TextEditor.cs

TextEditor.cs

private delegate IEnumerable<string> ManipulateHistory(IEnumerable<string> history);

ManipulateHistory append(string tail) =>
    history => history.Append(new[] { history.Last() + tail });

ManipulateHistory delete(int k) =>
    history => history.Append(new[] { delete(history.Last(), k) });

string delete(string last, int k) => last.Substring(0, last.Length - k);

ManipulateHistory print(int k) => history =>
{
    Console.WriteLine(history.Last()[k - 1].ToString());
    return history;
};

ManipulateHistory undo() =>
    history => history.Take(history.Length() - 1);


ManipulateHistory toCommand(string commandAndParameter)
{
    var parts = commandAndParameter.Split(" ");
    var command = parts[0];

    return command switch
    {
        "1" => append(parts[1]),
        "2" => delete(int.Parse(parts[1])),
        "3" => print(int.Parse(parts[1])),
        "4" => undo(),
    };
}

[Fact]
void pass()
{ 
    var input = new List<string>
    {
        "1 fg", // abcdefg 
        "3 6",  // abcdefg
        "2 5",  // abcdefg    ab   
        "4",    // abcdefg
        "3 7",  // abcdefg
        "4",    // abcde
        "3 4"   // 
    };

    IEnumerable<string> initial = new List<string>{"abcde"};

    var result = input.Select(toCommand).Aggregate(initial, (h, c) => c(h));

    Assert.Equal("abcde", result.Last());
}