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Posted 17 Apr 2015

Coercing AutoCompletion

, 17 Apr 2015
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efficient and secure selecting from large amounts of data


Presenting Data in order to choose an Item from is not solved sufficient yet. Imagine a Combobox with more than 300 Items - simply NoGo!
An Improvement would be a Textbox with AutoCompletion, because Autocompletion works as an easy-to-use filter, which reduces the data-amount very quickly.

But even Autocompletion has 2 disadvantages:

  • It is not data-safe: a user can ignore the autocompletion-suggestions, and input any rubbish he likes.
  • On large Data-Lists the user still looses the orientation, which keys are valid to continue reducing the abundance of choices to a human limit.

The concept of Coercing Autocompletion

  • display a dropdown-list of options, where the user can choose from, if he likes (standard-autocompletion-behavior)
  • ensure, that the user only can input valid Data. Reject invalid key-strokes immediately
  • give a preview of the next valid KeyStrokes
  • (optional) "WindForward": as long as only one KeyStroke is valid, autocompletion can input it automatically. This can reduce the nessecary keystrokes dramatically, but also can puzzle rapid-typing-users - So make it optional

Maybe theese concepts do not sound that "whoop! - world-changing". But assume a stable and easy-to-use standard-control, which provides theese concepts (and moreover databinding) - do you think anyone would ever use a combobox anymore? No - combobox, as we know it - perhaps/propably would die.

Several ways of looking at Data

Assume the following sorted data (some geman towns):


I put them into a uniform grid, and marked up some chars of specific importance:


At theese markup-points a user can decide, and each decision will reduce the range of further choice-options.

You can understand theese points as nodes of a decision-tree - let me draw in the available decision-pathes:


Moreover i can remove redundant chars from the Tree:


funny - isn't it?

But the most convenient view (to me) is visualizing a practical sample-selection: Lets select "Georgensgmünd", and see, how the decisions reduce choice-options in a few steps to one, which is the final selection:


After typing "G" one can choose any of the 11 towns. But marked up as "Decision-Node" on this level are only these 3: "n o r"
Choosing "o" reduces the choice-options to 5, and now there are only two Decision-Node: "e s"
Choosing the "e", reduces on to choice-options 3, and "b s t" are available as Decision-Nodes.
Choosing "s" finishes selection - only Option "Georgensgmünd" is left.

Proof of Concept: Sample-Application


My implementation is far away from beeing able to crowd out Combobox from Gui-Development: It misses Databinding-Support, the ValidNextKeys-Preview is not applicable everywhere, and it inherits a bug and some suboptimal behavior from Standard-Autocompletion. It is easy-to-use in fact, but - i admire: not (yet) as easy-to-use than combobox is...
But nevertheless i think, it already can do a good job in real-world-applications, who deal with selecting data from big data-amounts.
The sample-download is a kind of zip-code-finder, and it provides about 16000 german towns (and town-districts) to choose from.

My code-design is a kind of "Textbox-Behavior": You can attach it to a Textbox, give him data, and it will configure autocompletion, coercing, notify about Valid-Next-Keys and optional wind-forward-functionality.

Internally it simply let standard-autocompletion do its dropdown- and suggesting-job, and just extend functionality with coercion, displaying the available Chars  and stuff.
In other words: a really lightweight Solution - less than 200 lines of code.

Using the code

Public Class frmOrtFinder

   Private WithEvents _Coercer As New CoercingAutoCompletion

   Public Sub New()
      Dim data = OrteDts.Ort.Select(Function(rw) ", ".Between(rw.Name, rw.Bundesland, rw.PLZ))
      _Coercer.Textbox = TextBox1
   End Sub

   Private Sub _Coercer_AvailableCharsChanged(sender As Object, e As EventArgs) Handles _Coercer.AvailableCharsChanged
      lbAvailable.Text = _Coercer.AvailableChars
   End Sub

   Private Sub _Coercer_InputDone(sender As Object, e As EventArgs) Handles _Coercer.InputDone
      'busyness-logic: search the in the Textbox specified Data-Record and select it in Datagridview
      Dim fields = TextBox1.Text.Split({", "}, StringSplitOptions.None)
      For i = 0 To bsOrt.Count - 1
         If DirectCast(bsOrt(i), DataRowView).Row.ItemArray.Cast(Of String).SequenceEqual(fields) Then
            bsOrt.Position = i
            grdOrt.FirstDisplayedScrollingRowIndex = i
         End If
      Throw New Exception("Record not found - this should never occur!")
   End Sub
  • instantiate a coercer (line#3)
  • set its Textbox (line#9)
  • give data to it (line#10)
  • handle its Events (lines#13, #17)


