turnstile.c

#include <stdio.h>
#include <stdlib.h>
 
enum state {
    LOCKED,
    UNLOCKED
};
 
// 'c' for coin
// 'p' for push
enum state transition(char c, enum state current_state) {
    switch (current_state) {
        case LOCKED:
            if (c == 'c') return UNLOCKED;
            else if (c == 'p') return LOCKED;
            break;
 
        case UNLOCKED:
            if (c == 'c') return UNLOCKED;
            else if (c == 'p') return LOCKED;
            break;
    }
}
 
int main() {
    enum state current_state = LOCKED;
    char c;
    while ((c = getc(stdin)) == 'c' || c == 'p') {
        current_state = transition(c, current_state);
        printf("%c %s\n", c, current_state == LOCKED ? "LOCKED" : "UNLOCKED");
    }
 
    return 0;
}