Will following code be executed atomically?
const int oldId = id.exchange((id.load()+1) % maxId);
Where id is std::atomic<int>, and maxId is some integer value.
I searched google and stackoverflow for std::atomic modulo increment. And I found some topics but I can't find clear answer how to do that properly.
In my case even better would be to use:
const int newId = id.exchange((++id) % maxId);
But I am still not sure if it will be executed atomically.
No, this is not atomic, because the load() and the exchange() are separate operations, and nothing is preventing id from getting updated after the load, but before the exchange. In that case your exchange would write a value that has been calculated based on a stale input, so you end up with a missed update.
You can implement a modulo increment using a simple compare_exchange loop:
int val = id.load();
int newVal = (val + 1) % maxId;
while (!id.compare_exchange_weak(val, newVal) {
newVal = (val + 1) % maxId;
}
If the compare_exchange fails it performs a reload and populates val with the updated value. So we can re-calculate newVal and try again.
Edit:
The whole point of the compare-exchange-loop is to handle the case that between the load and the compare-exchange somebody might change id. The idea is to:
load the current value of id
calculate the new value
update id with our own value if and only if the value currently stored in id is the same one as we read in 1. If this is the case we are done, otherwise we restart at 1.
compare_exchange is allows us to perform the comparison and the conditional update in one atomic operation. The first argument to compare_exchange is the expected value (the one we use in our comparison). This value is passed by reference. So when the comparison fails, compare_exchange automatically reloads the current value and updates the provided variable (in our case val).
And since Peter Cordes pointed out correctly that this can be done in a do-while loop to avoid the code duplication, here it is:
int val = id.load();
int newVal;
do {
newVal = (val + 1) % maxId;
} while (!id.compare_exchange_weak(val, newVal);
Related
I am learning Dart, and I can't understand the logic behind this code if anyone can help:
Function applyMultiplier(num multiplier) {
return (num value) {
return value * multiplier;
};
}
final triple = applyMultiplier(3);
print(triple(6)); //output 18
There is an anonymous function inside a named function.
We assigned a function to a variable.
What I don't understand is how did we pass from triple to value. I can't understand the logic behind.
Well, the function applyMultiplier takes a num as argument and returns a function that itself returns the value it is given multiplied by another multiplier. final tripple = applyMultiplier(3) stores this function that is returned from applyMultiplier in the variable triple. Because the variable triple then stores a function it can also be used like a function.
Unfortunately I am not able to post the code I am debugging as it is not mine and I am bound not to show it... BUT I will describe it as detailed as possible.
There are 4 strategies base on 4 indicators, custom, and not-custom ones. So basically instead of 4 different EAs running in 4 different charts with the same 4 indicators each... The client asked me to optimise them by putting them all in one to run 4 into 1 EAs in the same chart.
EVERYTHING is the same. They are tested as well that they are the same. They open the same trades, on the same moments. Nothing is changed 100%. The only thing I did (for this part of the debugging, because obviously I had a lot more to do before that) is to copy functions and code. And I seperated all different strategies with an "if" as input
input bool strategy1enabled = true; etc... so he/she can disable/enable individual strategies if wanted.
everything works BUT....
All but 1 strategies, does not show the Comment on the trades.
All 4 use the same Buy/Sell/CloseOrder functions so I just input the values to keep the code shorter.
//---
bool OrdClose (int ticket_number, double lt, int slp)
{
return OrderClose(ticket_number,lt,iClose(NULL,0,0),slp,clrViolet);
}
//---
int Buy(double lt, int slp, int slss, int tpft, string cmt, int mgc)
{
return OrderSend(NULL,OP_BUY,lt,Ask,slp,Ask-slss*Point,Ask+tpft*Point,cmt,mgc,0,clrDarkBlue);
}
//---
int Sell(double lt, int slp, int slss, int tpft, string cmt, int mgc)
{
return OrderSend(NULL,OP_SELL,lt,Bid,slp,Bid+slss*Point,Bid-tpft*Point,cmt,mgc,0,clrDarkRed);
}
1 strategy just refuses to put comment. Any ideas why? When used seperated WITH THE SAME CODE and the EXACT SAME functions... comment shows...
