[ruby-core:105766] [Ruby master Feature#18254] Add an `offset` parameter to String#unpack and String#unpack1
From:
"matz (Yukihiro Matsumoto)" <noreply@...>
Date:
2021-10-25 02:08:45 UTC
List:
ruby-core #105766
Issue #18254 has been updated by matz (Yukihiro Matsumoto).
Sounds reasonable. Accepted.
Matz.
----------------------------------------
Feature #18254: Add an `offset` parameter to String#unpack and String#unpack1
https://bugs.ruby-lang.org/issues/18254#change-94280
* 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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