From 066675896deaa2dc654fc72aba7d749cd941454e Mon Sep 17 00:00:00 2001
From: "Sascha L. Teichmann" <sascha.teichmann@intevation.de>
Date: Mon, 18 Aug 2014 19:58:10 +0200
Subject: [PATCH] Dont rely on compiler smartness to replace / and % by
 shifting and masking. Make use of constants more consistence.

---
 common/coords.go | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/common/coords.go b/common/coords.go
index da10333..0ab4585 100644
--- a/common/coords.go
+++ b/common/coords.go
@@ -11,8 +11,8 @@ import (
 
 const (
 	numBitsPerComponent = 12
-	maxPositive         = 2048
-	modulo              = 4096
+	modulo              = 1 << numBitsPerComponent
+	maxPositive         = modulo / 2
 )
 
 type (
@@ -92,7 +92,9 @@ func InterleavedToXYZ(c int64) (x, y, z int16) {
 }
 
 func XYZToPlain(x, y, z int16) int64 {
-	return int64(z)<<24 + int64(y)<<12 + int64(x)
+	return int64(z)<<(2*numBitsPerComponent) +
+		int64(y)<<numBitsPerComponent +
+		int64(x)
 }
 
 func unsignedToSigned(i int16) int16 {
@@ -104,18 +106,19 @@ func unsignedToSigned(i int16) int16 {
 
 // To match C++ code.
 func pythonModulo(i int16) int16 {
+	const mask = modulo - 1
 	if i >= 0 {
-		return i % modulo
+		return i & mask
 	}
-	return modulo - (-i)%modulo
+	return modulo - -i&mask
 }
 
 // Only to match the C++ code.
 func PlainToXYZ(i int64) (x, y, z int16) {
 	x = unsignedToSigned(pythonModulo(int16(i)))
-	i = (i - int64(x)) / modulo
+	i = (i - int64(x)) >> numBitsPerComponent
 	y = unsignedToSigned(pythonModulo(int16(i)))
-	i = (i - int64(y)) / modulo
+	i = (i - int64(y)) >> numBitsPerComponent
 	z = unsignedToSigned(pythonModulo(int16(i)))
 	return
 }