From: "Eregon (Benoit Daloze) via ruby-core" <ruby-core@...>
Date: 2023-01-06T13:02:55+00:00
Subject: [ruby-core:111684] [Ruby master Feature#19314] String#bytesplice should support partial copy

Issue #19314 has been updated by Eregon (Benoit Daloze).


Ah, something I missed though is that with lazy substrings, there would still need to be a copy of the bytes to "unshare" the string when writing to it.
That copy would also be needed if the string was shared before (e.g. with .dup), but that's unknown in our case.

> It feels like a full memcpy/arraycopy which I don't think in general is a good idea for String.

To expand on that, I dislike that because it's using String as a byte array.
If anything, such operation should be supported on Array before String.

Now that we have IO::Buffer and there is https://docs.ruby-lang.org/en/master/IO/Buffer.html#method-i-copy, why not use that?

----------------------------------------
Feature #19314: String#bytesplice should support partial copy
https://bugs.ruby-lang.org/issues/19314#change-101080

* Author: shugo (Shugo Maeda)
* Status: Open
* Priority: Normal
----------------------------------------
String#bytesplice should support partial copy without temporary String objects.

For example, given `x = "0123456789"`, either of the following replaces the contents of `x` with `"0167856789"`:

```ruby
x.bytesplice(2, 3, x, 6, 3)
x.bytesplice(2..4, x, 6..8)
```

## Considerations

* What should be the return value?
    * The return value should be the whole source string for performance and consistency with `bytesplice(offset, len, s)`.
* Can the source and destination ranges overlap?
    * Yes.
* Can the source and destination lengths be different?
    * Yes.
* Can range form and offset/length form be mixed in the source and destination?
    * No.
* What should happen when any offset doesn't land on character boundary in text strings.
    * IndexError should be raised.
* Can the length be omitted in the destination?
    * Maybe yes, but it may be confusing.

## Use cases

* [Gapped buffer implementation for text editors](https://github.com/shugo/textbringer)
* [NAT implementation](https://github.com/kazuho/rat)
    * https://twitter.com/kazuho/status/1611279616098070532



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