Realtime telephone keypad decoder.

TelephoneKeypadDecoder.cs

using System;
using System.Threading.Tasks;

namespace GM.SP
{
	/// <summary>
	/// https://en.wikipedia.org/wiki/Telephone_keypad
	/// </summary>
	public class TelephoneKeypadDecoder : IDisposable
	{
		/// <summary>
		/// Threshold after which the multi-tap feature is reset.
		/// Value is in ms.
		/// https://en.wikipedia.org/wiki/Multi-tap
		/// </summary>
		public const int MULTITAP_THRESHOLD = 800;

		/// <summary>
		/// Occurs when a new key is decoded.
		/// </summary>
		public event EventHandler<NewKeyEventArgs> NewKey;

		private volatile int lastNumber;
		private char lastKey;
		private int lastKeyTime;
		private volatile uint lastKeyCount;
		private volatile bool wasLastKeyDefinite;

		private int multitapCount;

		private volatile bool isDisposed;

		public TelephoneKeypadDecoder()
		{
			// something that doesn't exist on the keypad
			lastNumber = -1;
			lastKey = '-';

			lastKeyTime = GetTime();
			lastKeyCount = 0;
			wasLastKeyDefinite = true;
			multitapCount = 0;
		}

		public void Dispose()
		{
			isDisposed = true;
		}

		public void NewNumber(int number)
		{
			uint myID = ++lastKeyCount;

			// check multi-tap
			{
				int time = GetTime();
				
				if(number != lastNumber) {
					multitapCount = 0;

					if(!wasLastKeyDefinite) {
						// we have to push previous key as definite
						NewKeyEventArgs definiteKeyArgs = new NewKeyEventArgs();
						definiteKeyArgs.Key = lastKey;
						definiteKeyArgs.IsDefinite = true;
						NewKey?.Invoke(this, definiteKeyArgs);
					}
				}else {
					if(time - lastKeyTime < MULTITAP_THRESHOLD)
						// the same number was pressed quicker than the multitap threshold
						++multitapCount;
					else
						multitapCount = 0;
				}

				lastKeyTime = time;
			}

			lastNumber = number;

			// decode key
			char key = DecodeKey(number, multitapCount);
			lastKey = key;

			// wait for multitap threshold amount of time
			Task.Delay(MULTITAP_THRESHOLD).ContinueWith(delegate
			{
				if(isDisposed)
					return;
				if(myID != lastKeyCount)
					// new numbers were present after this one
					return;

				lastNumber = -1;

				// invoke this key as the definite one
				wasLastKeyDefinite = true;
				NewKeyEventArgs definiteKeyArgs = new NewKeyEventArgs();
				definiteKeyArgs.Key = key;
				definiteKeyArgs.IsDefinite = true;
				NewKey?.Invoke(this, definiteKeyArgs);
			});

			// invoke the key as indefinite, because the definite one will be pushed later
			wasLastKeyDefinite = false;
			NewKeyEventArgs newKeyArgs = new NewKeyEventArgs();
			newKeyArgs.Key = key;
			newKeyArgs.IsDefinite = false;
			NewKey?.Invoke(this, newKeyArgs);
		}

		/// <summary>
		/// https://en.wikipedia.org/wiki/Telephone_keypad#Layout
		/// </summary>
		private char DecodeKey(int number,int multitap)
		{
			switch(number) {
				case 0:
				case 1:
					// use 0 and 1 for space
					return ' ';
				case 2:
					switch(multitapCount % 3) {
						case 0:
							return 'A';
						case 1:
							return 'B';
						case 2:
							return 'C';
					}
					break;
				case 3:
					switch(multitapCount % 3) {
						case 0:
							return 'D';
						case 1:
							return 'E';
						case 2:
							return 'F';
					}
					break;
				case 4:
					switch(multitapCount % 3) {
						case 0:
							return 'G';
						case 1:
							return 'H';
						case 2:
							return 'I';
					}
					break;
				case 5:
					switch(multitapCount % 3) {
						case 0:
							return 'J';
						case 1:
							return 'K';
						case 2:
							return 'L';
					}
					break;
				case 6:
					switch(multitapCount % 3) {
						case 0:
							return 'M';
						case 1:
							return 'N';
						case 2:
							return 'O';
					}
					break;
				case 7:
					switch(multitapCount % 4) {
						case 0:
							return 'P';
						case 1:
							return 'Q';
						case 2:
							return 'R';
						case 3:
							return 'S';
					}
					break;
				case 8:
					switch(multitapCount % 3) {
						case 0:
							return 'T';
						case 1:
							return 'U';
						case 2:
							return 'V';
					}
					break;
				case 9:
					switch(multitapCount % 4) {
						case 0:
							return 'W';
						case 1:
							return 'X';
						case 2:
							return 'Y';
						case 3:
							return 'Z';
					}
					break;
			}

			throw new NotImplementedException("Wrong number and/or multitap.");
		}

		private int GetTime()
		{
			return DateTime.UtcNow.Millisecond;
		}
	}

	public class NewKeyEventArgs : EventArgs
	{
		public char Key;

		/// <summary>
		/// If false, it means that this key is only a part of a multi-tap sequence and is therefore not yet definite.
		/// </summary>
		public bool IsDefinite;
	}
}