EDIT:
2021.05.04 18:30:48.670 The_Big_Holla_v1_8_EA CADJPY,H1: open #85710545 buy 0.06 CADJPY at 88.755 sl: 88.655 tp: 88.955 ok
2021.05.04 18:30:48.462 The_Big_Holla_v1_8_EA CADJPY,H1: Holla v4.9 || GreedInjectionMode
2021.05.04 18:30:48.462 The_Big_Holla_v1_8_EA CADJPY,H1: Holla v4.9 || GreedInjectionMode
Comment is passed properly and checked before being passed to function and before OrderSend within function:
The function:
int Sell(double lt, int slp, int slss, int tpft, string cmt, int mgc)
{
Print(cmt);
return OrderSend(NULL,OP_SELL,lt,Bid,slp,Bid+slss*Point,Bidtpft*Point,cmt,mgc,0,clrDarkRed);
}
How the function is called:
Print(EACommentInj);
ticket_val_inj = Buy(lotsizeInj,slippageInj,stoplossInj,takeprofitInj,EACommentInj,MagicInj);
This is how it is initialised and it NEVER changes. It is mentioned only where it is passed. Where I showed you above.
const string EACommentInjGreed = "Holla v4.9 || GreedInjectionMode Greed Mode";
Although this is undocumented, the "string comment=NULL" parameter of the trade function OrderSend() in MQL4 is limited to 31 characters. If this limit is exceeded then the string is rejected as a whole and treated as NULL.
In your code, just before the OrderSend() function, add the following line:
cmt=StringSubstr(cmt,0,31);
Functions in Dart are first-class objects, allowing you to pass them to other objects or functions.
void main() {
var shout = (msg) => ' ${msg.toUpperCase()} ';
print(shout("yo"));
}
This made me wonder if there was a way to modify a function a run time, just like an object, prior to passing it to something else. For example:
Function add(int input) {
return add + 2;
}
If I wanted to make the function a generic addition function, then I would do:
Function add(int input, int increment) {
return add + increment;
}
But then the problem would be that the object I am passing the function to would need to specify the increment. I would like to pass the add function to another object, with the increment specified at run time, and declared within the function body so that the increment cannot be changed by the recipient of the function object.
The answer seems to be to use a lexical closure.
From here: https://dart.dev/guides/language/language-tour#built-in-types
A closure is a function object that has access to variables in its
lexical scope, even when the function is used outside of its original
scope.
Functions can close over variables defined in surrounding scopes. In
the following example, makeAdder() captures the variable addBy.
Wherever the returned function goes, it remembers addBy.
/// Returns a function that adds [addBy] to the
/// function's argument.
Function makeAdder(int addBy) {
return (int i) => addBy + i;
}
void main() {
// Create a function that adds 2.
var add2 = makeAdder(2);
// Create a function that adds 4.
var add4 = makeAdder(4);
assert(add2(3) == 5);
assert(add4(3) == 7);
}
In the above cases, we pass 2 or 4 into the makeAdder function. The makeAdder function uses the parameter to create and return a function object that can be passed to other objects.
You most likely don't need to modify a closure, just the ability to create customized closures.
The latter is simple:
int Function(int) makeAdder(int increment) => (int value) => value + increment;
...
foo(makeAdder(1)); // Adds 1.
foo(makeAdder(4)); // Adds 2.
You can't change which variables a closure is referencing, but you can change their values ... if you an access the variable. For local variables, that's actually hard.
Mutating state which makes an existing closure change behavior can sometimes be appropriate, but those functions should be very precise about how they change and where they are being used. For a function like add which is used for its behavior, changing the behavior is rarely a good idea. It's better to replace the closure in the specific places that need to change behavior, and not risk changing the behavior in other places which happen to depend on the same closure. Otherwise it becomes very important to control where the closure actually flows.
If you still want to change the behavior of an existing global, you need to change a variable that it depends on.
Globals are easy:
int increment = 1;
int globalAdder(int value) => value + increment;
...
foo(globalAdd); // Adds 1.
increment = 2;
foo(globalAdd); // Adds 2.
I really can't recommend mutating global variables. It scales rather badly. You have no control over anything.
Another option is to use an instance variable to hold the modifiable value.
class MakeAdder {
int increment = 1;
int instanceAdd(int value) => value + increment;
}
...
var makeAdder = MakeAdder();
var adder = makeAdder.instanceAdd;
...
foo(adder); // Adds 1.
makeAdder.increment = 2;
foo(adder); // Adds 2.
That gives you much more control over who can access the increment variable. You can create multiple independent mutaable adders without them stepping on each other's toes.
