Revert binary database block position encoding

This reverts commits a2003b0d55 and 54ffe2e5de.

These weren't correct.  Add a black magic warning instead.

src/database.cpp

@@ -20,6 +20,14 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "database.h"
 #include "irrlichttypes.h"
 
+
+/****************
+ * Black magic! *
+ ****************
+ * The position hashing is very messed up.
+ * It's a lot more complicated than it looks.
+ */
+
 static inline s16 unsigned_to_signed(u16 i, u16 max_positive)
 {
 	if (i < max_positive) {
@@ -30,19 +38,32 @@ static inline s16 unsigned_to_signed(u16 i, u16 max_positive)
 }
 
 
+// Modulo of a negative number does not work consistently in C
+static inline s64 pythonmodulo(s64 i, s16 mod)
+{
+	if (i >= 0) {
+		return i % mod;
+	}
+	return mod - ((-i) % mod);
+}
+
+
 s64 Database::getBlockAsInteger(const v3s16 pos) const
 {
-	return (((u64) pos.Z) << 24) +
-		(((u64) pos.Y) << 12) +
-		((u64) pos.X);
+	return (u64) pos.Z * 0x1000000 +
+		(u64) pos.Y * 0x1000 +
+		(u64) pos.X;
 }
 
-v3s16 Database::getIntegerAsBlock(const s64 i) const
+
+v3s16 Database::getIntegerAsBlock(s64 i) const
 {
-        v3s16 pos;
-        pos.Z = unsigned_to_signed((i >> 24) & 0xFFF, 0x1000 / 2);
-        pos.Y = unsigned_to_signed((i >> 12) & 0xFFF, 0x1000 / 2);
-        pos.X = unsigned_to_signed((i      ) & 0xFFF, 0x1000 / 2);
-        return pos;
+	v3s16 pos;
+	pos.X = unsigned_to_signed(pythonmodulo(i, 4096), 2048);
+	i = (i - pos.X) / 4096;
+	pos.Y = unsigned_to_signed(pythonmodulo(i, 4096), 2048);
+	i = (i - pos.Y) / 4096;
+	pos.Z = unsigned_to_signed(pythonmodulo(i, 4096), 2048);
+	return pos;
 }
 

src/database.h

@@ -35,7 +35,7 @@ public:
 	virtual void saveBlock(MapBlock *block)=0;
 	virtual MapBlock* loadBlock(v3s16 blockpos)=0;
 	s64 getBlockAsInteger(const v3s16 pos) const;
-	v3s16 getIntegerAsBlock(const s64 i) const;
+	v3s16 getIntegerAsBlock(s64 i) const;
 	virtual void listAllLoadableBlocks(std::list<v3s16> &dst)=0;
 	virtual int Initialized(void)=0;
 	virtual ~Database() {};