Autocompleting console input

Yesterday I stumbled upon a Stack Overflow question that asked if it was possible to implement Tab-triggered auto-completion in a console application.

I thought that was an interesting idea, so I went ahead and wrote a small program that does just that, given a hard-coded string[] - I might eventually refactor it into its own class and reuse it in my own projects if I ever actually need auto-completion in a console app, but before I do that I'd like some feedback on the way it's implemented, given static aside, the logic itself is pretty much the way I'll have it regardless of whether it's in a dedicated class as part of something much bigger, or right there in a console application that does nothing but verify that the code works.

Pressing Tab when there's no input will change the current line to say Bar; because there's a word in data that starts with Bar, pressing Tab again will autocomplete to Barbec; another Tab will make it Barbecue, and then any subsequent Tab will have no effect, because nothing in the data starts with Barbecue - but then you could Backspace until the input is Ba, type a t to make it Bat, and when you press Tab then it autocompletes to Batman.

In other words, it all works exactly as it should. But does it look right?

```
class Program
{
static void Main(string[] args)
{
var data = new[]
{
"Bar",
"Barbec",
"Barbecue",
"Batman",
};

var builder = new StringBuilder();
var input = Console.ReadKey(intercept:true);

while (input.Key != ConsoleKey.Enter)
{
if (input.Key == ConsoleKey.Tab)
{
HandleTabInput(builder, data);
}
else
{
HandleKeyInput(builder, data, input);
}

input = Console.ReadKey(intercept:true);
}
Console.Write(input.KeyChar);
}

///
/// https://stackove

Testing

• Personally I think it’s conceptually easier to test this code if you are returning a string – rather than a void: what if you decided to change the out put device from a console window to your printer – it would not be easy to change that. What if sometimes you wanted a console window and other times you wanted to send the same thing via text message? I guess it’s easier to separate what is being done vs how it is being done. E.g. Output key is the abstract concept but this could be done a number of different ways: (i) to the console, or (ii) through a voice synthesiser etc. I wrote some tests but then i threw it once it broke when i changed the API.

Naming

• What is data? It’s harder to think of a more abstract name. I’ve changed this to keywords.

• Personally I like to see the type along with the variable declaration.

Refactoring the code:

-
I simplified and extracted some methods.

-
The The HandleKeyInput and the HandleTabInput both seem to be doing conceptually similar things: a key is inputted and then a result is obtained. If it’s a tab then do X, but if it’s a key then do Y. There’s a common abstraction here that can be extracted.

-
What I want to do is to make the two methods: HandleTabInput and HandleKeyInput to look and be absolutely identical.

-
Data (renamed to keywords) can be a field so there’s no need to pass it in as a variable.

-
I want to make two of your methods to basically be the same. They’re kinda doing this same things. After some refactoring this is what I got:

public void HandleTabInput(StringBuilder builder, ConsoleKeyInfo keyInput)
  {
          // Perform calculation
      string match = ExtractMatch(builder);

          // Alter the builder
      builder.Clear();
      builder.Append(match);

          // Print Results
      PrintMatch(match);
  }

And then this:

private void HandleKeyInput(StringBuilder builder, ConsoleKeyInfo keyInput)
    {
        if (keyInput.Key == ConsoleKey.Backspace && builder.ToString().Length > 0)
        {
                // Perform Calculation (nothing here)

                // Alter the builder
            builder.Remove(builder.Length - 1, 1);

                // Print Results
            ClearCurrentLine();                
            Console.Write(builder.ToString().Remove(builder.ToString().Length - 1));
        }
        else
        {
                // Perform calculation (nothing here)

                // Alter the Builder
            var key = keyInput.KeyChar;
            builder.Append(key);

                // Print Reuslts
            Console.Write(key);
        }
    }

We can abstract out the calculation, and then the printing. After some work this was the result. It’s starting to look pretty similar:

private void HandleKeyInput(StringBuilder builder, ConsoleKeyInfo keyInput)
    {
        if (keyInput.Key == ConsoleKey.Backspace && builder.ToString().Length > 0)
        {
            Program.KeyInput backSpaceKey = new Program.KeyInput.BackspaceInput(builder, keywords);
            backSpaceKey.AlterBuilder();
            backSpaceKey.PrintResult();                
        }
        else
        {
            KeyInput input = new KeyInput.StandardKeyInput(builder, keywords, keyInput.KeyChar);
            input.AlterBuilder();
            input.PrintResult();
        }
    }

You could probably tidy things up a little more.
The final Result:

public void RunProgram()
    {
        StringBuilder builder = new StringBuilder();
        ConsoleKeyInfo capturedCharacter = Console.ReadKey(intercept: true);

        while (EnterIsNotThe(capturedCharacter))
        {
            KeyInput key = KeyInput.GetKey(builder, keywords, capturedCharacter);
            builder = key.UpdateBuilder();
            key.Print();

            capturedCharacter = Console.ReadKey(intercept: true);
        }

        Console.Write(capturedCharacter.KeyChar);
    }

Concluding Thoughts

• It reads marginally better. Still If you want to reuse the code you’ll have to make changes, but at least it will be a little easier to do when the time comes.

• Encapsulation could be a little better – typically speaking you want the KeyInput class to be instantiated via the factory only.

A link to see the full code.

StackExchange Code Review Q#139172, answer score: 3