#include <WiFi.h>
#include <time.h>
#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

// ===== USER CONFIG =====
const char* WIFI_SSID     = "Wokwi-GUEST";
const char* WIFI_PASSWORD = "";

// India (IST UTC+5:30)
const long  GMT_OFFSET_SEC     = 19800;
const int   DAYLIGHT_OFFSET_SEC = 0;
// =======================

// OLED
#define SCREEN_WIDTH 128
#define SCREEN_HEIGHT 64
#define OLED_RESET    -1
Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, OLED_RESET);

const char* months[] = {"Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"};
const char* weekdays[] = {"S","M","T","W","T","F","S"};

bool getLocal(struct tm &out) {
  time_t now = time(nullptr);
  if (now < 1700000000) return false; // invalid
  localtime_r(&now, &out);
  return true;
}

int daysInMonth(int year, int mon) {
  tm t = {};
  t.tm_year = year - 1900;
  t.tm_mon  = mon + 1;   // next month
  t.tm_mday = 0;         // day 0 of next month = last day of this month
  mktime(&t);
  return t.tm_mday;
}

void drawCalendar(const tm& t) {
  display.clearDisplay();
  display.setTextSize(1);
  display.setTextColor(SSD1306_WHITE);

  // Month + Year header
  String header = String(months[t.tm_mon]) + "  " + String(t.tm_year + 1900);
  int16_t x1, y1; uint16_t w, h;
  display.getTextBounds(header, 0, 0, &x1, &y1, &w, &h);
  display.setCursor((SCREEN_WIDTH - w) / 2, 0);
  display.print(header);

  // Weekday header
  for (int i = 0; i < 7; i++) {
    display.setCursor(i * 18, 12);  // 18px column width
    display.print(weekdays[i]);
  }

  // First day of month
  tm first = {};
  first.tm_year = t.tm_year;
  first.tm_mon  = t.tm_mon;
  first.tm_mday = 1;
  mktime(&first);
  int startWday = first.tm_wday; // 0=Sun..6=Sat

  int dim = daysInMonth(t.tm_year + 1900, t.tm_mon);

  // Dates grid
  int day = 1;
  for (int week = 0; week < 6; week++) {
    for (int wd = 0; wd < 7; wd++) {
      int cellX = wd * 18;
      int cellY = 22 + week * 8;

      if (week == 0 && wd < startWday) continue;
      if (day > dim) continue;

      // Highlight today
      if (day == t.tm_mday) {
        display.fillRect(cellX, cellY, 16, 8, SSD1306_WHITE);
        display.setTextColor(SSD1306_BLACK);
        display.setCursor(cellX + 2, cellY);
        display.print(day);
        display.setTextColor(SSD1306_WHITE);
      } else {
        display.setCursor(cellX + 2, cellY);
        display.print(day);
      }
      day++;
    }
  }

  display.display();
}

void setup() {
  Wire.begin(); // SDA, SCL default for ESP32: GPIO21, GPIO22

  display.begin(SSD1306_SWITCHCAPVCC, 0x3C);
  display.clearDisplay();
  display.display();

  // WiFi
  WiFi.begin(WIFI_SSID, WIFI_PASSWORD);
  while (WiFi.status() != WL_CONNECTED) delay(500);

  // Time from NTP
  configTime(GMT_OFFSET_SEC, DAYLIGHT_OFFSET_SEC, "pool.ntp.org", "time.nist.gov");

  delay(2000); // wait for sync
}

void loop() {
  static int lastDay = -1;
  tm t;
  if (getLocal(t)) {
    if (t.tm_mday != lastDay) { // redraw only when day changes
      lastDay = t.tm_mday;
      drawCalendar(t);
    }
  }
  delay(10000); // check every 10s
}