State machine example

void handle_tcp_packet(struct TCB *tcb, const unsigned char *data, unsigned length)
{
    ...
    for (i=0; i<length; i++)
        tcb->state = http_parse(tcb->state, data[i], &tcb->http);
    ...
}

/* States: note that the parser assumes this order of states
 * as it sometimes returns "state + 1". */
enum {START=0, METHOD, INTER_MU, URI, INTER_UV, VERSION,
    NAME, VALUE, EOL, END_OF_HEADER=-1};
int http_parse(int state, unsigned char c, struct HttpParser *h)
{
    static const char WHITESPACE = " \n\r\f\t\v";

    /* ignore nuls and carriage-returns 'cause they complicate
     * stuff */
    if (c == '\0' || c == '\r')
        return state;

    /* Note that the order of states in the switches statement
     * is not the same as the order of states in the enum */
    switch (state) {
        case START:
            if (strchr(WHITESPACE, c)) {
                return state; /* no transition */
            } else {
                search_start(h, METHOD, c);
                return METHOD;
            }

            /* text fields*/
        case METHOD:
        case URL:
        case VERSION:
        case VALUE:
            if (c == '\n') {
                search_end(h);
                return EOL;
            } else if (strchr(WHITESPACE, c)) {
                search_end(h);
                return state+1;
            } else {
                search_continue(h, c);
                return state; /* no transition */
            }
        case NAME:
            if (c == '\n') {
                search_end(h);
                search_start(h, VALUE, '\0');
                search_end(h);
                return EOL;
            } else if (c == ':')) {
                search_end(h);
                search_start(h, VALUE, '\0');
                return VALUE;
            } else {
                search_continue(h, c);
                return state; /* no transition */
            }

            /* space fields */
        case INTER_MV:
        case INTER_UV:
            if (c == '\n') {
                return EOL;
            } else if (strchr(WHITESPACE, c)) {
                return state; /* no transition */
            } else {
                search_start(h, state+1, c);
                return state+1;
            }
        case POST_V:
            if (c == '\n') {
                return EOL;
            } else {
                return state; /* no transition */
            }
        case EOL:
            if (c == '\n') {
                http_request_end(h);
                return END_OF_HEADER;
            } else {
                search_start(h, NAME, c);
                return NAME;
            }
    }
}

void search_start(struct HTTP *h, int search_id, int c)
{
    h->ac_table = aho_corasic_searches[search_id];
    h->ac_state = 0;
    search_continue(h, c);
}
void search_continue(struct HTTP *h, unsigned char c)
{
    int x;
    int row = h->ac_state;
    int column = translate(h, c);

    h->ac_state = h->ac_table[row][column];

    x = ac_match(h->ac_table, h->ac_state);
    if (x)
        report_ids_event(x);
}
void search_end(struct HTTP *h)
{
    search_continue(h, -1);
}