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https://github.com/ottypes/text

OT type for plaintext
https://github.com/ottypes/text

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OT type for plaintext

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README 

        
# The Plaintext OT Type



**NOTE**: This OT type counts characters using UTF16 offsets instead of

unicode codepoints. This is slightly faster in javascript, but its

incompatible with ot implementations in other languages. For future

projects I recommend that you use

[ot-text-unicode](https://github.com/ottypes/text-unicode) instead.

ot-text-unicode also has full typescript type information and it supports ropes

for better performance string editing.



This OT type can be used to edit plaintext documents, like sourcecode or

markdown.



This project's [history is here](https://github.com/share/ShareJS/blob/0.6/src/types/text2.coffee).



For documentation on the spec this type implements, see [ottypes/docs](https://github.com/ottypes/docs).



## Spec



The plaintext OT type thinks of the document as a giant string, and edits index

into the string directly. This is different from most text editors, which break

up a document into an array of lines. For small documents on modern computers,

the conversion isn't particularly expensive. However, if you have giant

documents you should be using a rope library like

[jumprope](https://github.com/josephg/jumprope) or

[librope](https://github.com/josephg/librope).



Each operation describes a traversal over the document. The traversal can edit

the document as it goes.



For example, given the document:



```

"ABCDEFG"

```



You could apply the operation



```

[1, ' hi ', 2, {d:3}]

```



This operation will skip the first character, insert ' hi ', skip 2 more

characters then delete the next 3 characters. The result would be:



```

"A hi BCG"

```



### Operations



Operations are lists of components, which move along the document. Each

component is one of



- **Number N**: Skip forward *N* characters in the document

- **"str"**: Insert *"str"* at the current position in the document

- **{d:N}**: Delete *N* characters at the current position in the document



The operation does not have to skip the last characters in the document.



### Selections



The text type also has methods for manipulating selections.



Selection ranges are either a single number (the cursor position) or a pair of

[anchor, focus] numbers (aka [start, end]) of the selection range. Be aware

that end can be before start.



---



# Commentary



This is the 3rd iteration of ShareJS's plaintext type. It hasn't been changed

in a long time now.



The first iteration was similar, except it is invertable. Invertability is

nice, but I want to eventually build an arbitrary P2P OT system, and in a p2p

setting invertibillity becomes impractical to achieve. I don't want systems to

depend on it.



The second iteration made each component specify a location and an edit there.

Operations were lists of these edits. Because the components were not sorted,

if you transform two big operations by one another it requires M\*N

time to transform. The components could be sorted to fix this, but if you're

going to do that you may as well just make them sorted by design - which is

what the current text implementation does. I thought the individual edits style

was better because I expected it to be simpler, but when I implemented it I

found the implementation of each method was almost identical in size.



There is also a [C implementation of this

type](https://github.com/share/libot/blob/master/text.h) which is [insanely](h

ttps://dl.dropboxusercontent.com/u/2494815/ot%20apply%20bench%201.png) [fast](

https://dl.dropboxusercontent.com/u/2494815/ot%20apply%20bench%202.png). The

implementations are almost the same, except this javascript implemention

counts characters using 16 bit words and the C implementation counts

characters using unicode codepoints. If you have any characters in the astral

plane in your document (like emoji 😅), edit & cursor positions will be

misaligned between implementations. See [here for more

information](http://josephg.com/blog/string-length-lies). If you are building

a cross-platform application, use the newer [ot-text-

unicode](https://github.com/ottypes/text-unicode) instead.



---



# License



All code contributed to this repository is licensed under the standard MIT license:



Copyright 2011 ottypes library contributors



Permission is hereby granted, free of charge, to any person obtaining a copy

of this software and associated documentation files (the "Software"), to deal

in the Software without restriction, including without limitation the rights

to use, copy, modify, merge, publish, distribute, sublicense, and/or sell

copies of the Software, and to permit persons to whom the Software is

furnished to do so, subject to the following condition:



THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR

IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,

FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE

AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER

LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,

OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN

THE SOFTWARE.