Version 0 of the JSON OT type
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Fix typo in README about invertability
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README.md

JSON0 OT Type

The JSON OT type can be used to edit arbitrary JSON documents.

Features

The JSON OT type supports the following operations:

  • Insert/delete/move/replace items in a list, shuffling adjacent list items as needed
  • Object insert/delete/replace
  • Atomic numerical add operation
  • Embed arbitrary subtypes
  • Embedded string editing, using the old text0 OT type as a subtype

JSON0 is an invertable type - which is to say, all operations have an inverse operation which will undo the original op. As such, all operations which delete content have the content to be deleted inline in the operation.

But its not perfect - here's a list of things it cannot do:

  • Object-move
  • Set if null (object insert with first writer wins semantics)
  • Efficient list insert-of-many-items

It also has O(a * b) complexity when transforming large operations by one another (as opposed to O(a + b) which better algorithms can manage).

Operations

JSON operations are lists of operation components. The operation is a grouping of these components, applied in order.

Each operation component is an object with a p:PATH component. The path is a list of keys to reach the target element in the document. For example, given the following document:

{'a':[100, 200, 300], 'b': 'hi'}

An operation to delete the first array element (100) would be the following:

[{p:['a', 0], ld:100}]

The path (['a', 0]) describes how to reach the target element from the root. The first element is a key in the containing object and the second is an index into the array.

Summary of operations

op Description
{p:[path], na:x} adds x to the number at [path].
{p:[path,idx], li:obj} inserts the object obj before the item at idx in the list at [path].
{p:[path,idx], ld:obj} deletes the object obj from the index idx in the list at [path].
{p:[path,idx], ld:before, li:after} replaces the object before at the index idx in the list at [path] with the object after.
{p:[path,idx1], lm:idx2} moves the object at idx1 such that the object will be at index idx2 in the list at [path].
{p:[path,key], oi:obj} inserts the object obj into the object at [path] with key key.
{p:[path,key], od:obj} deletes the object obj with key key from the object at [path].
{p:[path,key], od:before, oi:after} replaces the object before with the object after at key key in the object at [path].
{p:[path], t:subtype, o:subtypeOp} applies the subtype op o of type t to the object at [path]
{p:[path,offset], si:s} inserts the string s at offset offset into the string at [path] (uses subtypes internally).
{p:[path,offset], sd:s} deletes the string s at offset offset from the string at [path] (uses subtypes internally).

Number operations

The only operation you can perform on a number is to add to it. Remember, you can always replace the number with another number by operating on the number's container.

Are there any other ways the format should support modifying numbers? Ideas:

  • Linear multiple as well (Ie, x = Bx + C)
  • MAX, MIN, etc? That would let you do timestamps...

I can't think of any good use cases for those operations...

Add

Usage:

{p:PATH, na:X}

Adds X to the number at PATH. If you want to subtract, add a negative number.


Lists and Objects

Lists and objects have the same set of operations (Insert, Delete, Replace, Move) but their semantics are very different. List operations shuffle adjacent list items left or right to make space (or to remove space). Object operations do not. You should pick the data structure which will give you the behaviour you want when you design your data model.

To make it clear what the semantics of operations will be, list operations and object operations are named differently. (li, ld, lm for lists and oi, od and om for objects).

Inserting, Deleting and Replacing in a list

Usage:

  • Insert: {p:PATH, li:NEWVALUE}
  • Delete: {p:PATH, ld:OLDVALUE}
  • Replace: {p:PATH, ld:OLDVALUE, li:NEWVALUE}

Inserts, deletes, or replaces the element at PATH.

The last element in the path specifies an index in the list where elements will be deleted, inserted or replaced. The index must be valid (0 <= new index <= list length). The indexes of existing list elements may change when new list elements are added or removed.

The replace operation:

{p:PATH, ld:OLDVALUE, li:NEWVALUE}

is equivalent to a delete followed by an insert:

{p:PATH, ld:OLDVALUE}
{p:PATH, li:NEWVALUE}

Given the following list:

[100, 300, 400]

applying the following operation:

[{p:[1], li:{'yo':'hi there'}}, {p:[3], ld:400}]

would result in the following new list:

[100, {'yo':'hi there'}, 300]

Moving list elements

You can move list items by deleting them and & inserting them back elsewhere, but if you do that concurrent operations on the deleted element will be lost. To fix this, the JSON OT type has a special list move operation.

