From c02f43044fc8595076f6997a22fe8674dbce2907 Mon Sep 17 00:00:00 2001
From: "Matthias P. Braendli" <matthias.braendli@mpb.li>
Date: Thu, 18 May 2023 15:49:22 +0200
Subject: Start encoding aprs434 compressed messages

---
 tracker-stm32/src/main.ino | 79 ++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 72 insertions(+), 7 deletions(-)

diff --git a/tracker-stm32/src/main.ino b/tracker-stm32/src/main.ino
index da82e01..ed1d570 100644
--- a/tracker-stm32/src/main.ino
+++ b/tracker-stm32/src/main.ino
@@ -4,9 +4,15 @@
 #include <SparkFun_u-blox_GNSS_Arduino_Library.h>
 #include <SD.h>
 
-File myFile;
+constexpr char *CALLSIGN = "HB9EGM";
+constexpr int SSID = 12;
+constexpr char SYMBOL_TABLE_IDENTIFIER = '/';
+constexpr char SYMBOL_CODE_BICYCLE = 'b';
 
+#if 0
+File myFile;
 constexpr int SD_CS = 17;
+#endif
 
 // SX1278 has the following connections:
 // NSS pin:   PD14 (Arduino 10)
@@ -18,8 +24,39 @@ SFE_UBLOX_GNSS gnss;
 
 long lastGnssPoll = 0;
 long lastPositionReport = 0;
-constexpr size_t POSITION_REPORT_LEN = 32;
-char positionReport[POSITION_REPORT_LEN+1];
+
+constexpr size_t MAX_REPORT_LEN = 32;
+size_t report_len = 0;
+uint8_t report[MAX_REPORT_LEN];
+
+const char* digits = " 0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ-./?@";
+
+#define callsign_int_t uint32_t
+callsign_int_t encodeCallsign(const char *callsign) {
+    // Encode CCCC according to aprs434.github.io/code/codec.cpp
+    const int b = 37;
+
+    callsign_int_t result;
+    callsign_int_t weight;
+    int i, j, k, ch;
+
+    result = 0, weight = 1;
+    for (i = j = strlen(callsign); i > 0; i--) {  // iterate over input string
+        ch = toupper(callsign[i-1]);
+        for (k = 0; k < strlen(digits); k++) {    // lookup char index
+            if (digits[k] == ch) {
+                break;
+            }
+        }
+        if (k == strlen(digits))                  // ignore if ch not found in digits
+            continue;
+        result = result + ((k) * weight);
+        weight = weight * b;
+    }
+
+    return result;                                // result can be pow(42,51)!
+}
+
 
 static char letterize(int x) {
     return (char) x + 65;
@@ -54,13 +91,14 @@ void setup() {
     Serial.begin(9600);
     pinMode(LED_BUILTIN, OUTPUT);
 
+#if 0
     pinMode(SD_CS, OUTPUT);
 
     if (!SD.begin(SD_CS)) {
         Serial.println("SD init failed!");
         return;
     }
-
+#endif
 
     Wire.begin();
 
@@ -175,12 +213,39 @@ void loop()
 
             digitalWrite(LED_BUILTIN, HIGH);
 
-            ssize_t len = snprintf(positionReport, POSITION_REPORT_LEN, "HB9EGM %s", locator);
+            // Encode LoRa message according to aprs434.github.io
+            const uint32_t callsign_EEEE = encodeCallsign(CALLSIGN);
+
+            report_len = 0;
+            // Callsign encoded as CCCC
+            report[report_len++] = (callsign_EEEE >> 24) & 0xFF;
+            report[report_len++] = (callsign_EEEE >> 16) & 0xFF;
+            report[report_len++] = (callsign_EEEE >> 8) & 0xFF;
+            report[report_len++] = callsign_EEEE & 0xFF;
+
+            // D SSID Path Code and Data Type Code
+
+            const int date_type_code = 0; // compressed geolocation
+            const int path_code = 2; // metropolitan mobile
+            report[report_len++] = SSID * 16 + path_code * 4 + date_type_code;
+
+            // / Symbol Table Identifier
+            report[report_len++] = SYMBOL_TABLE_IDENTIFIER;
+#warn "TODO longitude and latitude"
+            // XXXX Base91 Longitude
+            // YYYY Base91 Latitude
+            // $ Symbol Code
+            report[report_len++] = SYMBOL_CODE_BICYCLE;
+            // cs Course and Speed
 
             Serial.print(F("TX: "));
-            Serial.print(positionReport);
+            // This is the old custom report I used before adopting aprs434.github.io,
+            // it is just callsign SPACE locator sent as ASCII
+            Serial.print(CALLSIGN);
+            Serial.print(" ");
+            Serial.println(locator);
 
-            int state = radio.transmit(positionReport, len);
+            int state = radio.transmit(report, report_len);
 
             if (state == RADIOLIB_ERR_NONE) {
                 Serial.print(F(" RFM OK Datarate: "));
-- 
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