Implementation of M17 convolutional encoder

openrtx/include/protocols/M17/M17ConvolutionalEncoder.h

@@ -0,0 +1,114 @@
+/***************************************************************************
+ *   Copyright (C) 2021 by Federico Amedeo Izzo IU2NUO,                    *
+ *                         Niccolò Izzo IU2KIN                             *
+ *                         Frederik Saraci IU2NRO                          *
+ *                         Silvano Seva IU2KWO                             *
+ *                                                                         *
+ *   Adapted from original code written by Rob Riggs, Mobilinkd LLC        *
+ *                                                                         *
+ *   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 M17CONVOLUTIONALENCODER_H
+#define M17CONVOLUTIONALENCODER_H
+
+#ifndef __cplusplus
+#error This header is C++ only!
+#endif
+
+#include <cstdint>
+#include <cstddef>
+
+/**
+ * Convolutional encoder tailored on M17 protocol specifications, requiring a
+ * coder rate R = 1/2, a constraint length K = 5 and polynomials G1 = 0x19 and
+ * G2 = 0x17.
+ */
+class M17ConvolutionalEncoder
+{
+public:
+
+    /**
+     * Constructor.
+     */
+    M17ConvolutionalEncoder(){ }
+
+    /**
+     * Encode a given block of data using the M17 convolutional encoding scheme.
+     * Given the coder rate of 1/2, calling code must ensure that the destination
+     * buffer for the convolved data has twice the size of the original data block.
+     *
+     * \param data: pointer to source data block.
+     * \param convolved: pointer to a destination buffer for the convolved data.
+     * \param len: length of the source data block.
+     */
+    void encode(const void *data, void *convolved, const size_t len)
+    {
+        const uint8_t  *src  = reinterpret_cast< const uint8_t * >(data);
+        uint16_t       *dest = reinterpret_cast< uint16_t * >(convolved);
+
+        for(size_t i = 0; i < len; i++)
+        {
+            dest[i] = convolveByte(src[i]);
+        }
+    }
+
+    /**
+     * Flush the convolutional encoder, returning the remaining encoded data.
+     *
+     * \return the convolution encoding of the remaining content of the encoder
+     * memory.
+     */
+    uint16_t flush()
+    {
+        return convolveByte(0x00);
+    }
+
+    /**
+     * Reset the convolutional encoder memory to zero.
+     */
+    void reset()
+    {
+        memory = 0;
+    }
+
+private:
+
+    /**
+     * Compute the convolutional encoding of a byte, using the M17 encoding
+     * scheme.
+     *
+     * \param value: byte to be convolved.
+     * \return result of the convolutional encoding process.
+     */
+    uint16_t convolveByte(uint8_t value)
+    {
+        uint16_t result = 0;
+
+        for(uint8_t i = 0; i < 8; i++)
+        {
+            memory  = (memory << 1) | ((value & 0x80) >> 7);
+            memory &= 0x1F;
+            result  = (result << 1) | (__builtin_popcount(memory & 0x19) & 0x01);
+            result  = (result << 1) | (__builtin_popcount(memory & 0x17) & 0x01);
+            value <<= 1;
+        }
+
+        return __builtin_bswap16(result);
+    }
+
+    uint8_t memory = 0;    ///< Convolutional encoder memory.
+};
+
+#endif /* M17CONVOLUTIONALENCODER_H */