I want to sort my vector but I need to use a swap function in order to do that... Is there any pre-defined methods like Collections.swap(vector, index1, index2) in java2me?
Thats pretty simple actually
private static void swap(Vector src, int i, int j)
{
Object tmp = src.elementAt(i);
src.setElementAt(src.elementAt(j), i);
src.setElementAt(tmp, j);
}
Well, there's certainly a Collections.sort()
http://download.oracle.com/javame/config/cdc/ref-impl/cdc1.1.2/jsr218/java/util/Collections.html
Edit: And since I'm apparently blind, as Bala R points out below, there's a swap as well, exactly as you describe.
Related
I'd like to extend my instrumental Profiler in order to avoid it affect too much performances.
Im my current implementation, I'm using a ProfilerHelper taking one string, which is put whereever you want in the profiling f().
The ctor is starting the measurement and the dector is closing it, logging the Delta in an unordered_map entry, which is key is the string.
Now, I'd like to turn all of that into a faster stuff.
First of all, I'd like to create a string LUT (Look Up Table) contaning the f()s names at compile time, and turn the unordered_map to a plain vector which is paired by the string function LUT.
Now the question is: I've managed to create a LUT but std::string_view, but I cannot find a way to extend it at compile time.
A first rought trial sounds like this:
template<unsigned N>
constexpr auto LUT() {
std::array<std::string_view, N> Strs{};
for (unsigned n = 0; n < N; n++) {
Strs[n] = "";
}
return Strs;
};
constexpr std::array<std::string_view, 0> StringsLUT { LUT<0>() };
constexpr auto AddString(std::string_view const& Str)
{
constexpr auto Size = StringsLUT.size();
std::array<std::string_view, Size + 1> Copy{};
for (auto i = 0; i < Size; ++i)
Copy[i] = StringsLUT[i];
Copy[Size] = Str;
return Copy;
};
int main()
{
constexpr auto Strs = AddString(__builtin_FUNCTION());
//for (auto const Str : Strs)
std::cout << Strs[0] << std::endl;
}
So my idea should be to recall the AddString whenever needed in my f()s to be profiled, extending this list at compile time.
But of course I should take the returned Copy and replace the StringsLUT everytime, to land to a final StringsLUT with all the f() names inside it.
Is there a way to do that at compile time?
Sorry, but I'm just entering the magic "new" world of constexpr applied to LUT right in these days.
Tx for your support in advance.
What I want is extracting a value from vector using a variable scalar index.
Like _mm_extract_epi8 / _mm256_extract_epi8 but with non-immediate input.
(There are some results in the vector, the one with the given index is found out to be the true result, the rest are discarded)
Especially, if index is in a GPR, the easiest way is probably to store val to memory and then movzx it into another GPR. Sample implementation using C:
uint8_t extract_epu8var(__m256i val, int index) {
union {
__m256i m256;
uint8_t array[32];
} tmp;
tmp.m256 = val;
return tmp.array[index];
}
Godbolt translation (note that a lot of overhead happens for stack alignment -- if you don't have an aligned temporary storage area, you could just vmovdqu instead of vmovdqa): https://godbolt.org/z/Gj6Eadq9r
So far the best option seem to be using _mm_shuffle_epi8 for SSE
uint8_t extract_epu8var(__m128i val, int index) {
return (uint8_t)_mm_cvtsi128_si32(
_mm_shuffle_epi8(val, _mm_cvtsi32_si128(index)));
}
Unfortunately this does not scale well for AVX. vpshufb does not shuffle across lanes. There is a cross lane shuffle _mm256_permutevar8x32_epi32, but the resulting stuff seem to be complicated:
uint8_t extract_epu8var(__m256i val, int index) {
int index_low = index & 0x3;
int index_high = (index >> 2);
return (uint8_t)(_mm256_cvtsi256_si32(_mm256_permutevar8x32_epi32(
val, _mm256_zextsi128_si256(_mm_cvtsi32_si128(index_high))))
>> (index_low << 3));
}
Is there a fast (native) method to search for a sequence in a Uint8List?
///
/// Return index of first occurrence of seq in list
///
int indexOfSeq(Uint8List list, Uint8List seq) {
...
