Generator of pseudorandom binary sequence compliant with M17 specification

openrtx/include/protocols/M17/M17Prbs.h

@@ -0,0 +1,130 @@
+/***************************************************************************
+ *   Copyright (C) 2022 by Federico Amedeo Izzo IU2NUO,                    *
+ *                         Niccolò Izzo IU2KIN                             *
+ *                         Frederik Saraci IU2NRO                          *
+ *                         Silvano Seva IU2KWO                             *
+ *                                                                         *
+ *   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 3 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, see <http://www.gnu.org/licenses/>   *
+ ***************************************************************************/
+
+#ifndef M17PRBS_H
+#define M17PRBS_H
+
+#ifndef __cplusplus
+#error This header is C++ only!
+#endif
+
+#include <cstdint>
+
+namespace M17
+{
+
+/**
+ * Pseudo random number generator compliant with M17 PRBS9 specification.
+ */
+class PRBS9
+{
+public:
+
+    /**
+     * Constructor.
+     */
+    PRBS9() : state(1), syncCnt(0), synced(false) { }
+
+    /**
+     * Destructor.
+     */
+    ~PRBS9() { }
+
+    /**
+     * Reset the generator state.
+     */
+    void reset()
+    {
+        state   = 1;
+        syncCnt = 0;
+        synced  = false;
+    }
+
+    /**
+     * Generate a new bit from the PRBS9 sequence.
+     *
+     * @return a bit from the PRBS9 sequence.
+     */
+    bool generateBit()
+    {
+        bool result = ((state >> TAP_1) ^ (state >> TAP_2)) & 0x01;
+        state       = ((state << 1) | result) & MASK;
+        return result;
+    }
+
+    /**
+     * Syncronise the PRBS9 generator with an external stream of bits.
+     *
+     * @param bit: current bit of the external stream.
+     * @return true when the PRBS9 state is syncronised with the bit stream.
+     */
+    bool syncronize(const bool& bit)
+    {
+        if(synced) return true;
+
+        bool result = (bit ^ (state >> TAP_1) ^ (state >> TAP_2)) & 1;
+        state       = ((state << 1) | bit) & MASK;
+
+        syncCnt += 1;
+
+        if(result)
+        {
+            syncCnt = 0;
+            synced  = false;
+        }
+
+        if(syncCnt >= LOCK_COUNT)
+        {
+            synced = true;
+        }
+
+        return synced;
+    }
+
+    /**
+     * Validate the current bit of an external stream against the PRBS9 state
+     * and advance the generator state by one step. To have meaningful results
+     * the generator state has to be syncronised with the stream.
+     *
+     * @param bit: current bit of the external stream.
+     * @return true if the bit from the PRBS9 matches the external one, false
+     * otherwise.
+     */
+    bool validateBit(const bool& bit)
+    {
+        if(synced == false) return false;
+        return ((bit ^ generateBit()) == 0);
+    }
+
+private:
+
+    static constexpr uint16_t MASK       = 0x1FF;
+    static constexpr uint8_t  TAP_1      = 8;
+    static constexpr uint8_t  TAP_2      = 4;
+    static constexpr uint8_t  LOCK_COUNT = 18;
+
+    uint16_t state;
+    uint8_t  syncCnt;
+    bool     synced;
+};
+
+}      // namespace M17
+
+#endif // M17PRBS_H