Add support for presence (range) transformations

This change adds support for presence transformations, which are being
[added to ShareDB][1]. In order to support presence updates, this change
adds support for the optional `transformPresence` method, which simply
reuses the existing `transformCursor` method, but also:

  - applies changes to the `length` of a range
  - keeps existing metadata
  - returns `null` if no range has been provided

[1]: share/sharedb#322

lib/type.js

@@ -50,6 +50,21 @@ module.exports = {
 
     deserialize: function(ops) {
       return new Delta(ops);
-    }
+    },
+
+    transformPresence: function(range, op, isOwnOp) {
+      if (!range) {
+        return null;
+      }
+
+      const delta = new Delta(op);
+      const start = this.transformCursor(range.index, delta, isOwnOp);
+      const end = this.transformCursor(range.index + range.length, delta, isOwnOp);
+
+      return Object.assign({}, range, {
+        index: start,
+        length: end - start,
+      });
+    },
   }
 };

package.json

@@ -19,7 +19,7 @@
   },
   "license": "MIT",
   "scripts": {
-    "test": "mocha test/fuzzer.js --timeout 5000"
+    "test": "mocha test/**/*.js --timeout 5000"
   },
   "repository": {
     "type": "git",

test/presence.js

@@ -0,0 +1,87 @@
+const Delta = require("quill-delta");
+var richText = require('../lib/type').type;
+var expect = require('chai').expect;
+
+describe('presence', function() {
+  it('transforms a zero-length range by an op before it', function() {
+    const range = {index: 10, length: 0};
+    const op = new Delta().insert('foo');
+    const transformed = richText.transformPresence(range, op);
+    expect(transformed).to.eql({index: 13, length: 0});
+  });
+
+  it('does not transform a zero-length range by an op after it', function() {
+    const range = {index: 10, length: 0};
+    const op = new Delta().retain(20).insert('foo');
+    const transformed = richText.transformPresence(range, op);
+    expect(transformed).to.eql({index: 10, length: 0});
+  });
+
+  it('transforms a range with length by an op before it', function() {
+    const range = {index: 10, length: 3};
+    const op = new Delta().insert('foo');
+    const transformed = richText.transformPresence(range, op);
+    expect(transformed).to.eql({index: 13, length: 3});
+  });
+
+  it('transforms a range with length by an op that deletes part of it', function() {
+    const range = {index: 10, length: 3};
+    const op = new Delta().retain(9).delete(3);
+    const transformed = richText.transformPresence(range, op);
+    expect(transformed).to.eql({index: 9, length: 1});
+  });
+
+  it('transforms a range with length by an op that deletes the whole range', function() {
+    const range = {index: 10, length: 3};
+    const op = new Delta().retain(9).delete(5);
+    const transformed = richText.transformPresence(range, op);
+    expect(transformed).to.eql({index: 9, length: 0});
+  });
+
+  it('keeps extra metadata when transforming', function() {
+    const range = {index: 10, length: 0, meta: 'lorem ipsum'};
+    const op = new Delta().insert('foo');
+    const transformed = richText.transformPresence(range, op);
+    expect(transformed).to.eql({index: 13, length: 0, meta: 'lorem ipsum'});
+  });
+
+  it('returns null when no presence is provided', function() {
+    const op = new Delta().insert('foo');
+    const transformed = richText.transformPresence(undefined, op);
+    expect(transformed).to.be.null;
+  });
+
+  it('advances the cursor if inserting at own index', function() {
+    const range = {index: 10, length: 2};
+    const op = new Delta().retain(10).insert('foo');
+    const transformed = richText.transformPresence(range, op, true);
+    expect(transformed).to.eql({index: 13, length: 2});
+  });
+
+  it('does not advance the cursor if not own op', function () {
+    const range = {index: 10, length: 2};
+    const op = new Delta().retain(10).insert('foo');
+    const transformed = richText.transformPresence(range, op, false);
+    expect(transformed).to.eql({index: 10, length: 5});
+  });
+
+  it('accepts an array of ops rather than a Delta', function() {
+    const range = {index: 10, length: 0};
+    const op = [{insert: 'foo'}];
+    const transformed = richText.transformPresence(range, op);
+    expect(transformed).to.eql({index: 13, length: 0});
+  });
+
+  it('does nothing if no op is provided', function() {
+    const range = {index: 10, length: 0};
+    const transformed = richText.transformPresence(range, undefined);
+    expect(transformed).to.eql({index: 10, length: 0});
+  });
+
+  it('does not mutate the original range', function() {
+    const range = {index: 10, length: 0};
+    const op = new Delta().insert('foo');
+    richText.transformPresence(range, op);
+    expect(range).to.eql({index: 10, length: 0});
+  });
+});