From 7cded79b59a1c04c327e11d17e488e475a7ae09b Mon Sep 17 00:00:00 2001
From: Jared Boone <jboone@earfeast.com>
Date: Tue, 8 Dec 2015 10:35:54 -0800
Subject: [PATCH] Clean up CRC class/interface, make more like
 boost::crc_basic.

---
 firmware/application/app_ais.cpp | 19 +++----
 firmware/application/app_ert.cpp | 15 ++---
 firmware/common/crc.hpp          | 95 ++++++++++++++++----------------
 3 files changed, 62 insertions(+), 67 deletions(-)

diff --git a/firmware/application/app_ais.cpp b/firmware/application/app_ais.cpp
index 3633b6512..a7711818e 100644
--- a/firmware/application/app_ais.cpp
+++ b/firmware/application/app_ais.cpp
@@ -132,23 +132,18 @@ struct PacketTooLong {
 
 struct CRCCheck {
 	bool operator()(const ::Packet& packet) {
+		const size_t fcs_length = 16;
 		const size_t data_and_fcs_length = packet.size() - 7;
-		const size_t data_length = data_and_fcs_length - 16;
+		const size_t data_length = data_and_fcs_length - fcs_length;
 
 		CRCFieldReader field_crc { packet };
-		CRC<uint16_t> ais_fcs { 0x1021 };
+		CRC<uint16_t> ais_fcs { 0x1021, 0xffff, 0xffff };
 		
-		uint16_t crc_calculated = 0xffff;
-		
-		for(size_t i=0; i<data_length + 16; i+=8) {
-			if( i == data_length ) {
-				crc_calculated ^= 0xffff;
-			}
-			const uint8_t byte = field_crc.read(i, 8);
-			crc_calculated = ais_fcs.calculate_byte(crc_calculated, byte);
+		for(size_t i=0; i<data_length; i+=8) {
+			ais_fcs.process_byte(field_crc.read(i, 8));
 		}
-
-		return crc_calculated == 0x0000;
+		
+		return ais_fcs.checksum() == field_crc.read(data_length, fcs_length);
 	}
 };
 
diff --git a/firmware/application/app_ert.cpp b/firmware/application/app_ert.cpp
index f1d4aa050..970e8588f 100644
--- a/firmware/application/app_ert.cpp
+++ b/firmware/application/app_ert.cpp
@@ -87,22 +87,19 @@ bool Packet::crc_ok() const {
 bool Packet::crc_ok_scm() const {
 	CRC<uint16_t> ert_bch { 0x6f63 };
 	size_t start_bit = 5;
-	auto crc_calculated = ert_bch.calculate_byte(0x0000, reader_.read(0, start_bit));
+	ert_bch.process_byte(reader_.read(0, start_bit));
 	for(size_t i=start_bit; i<length(); i+=8) {
-		const uint8_t byte = reader_.read(i, 8);
-		crc_calculated = ert_bch.calculate_byte(crc_calculated, byte);
+		ert_bch.process_byte(reader_.read(i, 8));
 	}
-	return crc_calculated == 0x0000;
+	return ert_bch.checksum() == 0x0000;
 }
 
 bool Packet::crc_ok_idm() const {
-	CRC<uint16_t> ert_crc_ccitt { 0x1021 };
-	uint16_t crc_calculated = 0xffff;
+	CRC<uint16_t> ert_crc_ccitt { 0x1021, 0xffff, 0x1d0f };
 	for(size_t i=0; i<length(); i+=8) {
-		const uint8_t byte = reader_.read(i, 8);
-		crc_calculated = ert_crc_ccitt.calculate_byte(crc_calculated, byte);
+		ert_crc_ccitt.process_byte(reader_.read(i, 8));
 	}
-	return crc_calculated == 0x1d0f;
+	return ert_crc_ccitt.checksum() == 0x0000;
 }
 
