ACARS Rx App using libacars (#3029)

* Using same decode as libacrars
* Enable ACARS RX
* indent + fix warning
* fixed CRC calculations, length validation, message print
Co-authored-by: gullradriel <gullradriel@users.noreply.github.com>

firmware/application/external/acars_rx/acars_app.cpp

@@ -33,6 +33,76 @@ using namespace portapack;
 
 namespace ui::external_app::acars_rx {
 
+// ACARS frame field layout (0-based byte offsets):
+//   [0]          SOH framing byte   (skipped)
+//   [1..7]       Aircraft registration (7 chars)
+//   [8]          STX framing byte   (skipped)
+//   [9..10]      Label              (2 chars)
+//   [11]         Block ID           (1 char)
+//   [12..14]     Message number     (3 chars)
+//   [15..20]     Flight ID          (6 chars)
+//   [21..N-3]    Free-text payload  (variable)
+//   [N-2..N-1]   CRC-16/CCITT      (2 bytes, MSB first, NOT part of payload)
+//
+// Minimum valid frame: 21-byte fixed header + 2 CRC bytes = 23 bytes.
+// (A zero-length payload is legal per spec; we require at least 23 bytes.)
+static constexpr std::string::size_type kAcarsCrcLen = 2;
+static constexpr std::string::size_type kAcarsHeaderLen = 21;
+static constexpr std::string::size_type kAcarsMinLen = kAcarsHeaderLen + kAcarsCrcLen;
+
+// CRC-16/CCITT: poly 0x1021, init 0x0000, no reflection, no final XOR.
+// This is the variant used by ACARS (same as XMODEM CRC).
+static uint16_t acars_crc16(const std::string& data, std::string::size_type len) {
+    uint16_t crc = 0x0000;
+    for (std::string::size_type i = 0; i < len; ++i) {
+        crc ^= static_cast<uint16_t>(static_cast<uint8_t>(data[i])) << 8;
+        for (int bit = 0; bit < 8; ++bit) {
+            if (crc & 0x8000)
+                crc = static_cast<uint16_t>((crc << 1) ^ 0x1021);
+            else
+                crc = static_cast<uint16_t>(crc << 1);
+        }
+    }
+    return crc;
+}
+
+AcarsDecoded acars_decode(const std::string& raw) {
+    AcarsDecoded result;
+    if (raw.size() < kAcarsMinLen) {
+        result.txt = "ACARS message too short (" + std::to_string(raw.size()) +
+                     " bytes, need " + std::to_string(kAcarsMinLen) + ")";
+        return result;
+    }
+
+    // Verify CRC: computed over everything except the last 2 bytes,
+    // then compared against those 2 bytes (MSB first).
+    const std::string::size_type payload_len = raw.size() - kAcarsCrcLen;
+    const uint16_t computed = acars_crc16(raw, payload_len);
+    const uint16_t received = (static_cast<uint16_t>(static_cast<uint8_t>(raw[raw.size() - 2])) << 8) |
+                              static_cast<uint16_t>(static_cast<uint8_t>(raw[raw.size() - 1]));
+    result.crc_ok = (computed == received);
+
+    result.reg = raw.substr(1, 7);
+    result.label = raw.substr(9, 2);
+    result.block_id = raw[11];
+    result.msg_num = raw.substr(12, 3);
+    result.flight_id = raw.substr(15, 6);
+    // Payload sits between end of fixed header and the 2 trailing CRC bytes.
+    if (payload_len > kAcarsHeaderLen)
+        result.txt = raw.substr(kAcarsHeaderLen, payload_len - kAcarsHeaderLen);
+    return result;
+}
+
+std::string acars_format(const AcarsDecoded& msg) {
+    return std::string("ACARS Decoded Result\nCRC: ") + (msg.crc_ok ? "OK" : "FAIL") +
+           "\nRegistration: " + msg.reg +
+           "\nLabel: " + msg.label +
+           "\nBlockID: " + msg.block_id +
+           "\nMsgNum: " + msg.msg_num +
+           "\nFlightID: " + msg.flight_id +
+           "\nMessage: " + msg.txt;
+}
+
 void ACARSLogger::log_str(std::string msg) {
     log_file.write_entry(msg);
 }
@@ -77,17 +147,16 @@ void ACARSAppView::focus() {
 
 void ACARSAppView::on_packet(const ACARSPacketMessage* packet) {
     std::string console_info;
-
     if (packet->state == 255) {
         // got a packet, parse it, and display
         rtc::RTC datetime;
         rtc_time::now(datetime);
-        // todo parity error recovery
         console_info = to_string_datetime(datetime, HMS);
         console_info += ": ";
-        console_info += packet->message;
+        std::string message{packet->message, packet->message + packet->msg_len};
+        AcarsDecoded decoded = acars_decode(message);
+        console_info += acars_format(decoded);
         console.writeln(console_info);
-        // Log raw data whatever it contains
         if (logger && logging)
             logger->log_str(console_info);
     } else {