IR remote control relay module on an Arduino Uno

// IR remote control relay module: the 1 to 4 keys of the handset switch the
// four channels of a relay board, each running a 12 V lamp, and 0 turns them
// all off. The first step of most home automation builds.
//
//   pin 2        the VS1838B receiver's OUT (VCC 5V, GND)
//   pins 4 to 7  the relay board's IN1 to IN4 (VCC 5V, GND)
//   each relay   COM to the 12 V adapter's +, NO to its lamp, the lamp's
//                other lead to the adapter's -
//   adapter -    also to GND, so the 12 V side shares the Uno's ground
//
// The handset's IR pin is wired to the receiver's: in Mokxi two parts only
// meet on a net, so the beam is a wire. Click a key on the handset.
//
// IRremote does the decoding. Each key of the common 21-key NEC handset
// sends a command byte: 1 is 0x0C, 2 is 0x18, 3 is 0x5E, 4 is 0x08 and 0 is
// 0x16. Hold a key and the handset sends repeat frames; a toggle must ignore
// them, or the lamp flickers on and off while the key is held.
//
// The relay board is active low, like nearly every board sold: LOW pulls a
// relay in. Each pin is written HIGH before it becomes an output, or every
// relay clicks on for an instant at power-up.
//
// The lamps here are 12 V. A relay board's contacts are rated for mains, but
// mains wiring belongs in a closed box and is not something to learn on.

#define DECODE_NEC
#include <IRremote.hpp>

const int RECEIVER_PIN = 2;
const int RELAYS[4] = {4, 5, 6, 7};
const uint8_t KEYS[4] = {0x0C, 0x18, 0x5E, 0x08};  // 1, 2, 3, 4
const uint8_t KEY_0 = 0x16;

bool on[4] = {false, false, false, false};

void set(int i, bool state) {
  on[i] = state;
  digitalWrite(RELAYS[i], state ? LOW : HIGH);  // active low
  Serial.print("relay ");
  Serial.print(i + 1);
  Serial.println(state ? " ON" : " OFF");
}

void setup() {
  for (int i = 0; i < 4; i++) {
    digitalWrite(RELAYS[i], HIGH);  // off before it is an output
    pinMode(RELAYS[i], OUTPUT);
  }
  Serial.begin(9600);
  IrReceiver.begin(RECEIVER_PIN, ENABLE_LED_FEEDBACK);
  Serial.println("IR relays: keys 1 to 4 toggle a relay, 0 turns all off");
}

void loop() {
  if (!IrReceiver.decode()) return;
  bool repeat = IrReceiver.decodedIRData.flags & IRDATA_FLAGS_IS_REPEAT;
  uint8_t command = IrReceiver.decodedIRData.command;
  IrReceiver.resume();
  if (repeat) return;

  if (command == KEY_0) {
    for (int i = 0; i < 4; i++) set(i, false);
    return;
  }
  for (int i = 0; i < 4; i++) {
    if (command == KEYS[i]) {
      set(i, !on[i]);
      return;
    }
  }
  Serial.print("key 0x");
  Serial.print(command, HEX);
  Serial.println(" is not used");
}