Add the CRC and callsign encoding.

M17Utils.cpp

@@ -0,0 +1,83 @@
+/*
+ *   Copyright (C) 2020 by Jonathan Naylor G4KLX
+ *
+ *   This program is free software; you can redistribute it and/or modify
+ *   it under the terms of the GNU General Public License as published by
+ *   the Free Software Foundation; either version 2 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program 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 General Public License for more details.
+ *
+ *   You should have received a copy of the GNU General Public License
+ *   along with this program; if not, write to the Free Software
+ *   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+#include "M17Utils.h"
+
+#include <cassert>
+
+void CM17Utils::encodeCallsign(const std::string& callsign, unsigned char* encoded)
+{
+	assert(encoded != NULL);
+
+    std::string call = callsign;
+    call.resize(8U, ' ');
+
+    uint64_t enc = 0ULL;
+    for (int i = 7U; i >= 0; i--) {
+        char c = call.at(i);
+
+        enc *= 40ULL;
+
+        // If speed is more important than code space, you can replace this with a lookup into a 256 byte array.
+        if (c >= 'A' && c <= 'Z') // 1-26
+            enc += c - 'A' + 1ULL;
+        else if (c >= '0' && c <= '9') // 27-36
+            enc += c - '0' + 27ULL;
+        else if (c == '-') // 37
+            enc += 37ULL;
+
+        // These are just place holders. If other characters make more sense, change these.
+        // Be sure to change them in the decode array below too.
+        else if (c == '/') // 38
+            enc += 38ULL;
+        else if (c == '.') // 39
+            enc += 39ULL;
+        else
+            // Invalid character or space, represented by 0, decoded as a space.
+            enc += 0ULL;
+    }
+
+    encoded[0U] = enc >> 40;
+    encoded[1U] = enc >> 32;
+    encoded[2U] = enc >> 24;
+    encoded[3U] = enc >> 16;
+    encoded[4U] = enc >> 8;
+    encoded[5U] = enc >> 0;
+}
+
+void CM17Utils::decodeCallsign(const unsigned char* encoded, std::string& callsign)
+{
+    assert(encoded != NULL);
+
+    callsign.empty();
+
+    uint64_t enc = (uint64_t(encoded[5U]) << 40) +
+                   (uint64_t(encoded[4U]) << 32) +
+                   (uint64_t(encoded[3U]) << 24) +
+                   (uint64_t(encoded[2U]) << 16) +
+                   (uint64_t(encoded[1U]) << 8)  +
+                   (uint64_t(encoded[0U]) << 0);
+
+    if (enc >= 262144000000000ULL) // 40^9
+        return;
+
+    while (enc > 0ULL) {
+        callsign += " ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-/."[enc % 40ULL];
+        enc /= 40ULL;
+    }
+}