Add bit unstuffing algorithm.

For use in AIS. Probably useful elsewhere, too...

firmware/baseband/symbol_coding.hpp

@@ -23,6 +23,7 @@
 #define __SYMBOL_CODING_H__
 
 #include <cstdint>
+#include <cstddef>
 
 namespace symbol_coding {
 
@@ -38,6 +39,32 @@ private:
 	uint_fast8_t last { 0 };
 };
 
+class Unstuff {
+public:
+	uint_fast8_t is_stuffing_bit(const uint_fast8_t symbol) {
+		history = (history << 1) | (symbol & 1);
+		return (history & mask) == match;
+	}
+
+	void configure(
+		const uint32_t pattern,
+		const size_t length
+	) {
+		match = pattern;
+		mask = (1U << length) - 1;
+		reset();
+	}
+
+	void reset() {
+		history = 0;
+	}
+
+private:
+	uint32_t history { 0 };
+	uint32_t match { 0b111110 };
+	uint32_t mask  { 0b111111 };
+};
+
 } /* namespace symbol_coding */
 
 #endif/*__SYMBOL_CODING_H__*/