It is quite commonly known that data-value 6 pistons can cause update suppression or that data-value 6 hoppers and droppers crash the game. Although I will not go into the code of any specific block, this document should explain why these exceptions happen.
The facing directions are typically represented in 3 bits as follows:
- |
+ |
|
|---|---|---|
Y |
0b00_0 or 0 |
0b00_1 or 1 |
Z |
0b01_0 or 2 |
0b01_1 or 3 |
X |
0b10_0 or 4 |
0b10_1 or 5 |
This value is, in most cases, going to be located in the 3 least significant bits of a block's metadata and will be extracted by a bitwise AND. Simply put, the game will always take the first 3 bits of the metadata and this is no matter if the value is greather than 5.
While orientation 7 does not correspond to any real direction, it is an intended facing values. Blocks using these facing values will be placed by the player with orientation 7 and initialize them based on the players position right after the placement.
There is a class that basically holds a collection of utility look-up tables.
0 (Y-) |
1 (Y+) |
2 (Z-) |
3 (Z+) |
4 (X-) |
5 (X+) |
|
|---|---|---|---|---|---|---|
Facing.offsetsYForSide |
-1 |
+1 |
0 |
0 |
0 |
0 |
Facing.offsetsZForSide |
0 |
0 |
-1 |
+1 |
0 |
0 |
Facing.offsetsXForSide |
0 |
0 |
0 |
0 |
-1 |
+1 |
0 (Y-) |
1 (Y+) |
2 (Z-) |
3 (Z+) |
4 (X-) |
5 (X+) |
|
|---|---|---|---|---|---|---|
Facing.oppositeSide |
1 |
0 |
3 |
2 |
5 |
4 |
0 (Y-) |
1 (Y+) |
2 (Z-) |
3 (Z+) |
4 (X-) |
5 (X+) |
|
|---|---|---|---|---|---|---|
Facing.facings |
"DOWN" |
"UP" |
"NORTH" |
"SOUTH" |
"WEST" |
"EAST" |
Note that the Facing.facings look-up table is never used anywhere in the game's code.
There is no entry for the orientations 6 and 7. This makes one wonder: What would happen if the game were to try and look-up an offset for one of those orientations? What if we made, let's say, a piston with metadata 6? (Pistons with orientation 7, for example, will just refuse to update.)
In a situation where a program attempts to access an index of an array that simply is not there, Java will throw a ArrayIndexOutOfBoundsException. This can cause a game crash or update suppression in case the exception is caught.
TODO