Odd behaviour regarding BlockReplacedState, BlockReplacedState.getBlock() and slabs

For my current plugin I need to know when a slab turns into a double slab which has caused me to fiddle around with BlockPlaceEvent.getBlockReplacedState() and BlockPlaceEvent.getBlockReplacedState().getBlock().

While printing out some debug info to see how this event is processed I noticed an odd behaviour when I placed a different type of slab over an existing one.

Allow me to explain:

When I place a single stone block I get the following output:

Code:

[Action]: [MaterialID].[Metadata] at [Coordinates]
Placed: 4.0 at X:-40 Y:66 Z:117

ReplacedState: 0.0 at X:-40 Y:66 Z:117
ReplacedStateBlock: 4.0 at X:-40 Y:66 Z:117

I can see that I placed a stone (ID 4) and replaced air (ID 0). The getBlockReplacedState().getBlock() is stone again, I take it this is the new block at the BlockReplaceState location.


When I place two stone slabs overeach other I get the following output:

Code:

(First slab)
Placed: 44.0 at X:-40 Y:66 Z:115
ReplacedState: 0.0 at X:-40 Y:66 Z:115
ReplacedStateBlock: 44.0 at X:-40 Y:66 Z:115
(Second slab)
Placed: 0.0 at X:-40 Y:67 Z:115
ReplacedState: 44.0 at X:-40 Y:66 Z:115
ReplacedStateBlock: 43.0 at X:-40 Y:66 Z:115

The first part is obvious: a stone slab replaces air. In the second part I see myself placing air at the location above the first slab (which would explain why you can't turn a single slab into a double slab when something is above it - you can't place the air). I also notice that I'm replacing a single slab (ID 44) one field below and that the getBlockReplacedState().getBlock() is now a double slab (ID 43) at the location where the single slab was replaced.


However it becomes a bit odd when I place a different type of slab above the first one:

Code:

(first slab: stone)
Placed: 44.0 at X:-40 Y:66 Z:113
ReplacedState: 0.0 at X:-40 Y:66 Z:113
ReplacedStateBlock: 44.0 at X:-40 Y:66 Z:113
(second slab: cobblestone)
Placed: 44.3 at X:-40 Y:67 Z:113
ReplacedState: 44.0 at X:-40 Y:66 Z:113
ReplacedStateBlock: 44.0 at X:-40 Y:66 Z:113

According to the output I'm now placing a cobblestone slab above the first (which is what actually happens), yet I don't replace the air in the new location but the single stone slab below, that isn't actally modified. If I place a normal stone above the slab the behaviour is as expected:

Code:

(stone slab)
Placed: 44.0 at X:-40 Y:66 Z:111
ReplacedState: 0.0 at X:-40 Y:66 Z:111
ReplacedStateBlock: 44.0 at X:-40 Y:66 Z:111
(stone placed above)
Placed: 4.0 at X:-40 Y:67 Z:111
ReplacedState: 0.0 at X:-40 Y:67 Z:111
ReplacedStateBlock: 4.0 at X:-40 Y:67 Z:111

While this isn't exactly a problem for my plugin it still makes me wonder:

Is this behaviour "normal" for minecraft or have I actually found a bug in bukkit?

Edit: I forgot to mention: I'm running craftbukkit 516
Edit2: Just tested it with craftbukkit 556 and bukkit 461 - same behaviour

 

I believe this is due to the way CraftBukkit is selecting what block to return as the replacedBlock. If you look in net.minecraft.server.ItemBlock.java you will see this at line 72 - see how it does not discriminate based on the block trying to be placed? This is probably a hangover from when there was only one type of halfblock

Code:

if (typeId == Block.SNOW.id || (typeId == Block.STEP.id && itemstack.id == Block.STEP.id && faceClicked == BlockFace.UP))
                replacedBlock = blockClicked;

What they should do here is add a check to make sure the itemstack data is the same as the blockClicked data, so that it doesn't cause the odd behaviour you are describing.

EDIT: maybe like this - I think itemstack.damage stores the metadata for blocks? I think I read that somewhere a while ago, if so the below code should fix the problem.

Code:

if (typeId == Block.SNOW.id || (typeId == Block.STEP.id && itemstack.id == Block.STEP.id && faceClicked == BlockFace.UP && blockClicked.getData() == itemstack.damage))
    replacedBlock = blockClicked;

 

I see. I guess this would count as a "bug" of sorts then...
Well it's not a big deal, for now I will just assume that no person in the right mind will walk around placing air.

 

I will log it on leaky if you like, to get it addressed, and link back to this thread.

http://leaky.bukkit.org/issues/580

 

Your help is much appreciated.
Shame on me - I didn't know where to report bugs

 

That's ok, that was my first one!