#define DATA_PIN 13
#define STORAGE_PIN 12
#define CLOCK_PIN 11

int segments[] = {
  1 << 1,  // top mid
  1 << 2,  // top right
  1 << 5,  // bot right
  1 << 6,  // bot mid
  1 << 7,  // bot left
  1 << 4,  // top left
  1 << 3,  // mid mid
};

int digits[] = {
  segments[0] | segments[1] | segments[2] | segments[3] | segments[4] | segments[5],  // 0 
  segments[1] | segments[2], // 1
  segments[0] | segments[1] | segments[6] | segments[4] | segments[3], // 2
  segments[0] | segments[1] | segments[2] | segments[3] | segments[6], // 3
  segments[1] | segments[2] | segments[5] | segments[6], // 4 
  segments[0] | segments[2] | segments[3] | segments[5] | segments[6], // 5
  segments[0] | segments[2] | segments[3] | segments[4] | segments[5] | segments[6], // 6
  segments[0] | segments[1] | segments[2], // 7
  segments[0] | segments[1] | segments[2] | segments[3] | segments[4] | segments[5] | segments[6], // 8
  segments[0] | segments[1] | segments[2] | segments[3] | segments[5] | segments[6], // 9
};

void setup() {
  pinMode(DATA_PIN, OUTPUT);
  pinMode(STORAGE_PIN, OUTPUT);
  pinMode(CLOCK_PIN, OUTPUT);
}

#define print(left, right) do { \
  shiftOut(DATA_PIN, CLOCK_PIN, MSBFIRST, digits[right]); \
  shiftOut(DATA_PIN, CLOCK_PIN, MSBFIRST, digits[left]); \
  digitalWrite(STORAGE_PIN, LOW); \
  digitalWrite(STORAGE_PIN, HIGH); \
} while(false)

void loop() {
  for (int i = 99; i >= 0; i--) {
    print(i / 10, i % 10);
    delay(500);
  }
}