[ruby-core:105792] [Ruby master Feature#18254] Add an `offset` parameter to String#unpack and String#unpack1

Issue #18254 has been updated by mame (Yusuke Endoh).


@byroot Thank you for adding documentation. I agree with merging.

> there should be a way to know how many bytes an invocation of String#unpack consumes.

In fact, some committers discussed this point at the dev-meeting. However, in many cases, it is trivial (or able to calculate) for a programmer how many bytes are consumed. Also, it looks difficult to provide the feature by just extending the current API design of `String#unpack`. So, matz concluded that those who really wants the feaature should create another ticket with use case discussion and a concrete API proposal.

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Feature #18254: Add an `offset` parameter to String#unpack and String#unpack1
https://bugs.ruby-lang.org/issues/18254#change-94310

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
----------------------------------------
When working with binary protocols it's common to have to first unpack some kind of header or type prefix, and then based on that unpack another part of the string.

For instance here's [a code snippet from Dalli, the most common Memcached client](https://github.com/petergoldstein/dalli/blob/76b79d78cda13562da17bc99f92edcedf1873994/lib/dalli/protocol/binary.rb#L156-L184):

```ruby
while buf.bytesize - pos >= 24
  header = buf.slice(pos, 24)
  (key_length, _, body_length, cas) = header.unpack(KV_HEADER)

  if key_length == 0
    # all done!
    @multi_buffer = nil
    @position = nil
    @inprogress = false
    break

  elsif buf.bytesize - pos >= 24 + body_length
    flags = buf.slice(pos + 24, 4).unpack1("N")
    key = buf.slice(pos + 24 + 4, key_length)
    value = buf.slice(pos + 24 + 4 + key_length, body_length - key_length - 4) if body_length - key_length - 4 > 0

    pos = pos + 24 + body_length

    begin
      values[key] = [deserialize(value, flags), cas]
    rescue DalliError
    end

  else
    # not enough data yet, wait for more
    break
  end
end
@position = pos
```

### Proposal

If `unpack` and `unpack1` had an `offset:` parameter, it would allow this kind of code to extract the fields it needs without allocating and copying as much strings, e.g.:

```ruby
flags = buf.slice(pos + 24, 4).unpack1("N")
```

could be:

```ruby
buf.unpack1("N", offset: pos + 24)
```




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