FASTCOVER parameters incorrect when training dictionary with small size

[Ullaakut]

Describe the bug

Training dictionaries with the zstd binary used to work just fine for me when I used high dictionary sizes, but I now encounter an issue where, when running the following command:

zstd --train --maxdict 256 samples/updates/* -o ./dictionary/updates

I get the following output:

$ zstd --train --maxdict 256 samples/updates/* -o ./dictionary/updates
Trying 5 different sets of parameters
Failed to select dictionary
20%       FASTCOVER parameters incorrect
FASTCOVER parameters incorrect
FASTCOVER parameters incorrect
FASTCOVER parameters incorrect
dictionary training failed : Error (generic)

Along with an exit status at 1. This seems to be due to the low maxdict size, even though I set it explicitly at the minimum allowed value, 256.

Using a maxdict size of 512 for example, seems to solve the issue, at least partly:

$ zstd --train --maxdict 512 samples/updates/* -o ./dictionary/updates
Trying 5 different sets of parameters
20%       FASTCOVER parameters incorrect
FASTCOVER parameters incorrect
FASTCOVER parameters incorrect
FASTCOVER parameters incorrect
k=50
d=8
f=20
steps=4
split=75
accel=1
Save dictionary of size 512 into file ./dictionary/updates

There are still errors about the FASTCOVER parameters, but the dictionary is generated successfully this time.

To Reproduce

Steps to reproduce the behavior:

1. Attempt to train a dictionary using a maxdict parameter at 256.

Expected behavior

A size 256 dictionary should be generated and saved, given enough samples.

Desktop (please complete the following information):

• OS: Ubuntu under Windows Linux Subsystem
• Version: 20.04
• Compiler: Installed pre-compiled package
• Flags: --train, --maxdict 256, -o <PATH>.

[terrelln]

We'll look into this for the next release, but for now, I suggest a workaround of selecting a slightly larger --maxdict, as you've already done.

If you truly need a smaller dictionary, you can take the prefix of a larger dictionary.

[terrelln]

This should be fixed by #2887. If you find it isn't fixed in the latest dev branch, please reopen the issue.