From 90a7327cd5c49434d25eeb758191e26a177b863a Mon Sep 17 00:00:00 2001
From: Jared Boone <jboone@earfeast.com>
Date: Tue, 8 Dec 2015 14:15:15 -0800
Subject: [PATCH] Clean up FieldReader and BitRemap types.

---
 firmware/application/app_ais.cpp |  8 +-------
 firmware/application/app_ais.hpp | 12 +++---------
 firmware/application/app_ert.hpp |  8 +-------
 firmware/common/field_reader.hpp | 15 +++++++++++++++
 4 files changed, 20 insertions(+), 23 deletions(-)

diff --git a/firmware/application/app_ais.cpp b/firmware/application/app_ais.cpp
index 6f2b3a30a..5c387e5bd 100644
--- a/firmware/application/app_ais.cpp
+++ b/firmware/application/app_ais.cpp
@@ -33,13 +33,7 @@ using namespace portapack;
 namespace baseband {
 namespace ais {
 
-struct CRCBitRemap {
-	constexpr size_t operator()(const size_t& bit_index) const {
-		return bit_index;
-	}
-};
-
-using CRCFieldReader = ::FieldReader<::Packet, CRCBitRemap>;
+using CRCFieldReader = ::FieldReader<::Packet, BitRemapNone>;
 
 struct PacketLengthRange {
 	constexpr PacketLengthRange(
diff --git a/firmware/application/app_ais.hpp b/firmware/application/app_ais.hpp
index 160aec45c..8cba0dacb 100644
--- a/firmware/application/app_ais.hpp
+++ b/firmware/application/app_ais.hpp
@@ -43,14 +43,6 @@ using namespace lpc43xx;
 namespace baseband {
 namespace ais {
 
-struct BitRemap {
-	constexpr size_t operator()(const size_t bit_index) const {
-		return bit_index ^ 7;
-	}
-};
-
-using FieldReader = ::FieldReader<::Packet, BitRemap>;
-
 struct DateTime {
 	uint16_t year;
 	uint8_t month;
@@ -98,9 +90,11 @@ public:
 	bool crc_ok() const;
 
 private:
+	using Reader = FieldReader<::Packet, BitRemapByteReverse>;
+	
 	const ::Packet packet_;
 	const rtc::RTC received_at_;
-	const FieldReader field_;
+	const Reader field_;
 
 	const size_t fcs_length = 16;
 
diff --git a/firmware/application/app_ert.hpp b/firmware/application/app_ert.hpp
index 0053dbce7..bb78254fe 100644
--- a/firmware/application/app_ert.hpp
+++ b/firmware/application/app_ert.hpp
@@ -38,12 +38,6 @@ using namespace lpc43xx;
 
 namespace ert {
 
-struct BitRemap {
-	constexpr size_t operator()(const size_t bit_index) const {
-		return bit_index;
-	}
-};
-
 using ID = uint32_t;
 using Consumption = uint32_t;
 
@@ -76,7 +70,7 @@ public:
 	bool crc_ok() const;
 
 private:
-	using Reader = FieldReader<ManchesterDecoder, BitRemap>;
+	using Reader = FieldReader<ManchesterDecoder, BitRemapNone>;
 
 	const ::Packet packet_;
 	const rtc::RTC received_at_;
diff --git a/firmware/common/field_reader.hpp b/firmware/common/field_reader.hpp
index 4a1e05d6f..03d0a24d0 100644
--- a/firmware/common/field_reader.hpp
+++ b/firmware/common/field_reader.hpp
@@ -25,6 +25,18 @@
 #include <cstdint>
 #include <cstddef>
 
+struct BitRemapNone {
+	constexpr size_t operator()(const size_t& bit_index) const {
+		return bit_index;
+	}
+};
+
+struct BitRemapByteReverse {
+	constexpr size_t operator()(const size_t bit_index) const {
+		return bit_index ^ 7;
+	}
+};
+
 template<typename T, typename BitRemap>
 class FieldReader {
 public:
@@ -34,6 +46,9 @@ public:
 	{
 	}
 
+	/* The "start_bit" winds up being the MSB of the returned field value. */
+	/* The BitRemap functor determines which bits are read from the source
+	 * packet. */
 	uint32_t read(const size_t start_bit, const size_t length) const {
 		uint32_t value = 0;
 		for(size_t i=start_bit; i<(start_bit + length); i++) {