Created calculateAirtime and made airtime public (TheThingsNetwork#162)

* created a calculateAirtime to inform the user and let him work with the airtime, called calculateAirtime in Trackaritime, added calculateAirtime in keywords.txt and in docs

* fix docs

Author: Nicolasdejean

docs/TheThingsNetwork.md

@@ -133,3 +133,14 @@ bool provision(const char *appEui, const char *appKey);
 
 - `const char *appEui`: Application Identifier for the device.
 - `const char *appKey`: Application Key assigned to the device.
+
+## Method: calculateAirtime
+Calculate the uplink time of a message.
+
+```c
+float calculateAirtime(size_t payloadSize);
+```
+
+- `size_t payloadSize`: number of bytes you want to send.
+
+Returns the uplink time of the message in seconds.

keywords.txt

@@ -20,6 +20,8 @@ join                    KEYWORD2
 personalize             KEYWORD2
 sendBytes               KEYWORD2
 poll                    KEYWORD2
+calculateAirtime        KEYWORD2
+airtime                 KEYWORD2
 
 encodeSensorData        KEYWORD2
 decodeAppData           KEYWORD2

src/TheThingsNetwork.cpp

@@ -494,6 +494,17 @@ int8_t TheThingsNetwork::poll(port_t port, bool confirm) {
   return sendBytes(payload, 1, port, confirm);
 }
 
+float TheThingsNetwork::calculateAirtime(size_t payloadSize) {
+  payloadSize = 13 + payloadSize;
+
+  float Tsym = pow(2, this->info.sf) / this->info.band;
+  float Tpreamble = (this->info.ps + 4.25) * Tsym;
+  uint16_t payLoadSymbNb = 8 + (max(ceil((8 * payloadSize - 4 * this->info.sf + 28 + 16 - 20 * this->info.header) / (4 * (this->info.sf - 2 * this->info.de))) * (this->info.cr + 4), 0));
+  float Tpayload = payLoadSymbNb * Tsym;
+  float Tpacket = Tpreamble + Tpayload;
+  return (Tpacket / 1000);
+}
+
 void TheThingsNetwork::fillAirtimeInfo() {
   this->info = {0, 0, 0, 0, 0, 0};
 
@@ -516,14 +527,7 @@ void TheThingsNetwork::fillAirtimeInfo() {
 }
 
 void TheThingsNetwork::trackAirtime(size_t payloadSize) {
-  payloadSize = 13 + payloadSize;
-
-  float Tsym = pow(2, this->info.sf) / this->info.band;
-  float Tpreamble = (this->info.ps + 4.25) * Tsym;
-  uint16_t payLoadSymbNb = 8 + (max(ceil((8 * payloadSize - 4 * this->info.sf + 28 + 16 - 20 * this->info.header) / (4 * (this->info.sf - 2 * this->info.de))) * (this->info.cr + 4), 0));
-  float Tpayload = payLoadSymbNb * Tsym;
-  float Tpacket = Tpreamble + Tpayload;
-  this->airtime = this->airtime + (Tpacket / 1000);
+  this->airtime = this->airtime + calculateAirtime(payloadSize);
 }
 
 void TheThingsNetwork::showStatus() {

src/TheThingsNetwork.h

@@ -42,7 +42,6 @@ class TheThingsNetwork
     Stream* debugStream;
     bool model_EU;
     airtime_t info;
-    float airtime;
     ttn_fp_t fp;
     uint8_t sf;
     uint8_t fsb;
@@ -74,6 +73,8 @@ class TheThingsNetwork
 
   public:
     TheThingsNetwork(Stream& modemStream, Stream& debugStream, ttn_fp_t fp, uint8_t sf = TTN_DEFAULT_SF, uint8_t fsb = TTN_DEFAULT_FSB);
+    float airtime;
+    float calculateAirtime(size_t payload);
     void showStatus();
     void onMessage(void (*cb)(const byte* payload, size_t length, port_t port));
     bool provision(const char *appEui, const char *appKey);