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cheers,
Chris Maunder
The Code Project Co-founder
Microsoft C++ MVP
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cheers,
Chris Maunder
The Code Project Co-founder
Microsoft C++ MVP
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Imagine you have a switch with anything between 3 and a silly amount of comparisons, at which size does a lookup dictionary with delegates get faster?
I'm fully aware there are no exact answers to this, I just want some elaboration on what's affecting the performance.
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Let me add to that question - not using a lookup dictionary, but a 2D array:
I regularly see people claim that they program in a 'state machine' fashion (breaking numerous rules for state machine programming, but that's not the question here), essentially as a switch or sequence of if-else on the event, each switch/else alternative being another switch/if-else on the current state. I find that coding style terrible, impossible to maintain.
My coding style for state machines is creating a 2D array of Action and Output delegate references and a NextState value, possibly headed by a predicate delegate reference (so the array becomes a 2,5D one). In the worst case, three delegates must be called, plus one per failing predicate. In the simplest case, a single delegate is called (no predicate, no output).
Obviously, there is also the initial indexing of the state table on event and state, and if the entry has a chain of predicated alternatives, the code to iterate over them. This is part of the basic transition mechanism, unrelated to the specific table/transition.
This way of coding state machines has so many advantages that I will be very reluctant to change it. Yet I wonder: Is this indexing and delegate calling a CPU costly way of doing it, compared to nesting of switch / if-else in 2-3 levels? Are there performance pitfalls I should be aware of when indexing / calling delegates?
Religious freedom is the freedom to say that two plus two make five.
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Jörgen Andersson wrote: between 3 and a silly amount of comparisons, a
Presumably you mean cases.
Jörgen Andersson wrote: y with delegates get faster?
At the point where I have profiled the application (not the code) with realistic production data and determined that the specific code is in fact a performance bottle neck.
At that point then I would look at the design not the code to determine if there was some completely different way to do it.
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Interesting question, to which I wouldn't like to guess the answer, but here's what I'd consider.
Firstly, what is it that is being switched? If its something primitive like an integer, the switch code would I expect boil down to a collection of CMP and BEQ instructions (compare and branch). These would be stupidly fast, and because they are consecutive in memory are likely to benefit from CPU caching, so in that instance, an awful lot of switch cases could be compared in the time of a dictionary look up.
If you are switching on strings though, things get more complicated. To do the dictionary look up, first the string needs to be hashed to give a bucket index, then an equality check is needed to make sure it matches. The switch statement doesn't need to do the hash, but has multiple equality checks to do, so I suspect the answer here boils down to the ratio of time taken to hash vs. time taken to do an equality check. So then you get into the realms of how similar are the strings? To check for equality, if the first character is different you can just bail out and fail the test quickly, but if its the last you have to go through every character before you can pass or fail the test.
It'd be interesting to profile this, but somehow the idea of creating a switch statement with hundreds/thousands of cases sounds unpleasant, I have no idea whether a compiler would accept it and would be completely impossible to work with.
Regards,
Rob Philpott.
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Rob Philpott wrote: Firstly, what is it that is being switched?
It's strings.
Rob Philpott wrote: So then you get into the realms of how similar are the strings?
They are quite similar I'm afraid, as the words I'm parsing starts with the category.
So one of the larger switches will be 60+ words where the first difference is at the 19th position.
And as the files that will be parsed are in between GB and TB in size it will probably be worth some optimization.
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Jörgen Andersson wrote: So one of the larger switches will be 60+ words where the first difference is at the 19th position.
Perhaps you could break that down a bit? Validate that the input contains at least 19 characters, then switch on the 19th character to decide which path to take.
if (input.Length >= 19)
{
return input[18] switch
{
'A' => ProcessA(input.AsSpan(19)),
'B' => ProcessB(input.AsSpan(19)),
...
};
}
You could even do that with a list pattern[^], although the repeated discards would look quite messy. 
return input switch
{
[_, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, 'A', ..] => ProcessA(input.AsSpan(19)),
...
};
"These people looked deep within my soul and assigned me a number based on the order in which I joined."
- Homer
modified 20hrs ago.
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C# has gone a bit bonkers hasn't it? I still have to look up how to do this 'new' stuff. Although I do like being able to create empty arrays with [] etc.
Oh, and I think you're out-by-one: return input[18] switch
Regards,
Rob Philpott.
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List pattern looks interesting
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Ah ok, so is it that you've got these large files to process and you're trying to optimise the switching (state changes) for speed? Which approach are you using at the moment (switch vs. array lookup/not dictionary, sorry just read your update)?
