caze transition table from https://ideone.com/DdKmwq

transition.c

#include <stdio.h>
#include <stdlib.h>
 
typedef void effector(int);
typedef struct transition transition;
struct transition {
  int state;
  effector *effect;
};
 
enum sigil { Letter, EndOfFile };
typedef enum sigil sigil;
 
sigil sigilify(int c) {
  switch (c) {
    case EOF:  return EndOfFile;
    default:   return Letter;
  }
}
 
void push   (int c);
void pop    (int c);
void swap   (int c);
void halt   (int c);
 
#define T(state, sigil) (((state) << 1) | sigil)
 
int main() {
  transition state = { 0, push };
  transition table[] = {
    [T(0, Letter)]={ 1, push },    [T(0, EndOfFile)]={ 0, halt },
    [T(1, Letter)]={ 0, swap },    [T(1, EndOfFile)]={ 0, pop  },
  };
 
  for (;;) {
    int c = getchar();
    (state = table[T(state.state, sigilify(c))]).effect(c);
  }
}
 
int b;
 
void push   (int c) { b = c; }
void pop    (int c) { putchar(b); }
void swap   (int c) { putchar(c); pop(0); }
void halt   (int c) { exit(EXIT_SUCCESS); }