[ruby-core:117331] [Ruby master Feature#20394] Add an offset parameter to `String#to_i`

Issue #20394 has been updated by Dan0042 (Daniel DeLorme).


byroot (Jean Boussier) wrote in #note-3:
> We tried this, but it was way slower (https://github.com/redis-rb/redis-client/pull/150), we haven't dug much as to why. But generally I'd love if I could just rely on the internal IO buffer instead of having to maintain a buffer string an an offset, but we're far from there I'm afraid.

The idea of an offset in a string is perfectly represented via the cursor in IO/StringIO, so it would be ideal to just use that instead of adding `offset` kwargs to various core methods.

If the only issue is performance I tried to get an idea of the problem with some cursory benchmarking for #getbyte and...

```ruby
require "stringio"
fd = File.open("1.2MB-file")
str = fd.read
io = StringIO.new(str)

bm{ fd.rewind; nil while fd.getbyte } #=> 0.0270s
bm{ io.rewind; nil while io.getbyte } #=> 0.0195s
bm{ i=0; i+=1 while str.getbyte(i)  } #=> 0.0237s
```

It doesn't seem like String#getbyte is much faster than File#getbyte, and StringIO#getbyte is fastest of all. Maybe the result would be different if using sockets? In that case it might be worth buffering to a StringIO.

Of course even with that it's not ideal to call #getbyte multiple times to build an integer, so maybe we could have something like `IO#get_i(base=10)` which reads as many numeric characters as possible and return them as an integer. Returns `nil` if no numeric characters at cursor. WDYT?


----------------------------------------
Feature #20394: Add an offset parameter to `String#to_i`
https://bugs.ruby-lang.org/issues/20394#change-107475

* Author: byroot (Jean Boussier)
* Status: Open
----------------------------------------
### Context

I maintain the `redis-client` gem, and it comes with an optional swapable implementation in C that binds the `hiredis` C client, [which used to performs up to 5 times faster in some cases](https://github.com/redis-rb/redis-client/commit/9fabd57c6786a03fe0c6021eab5b181d9316d9d7).

I recently paired with @tenderlovemaking to try to close this gap, or even try to make the pure Ruby version faster, and we came up with several optimizations that now almost make both version on par (assuming YJIT is enabled).

An important source of performance loss, is that the Redis protocol is line based and to parse it in Ruby requires to slice a lot of small strings from the buffer. To give an example, here's how an Array with two String (`["foo", "plop"]`) is serialized in RESP3 (Redis protocol):

```
*2\r\n
$3\r\n
foo\r\n
$4\r\n
plop\r\n
```

>From this you can understand that a big hotspot in the parser is essentially `Integer(gets)`.

With @tenderlovemaking we managed to get [a fairly significant perf boost](https://github.com/redis-rb/redis-client/commit/41b3abe94243d2598211d448c4e457a3585ff9d5#diff-a8b5ce23fb9396492f56bf0bd23090910918a488416cfb488cef8b5b34877328) by avoiding these string allocation using `String#getbyte` and [basically implementing a rudimentary `String#to_i(offset: )` in Ruby](https://github.com/redis-rb/redis-client/commit/41b3abe94243d2598211d448c4e457a3585ff9d5#diff-5f15c6483e788ee14f367f65fb951800d52341726f528bcddff1e2cd3e62cab9R105-R115).

But while the gains are huge with YJIT enabled, they are much more tame with the interpreter. And it feels a bit wrong to have to implement this sorts of things for performance reasons.

### `String#to_i(offset: )`

Similar to `String#unpack(offset:)` ([Feature #18254]), I believe `String#to_i(offset: )` would be useful.

### Alternative new `String#unpack` format

Another possibility would be to add a new format to `String#pack` `String#unpack` for decimal numbers. It sounds a bit weird at first, but given it supports things like Base64 and hexadecimal, perhaps it's not that much of a stretch?







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