// Install from the Library Manager:
// "LiquidCrystal I2C"
// Board: "ESP32 Dev Module"
#include <WiFi.h>
#include <WiFiClientSecure.h>
#include <HTTPClient.h>
#include <Wire.h>
#include <LiquidCrystal_I2C.h>

// Your Wi-Fi network's name and password
const char* WIFI_NAME = "Network_Name";
const char* WIFI_PASSWORD = "Password";
// The Central Bank's daily rates list
const char* RATES_URL = "https://www.tcmb.gov.tr/kurlar/today.xml";
// Rates come out on workdays at 15:30;
// a look every half hour is enough (ms)
const unsigned long FETCH_EVERY = 30UL * 60 * 1000;
// If they fail, try again after this
const unsigned long RETRY_AFTER = 60000;
// Time between screen pages (ms)
const unsigned long PAGE_EVERY = 4000;
// The ESP32's I2C pins
const int SDA_PIN = 21;
const int SCL_PIN = 22;
// Ask about the password after this (ms)
const unsigned long WIFI_HINT_AFTER = 15000;

// I2C address: 0x27 on most, 0x3F on some
LiquidCrystal_I2C lcd(0x27, 16, 2);

// The last selling rates read; 0: none
float usd = 0;
float eur = 0;
float gbp = 0;
// The rates' date, e.g. "30.10.2026"
String rateDate = "";
unsigned long nextFetch = 0;
unsigned long lastPage = 0;
int page = 0;

// Writes a line and blanks the rest
// (no flicker)
void printLine(int row, const String& text) {
  lcd.setCursor(0, row);
  lcd.print(text);
  for (int n = text.length(); n < 16; n++) {
    lcd.print(' ');
  }
}

// Finds a currency's selling rate
// (ForexSelling); 0 if it is missing
float sellingRate(const String& xml, const char* code) {
  int at = xml.indexOf(String("CurrencyCode=\"") + code + "\"");
  if (at < 0) return 0;
  int start = xml.indexOf("<ForexSelling>", at);
  if (start < 0) return 0;
  start += 14;
  int end = xml.indexOf('<', start);
  return xml.substring(start, end).toFloat();
}

// Downloads the list; if it fails, the
// old rates stay on screen
bool fetchRates() {
  WiFiClientSecure client;
  // The certificate is not checked: only
  // a public list is being read
  client.setInsecure();
  HTTPClient http;
  http.begin(client, RATES_URL);
  // Ask for it plain: to the ESP32's own
  // request TCMB answers with gzip
  http.setAcceptEncoding("identity");
  int status = http.GET();
  if (status != 200) {
    Serial.print("Could not get rates, HTTP ");
    Serial.println(status);
    http.end();
    return false;
  }
  String xml = http.getString();
  http.end();

  // The list gives its date at the top:
  // Tarih="30.10.2026"
  int at = xml.indexOf("Tarih=\"");
  if (at >= 0) {
    int end = xml.indexOf('"', at + 7);
    rateDate = xml.substring(at + 7, end);
  }
  float newUsd = sellingRate(xml, "USD");
  if (newUsd == 0) {
    // Not the rates list: its start shows
    // what went wrong
    Serial.print("Unexpected answer (");
    Serial.print(xml.length());
    Serial.print(" bytes): ");
    Serial.println(xml.substring(0, 160));
    return false;
  }
  usd = newUsd;
  eur = sellingRate(xml, "EUR");
  gbp = sellingRate(xml, "GBP");

  Serial.print(rateDate);
  Serial.print("  USD ");
  Serial.print(usd, 4);
  Serial.print("  EUR ");
  Serial.print(eur, 4);
  Serial.print("  GBP ");
  Serial.println(gbp, 4);
  return usd > 0;
}

// "USD/TL   49.8644": exactly 16 chars
String rateLine(const char* code, float rate) {
  char text[17];
  snprintf(text, sizeof text, "%s/TL  %8.4f", code, rate);
  return String(text);
}

void showPage() {
  if (usd == 0) {
    printLine(0, "No rates yet");
    printLine(1, "Will try again");
    return;
  }
  if (page == 0) {
    printLine(0, rateLine("USD", usd));
    printLine(1, rateLine("EUR", eur));
  } else {
    printLine(0, rateLine("GBP", gbp));
    printLine(1, "TCMB " + rateDate);
  }
}

void setup() {
  Serial.begin(115200);
  Wire.begin(SDA_PIN, SCL_PIN);
  lcd.init();
  lcd.backlight();
  lcd.print("Waiting for WiFi");

  Serial.print("Connecting to ");
  Serial.println(WIFI_NAME);
  WiFi.begin(WIFI_NAME, WIFI_PASSWORD);
  unsigned long start = millis();
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
    if (millis() - start > WIFI_HINT_AFTER) {
      // No luck: check name, password
      lcd.setCursor(0, 1);
      lcd.print("Name/password?");
    }
  }
  Serial.println();
  Serial.println("Connected");
  printLine(0, "Getting rates");
  printLine(1, "");
}

void loop() {
  // At start-up, then every half hour
  if ((long)(millis() - nextFetch) >= 0) {
    bool ok = fetchRates();
    nextFetch = millis() + (ok ? FETCH_EVERY : RETRY_AFTER);
    showPage();
  }
  // Two pages in turn: USD and EUR, then
  // GBP and the rates' date
  if (millis() - lastPage > PAGE_EVERY) {
    lastPage = millis();
    page = 1 - page;
    showPage();
  }
}