}
EDIT: Changed List<int> into Uint8List
No. There is no built-in way to search for a sequence of elements in a list.
I am also not aware of any dart:ffi based implementations.
The simplest approach would be:
extension IndexOfElements<T> on List<T> {
int indexOfElements(List<T> elements, [int start = 0]) {
if (elements.isEmpty) return start;
var end = length - elements.length;
if (start > end) return -1;
var first = elements.first;
var pos = start;
while (true) {
pos = indexOf(first, pos);
if (pos < 0 || pos > end) return -1;
for (var i = 1; i < elements.length; i++) {
if (this[pos + i] != elements[i]) {
pos++;
continue;
}
}
return pos;
}
}
}
This has worst-case time complexity O(length*elements.length). There are several more algorithms with better worst-case complexity, but they also have larger constant factors and more expensive pre-computations (KMP, BMH). Unless you search for the same long list several times, or do so in a very, very long list, they're unlikely to be faster in practice (and they'd probably have an API where you compile the pattern first, then search with it.)
You could use dart:ffi to bind to memmem from string.h as you suggested.
We do the same with binding to malloc from stdlib.h in package:ffi (source).
final DynamicLibrary stdlib = Platform.isWindows
? DynamicLibrary.open('kernel32.dll')
: DynamicLibrary.process();
final PosixMalloc posixMalloc =
stdlib.lookupFunction<Pointer Function(IntPtr), Pointer Function(int)>('malloc');
Edit: as lrn pointed out, we cannot expose the inner data pointer of a Uint8List at the moment, because the GC might relocate it.
One could use dart_api.h and use the FFI to pass TypedData through the FFI trampoline as Dart_Handle and use Dart_TypedDataAcquireData from the dart_api.h to access the inner data pointer.
(If you want to use this in Flutter, we would need to expose Dart_TypedDataAcquireData and Dart_TypedDataReleaseData in dart_api_dl.h https://github.com/dart-lang/sdk/issues/40607 I've filed https://github.com/dart-lang/sdk/issues/44442 to track this.)
Alternatively, could address https://github.com/dart-lang/sdk/issues/36707 so that we could just expose the inner data pointer of a Uint8List directly in the FFI trampoline.
Imagine a gfor with a seq j...
If I need to use the value of the instance j as a index, who can I do that?
something like:
vector<double> a(n);
gfor(seq j, n){
//Do some calculation and save this on someValue
a[j] = someValue;
}
Someone can help me (again) ?
Thanks.
I've found a solution for this...
if someone had a better option, feel free to post...
First, create a seq with the same size of your gfor instances.
Then, convert that seq in a array.
Now, take the value of that line on array (it's equals the index)
seq sequencia(0, 200);
af::array sqc = sequencia;
//Inside the gfor loop
countLoop = (int) sqc(j).scalar<float>();
Your approach works, but breaks gfors parallelization as converting the index to a scalar forces it to be written from the gpu back to the host, slamming the breaks on the GPU.
You want to do it more like this :
af::array a(200);
gfor(seq j, 200){
//Do some calculation and save this on someValue
a[j] = af::array(someValue); // for someValue a primitive type, say float
}
// ... Now we're safe outside the parallel loop, let's grab the array results
float results[200];
a.host(results) // Copy array from GPU to host, populating a c-type array
I'm trying to write what I would think of as an extremely simple piece of code in Rascal: Testing if list A contains list B.
Starting out with some very basic code to create a list of strings
public list[str] makeStringList(int Start, int End)
{
return [ "some string with number <i>" | i <- [Start..End]];
}
public list[str] toTest = makeStringList(0, 200000);
My first try was 'inspired' by the sorting example in the tutor:
public void findClone(list[str] In, str S1, str S2, str S3, str S4, str S5, str S6)
{
switch(In)
{
case [*str head, str i1, str i2, str i3, str i4, str i5, str i6, *str tail]:
{
if(S1 == i1 && S2 == i2 && S3 == i3 && S4 == i4 && S5 == i5 && S6 == i6)
{
println("found duplicate\n\t<i1>\n\t<i2>\n\t<i3>\n\t<i4>\n\t<i5>\n\t<i6>");
}
fail;
}
default:
return;
}
}
Not very pretty, but I expected it to work. Unfortunately, the code runs for about 30 seconds before crashing with an "out of memory" error.
