/*
  WiFiTelnetToSerial - Example Transparent UART to Telnet Server for esp8266

  Copyright (c) 2015 Hristo Gochkov. All rights reserved.
  This file is part of the ESP8266WiFi library for Arduino environment.

  This library is free software; you can redistribute it and/or
  modify it under the terms of the GNU Lesser General Public
  License as published by the Free Software Foundation; either
  version 2.1 of the License, or (at your option) any later version.

  This library is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public
  License along with this library; if not, write to the Free Software
  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
*/
#include <ESP8266WiFi.h>

#include <algorithm> // std::min

#ifndef STASSID
#define STASSID "your-ssid"
#define STAPSK  "your-password"
#endif

/*
    SWAP_PINS:
   0: use Serial1 for logging (legacy example)
   1: configure Hardware Serial port on RX:GPIO13 TX:GPIO15
      and use SoftwareSerial for logging on
      standard Serial pins RX:GPIO3 and TX:GPIO1
*/

#define SWAP_PINS 0

/*
    SERIAL_LOOPBACK
    0: normal serial operations
    1: RX-TX are internally connected (loopback)
*/

#define SERIAL_LOOPBACK 0

#define BAUD_SERIAL 115200
#define BAUD_LOGGER 115200
#define RXBUFFERSIZE 1024

////////////////////////////////////////////////////////////

#if SERIAL_LOOPBACK
#undef BAUD_SERIAL
#define BAUD_SERIAL 3000000
#include <esp8266_peri.h>
#endif

#if SWAP_PINS
#include <SoftwareSerial.h>
SoftwareSerial* logger = nullptr;
#else
#define logger (&Serial1)
#endif

#define STACK_PROTECTOR  512 // bytes

//how many clients should be able to telnet to this ESP8266
#define MAX_SRV_CLIENTS 2
const char* ssid = STASSID;
const char* password = STAPSK;

const int port = 23;

WiFiServer server(port);
WiFiClient serverClients[MAX_SRV_CLIENTS];

void setup() {

  Serial.begin(BAUD_SERIAL);
  Serial.setRxBufferSize(RXBUFFERSIZE);

#if SWAP_PINS
  Serial.swap();
  // Hardware serial is now on RX:GPIO13 TX:GPIO15
  // use SoftwareSerial on regular RX(3)/TX(1) for logging
  logger = new SoftwareSerial(3, 1);
  logger->begin(BAUD_LOGGER);
  logger->println("\n\nUsing SoftwareSerial for logging");
#else
  logger->begin(BAUD_LOGGER);
  logger->println("\n\nUsing Serial1 for logging");
#endif
  logger->println(ESP.getFullVersion());
  logger->printf("Serial baud: %d (8n1: %d KB/s)\n", BAUD_SERIAL, BAUD_SERIAL * 8 / 10 / 1024);
  logger->printf("Serial receive buffer size: %d bytes\n", RXBUFFERSIZE);

#if SERIAL_LOOPBACK
  USC0(0) |= (1 << UCLBE); // incomplete HardwareSerial API
  logger->println("Serial Internal Loopback enabled");
#endif

  WiFi.mode(WIFI_STA);
  WiFi.begin(ssid, password);
  logger->print("\nConnecting to ");
  logger->println(ssid);
  while (WiFi.status() != WL_CONNECTED) {
    logger->print('.');
    delay(500);
  }
  logger->println();
  logger->print("connected, address=");
  logger->println(WiFi.localIP());

  //start server
  server.begin();
  server.setNoDelay(true);

  logger->print("Ready! Use 'telnet ");
  logger->print(WiFi.localIP());
  logger->printf(" %d' to connect\n", port);
}

void loop() {
  //check if there are any new clients
  if (server.hasClient()) {
    //find free/disconnected spot
    int i;
    for (i = 0; i < MAX_SRV_CLIENTS; i++)
      if (!serverClients[i]) { // equivalent to !serverClients[i].connected()
        serverClients[i] = server.available();
        logger->print("New client: index ");
        logger->print(i);
        break;
      }

    //no free/disconnected spot so reject
    if (i == MAX_SRV_CLIENTS) {
      server.available().println("busy");
      // hints: server.available() is a WiFiClient with short-term scope
      // when out of scope, a WiFiClient will
      // - flush() - all data will be sent
      // - stop() - automatically too
      logger->printf("server is busy with %d active connections\n", MAX_SRV_CLIENTS);
    }
  }

  //check TCP clients for data
#if 1
  // Incredibly, this code is faster than the bufferred one below - #4620 is needed
  // loopback/3000000baud average 348KB/s
  for (int i = 0; i < MAX_SRV_CLIENTS; i++)
    while (serverClients[i].available() && Serial.availableForWrite() > 0) {
      // working char by char is not very efficient
      Serial.write(serverClients[i].read());
    }
#else
  // loopback/3000000baud average: 312KB/s
  for (int i = 0; i < MAX_SRV_CLIENTS; i++)
    while (serverClients[i].available() && Serial.availableForWrite() > 0) {
      size_t maxToSerial = std::min(serverClients[i].available(), Serial.availableForWrite());
      maxToSerial = std::min(maxToSerial, (size_t)STACK_PROTECTOR);
      uint8_t buf[maxToSerial];
      size_t tcp_got = serverClients[i].read(buf, maxToSerial);
      size_t serial_sent = Serial.write(buf, tcp_got);
      if (serial_sent != maxToSerial) {
        logger->printf("len mismatch: available:%zd tcp-read:%zd serial-write:%zd\n", maxToSerial, tcp_got, serial_sent);
      }
    }
#endif

  // determine maximum output size "fair TCP use"
  // client.availableForWrite() returns 0 when !client.connected()
  size_t maxToTcp = 0;
  for (int i = 0; i < MAX_SRV_CLIENTS; i++)
    if (serverClients[i]) {
      size_t afw = serverClients[i].availableForWrite();
      if (afw) {
        if (!maxToTcp) {
          maxToTcp = afw;
        } else {
          maxToTcp = std::min(maxToTcp, afw);
        }
      } else {
        // warn but ignore congested clients
        logger->println("one client is congested");
      }
    }

  //check UART for data
  size_t len = std::min((size_t)Serial.available(), maxToTcp);
  len = std::min(len, (size_t)STACK_PROTECTOR);
  if (len) {
    uint8_t sbuf[len];
    size_t serial_got = Serial.readBytes(sbuf, len);
    // push UART data to all connected telnet clients
    for (int i = 0; i < MAX_SRV_CLIENTS; i++)
      // if client.availableForWrite() was 0 (congested)
      // and increased since then,
      // ensure write space is sufficient:
      if (serverClients[i].availableForWrite() >= serial_got) {
        size_t tcp_sent = serverClients[i].write(sbuf, serial_got);
        if (tcp_sent != len) {
          logger->printf("len mismatch: available:%zd serial-read:%zd tcp-write:%zd\n", len, serial_got, tcp_sent);
        }
      }
  }
}
