#include <Wire.h>
#include <Adafruit_SSD1306.h>

#define W 128
#define H 64
#define B 4

#define MX (W/B)
#define MY (H/B)

Adafruit_SSD1306 d(W, H, &Wire, -1);

#define X 26
#define Y 27
#define BTN 15

int sx[50], sy[50];
int len = 3;
int dx = 1, dy = 0;
int fx, fy;

unsigned long t = 0;

void food() {
  fx = random(MX);
  fy = random(MY);
}

void resetGame() {
  len = 3;
  dx = 1; dy = 0;

  sx[0]=10; sy[0]=10;
  sx[1]=9;  sy[1]=10;
  sx[2]=8;  sy[2]=10;

  food();
}

void setup() {
  pinMode(BTN, INPUT_PULLUP);
  Wire.begin();
  d.begin(SSD1306_SWITCHCAPVCC, 0x3C);
  randomSeed(analogRead(0));
  resetGame();
}

void loop() {
  if (!digitalRead(BTN)) resetGame();

  int vx = analogRead(X);
  int vy = analogRead(Y);

  if (vx < 1000 && dx == 0) { dx=-1; dy=0; }
  if (vx > 3000 && dx == 0) { dx=1; dy=0; }
  if (vy < 1000 && dy == 0) { dx=0; dy=1; }
  if (vy > 3000 && dy == 0) { dx=0; dy=-1; }

  if (millis() - t > 160) {
    t = millis();

    for (int i=len-1;i>0;i--) {
      sx[i]=sx[i-1];
      sy[i]=sy[i-1];
    }

    sx[0]+=dx;
    sy[0]+=dy;

    if (sx[0]<0||sx[0]>=MX||sy[0]<0||sy[0]>=MY) {
      resetGame(); return;
    }

    for (int i=1;i<len;i++) {
      if (sx[0]==sx[i] && sy[0]==sy[i]) {
        resetGame(); return;
      }
    }

    if (sx[0]==fx && sy[0]==fy) {
      if (len<50) len++;
      food();
    }

    draw();
  }
}

void draw() {
  d.clearDisplay();

  d.fillRect(fx*B, fy*B, B, B, 1);

  for (int i=0;i<len;i++) {
    d.fillRect(sx[i]*B, sy[i]*B, B, B, 1);
  }

  d.display();
}