 } /* namespace ert */
diff --git a/firmware/common/crc.hpp b/firmware/common/crc.hpp
index ded1934a7..ab10455dc 100644
--- a/firmware/common/crc.hpp
+++ b/firmware/common/crc.hpp
@@ -25,61 +25,74 @@
 #include <cstddef>
 #include <cstdint>
 
+/* Inspired by
+ * http://www.barrgroup.com/Embedded-Systems/How-To/CRC-Calculation-C-Code
+ *
+ * ...then munged into a shape resembling boost::crc_basic.
+ * http://www.boost.org/doc/libs/release/libs/crc/
+ */
+
 template<typename T>
 class CRC {
 public:
 	constexpr CRC(
-		const T polynomial/*,
-		const T initial*/
-	) : polynomial { polynomial }/*,
-		initial { initial }*/
+		const T truncated_polynomial,
+		const T initial_remainder = 0,
+		const T final_xor_value = 0
+	) : truncated_polynomial { truncated_polynomial },
+		initial_remainder { initial_remainder },
+		final_xor_value { final_xor_value },
+		remainder { initial_remainder }
 	{
-		// CRC LSB must always be 1
 	}
-/*
-	template<typename U>
-	T calculate(const U& bits, const size_t length) {
-		if( length > bits.size() ) {
-			// Exception.
-			return 0;
-		}
 
-		T crc = 0;
-
-		for(size_t i=0; i<length; i++) {
-			crc = feed_bit(crc, bits[i]);
-		}
-
-		return crc;
+	T get_initial_remainder() const {
+		return initial_remainder;
 	}
-*/
-	T calculate_byte(const T crc_in, const uint8_t data) {
-		/* Inspired by
-		 * http://www.barrgroup.com/Embedded-Systems/How-To/CRC-Calculation-C-Code
-		 */
-		T remainder = crc_in;
+
+	void reset(T new_initial_remainder) {
+		remainder = new_initial_remainder;
+	}
+
+	void reset() {
+		remainder = initial_remainder;
+	}
+
+	void process_bit(bool bit) {
+		remainder ^= bit << (width() - 1);
+		if( remainder & top_bit() ) {
+			remainder = (remainder << 1) ^ truncated_polynomial;
+		} else {
+			remainder = (remainder << 1);
+		}
+	}
+
+	void process_byte(const uint8_t data) {
 		remainder ^= data << (width() - 8);
 		for(size_t bit=0; bit<8; bit++) {
 			if( remainder & top_bit() ) {
-				remainder = (remainder << 1) ^ polynomial;
+				remainder = (remainder << 1) ^ truncated_polynomial;
 			} else {
 				remainder = (remainder << 1);
 			}
 		}
-		return remainder;
 	}
-/*
-	T calculate(const uint8_t* const data, const size_t length) {
-		T remainder = initial;
-		for(size_t byte=0; byte<length; ++byte) {
-			remainder = calculate_byte(remainder, data[byte]);
+
+	void process_bytes(const uint8_t* const data, const size_t length) {
+		for(size_t i=0; i<length; i++) {
+			process_byte(data[i]);
 		}
-		return remainder;
 	}
-*/
+
+	T checksum() const {
+		return remainder ^ final_xor_value;
+	}
+
 private:
-	const T polynomial;
-//	const T initial;
+	const T truncated_polynomial;
+	const T initial_remainder;
+	const T final_xor_value;
+	T remainder;
 
 	static constexpr size_t width() {
 		return 8 * sizeof(T);
@@ -88,16 +101,6 @@ private:
 	static constexpr T top_bit() {
 		return 1U << (width() - 1);
 	}
-/*
-	T feed_bit(const T crc_in, const uint_fast8_t bit) {
-		T crc_out = crc_in ^ (bit & 1);
-		if( crc_in & top_bit() ) {
-			return ((crc_in << 1) | (bit & 1)) ^ polynomial;
-		} else {
-			return (crc_in << 1) | (bit & 1);
-		}
-	}
-*/
 };