From def14493cb660b9503f6493f88f9935d4119f9ba Mon Sep 17 00:00:00 2001
From: Adrian Conlon <Adrian.conlon@gmail.com>
Date: Sat, 2 Dec 2017 19:01:07 +0000
Subject: [PATCH] Allow fallback for where intrinsics aren't available.

Signed-off-by: Adrian Conlon <Adrian.conlon@gmail.com>
---
 inc/EightBitCompilerDefinitions.h | 36 +++++++++++++++++++++++++++++--
 1 file changed, 34 insertions(+), 2 deletions(-)

diff --git a/inc/EightBitCompilerDefinitions.h b/inc/EightBitCompilerDefinitions.h
index 2cbe43d..965743f 100644
--- a/inc/EightBitCompilerDefinitions.h
+++ b/inc/EightBitCompilerDefinitions.h
@@ -1,5 +1,30 @@
 #pragma once
 
+namespace EightBit {
+	int countBits(uint8_t value);
+	bool oddParity(uint8_t value);
+}
+
+/*
+Published in 1988, the C Programming Language 2nd Ed.
+(by Brian W.Kernighan and Dennis M.Ritchie) mentions
+this in exercise 2 - 9. On April 19, 2006 Don Knuth pointed
+out to me that this method "was first published by Peter
+Wegner in CACM 3 (1960), 322.
+(Also discovered independently by Derrick Lehmer and published
+in 1964 in a book edited by Beckenbach.)"
+*/
+inline int EightBit::countBits(uint8_t value) {
+	int count; // c accumulates the total bits set in value
+	for (count = 0; value; ++count)
+		value &= value - 1; // clear the least significant bit set
+	return count;
+}
+
+inline bool EightBit::oddParity(uint8_t value) {
+	return countBits(value) % 2;
+}
+
 #ifdef _MSC_VER
 
 #	include <intrin.h>
@@ -7,7 +32,7 @@
 #	define LIKELY(x)	(x)
 #	define UNLIKELY(x)	(x)
 
-#	define EIGHTBIT_PARITY(x)	(__popcnt(value) % 2)
+#	define EIGHTBIT_PARITY(x)	(__popcnt(x) % 2)
 
 #	define UNREACHABLE __assume(0)
 
@@ -23,5 +48,12 @@
 #	define UNREACHABLE __builtin_unreachable();
 
 #else
-#	error Unknown compiler
+
+#	define LIKELY(x)	(x)
+#	define UNLIKELY(x)	(x)
+
+#	define EIGHTBIT_PARITY(x)	EightBit::oddParity(x)
+
+#	define UNREACHABLE
+
 #endif