Sample table-driven finite-state machine

We have interviewees write sample code, but now we want them to modify some simple working code. I came up with the following proposal, which is a syntax-directed interpreter implemented with a table-driven finite state machine. The interpreter simply accepts whitespace-delimited text and outputs "Hello" when it sees the case-insensitive command "HELLO". Any other input is an error.

This is a highly-simplified version of some C code converted to C# for the interview:

enum Lexeme
{
    Error,
    EOF,
    Whitespace,
    H, E, L, O
}

static Lexeme LexemeOfIntChar(int cIn)
{
    if (cIn = 0) && (currentState != 7))
    {
        Lexeme t = LexemeOfIntChar(Console.Read());
        currentState = transition[currentState, (int)t];
        switch (currentState)
        {
            case -1: Console.WriteLine("Invalid character!"); break;
            case 6: Console.WriteLine("Hello"); break;  // 6 = saw 'HELLO' with terminal symbol
        }
    }
}

The proposed exercise is to extend this interpreter to accept a "HELP" command that will print out:


"usage 'HelloScript <scriptfile'

This can be done in about a half-dozen changes.

A co-worker I asked this about was able to complete it in under 15 minutes, but wanted to argue about terminology, implementation, et al.

The intent is for it to be a simple exercise only to see if interviewees can understand and maintain existing code. How understandable and maintainable is it?

If I were a candidate, I'd judge what I think the quality of your code looks like based on this simple, and I'd have some concerns:

• Your transition table depends on the values of the Lexeme enum not changing (say by inserting additional values). If this is the case I'd like there to be a comment on enum saying order is significant or having the values explicitly given orders. Otherwise I'd expect the actual numbers to not matter.

• Your code contains a transition table. That's hard to read and hard to modify. Why would you do that?

-
You've two extra sets of parens here:

while ((currentState >= 0) && (currentState != 7))

• Your states and done via magic numbers and not an enum.

• The check for -1 feels like it's in the wrong place. It is handling an error condition. But the other element of the switch isn't an error condition.

StackExchange Code Review Q#5044, answer score: 4