Fix isInfixOf and breakSubString in Word16

[hasufell]

Fixes #195

@Bodigrim @sol

[sol]

This looks nice.

[sol]

We repeatedly use Data.ByteString.Short.breakSubstring and Data.ByteString.Short.splitAt which both copy. See: #199 (comment)

I think we will get more predictable performance characteristics if we convert to a ByteString first, calculate the offset, and then do a final splitAt on the original input.

@Bodigrim any more thoughts on this?

[hasufell]

We repeatedly use Data.ByteString.Short.breakSubstring and Data.ByteString.Short.splitAt which both copy. See: #199 (comment)

I think we will get more predictable performance characteristics if we convert to a ByteString first, calculate the offset, and then do a final splitAt on the original input.

@Bodigrim any more thoughts on this?

I don't think I agree.

The point of ShortByteString is that it doesn't use pinned memory and doesn't contribute to memory fragmentation. Even if the ByteString here will be temporary, I find it morally wrong to use it inside the ShortByteString module.

[Bodigrim]

As discussed in haskell/bytestring#307, I'm not convinced that the algorithm behind Data.ByteString.{,Short.}.breakSubstring is any better than memcmp.

Looking at the difficulties with appending/slicing of ShortByteString to avoid quadratic performance, I think running compareByteArrays# in a loop with even indices is the simplest and most robust solution here.

[hasufell]

@Bodigrim done

[hasufell]

Unfortunately, bytestring doesn't expose this internal and we need it to to be compatible with GHC-8.2.2 and lower.