I then tried a better looking alternative:
public void findClone2(list[str] In, list[str] whatWeSearchFor)
{
for ([*str head, *str mid, *str end] := In)
if (mid == whatWeSearchFor)
println("gotcha");
}
with approximately the same result (seems to run a little longer before running out of memory)
Finally, I tried a 'good old' C-style approach with a for-loop
public void findClone3(list[str] In, list[str] whatWeSearchFor)
{
cloneLength = size(whatWeSearchFor);
inputLength = size(In);
if(inputLength < cloneLength) return [];
loopLength = inputLength - cloneLength + 1;
for(int i <- [0..loopLength])
{
isAClone = true;
for(int j <- [0..cloneLength])
{
if(In[i+j] != whatWeSearchFor[j])
isAClone = false;
}
if(isAClone) println("Found clone <whatWeSearchFor> on lines <i> through <i+cloneLength-1>");
}
}
To my surprise, this one works like a charm. No out of memory, and results in seconds.
I get that my first two attempts probably create a lot of temporary string objects that all have to be garbage collected, but I can't believe that the only solution that worked really is the best solution.
Any pointers would be greatly appreciated.
My relevant eclipse.ini settings are
-XX:MaxPermSize=512m
-Xms512m
-Xss64m
-Xmx1G
We'll need to look to see why this is happening. Note that, if you want to use pattern matching, this is maybe a better way to write it:
public void findClone(list[str] In, str S1, str S2, str S3, str S4, str S5, str S6) {
switch(In) {
case [*str head, S1, S2, S3, S4, S5, S6, *str tail]: {
println("found duplicate\n\t<S1>\n\t<S2>\n\t<S3>\n\t<S4>\n\t<S5>\n\t<S6>");
}
default:
return;
}
}
If you do this, you are taking advantage of Rascal's matcher to actually find the matching strings directly, versus your first example in which any string would match but then you needed to use a number of separate comparisons to see if the match represented the combination you were looking for. If I run this on 110145 through 110150 it takes a while but works and it doesn't seem to grow beyond the heap space you allocated to it.
Also, is there a reason you are using fail? Is this to continue searching?
It's an algorithmic issue like Mark Hills said. In Rascal some short code can still entail a lot of nested loops, almost implicitly. Basically every * splice operator on a fresh variable that you use on the pattern side in a list generates one level of loop nesting, except for the last one which is just the rest of the list.
In your code of findClone2 you are first generating all combinations of sublists and then filtering them using the if construct. So that's a correct algorithm, but probably slow. This is your code:
void findClone2(list[str] In, list[str] whatWeSearchFor)
{
for ([*str head, *str mid, *str end] := In)
if (mid == whatWeSearchFor)
println("gotcha");
}
You see how it has a nested loop over In, because it has two effective * operators in the pattern. The code runs therefore in O(n^2), where n is the length of In. I.e. it has quadratic runtime behaviour for the size of the In list. In is a big list so this matters.
In the following new code, we filter first while generating answers, using fewer lines of code:
public void findCloneLinear(list[str] In, list[str] whatWeSearchFor)
{
for ([*str head, *whatWeSearchFor, *str end] := In)
println("gotcha");
}
The second * operator does not generate a new loop because it is not fresh. It just "pastes" the given list values into the pattern. So now there is actually only one effective * which generates a loop which is the first on head. This one makes the algorithm loop over the list. The second * tests if the elements of whatWeSearchFor are all right there in the list after head (this is linear in the size of whatWeSearchFor and then the last *_ just completes the list allowing for more stuff to follow.
It's also nice to know where the clone is sometimes:
public void findCloneLinear(list[str] In, list[str] whatWeSearchFor)
{
for ([*head, *whatWeSearchFor, *_] := In)
println("gotcha at <size(head)>");
}
Rascal does not have an optimising compiler (yet) which might possibly internally transform your algorithms to equivalent optimised ones. So as a Rascal programmer you are still asked to know the effect of loops on your algorithms complexity and know that * is a very short notation for a loop.