Changeset Library

Stefan edited this page Jan 23, 2016 · 6 revisions

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you can try all this commands live on the node.js console!

$ cd etherpad-lite
$ node

Changesets

> var Changeset = require("./src/static/js/Changeset");

Lets require the Changeset Library. This code is still 99% from the old Etherpad

> var cs = "Z:z>1|2=m=b*0|1+1$\n";

This is a Changeset. It's just a string that is very difficult to read in this form. Thankfully the Changeset Library gives us some tools to read it. A changeset describes the diff between two revisions of the document. The Browser sends changesets to the server and the server sends them to the clients to update them. Changesets also get saved into the history of a pad which allows Etherpad to go back to every revision from the past.

As an example let's look into this changeset

> var unpacked = Changeset.unpack(cs);
{ oldLen: 35, newLen: 36, ops: '|2=m=b*0|1+1', charBank: '\n' }

We unpacked the Changeset. oldLen is the original length. newLen is the Length of the text after the Changeset was applied. We will get to know ops and charBank next.

> var opiterator = Changeset.opIterator(unpacked.ops);
> console.log(opiterator)
{ next: [Function: next],
  hasNext: [Function: hasNext],
  lastIndex: [Function: lastIndex] }

We created a operator iterator. This iterator allows us to iterate over all operators that are in the Changeset. Let's see how an operator looks like

  
> opiterator.next()
{ opcode: '=',
  chars: 22,
  lines: 2,
  attribs: '' }
> opiterator.next()
{ opcode: '=',
  chars: 11,
  lines: 0,
  attribs: '' }
> opiterator.next()
{ opcode: '+',
  chars: 1,
  lines: 1,
  attribs: '*0' }
> opiterator.next()
{ opcode: '',
  chars: 0,
  lines: 0,
  attribs: '' }
> opiterator.hasNext()
false

There are 3 operators in this Changeset. There are 3 types of operators: =,-,+. Operators describe different changes on the text beginning at the first Character of the Text.

The = operator doesn't change the text, but it may change the attributes of the text (For example make it bold).

The - operator removes text.

Finally the + Operator adds text with attributes.

The opcode tells us the type of the operator. chars and lines is the length of the text these operators apply to. attribs is which attributes this text has.

An an example, The third operator adds one character with the attribute 0. It takes this one character out of the charBank you have seen above

APool (Documentation deprecated)

> var AttributePoolFactory = require("./utils/AttributePoolFactory");
> var apool = AttributePoolFactory.createAttributePool();
> console.log(apool)
{ numToAttrib: {},
  attribToNum: {},
  nextNum: 0,
  putAttrib: [Function],
  getAttrib: [Function],
  getAttribKey: [Function],
  getAttribValue: [Function],
  eachAttrib: [Function],
  toJsonable: [Function],
  fromJsonable: [Function] }

This creates an empty apool. The apool saves which attributes were used during the history of a pad. There is one apool for each pad. The apool only saves the attributes that were really used, it doesn't save unused attributes. Let's fill this apool with some values

> apool.fromJsonable({"numToAttrib":{"0":["author","a.kVnWeomPADAT2pn9"],"1":["bold","true"],"2":["italic","true"]},"nextNum":3});
> console.log(apool)
{ numToAttrib: 
   { '0': [ 'author', 'a.kVnWeomPADAT2pn9' ],
     '1': [ 'bold', 'true' ],
     '2': [ 'italic', 'true' ] },
  attribToNum: 
   { 'author,a.kVnWeomPADAT2pn9': 0,
     'bold,true': 1,
     'italic,true': 2 },
  nextNum: 3,
  putAttrib: [Function],
  getAttrib: [Function],
  getAttribKey: [Function],
  getAttribValue: [Function],
  eachAttrib: [Function],
  toJsonable: [Function],
  fromJsonable: [Function] }

We used the fromJsonable function to fill the empty apool with values. the fromJsonable and toJsonable functions are used to serialize and deserialize an apool. You can see that it stores the relation between numbers and attributes. So for example the attribute 1 is the attribute bold and vise versa. An attribute is always a key value pair. For stuff like bold and italic its just 'italic':'true'. For authors its author:$AUTHORID. So a character can be bold and italic. But it can't belong to multiple authors.

> apool.getAttrib(1)
[ 'bold', 'true' ]

Simple example of how to get the key value pair for the attribute 1

AText

> var atext = {"text":"bold text\nitalic text\nnormal text\n\n","attribs":"*0*1+9*0|1+1*0*1*2+b|1+1*0+b|2+2"};
> console.log(atext)
{ text: 'bold text\nitalic text\nnormal text\n\n',
  attribs: '*0*1+9*0|1+1*0*1*2+b|1+1*0+b|2+2' }

This is an atext. An atext has two parts: text and attribs. The text is just the text of the pad as a string. We will look closer at the attribs at the next steps...

> var opiterator = Changeset.opIterator(atext.attribs)
> console.log(opiterator)
{ next: [Function: next],
  hasNext: [Function: hasNext],
  lastIndex: [Function: lastIndex] }
> opiterator.next()
{ opcode: '+',
  chars: 9,
  lines: 0,
  attribs: '*0*1' }
> opiterator.next()
{ opcode: '+',
  chars: 1,
  lines: 1,
  attribs: '*0' }
> opiterator.next()
{ opcode: '+',
  chars: 11,
  lines: 0,
  attribs: '*0*1*2' }
> opiterator.next()
{ opcode: '+',
  chars: 1,
  lines: 1,
  attribs: '' }
> opiterator.next()
{ opcode: '+',
  chars: 11,
  lines: 0,
  attribs: '*0' }
> opiterator.next()
{ opcode: '+',
  chars: 2,
  lines: 2,
  attribs: '' }

The attribs are again a bunch of operators like .ops in the changeset was. But these operators are only + operators. They describe which part of the text has which attributes.

For more information see ./doc/easysync/easysync-notes.txt in the source.