I suppose another difference is that switch statements are compile timed things, turned into code, whereas dictionaries are created and used at runtime. Does this mean the switch/state change logic is fixed in advance?
I think the only well to tell really is try both methods and see which is quicker (if noticeably so). What I would say is I'd expect them both to be fast, so is this really the bottleneck to performance gains, or could something else be optimised? Multithreading/pipelining etc. TPL Dataflow (if you're in .NET) is good for this.
Regards,
Rob Philpott.
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Rob Philpott wrote: so is it that you've got these large files to process and you're trying to optimise the switching (state changes) for speed
Indeed.
Rob Philpott wrote: Which approach are you using at the moment
I've set up the parsing using switches just to make sure it works, but it's painfully slow so I'm looking at refactoring it at the moment.
Rob Philpott wrote: Does this mean the switch/state change logic is fixed in advance?
This is where it gets funny.
In theory yes. But changes might happen every now and then
These files are supplied by a government entity. And while we're allowed to get the data (which is actually only a subset), we're not allowed to take part of the documentation (no, really).
And they can't be bothered to make separate documentation for our subset (without us paying an extortion fee that is).
So I've added logics that tells me when they've added or removed attributes.
Oddly enough, I'm having fun tinkering with these files, mostly.
Multithreading is the next logical step, but I want to get as far as possible without using brute force before that.
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I believe I might have a generic answer to my question.
This is a simple implementation of IDictionary using a singly linked list. It is smaller and faster than a Hashtable if the number of elements is 10 or less. This should not be used if performance is important for large numbers of elements.
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Maybe, that thing is old, predating generics so there might be some boxing overhead depending on what you stick in it, unless they've done a generic version of it.
It's hard to comment from this distance, but if the state machine might change, surely its better to model it at runtime so you just need to adjust some static data rather than go back to source...
Profiling is always a good option, to see where the bottlenecks lie. Anyway, best of luck!
Regards,
Rob Philpott.
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Jörgen Andersson wrote: It's strings.
For a dictionary then you are going to need to compute the hash.
Jörgen Andersson wrote: And as the files
And then you must compute the hash for each of those.
I suspect this really depends on the size and probably the standard deviation of the sizes for each string.
I haven't thought this through and certainly have not profiled it but a tree might be better. The sparse tree is built with each fork having one character. Next level has 26 (or whatever size your set is) characters.
Keep in mind that a hash requires sequencing through each character. So a tree is somewhat similar to that EXCEPT when you reach the end (leaf of tree) you have already reached your delegate. So no further operations to look up.
If each level has the entire character set you can use an array and do a direct look up to the next level (the character is the index into the array.)
Carefully calculate the memory space. You could use a sparse tree but that will slow it down.
And maybe you should look at unmanaged code. Specifically C++.
One advantage to C++ (and C) is that you can force a string to be treated as a numeric. So a value like "ABCD" can be cast directly to a 32 bit unsigned integer. And of course you could use 64 bit also.
The problem with that of course is that you must then deal with 4/8 character size blocks only.
Jörgen Andersson wrote: be worth some optimization.
You should actually profile the application. Not specific code but the entire application. If you want it to be fast then you should find the exact places where it is slow.
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П.... не копил Editor3D_Render_Control на 2022 MSVStuio
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write your question in English and NOT with kyrillic letters ...
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I'm sorry, your question makes no sense. First of all, this is an English speaking site so please post questions in English. Second, when I translated your question, it said:
P.... didn’t save Editor3D_Render_Control for 2022 MS VStuio What does that even mean?
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What did you use to translate? Google produces "screw up" instead of "save" for me. 
"These people looked deep within my soul and assigned me a number based on the order in which I joined."
- Homer
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If you mean this control: Editor3D: A Windows.Forms Render Control with interactive 3D Editor in C#[^] then don't post this under a "generic" forum like C# - if you got the code from an article, then there is a "Add a Comment or Question" button at the bottom of that article, which causes an email to be sent to the author. They are then alerted that you wish to speak to them.
Posting this here relies on them "dropping by" and realising it is for them.
"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony
"Common sense is so rare these days, it should be classified as a super power" - Random T-shirt
AntiTwitter: @DalekDave is now a follower!
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In VS 2022 I can not find any templates for WinUI. I installed Windows SDK and WinUI controls but I can not find any templates. Any idea why ?
Thanks
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Did you install the correct workloads by following the link in point 1?
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