C# Coding Challenge – Old Phone Pad

The original challenge PDF asks for a method that converts “old phone keypad” input (digits, pauses, backspace *, send #) into text, plus tests and documentation.

The AI usage can be seen at https://chatgpt.com/share/691b01cc-47e0-8000-9e4f-a32716cb1a3e

Problem summary

• Keys 2–9 map to letters (2 → ABC, 3 → DEF, …, 9 → WXYZ).
• Repeating a key cycles through its letters (for key 2: 2# → A, 22# → B, 222# → C).
• A space separates sequences on the same key (e.g. 222 2 22# → CAB).
• * acts as backspace, deleting the last output character.
• # sends the message and terminates input; it is not part of the output.

Examples from the PDF (covered by tests):

• OldPhonePad("33#") → E
• OldPhonePad("227*#") → B
• OldPhonePad("4433555 555666#") → HELLO
• OldPhonePad("8 88777444666*664#") → TURING

Implementation

• Core logic: OldPhonePad/OldPhonePad.cs (OldPhonePadApp.OldPhonePad.Convert(string input)).
• Console entry point: OldPhonePad/Program.cs:
  • Writes Enter key sequence:
  • Reads a line (e.g. 4433555 555666#) and prints the decoded output.

How to build and run

From the root:

• Build the app project:
  dotnet build OldPhonePad/OldPhonePad.csproj

• Run the app project:
  dotnet run --project OldPhonePad/OldPhonePad.csproj

Example:

• Input: 4433555 555666#
• Output: HELLO

Tests and coverage

From the repository root:

• Run tests with line/branch coverage (coverage.runsettings + XPlat Code Coverage):
  dotnet test OldPhonePad.Tests/OldPhonePad.Tests.csproj --collect:"XPlat Code Coverage" --settings coverage.runsettings

  This produces OldPhonePad.Tests/TestResults/<guid>/coverage.info (LCOV).

• (Optional) Generate an HTML report with ReportGenerator:

  • Install once (if needed):
    dotnet tool install -g dotnet-reportgenerator-globaltool
  • Generate report:
    reportgenerator -reports:"OldPhonePad.Tests/TestResults/*/coverage.info" -targetdir:"coveragereport" -reporttypes:Html

Open coveragereport/index.html to inspect coverage.