WIP: initial implementation of cursors

backend/op_set.js

@@ -141,6 +141,22 @@ function patchList(opSet, objectId, index, elemId, action, ops) {
   return [opSet, [edit]]
 }
 
+// find the index of the closest preceding list element
+function findClosestListIndex(opSet, objectId, elemId) {
+  const elemIds = opSet.getIn(['byObject', objectId, '_elemIds'])
+
+  let prevId = elemId
+  while (true) {
+    index = -1
+    prevId = getPrevious(opSet, objectId, prevId)
+    if (!prevId) break
+    index = elemIds.indexOf(prevId)
+    if (index >= 0) break
+  }
+
+  return index
+}
+
 function updateListElement(opSet, objectId, elemId) {
   const ops = getFieldOps(opSet, objectId, elemId)
   const elemIds = opSet.getIn(['byObject', objectId, '_elemIds'])
@@ -155,17 +171,7 @@ function updateListElement(opSet, objectId, elemId) {
 
   } else {
     if (ops.isEmpty()) return [opSet, []] // deleting a non-existent element = no-op
-
-    // find the index of the closest preceding list element
-    let prevId = elemId
-    while (true) {
-      index = -1
-      prevId = getPrevious(opSet, objectId, prevId)
-      if (!prevId) break
-      index = elemIds.indexOf(prevId)
-      if (index >= 0) break
-    }
-
+    index = findClosestListIndex(opSet, objectId, elemId)
     return patchList(opSet, objectId, index + 1, elemId, 'insert', ops)
   }
 }
@@ -569,5 +575,5 @@ function listIterator(opSet, listId, context) {
 module.exports = {
   init, addChange, getMissingChanges, getChangesForActor, getMissingDeps,
   getObjectFields, getObjectField, getObjectConflicts, getFieldOps,
-  listElemByIndex, listLength, listIterator, ROOT_ID
+  listElemByIndex, listLength, listIterator, findClosestListIndex, ROOT_ID
 }

frontend/index.js

@@ -3,6 +3,7 @@ const { ROOT_ID, isObject, copyObject } = require('../src/common')
 const uuid = require('../src/uuid')
 const { applyDiffs, updateParentObjects, cloneRootObject } = require('./apply_patch')
 const { rootObjectProxy } = require('./proxies')
+const { findClosestListIndex } = require('../backend/op_set')
 const { Context } = require('./context')
 const { Text } = require('./text')
 const { Table } = require('./table')
@@ -492,10 +493,34 @@ function getElementIds(list) {
   return list[ELEM_IDS]
 }
 
+/**
+ * Finds the latest integer index of a cursor object.
+ * If the character was deleted, returns -1.
+ * 
+ * todos:
+ * - consider returning the closest character if deleted
+ * - consider optimizing the linear search
+ */
+function findCursorIndex (cursor, doc, findClosest) {
+  if(cursor.textId === undefined || cursor.elemId === undefined) {
+    throw new TypeError('Invalid cursor object')
+  }
+
+  const backend = getBackendState(doc)
+  const elemIds = backend.getIn(['opSet', 'byObject', cursor.textId, '_elemIds'])
+  let index = elemIds.indexOf(cursor.elemId)
+
+  if (index === -1 && findClosest) {
+    index = findClosestListIndex(backend.getIn(['opSet']), cursor.textId, cursor.elemId)
+  }
+
+  return index
+}
+
 module.exports = {
   init, from, change, emptyChange, applyPatch,
   canUndo, undo, canRedo, redo,
   getObjectId, getObjectById, getActorId, setActorId, getConflicts,
-  getBackendState, getElementIds,
-  Text, Table, Counter
+  getBackendState, getElementIds, findCursorIndex,
+  Text, Table, Counter,
 }

frontend/text.js

@@ -27,6 +27,18 @@ class Text {
     return this.elems[index].elemId
   }
 
+  /**
+   * Returns a cursor that points to a specific point in the text.
+   * For now, represented by a plain object.
+   */
+  getCursorAt (index) {
+    return {
+      // todo: are there any points in the lifecycle where the Text object doesn't have an ID?
+      textId: this[OBJECT_ID], 
+      elemId: this.getElemId(index)
+    }
+  }
+
   /**
    * Iterates over the text elements character by character, including any
    * inline objects.

test/text_test.js

@@ -365,6 +365,52 @@ describe('Automerge.Text', () => {
     })
   })
 
+  describe('cursors', () => {
+    let s1
+    beforeEach(() => {
+      s1 = Automerge.change(Automerge.init(), doc => {
+        doc.text = new Automerge.Text('hello world')
+        doc.cursor = doc.text.getCursorAt(2)
+      })
+    })
+
+    it('can retrieve the initial index on the cursor', () => {
+      assert.deepStrictEqual(Automerge.Frontend.findCursorIndex(s1.cursor, s1), 2)
+    })
+
+    it('updates the cursor index when text is updated', () => {
+      s1 = Automerge.change(s1, doc => {
+        doc.text.insertAt(0, 'a', 'b', 'c')
+      }) 
+
+      assert.deepStrictEqual(Automerge.Frontend.findCursorIndex(s1.cursor, s1), 5)   
+    })
+
+    it('throws an error if a non-cursor is passed in', () => {
+      s1 = Automerge.change(s1, doc => {
+        doc.value = "random string"
+      }) 
+
+      assert.throws(() => Automerge.Frontend.findCursorIndex(s1.value, s1), /Invalid cursor object/) 
+    })
+
+    it('returns -1 by default if character was deleted', () => {
+      s1 = Automerge.change(s1, doc => {
+        doc.text.deleteAt(2)
+      })  
+
+      assert.deepStrictEqual(Automerge.Frontend.findCursorIndex(s1.cursor, s1), -1) 
+    })
+
+    it('returns closest index if character was deleted and findClosest is set to true', () => {
+      s1 = Automerge.change(s1, doc => {
+        doc.text.deleteAt(2)
+      })  
+
+      assert.deepStrictEqual(Automerge.Frontend.findCursorIndex(s1.cursor, s1, true), 1) 
+    })
+  })
+
   describe('non-textual control characters', () => {
     let s1
     beforeEach(() => {