alertcondition() and alert() in MQL5: pop-up, push and e-mail

//@version=6
indicator("RSI alerts", overlay = false)

len = input.int(14, "RSI length", minval = 1)
ob = input.float(70, "Overbought")
os = input.float(30, "Oversold")

rsi = ta.rsi(close, len)
crossOb = ta.crossover(rsi, ob)
crossOs = ta.crossunder(rsi, os)

plot(rsi, "RSI", color.purple, 2)
hline(ob, "Overbought", color.gray)
hline(os, "Oversold", color.gray)

// 1. alertcondition(): a choice in TradingView's "Create alert" dialog (constant message, {{placeholders}})
alertcondition(crossOb, "RSI overbought", "RSI crossed above the overbought level on {{ticker}} ({{interval}}), close {{close}}")
alertcondition(crossOs, "RSI oversold", "RSI crossed below the oversold level on {{ticker}} ({{interval}}), close {{close}}")

// 2. alert(): fires from the code (one "Any alert() function call" alert), with a message built at run time
if crossOb or crossOs
    alert("RSI(" + str.tostring(len) + ") at " + str.tostring(rsi, "#.0") + " on " + syminfo.ticker, alert.freq_once_per_bar_close)

//+------------------------------------------------------------------+
//| alertcondition() and alert() in MQL5: RSI alerts coded in the    |
//| indicator (pop-up, push notification, e-mail).                   |
//| Custom indicator: save in MQL5/Indicators and compile (F7).      |
//+------------------------------------------------------------------+
#property indicator_separate_window
#property indicator_minimum 0
#property indicator_maximum 100
#property indicator_buffers 1
#property indicator_plots   1

#property indicator_label1  "RSI"
#property indicator_type1   DRAW_LINE
#property indicator_color1  clrMediumPurple
#property indicator_width1  2

enum ENUM_ALERT_FREQ
  {
   FREQ_BAR_CLOSE,   // Once per bar close (alert.freq_once_per_bar_close)
   FREQ_ONCE_PER_BAR // Once per bar, as soon as it happens (alert.freq_once_per_bar)
  };

input int             InpLen   = 14;             // RSI length
input double          InpOb    = 70;             // Overbought
input double          InpOs    = 30;             // Oversold
input ENUM_ALERT_FREQ InpFreq  = FREQ_BAR_CLOSE; // When to alert
input bool            InpPopup = true;           // Pop-up alert
input bool            InpPush  = false;          // Push notification to the MetaTrader app
input bool            InpMail  = false;          // E-mail (set up in Tools > Options > Email)

double   RsiBuf[];
int      rsiHandle    = INVALID_HANDLE;
datetime lastAlertBar = 0;

int OnInit()
  {
   if(InpLen < 1 || InpOs >= InpOb)
      return INIT_PARAMETERS_INCORRECT;
   SetIndexBuffer(0, RsiBuf, INDICATOR_DATA);
   PlotIndexSetDouble(0, PLOT_EMPTY_VALUE, EMPTY_VALUE);
   // hline(ob) / hline(os): levels of the indicator window
   IndicatorSetInteger(INDICATOR_LEVELS, 2);
   IndicatorSetDouble(INDICATOR_LEVELVALUE, 0, InpOb);
   IndicatorSetDouble(INDICATOR_LEVELVALUE, 1, InpOs);
   IndicatorSetInteger(INDICATOR_DIGITS, 1);
   IndicatorSetString(INDICATOR_SHORTNAME, StringFormat("RSI alerts (%d)", InpLen));

   rsiHandle = iRSI(_Symbol, _Period, InpLen, PRICE_CLOSE);
   if(rsiHandle == INVALID_HANDLE)
      return INIT_FAILED;
   return INIT_SUCCEEDED;
  }

void OnDeinit(const int reason)
  {
   IndicatorRelease(rsiHandle);
  }

// Sends the message by every channel switched on in the inputs
void Notify(const string msg)
  {
   Print(msg);
   if(InpPopup)
      Alert(msg);
   if(InpPush && !SendNotification(msg))
      Print("Push notification failed, error ", GetLastError());
   if(InpMail && !SendMail("RSI alert " + _Symbol, msg))
      Print("E-mail failed, error ", GetLastError());
  }

int OnCalculate(const int rates_total,
                const int prev_calculated,
                const datetime &time[],
                const double &open[],
                const double &high[],
                const double &low[],
                const double &close[],
                const long &tick_volume[],
                const long &volume[],
                const int &spread[])
  {
   if(rates_total < 3 || BarsCalculated(rsiHandle) < rates_total)
      return prev_calculated;

   int start = prev_calculated > 0 ? prev_calculated - 1 : 0;
   int count = rates_total - start;
   double rsi[];
   if(CopyBuffer(rsiHandle, 0, 0, count, rsi) != count)
      return prev_calculated;
   for(int k = 0; k < count; k++)
      RsiBuf[start + k] = rsi[k];

   // Pine alerts only fire in real time: skip the history, and the bar that closed before we started
   if(prev_calculated == 0)
     {
      lastAlertBar = time[rates_total - 2];
      return rates_total;
     }

   // Bar close: judge the bar that just closed. Once per bar: judge the forming bar, first time only
   int bar = InpFreq == FREQ_BAR_CLOSE ? rates_total - 2 : rates_total - 1;
   if(time[bar] == lastAlertBar)
      return rates_total;

   string what = "";
   if(RsiBuf[bar] > InpOb && RsiBuf[bar - 1] <= InpOb)       // ta.crossover(rsi, ob)
      what = "crossed above the overbought level";
   else if(RsiBuf[bar] < InpOs && RsiBuf[bar - 1] >= InpOs)  // ta.crossunder(rsi, os)
      what = "crossed below the oversold level";
   if(what == "")
      return rates_total;

   lastAlertBar = time[bar];
   // {{ticker}}, {{interval}} and {{close}} become _Symbol, the timeframe and close[bar]
   Notify(StringFormat("RSI(%d) %s on %s %s: RSI %.1f, close %s", InpLen, what, _Symbol,
                       StringSubstr(EnumToString((ENUM_TIMEFRAMES)_Period), 7), RsiBuf[bar], DoubleToString(close[bar], _Digits)));
   return rates_total;
  }
//+------------------------------------------------------------------+