Portable calculator built using app.nit

• calculator_logic defines CalculatorContext with all the business logic of a calculator. It takes as input operations and numbers, and outputs the text to display.
• calculator implements the portable graphical interface using the app.nit framework
• scientific_calculator refines calculator to add scientific operations.
• android_calculator refines calculator to get a nicer aesthetic on Android.
• ios_calculator refines calculator to get a nicer aesthetic on iOS.
• calculator_test test CalculatorContext as a black box.

• Compile and run on the desktop (GNU/Linux and OS X) with:

   make
   bin/calculator
  

• Compile for Android and install on a device or emulator with:

   make bin/android.apk
   adb install -r bin/calculator.apk
  

• Compile for iOS and run on the simulator with:

   make bin/android.app
   ios-sim launch bin/calculator.app