To modify a local variable, you need someone to give you access to it, from inside the function where the variable is visible.
int Function(int) makeAdder(void Function(void Function(int)) setIncrementCallback) {
var increment = 1;
setIncrementCallback((v) {
increment = v;
});
return (value) => value + increment;
}
...
void Function(int) setIncrement;
int Function(int) localAdd = makeAdder((inc) { setIncrement = inc; });
...
foo(localAdd); // Adds 1.
setIncrement(2);
foo(localAdd); // Adds 2.
This is one way of passing back a way to modify the local increment variable.
It's almost always far too complicated an approach for what it gives you, I'd go with the instance variable instead.
Often, the instance variable will actually represent something in your model, some state which can meaningfully change, and then it becomes predictable and understandable when and how the state of the entire model changes, including the functions referring to that model.
Using partial function application
You can use a partial function application to bind arguments to functions.
If you have something like:
int add(int input, int increment) => input + increment;
and want to pass it to another function that expects to supply fewer arguments:
int foo(int Function(int input) applyIncrement) => applyIncrement(10);
then you could do:
foo((input) => add(input, 2); // `increment` is fixed to 2
foo((input) => add(input, 4); // `increment` is fixed to 4
Using callable objects
Another approach would be to make a callable object:
class Adder {
int increment = 0;
int call(int input) => input + increment;
}
which could be used with the same foo function above:
var adder = Adder()..increment = 2;
print(foo(adder)); // Prints: 12
adder.increment = 4;
print(foo(adder)); // Prints: 14
I am just starting with Spring Reactor and want to implement something that I would call 'standard pagination', don't know if there is technical term for this. Basically no matter what start and end date is passed to method, I want to return same amound of data, evenly distributed.
This will be used for some chart drawing in the future.
I figured out rough copy with algorithm that does exactly that, unfortunatelly before I can filter results I need to either count() or take last index() and block to get this number.
This block is surelly not the reactive way to do this, also it makes flux to call DB twice for data (or am I missing something?)
Is there any operator than can help me and get result from count() somehow down the stream for further usage, it would need to compute anyway before stream can be processed, but to get rid of calling DB two times?
I am using mongoDB reactive driver.
Flux<StandardEntity> results = Flux.from(
mongoCollectionManager.getCollection(channel)
.find( and(gte("lastUpdated", begin), lte("lastUpdated", end))))
.map(d -> new StandardEntity(d.getString("price"), d.getString("lastUpdated")));
Long lastIndex = results
.count()
.block();
final double standardPage = 10.0D;
final double step = lastIndex / standardPage;
final double[] counter = {0.0D};
return
results
.take(1)
.mergeWith(
results
.skip(1)
.filter(e -> {
if (lastIndex > standardPage)
if (counter[0] >= step) {
counter[0] = counter[0] - step + 1;
return true;
} else {
counter[0] = counter[0] + 1;
return false;
}
else
return true;
}));
Example code that produces the problem:
import std.stdio, core.thread;
void main() {
ThreadGroup tg = new ThreadGroup();
foreach (int i; 1 .. 5)
tg.create( () => writeln(i) );
tg.joinAll();
}
Sample output:
3
5
5
5
5
(The expected output was the integers 1 through 5.)
I don't get why this is happening -- i is not a reference type, nor is there a reference to it being used in the delegate, so why does each thread use the value of i as whatever value it happens to have when the thread is scheduled, instead of the presumably pass-by-value value it's given?
I've made a handful of lame attempts like this, but nothing's been successful:
foreach (int i; 1 .. 5) {
scope j = i;
tg.create( () => writeln(j) );
}
I'm curious as to why this doesn't work, either. Is it not declaring a fresh j each time? Why does each thread refer to the same j (whose value is usually 5 by the time the threads get scheduled)?
so why does each thread use the value of i as whatever value it happens to have when the thread is scheduled, instead of the presumably pass-by-value value it's given?
It's pass-by-value as far as the loop body goes, however that does not apply to the threads created in it. The threads will still refer to i by its address.
To fix this problem, you need to create a closure inside the loop:
import std.stdio, core.thread;
void main() {
ThreadGroup tg = new ThreadGroup();
foreach (int i; 1 .. 5)
(i =>
tg.create( () => writeln(i) )
)(i);
tg.joinAll();
}
The lambda parameter will be stored in the closure, giving each thread its own copy.