Shunt CRC XOR table generation to compile time.

Numeric/CRC.hpp

@@ -8,6 +8,9 @@
 
 #pragma once
 
+#include "Carry.hpp"
+
+#include <array>
 #include <cstdint>
 #include <vector>
 
@@ -26,30 +29,41 @@ constexpr uint8_t reverse_byte(uint8_t byte) {
 }
 
 /*! Provides a class capable of generating a CRC from source data. */
-template <typename IntType, IntType reset_value, IntType output_xor, bool reflect_input, bool reflect_output>
+template <
+	typename IntType,
+	IntType polynomial,
+	IntType reset_value,
+	IntType output_xor,
+	bool reflect_input,
+	bool reflect_output
+>
 class Generator {
 public:
 	/*!
 		Instantiates a CRC16 that will compute the CRC16 specified by the supplied
 		@c polynomial and @c reset_value.
 	*/
-	constexpr Generator(IntType polynomial) noexcept: value_(reset_value) {
-		const IntType top_bit = IntType(~(IntType(~0) >> 1));
-		for(int c = 0; c < 256; c++) {
-			IntType shift_value = IntType(c << multibyte_shift);
-			for(int b = 0; b < 8; b++) {
-				IntType exclusive_or = (shift_value&top_bit) ? polynomial : 0;
-				shift_value = IntType(shift_value << 1) ^ exclusive_or;
-			}
-			xor_table[c] = shift_value;
-		}
-	}
+	constexpr Generator() noexcept: value_(reset_value) {}
 
 	/// Resets the CRC to the reset value.
 	void reset() { value_ = reset_value; }
 
 	/// Updates the CRC to include @c byte.
 	void add(uint8_t byte) {
+		static constexpr std::array<IntType, 256> xor_table = [] {
+			std::array<IntType, 256> table{};
+			constexpr IntType top_bit = Numeric::top_bit<IntType>();
+			for(size_t c = 0; c < 256; c++) {
+				IntType shift_value = IntType(c << multibyte_shift);
+				for(int b = 0; b < 8; b++) {
+					const IntType exclusive_or = (shift_value & top_bit) ? polynomial : 0;
+					shift_value = IntType(shift_value << 1) ^ exclusive_or;
+				}
+				table[c] = shift_value;
+			}
+			return table;
+		} ();
+
 		if constexpr (reflect_input) byte = reverse_byte(byte);
 		value_ = IntType((value_ << 8) ^ xor_table[(value_ >> multibyte_shift) ^ byte]);
 	}
@@ -100,7 +114,6 @@ public:
 
 private:
 	static constexpr int multibyte_shift = (sizeof(IntType) * 8) - 8;
-	IntType xor_table[256];
 	IntType value_;
 };
 
@@ -108,15 +121,11 @@ private:
 	Provides a generator of 16-bit CCITT CRCs, which amongst other uses are
 	those used by the FM and MFM disk encodings.
 */
-struct CCITT: public Generator<uint16_t, 0xffff, 0x0000, false, false> {
-	CCITT(): Generator(0x1021) {}
-};
+using CCITT = Generator<uint16_t, 0x1021, 0xffff, 0x0000, false, false>;
 
 /*!
 	Provides a generator of "standard 32-bit" CRCs.
 */
-struct CRC32: public Generator<uint32_t, 0xffffffff, 0xffffffff, true, true> {
-	CRC32(): Generator(0x04c11db7) {}
-};
+using CRC32 = Generator<uint32_t, 0x04c11db7, 0xffffffff, 0xffffffff, true, true>;
 
 }

Storage/Tape/Parsers/Acorn.cpp

@@ -14,7 +14,7 @@ namespace {
 constexpr int PLLClockRate = 1920000;
 }
 
-Parser::Parser(): crc_(0x1021) {
+Parser::Parser() {
 	shifter_.set_delegate(this);
 }
 

Storage/Tape/Parsers/Acorn.hpp

@@ -58,7 +58,7 @@ private:
 	void process_pulse(const Storage::Tape::Pulse &) override;
 
 	bool did_update_shifter(int new_value, int length);
-	CRC::Generator<uint16_t, 0x0000, 0x0000, false, false> crc_;
+	CRC::Generator<uint16_t, 0x1021, 0x0000, 0x0000, false, false> crc_;
 	Shifter shifter_;
 };