Initial BER sync recovery.

include/m17cxx/Util.h

@@ -328,6 +328,9 @@ struct PRBS9
     uint8_t sync_count = 0;
     uint32_t bit_count = 0;
     uint32_t err_count = 0;
+    std::array<uint8_t, 16> history;
+    size_t hist_count = 0;
+    size_t hist_pos = 0;
 
     // PRBS generator.
     bool operator()()
@@ -352,6 +355,23 @@ struct PRBS9
                     synced = true;
                     err_count = 0;
                     bit_count = LOCK_COUNT;
+                    history.fill(0);
+                    hist_count = 0;
+                    hist_pos = 0;
+                    sync_count = 0;
+                }
+            }
+        } else if (hist_count > 25) {
+            result = (bit ^ (state >> TAP_1) ^ (state >> TAP_2)) & 1;
+            state = ((state << 1) | bit) & MASK;
+            if (result) {
+                sync_count = 0;
+            } else {
+                if (++sync_count == LOCK_COUNT) {
+                    history.fill(0);
+                    hist_count = 0;
+                    hist_pos = 0;
+                    sync_count = 0;
                 }
             }
         } else {
@@ -360,7 +380,15 @@ struct PRBS9
             state = ((state << 1) | result) & MASK;
 
             bit_count += 1;
-            err_count += (result != bit);
+            hist_count -= (history[hist_pos >> 3] & (1 << (hist_pos & 7))) != 0;
+            if (result != bit) {
+                err_count += 1;
+                hist_count += 1;
+                history[hist_pos >> 3] |= (1 << (hist_pos & 7));
+            } else {
+                history[hist_pos >> 3] &= ~(1 << (hist_pos & 7));
+            }
+            if (++hist_pos == 128) hist_pos = 0;
         }
         return result;
     }
@@ -378,6 +406,9 @@ struct PRBS9
         sync_count = 0;
         bit_count = 0;
         err_count = 0;
+        history.fill(0);
+        hist_count = 0;
+        hist_pos = 0;
     }
 };
 

tests/UtilTest.cpp

@@ -217,18 +217,32 @@ TEST_F(UtilTest, PRBS9)
     }
 }
 
+#include <iomanip>
+
 TEST_F(UtilTest, PRBS9_FULL)
 {
     mobilinkd::PRBS9 prbs_generator;
     mobilinkd::PRBS9 prbs_validator;
 
+    uint16_t byte = 0;
+    uint16_t bits = 0;
     for (size_t i = 0; i != 1000; ++i) {
         bool n = prbs_generator();
+        byte <<= 1;
+        byte |= n;
+        if (++bits == 8) {
+            std::cout << std::hex << std::setw(2) << byte << " ";
+            byte = 0;
+            bits = 0;
+        }
+
         if (i == 499) n = !n;
         else if (i == 510) n = !n;
         prbs_validator(n);
     }
 
+    std::cout << std::endl;
+
     ASSERT_TRUE(prbs_validator.sync());
     EXPECT_EQ(prbs_validator.bits(), 1000);
     EXPECT_EQ(prbs_validator.errors(), 2);