I separated the concern "Textbox-Behavior" from "Data-Parsing" - first a glance at the Textbox-Part:

Public Class CoercingAutoCompletion

   Public Event AvailableCharsChanged As EventHandler
   Public Event InputDone As EventHandler


   Public Property Textbox As TextBox
         Return _TextBox
      End Get
      Set(value As TextBox)
         If _TextBox Is value Then Return
         If _TextBox IsNot Nothing Then Throw New Exception("Textbox already set!")
         _TextBox = value
         _TextBox = _TextBox
         _TextBox.AutoCompleteMode = AutoCompleteMode.SuggestAppend
         _TextBox.AutoCompleteSource = AutoCompleteSource.CustomSource
         _TextBox.AutoCompleteCustomSource = _AutoTextSource
      End Set
   End Property


   'improvements for standard-autocompletion: coerce valid inputs, notify about available Chars, selectionstart-wind-forward if wanted
   Private Sub _TextBox_KeyUp(sender As Object, e As KeyEventArgs) Handles _TextBox.KeyUp
      With _TextBox
         Dim i = .SelectionStart
         Select Case e.KeyCode
            Case Keys.Up, Keys.Down ' .Up/.Down choose text from Autocompletion-DropDown, try keep previous SelectionStart
               i = Math.Min(i, _UnselectedText.Length)
               _DataParser.Parse(.Text.Substring(0, i))
               _DataParser.SelStartPropose = i
            Case Keys.Left, Keys.Right
               i = _UnselectedText.Length + e.KeyCode - 38
               _DataParser.Parse(.Text.Substring(0, i))
               If e.KeyCode = Keys.Left Then _DataParser.SelStartPropose = i
            Case Else
               _DataParser.Parse(.Text.Substring(0, i))
               If e.KeyCode = Keys.Back Then _DataParser.SelStartPropose = i
               If _DataParser.FirstDivergence < i Then
                  .Text = .Text.Substring(0, _DataParser.FirstDivergence)
               End If
         End Select
         ImproveSelection()  'improves the Selection done right before by standard-Autocompletion, raises AvailableCharsChanged
      End With
   End Sub

   'Ensures that the Textbox can't be left without valid providing valid Input
   Private Sub _TextBox_Leave(sender As Object, e As EventArgs) Handles _TextBox.Leave
      With _TextBox
         _TextBox.Text = _DataParser.FirstBestMatch
         RaiseEvent InputDone(Me, EventArgs.Empty)
      End With
   End Sub

Mainly it configures AutoCompletion and then observes the Textbox_KeyUp-Event. And there is to distinguish between up/down, left/right, backspace and general Key-Strokes.

  • Up/Down address standard-autocompletion to select next/previous entry from Dropdown-list. Therefore the text is to be re-parsed, and try restore previous selection
  • Left/Right shall move the selection-start, but in every state an autocompletion-selection must reach to the end of text: that's the meaning of suggest: it immediately vanishes, when the user types anything else.
  • Backspace equals Cursor-Left-Keystroke, but Autocompletion removes its suggestion (instead of re-suggesting better - i can't help that)
  • other keys also cause re-parsing, and on invalid input the text gets shortened back to validity (line#22).

So far - maybe more interesting is the DataParser - concern:

Private _RawWords As Object()

Public Sub Refill(autoSource As AutoCompleteStringCollection, texts As IEnumerable(Of String))
   Dim txts = texts.ToArray
   Array.Sort(txts, StringComparer.OrdinalIgnoreCase)
   Dim bf = BindingFlags.Instance Or BindingFlags.NonPublic
   Dim inf = GetType(AutoCompleteStringCollection).GetField("data", bf)
   Dim data = inf.GetValue(autoSource)
   inf = GetType(ArrayList).GetField("_items", bf)
End Sub

Refill() fills the AutoCompleteCollection and then hack it with reflection, to get access to the underlying Data-Array. Thats why my solution takes no additional Memory to what standard-Autocompletion takes anyway.
Note that i use StringComparer.OrdinalIgnoreCase for sorting. The same Comparer is in use when searching the data with BinarySearch:

Private Function FindBestMatch(txt As String) As Integer
   Dim i = Array.BinarySearch(_RawWords, txt, StringComparer.OrdinalIgnoreCase)
   Return If(i < 0, Not i, i)
End Function