Usage:

{p:PATH, lm:NEWINDEX}

Moves the list element specified by PATH to a different place in the list, with index NEWINDEX. Any elements between the old index and the new index will get new indicies, as appropriate.

The new index must be 0 <= index < list length. The new index will be interpreted after the element has been removed from its current position. Given the following data:

['a', 'b', 'c']

the following operation:

[{p:[1], lm:2}]

will result in the following data:

['a', 'c', 'b']

Inserting, Deleting and Replacing in an object

Usage:

  • Insert: {p:PATH, oi:NEWVALUE}
  • Delete: {p:PATH, od:OLDVALUE}
  • Replace: {p:PATH, od:OLDVALUE, oi:NEWVALUE}

Set the element indicated by PATH from OLDVALUE to NEWVALUE. The last element of the path must be the key of the element to be inserted, deleted or replaced.

When inserting, the key must not already be used. When deleting or replacing a value, OLDVALUE must be equal to the current value the object has at the specified key.

As with lists, the replace operation:

{p:PATH, od:OLDVALUE, oi:NEWVALUE}

is equivalent to a delete followed by an insert:

{p:PATH, od:OLDVALUE}
{p:PATH, oi:NEWVALUE}

There is (unfortunately) no equivalent for list move with objects.


Subtype operations

Usage:

{p:PATH, t:SUBTYPE, o:OPERATION}

PATH is the path to the object that will be modified by the subtype. SUBTYPE is the name of the subtype, e.g. "text0". OPERATION is the subtype operation itself.

To register a subtype, call json0.registerSubtype with another OT type. Specifically, a subtype is a JavaScript object with the following methods:

  • apply
  • transform
  • compose
  • invert

See the OT types documentation for details on these methods.

Text subtype

The old string operations are still supported (see below) but are now implemented internally as a subtype using the text0 type. You can either continue to use the original si and sd ops documented below, or use the text0 type as a subtype yourself.

To edit a string, create a text0 subtype op. For example, given the following object:

{'key':[100,'abcde']}

If you wanted to delete the 'd' from the string 'abcde', you would use the following operation:

[{p:['key',1], t: 'text0', o:[{p:3, d:'d'}]}

Note the path. The components, in order, are the key to the list, and the index to the 'abcde' string. The offset to the 'd' character in the string is given in the subtype operation.

Insert into a string

Usage:

{p:PATH, t:'text0', o:[{p:OFFSET, i:TEXT}]}

Insert TEXT to the string specified by PATH at the position specified by OFFSET.

Delete from a string

Usage:

{p:PATH, t:'text0', o:[{p:OFFSET, d:TEXT}]}

Delete TEXT in the string specified by PATH at the position specified by OFFSET.


String operations

These operations are now internally implemented as subtype operations using the text0 type, but you can still use them if you like. See above.

If the content at a path is a string, an operation can edit the string in-place, either deleting characters or inserting characters.

To edit a string, add the string offset to the path. For example, given the following object:

{'key':[100,'abcde']}

If you wanted to delete the 'd' from the string 'abcde', you would use the following operation:

[{p:['key',1,3],sd:'d'}]

Note the path. The components, in order, are the key to the list, the index to the 'abcde' string, and then the offset to the 'd' character in the string.

Insert into a string

Usage:

{p:PATH, si:TEXT}

Insert TEXT at the location specified by PATH. The path must specify an offset in a string.

Delete from a string

Usage:

{p:PATH, sd:TEXT}

Delete TEXT at the location specified by PATH. The path must specify an offset in a string. TEXT must be contained at the location specified.


Commentary

This library was written a couple of years ago by Jeremy Apthorp. It was originally written in coffeescript as part of ShareJS, and then it got pulled out into the share/ottypes library and its finally landed here.

The type uses the list-of-op-components model, where each operation makes a series of individual changes to a document. Joseph now thinks this is a terrible idea because it doesn't scale well to large operations - it has N2 instead of 2N complexity.

Jeremy and Joseph have talked about rewriting this library to instead make each operation be a sparse traversal of the document. But it was obnoxiously difficult to implement JSON OT correctly in the first place - it'll probably take both of us thinking about nothing else for a few weeks to make that happen.

When it was written, the embedded text0 type was sharejs's text type. Its since been rewritten to make each operation be a traversal, but the JSON OT type still embeds the old type. As such, that old text type is included in this repository. If you want to use text0 in your own project, I'd be very happy to pull it out of here and make it its own module. However, I recommend that you just use the new text type. Its simpler and faster.


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.