Let's say we have some bytes, represented as a slice of byte, which in Go lingo means []byte. If we want to verify whether these bytes contain a certain subset of bytes, and we're particularly interested in knowing where in the slice they are located we can use the function Index, provided in the package bytes.

func Index(s, sep []byte) int

The way it is implemented is as follows:

  • If the lenght of the separator is 0, then it returns 0. Nothing more to do
  • If it is 1, it calls another function called IndexByte to do the work. This is also an assembly function. One of its variants can be found here
  • If the sizes between sep and s match, then it checks whether they are equal or not. For that, it calls Equal, which we have already mentioned.
  • If the separator length is bigger than one, then it's no longer a byte. It then goes to a more complex logic involving bytealg.Index.

Besides the full implementation, sometimes it's usefule to follow how things work under the hood. And in such a low level package, we can find the need of working with assembly and targetting the different architectures. Check bytealg package for an overview.