doing it. In this example there is function which returns only elements which are followed by same value i.e. filters given list. Typically there are very different ways how do similar thing which depends of purpose of it. List is basic functional structure and you can do amazing things with it as Lisp shows us but you have to remember it is not array.
Each time you need access i-th element repeatable it indicates you are using wrong data structure. You can build up different data structures form lists and tuples in Erlang which can serve your purposes better. So when you face problem to compare i-th with i+1-th element you should stop and think. What is purpose of it? Do you need perform some stream data transformation as Mazen Harake does or You need random access? If second you should use different data structure (array for example). Even then you should think about your task characteristics. If you will be mostly read and almost never write then you can use list_to_tuple(L) and then read using element/2. When you need write occasionally you will start thinking about partition it to several tuples and as your write ratio will grow you will end up with array implementation.
So you can use lists:nth/2 if you will do it only once or several times but on short list and you are not performance freak as I'm. You can improve it using [X1,X2|_] = lists:nthtail(I-1, L) (L = lists:nthtail(0,L) works as expected). If you are facing bigger lists and you want call it many times you have to rethink your approach.
P.S.: There are many other fascinating data structures except lists and trees. Zippers for example.
Should I use unsigned integers for my count class members?
Answer
For example, assume a class
TList <T> = class
private
FCount : Cardinal;
public
property Count : Cardinal read FCount;
end;
That does make sense, doesn't it? The number of items stored in a list can't be negative, so why not use an unsigned integer type for it? I think it's in general a good principle to always use the least general (ergo the most special) type possible.
Now, iterating over a list looks like this:
for I := 0 to List.Count - 1 do
Writeln (List [I]);
When the number of items stored in the list is zero, the compiler tries to evaluate
List.Count - 1
which results in a nice Integer overflow (underflow to be exact). Combined with the fact that the debugger does not show the appropriate location where the exception occured, this was very hard to find for me.
Let me add that if you have overflow checking turned off, the resulting errors will be even harder to track, because then you will often access memory that doesn't belong to you - and that results in undefined behaviour.
I will be using plain Integers for all my count members from now on to avoid situations like this.
If that's complete nonsense, please point it out to me :)
(I just spent an hour tracking an integer overflow in my code, so I decided to share that - most people on here will know that of course, but perhaps I can save someone some time.)
No, definitely not. Delphi idiom is to use integers here. Don't fight the language.
In a 32 bit environment you'll not have more elements in the list, except if you try to build a bitmap.
Let's be clear: every programmer who is going to have to use your code is going to hate you for using a Cardinal instead of an integer.
Unsigned integers are almost always more trouble than they're worth because you usually end up mixing signed and unsigned integers in expressions at some point. That means that the type will need to be widened (and probably have a performance hit) to get correct semantics (ideally the compiler does this as per language definition), or else you'll need to be very careful in your range checking.
Take C/C++ for example: size_t is the type of the integer for memory sizes and allocation, and is unsigned, but ptrdiff_t is the type for the offset you get when you subtract one pointer from another, and necessarily is signed. Want to know how many elements you've allocated in an array? Perhaps you subtract the first element address from the last+1 element address and divide by sizeof(element-type)? Well, now you've just mixed signed and unsigned integers.
Regarding your statement that "I think it's in general a good principle to always use the least general (ergo the most special) type possible." - actually I think it's a good principle to use the data type that will cause you least angst and trouble.
Generally for me that's a signed int since:
I don't usually have lists with 231 or more elements in them.
You shouldn't have lists that big either :-)
I don't like the hassle of having special edge cases in my code.
But it's really a style issue. If 'purity' of code is more important to you than brevity of code, your method is best (with modifications to catch the edge cases). Myself, I prefer brevity since edge cases tend to clutter the code and reduce understanding.
Don't.
It's not just going against a programming idiom, it's an explicit request to the compiler to use unsigned arithmetic, that is either prone to anomalous behaviors (if you don't guard against overflows) or to irrelevant runtime exceptions (if you guard against overflows, a temporary overflow will be fatal, f.i when you subtract before adding, even if the final result is positive, and I'm referring to the CPU opcode-level ordering of operations, which may not bear a trivial relationship to what you have in your code).
Keep in mind "unsigned" does not translate to "positive", it translates to "doesn't have a sign", which is different. The term "unsigned" was picked for good reason (and naming it "Cardinal" in Delphi was a poor choice IMO).
Unsigned types are for raw storage specifications, bitwise operations, ASM code, embedded controllers and other specialty uses. When you're doing high-level programming, you should forget you ever heard about unsigned types.
Moral: use iterators and foreach when you can, because it avoids this question altogether.
Boundary conditions frequently present problems. Allowing for a type that can go negative may just shift the issue. Perhaps it shifts it in a way that's easier to debug, perhaps not. I started off using integers for counting loops like that, but later on switched to cardinals to help me catch errors.