From a147d56ce692c02eea71ecd28945abc8b575f329 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 31 Dec 2016 14:15:20 -0500
Subject: [PATCH] Switched to a table-based implementation of CRC generation,
 adding construction cost to cheapen running cost.

---
 NumberTheory/CRC.hpp | 27 +++++++++++++++++----------
 1 file changed, 17 insertions(+), 10 deletions(-)

diff --git a/NumberTheory/CRC.hpp b/NumberTheory/CRC.hpp
index 65bb0477e..e70fc91f1 100644
--- a/NumberTheory/CRC.hpp
+++ b/NumberTheory/CRC.hpp
@@ -16,23 +16,30 @@ namespace NumberTheory {
 class CRC16 {
 	public:
 		CRC16(uint16_t polynomial, uint16_t reset_value) :
-			reset_value_(reset_value), value_(reset_value), polynomial_(polynomial) {}
+			reset_value_(reset_value), value_(reset_value)
+		{
+			for(int c = 0; c < 256; c++)
+			{
+				uint16_t shift_value = (uint16_t)(c << 8);
+				for(int b = 0; b < 8; b++)
+				{
+					uint16_t exclusive_or = (shift_value&0x8000) ? polynomial : 0x0000;
+					shift_value = (uint16_t)(shift_value << 1) ^ exclusive_or;
+				}
+				xor_table[c] = (uint16_t)shift_value;
+			}
+		}
 
 		inline void reset() { value_ = reset_value_; }
-		inline void add(uint8_t value) {
-			// TODO: go table based
-			value_ ^= (uint16_t)value << 8;
-			for(int c = 0; c < 8; c++)
-			{
-				uint16_t exclusive_or = (value_&0x8000) ? polynomial_ : 0x0000;
-				value_ = (uint16_t)(value_ << 1) ^ exclusive_or;
-			}
+		inline void add(uint8_t byte) {
+			value_ = (uint16_t)((value_ << 8) ^ xor_table[(value_ >> 8) ^ byte]);
 		}
 		inline uint16_t get_value() const {	return value_; }
 		inline void set_value(uint16_t value) { value_ = value; }
 
 	private:
-		const uint16_t reset_value_, polynomial_;
+		const uint16_t reset_value_;
+		uint16_t xor_table[256];
 		uint16_t value_;
 };