Is it appropriate to mention that authors refused to provide code samples when asked? I work in a sub field of computer science. There was a recent paper (2020) that was published in a reputable conference that claimed to achieve state of the art performance on a specific data set. This data set is a hot dataset, and my research group has many papers on that specific data set (improving state of the art). I asked the authors very politely if they can share their code samples for their fairly complicated algorithm, to which they rejected and told me that it is not available. 

One of my colleagues took 2 weeks to code up the algorithm, and verified its performance, as well as the claimed performance. Meanwhile, we reached a new breakthrough on the dataset and achieved a new state of the art. We are currently writing a draft. 

Now, I can't help but be left with a bad taste from this whole experience. I don't see why they wouldn't share their code especially if they claim a state of the art on a very important data set. Most papers have to be compared with the state of the art to some extent, and so essentially, they're slowing down all researchers in this specific field to recreate their own work prior to publishing their own research. 

Now my question is, would be advisable to add a note that the implementation of THEIR algorithm is our implementation, and that we reached out to them for their code and they declined?
would be advisable to add a note that the implementation of THEIR algorithm is our implementation


Absolutely yes. This provides important context for your experimentation and as such, is valuable information for the reader.

Even better, you could make your implementation of their algorithm publicly available, so that future groups of authors will not encounter the same situation as you did (and will cite your paper).


  and that we reached out to them for their code and they declined?


That is the more delicate question to ask. It would surely be legitimate to do so, as it would represent the situation accurately. But such a statement has a social implication, which is that the authors appear in a slightly negative light (perhaps rightfully so). Proceed with caution.