Removing comments with a sliding window without nested while loops

Question

I\'m trying to remove comments and strings from a c file with c code. I\'ll just stick to comments for the examples. I have a sliding window so I only have character n

Answer 1

The algorithm written with one while loop could look like this:

while ((c = getchar()) != EOF)
{
    ... // looking at the byte that was just read

    if (...) // the symbol is not inside a comment
    {
        putchar(c);
    }
}

To decide whether the input char belongs to a comment, you can use a state machine. In the following example, it has 4 states; there are also rules for traversing to next state.

int state = 0;
int next_state;
while ((c = getchar()) != EOF)
{
    switch (state)
    {
        case 0: next_state = (c == '/' ? 1 : 0); break;
        case 1: next_state = (c == '*' ? 2 : c == '/' ? 1 : 0); break;
        case 2: next_state = (c == '*' ? 3 : 2); break;
        case 3: next_state = (c == '/' ? 0 : c == '*' ? 3 : 2); break;
        default: next_state = state; // will never happen
    }

    if (state == 1 && next_state == 0)
    {
        putchar('/'); // for correct output when a slash is not followed by a star
    }
    if (state == 0 && next_state == 0)
    {
        putchar(c);
    }
    state = next_state;
}

The example above is very simple: it doesn't work correctly for /* in non-comment contexts like in C strings; it doesn't support // comments, etc.