A binary search is very efficient: Eg. with only 20 internal Iterations it finds an item in a range of about 1 million sorted items. The Return-Value is tricky: If the search finds a matching item it returns its Position as a value >= 0. Otherwise binarysearch doesn't simply return -1, but it returns a negative number - the Bit-Complement of the Position, where a matching item should be (also understand as: "insert-position"). 
And that is, what Dataparser needs to know: the insert-position of the input (although it doesn't insert).
Assume the input "geo", and look back to <figure4>: FindBestMatch() would come up with line#4, and that is the starting-line from where is to look up for the next Decision-Nodes.
Then next we must find the ending-line of the particular "option-range".
Once again apply FindBestMatch()-binarysearch: But first append Char.MaxValue to the input "geo", to ensure, the modified input "geo^" will retrieve the first line behind the option-range - namely line#9.
In summary we figured out, that the first "Bestmatch" to "geo" is at line#4 - "Georgenberg", and the last BestMatch is at line#8 - "Georgsmühle". From this "option-range" we must collect the "decision-nodes", which define the next level of selection.
But how find the correct column of the next available Decision-Nodes?
Not that complicated: Just compare the first line of the option-range with its last line. The occurance of the first difference will give us the x-position we're looking for - in the sample its column#6, because "Georgenberg" and "Georgsmühle" diverge from column#6 on.
Now loop from firstBestmatchPosition to lastBestMatchPosition and collect distinct(!) the chars at column#6. These are: "e s" - the decision-nodes, and the "available-chars" of a wind-forward-coercing-autocompletion.

The story in code:

'figure out _FirstMatchPos, _LastMatchPos, FirstbestMatch, FirstDivergence, _DecisionColumn, SelectionStart
Public Sub Parse(txt As String)
   _FirstMatchPos = FindBestMatch(txt) ' first cut - possibly incorrect, on invalid input
   FirstBestMatch = Words(_FirstMatchPos)
   FirstDivergence = GetDivergence(txt, FirstBestMatch, 0)
   If FirstDivergence < txt.Length AndAlso _FirstMatchPos > 0 Then 'signals invalid input
      'depending on the sort-order of the first invalid char FindBestMatch() may have retrieved the position **after** the option-range
      Dim div2 = GetDivergence(txt, Words(_FirstMatchPos - 1), 0)
      If div2 < FirstDivergence Then
         _LastMatchPos = FindBestMatch(txt.Substring(0, FirstDivergence) & Char.MaxValue) - 1
         '_FirstMatchPos actually incorrect, and contains the position (+1) that _LastMatchPos should have. Make up that fault
         _LastMatchPos = _FirstMatchPos - 1
         FirstDivergence = div2
         _FirstMatchPos = FindBestMatch(txt.Substring(0, FirstDivergence))
         FirstBestMatch = Words(_FirstMatchPos)
      End If
      _LastMatchPos = FindBestMatch(txt & Char.MaxValue) - 1 'Position of the last word which fits to FirstDivergence
   End If
   _DecisionColumn = GetDivergence(FirstBestMatch, Words(_LastMatchPos), FirstDivergence)
   'SelectionStart is proposal - can be changed by the caller
   SelectionStart = If(FirstDivergence >= txt.Length AndAlso WindForward, _DecisionColumn, FirstDivergence)
End Sub

Private Function GetDivergence(s0 As String, s1 As String, start As Integer) As Integer
   For i = start To s0.Length - 1
      If Char.ToUpper(s0(i)) <> Char.ToUpper(s1(i)) Then Return i
   Return s0.Length
End Function

A complication occurs on invalid inputs: FindBestmatch() will either return the firstBestmatch or the lastBestmatch - this depends on the sort-order of the invalid char.
Assume invalid input "Geoo": the binary-searched insert-position is line#4, because "Geoo" is smaller than "Georgenberg".
But another invalid input "Geot" will retrieve line#9, because "Geot" is bigger than "Georgsdorf". So on invalid input there is additional to re-check, whether the line was the first or the last of the option-range.


To me it's less important to introduce a particular tricky Textbox-Behavior-Implementation. More importent i see the introduction of the concept "Coerce-Windforward-Autocompletion" in general - not my contemplations about Data-structures or explain-code-tryals.

Even my implementation is rather poor, compared with what my fantasy imagines:
May be a Wpf-control, where on GotFocus the Valid-Next-Chars pop up as kind of tooltip, and stuff like that.
Or they get marked up within the DropDown-List.
Or - keeping good-ol' combo alive - why not extend ComboBox either with coerce-wind-forward-autocompletion?
And of course Datagrid-Coercing-Columns must be created either ...


  • the sample-app's data is taken from GeoNames, under the CreativeCommons-License, which means: for free, as long as i mention them as the author - which i comply willingly.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


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