beginning of pager decoder

decoder_modules/pager_decoder/src/pocsag/pocsag.cpp

@@ -0,0 +1,140 @@
+#include "pocsag.h"
+#include <string.h>
+#include <utils/flog.h>
+
+#define POCSAG_FRAME_SYNC_CODEWORD  ((uint32_t)(0b01111100110100100001010111011000))
+#define POCSAG_IDLE_CODEWORD_DATA   ((uint32_t)(0b011110101100100111000))
+#define POCSAG_BATCH_BIT_COUNT      (POCSAG_BATCH_CODEWORD_COUNT*32)
+
+#define POCSAG_GEN_POLY             ((uint32_t)(0b11101101001))
+
+namespace pocsag {
+    const char NUMERIC_CHARSET[] = {
+        '0',
+        '1',
+        '2',
+        '3',
+        '4',
+        '5',
+        '6',
+        '7',
+        '8',
+        '9',
+        '*',
+        'U',
+        ' ',
+        '-',
+        ']',
+        '['
+    };
+
+    void Decoder::process(uint8_t* symbols, int count) {
+        for (int i = 0; i < count; i++) {
+            // Get symbol
+            uint32_t s = symbols[i];
+
+            // If not sync, try to acquire sync (TODO: sync confidence)
+            if (!synced) {
+                // Append new symbol to sync shift register
+                syncSR = (syncSR << 1) | s;
+
+                // Test for sync
+                synced = (distance(syncSR, POCSAG_FRAME_SYNC_CODEWORD) <= POCSAG_SYNC_DIST);
+
+                // Go to next symbol
+                continue;
+            }
+
+            // TODO: Flush message on desync
+
+            // Append bit to batch
+            batch[batchOffset >> 5] |= (s << (31 - (batchOffset & 0b11111)));
+            batchOffset++;
+
+            // On end of batch, decode and reset
+            if (batchOffset >= POCSAG_BATCH_BIT_COUNT) {
+                decodeBatch();
+                batchOffset = 0;
+                synced = false;
+                memset(batch, 0, sizeof(batch));
+            }
+        }
+    }
+
+    int Decoder::distance(uint32_t a, uint32_t b) {
+        uint32_t diff = a ^ b;
+        int dist = 0;
+        for (int i = 0; i < 32; i++) {
+            dist += (diff >> i ) & 1;
+        }
+        return dist;
+    }
+
+    bool Decoder::correctCodeword(Codeword in, Codeword& out) {
+
+
+        return true; // TODO
+    }
+
+    void Decoder::flushMessage() {
+        if (!msg.empty()) {
+            onMessage(addr, msgType, msg);
+            msg.clear();
+        }
+    }
+
+    void Decoder::decodeBatch() {
+        for (int i = 0; i < POCSAG_BATCH_CODEWORD_COUNT; i++) {
+            // Get codeword
+            Codeword cw = batch[i];
+
+            // Correct errors. If corrupted, skip
+            if (!correctCodeword(cw, cw)) { continue; }
+            // TODO: End message if two consecutive are corrupt
+
+            // Get codeword type
+            CodewordType type = (CodewordType)((cw >> 31) & 1);
+            if (type == CODEWORD_TYPE_ADDRESS && (cw >> 11) == POCSAG_IDLE_CODEWORD_DATA) {
+                type = CODEWORD_TYPE_IDLE;
+            }
+
+            // Decode codeword
+            if (type == CODEWORD_TYPE_IDLE) {
+                // If a non-empty message is available, send it out and clear
+                flushMessage();
+                flog::debug("[{}:{}]: IDLE", (i >> 1), i&1);
+            }
+            else if (type == CODEWORD_TYPE_ADDRESS) {
+                // If a non-empty message is available, send it out and clear
+                flushMessage();
+
+                // Decode message type
+                msgType = (MessageType)((cw >> 11) & 0b11);
+
+                // Decode address and append lower 8 bits from position
+                addr = ((cw >> 13) & 0b111111111111111111) << 3;
+                addr |= (i >> 1);
+            }
+            else if (type == CODEWORD_TYPE_MESSAGE) {
+                // Extract the 20 data bits
+                uint32_t data = (cw >> 11) & 0b11111111111111111111;
+
+                // Decode data depending on message type
+                if (msgType == MESSAGE_TYPE_NUMERIC) {
+                    // Numeric messages pack 5 characters per message codeword
+                    msg += NUMERIC_CHARSET[(data >> 16) & 0b1111];
+                    msg += NUMERIC_CHARSET[(data >> 12) & 0b1111];
+                    msg += NUMERIC_CHARSET[(data >> 8) & 0b1111];
+                    msg += NUMERIC_CHARSET[(data >> 4) & 0b1111];
+                    msg += NUMERIC_CHARSET[data & 0b1111];
+                }
+                else if (msgType == MESSAGE_TYPE_ALPHANUMERIC) {
+                    
+                }
+
+                // Save last data
+                lastMsgData = data;
+            }
+        }
+    }
+}