A simple text editor class that stores an internal string when instantiated. This string can be manipulated via formatted input.
# Parses input and prints result of each `print` command on a new line.
$ cat input.txt | ruby main_script.rb
c
y
a
#or
$ ruby main_script.rb input.txt
c
y
amain_script.rb parses the input into command codes ('1', '2', '3', or '4') and their arguments. It then uses a case statement to map the command codes to their respective commands ('append', 'delete', 'print', and 'undo').
In order to make the script easier to reason about and easier to test, I encapsulate all four actions in a service object called TextEditor. TextEditor uses instance variables to store the text to be manipulated (initially an empty string called @text) and the history of executed commands (initially an empty array called @history). The four input commands are implemented as four methods in TextEditor: #append, #delete, #print, and #undo. main_script.rb creates an instance of TextEditor and calls these four methods on it as appropriate.
The implementations of #append, #delete, and #print are all fairly straightforward; the only thing I'll note is that the parsed arguments that get passed to #delete and #print need to be ints. #undo is a bit trickier. To implement #undo, I use an array to store the history of executed commands. I treat this array as a stack by only ever calling two methods on it: #push and #pop. When a method is called that manipulates @text (excluding #undo since I don't want to undo #undos), I push the inverse of its manipulation onto @history. For example, if append('world') is called, I push ['2', 5] onto @history, where '2' represents the 'delete' command, and 5 is the argument I would pass to it (since 'world' is five letters long). This allows me to simply implement #undo by popping the latest @text-manipulating action off of @history, checking its command code, and then calling the appropriate method with the given argument. The catch to this approach is that I don't want the action performed while undoing a previous action to be pushed onto the history stack, since this would result in an infinite loop of an action and its inverse action being performed.
I solve this problem with a keyword argument called store. When store: false is passed along with the method's usual argument, the performed action will not be stored in @history. The store argument defaults to true so if no value is passed for it, the performed action will be stored. I use a keyword argument here to enhance clarity. It's not obvious from the method names (#append and #delete) what that second argument is used for, so even though a keyword argument is less terse, it's worth using in this case.