static int const buttonPin = 0;
 static int const relayPin  = 12;  // active high
 static int const ledPin    = 13;  // active low

 static uint8_t prevButtonVal;
 static uint8_t curOutVal;

 static inline bool isRaisingEdge(uint8_t prevVal, uint8_t curVal) {
   return prevVal == LOW && curVal == HIGH;
 }

 void setup() {
   pinMode(buttonPin, INPUT);
   pinMode(ledPin, OUTPUT);
   pinMode(relayPin, OUTPUT);

   prevButtonVal = digitalRead(buttonPin);
   digitalWrite(relayPin, LOW);
   digitalWrite(ledPin, HIGH);
   curOutVal = LOW;
 }

 void loop() {
   uint8_t curButtonVal = digitalRead(buttonPin);
   if (isRaisingEdge(prevButtonVal, curButtonVal)) {
     curOutVal = curOutVal == HIGH ? LOW : HIGH;
     digitalWrite(relayPin, curOutVal);
     digitalWrite(ledPin, curOutVal == HIGH ? LOW : HIGH);
   }
   prevButtonVal = curButtonVal;
 }