Zstd doesn't always create RLE blocks

[terrelln]

cat <(head -c 1GB /dev/zero) <(echo Hello world) <(head -c 1GB /dev/zero) > file
zstd file

There is a small percent of blocks that aren't emitted as RLE blocks, and instead are emitted as a block with a single sequence.

[Cyan4973]

Yes, this is a known issue. There is a heuristic that attempts to determine when to test a block for conversion to RLE, but it does not always trigger. We could likely improve this by relaxing the condition, thereby increasing the likelihood that the RLE test is executed.

[terrelln]

I think there could be a small improvement to the heuristic. The blocks found are a single match. with 0 literals.

[Cyan4973]

I believed the heuristic is based on the compressed size of the block,
i.e. if the compressed size is "small enough", then it's a hint that it might be a candidate for RLE.

Blocks consisting of a single match with 0 literals should fit in this description.
I'm surprised if they are not.