Testing if position collides with block

Hello!
I am developing a Class that simplifies RayCasting. Everything works fine but you can't cast a ray through stair pieces. That's because I am only getting the bounding box of the block:

Get AxisAlignedBB (open) Get AxisAlignedBB (close)

Code:

 // BlockPosition is the NMS Class similar to org.bukkit.block.Block
private AxisAlignedBB getBBox(BlockPosition pos) {
    IBlockData data = world.getType(pos);
    Block block = data.getBlock();
    return new AxisAlignedBB((double)pos.getX()+block.B(), (double)pos.getY()+block.D(), (double)block.F(),
            (double)pos.getX()+block.C(), (double)pos.getY()+block.E(), (double)pos.getZ()+block.G());
}

The BoundingBox of a block is visible when looking at a block (the black wireframe surrounding the block).
But the BoundingBox it not the collision box. In most cases it is, of course, but stairs have a more complex collision box. It is made out of multiple boxes.

So my problem is:
How do I get the full collision box / boxes of a block?

 

I'm currently decompiling bukkit/spigot 1.8.8 and I found this code
in Entity.class (net.minecraft.server):

net.minecraft.server.v1_8_R3.Entity#checkBlockCollisions (open) net.minecraft.server.v1_8_R3.Entity#checkBlockCollisions (close)

Code:

protected void checkBlockCollisions() {
    final BlockPosition blockposition = new BlockPosition(this.getBoundingBox().a + 0.001, this.getBoundingBox().b + 0.001, this.getBoundingBox().c + 0.001);
    final BlockPosition blockposition2 = new BlockPosition(this.getBoundingBox().d - 0.001, this.getBoundingBox().e - 0.001, this.getBoundingBox().f - 0.001);
    if (this.world.areChunksLoadedBetween(blockposition, blockposition2)) {
        for (int i = blockposition.getX(); i <= blockposition2.getX(); ++i) {
            for (int j = blockposition.getY(); j <= blockposition2.getY(); ++j) {
                for (int k = blockposition.getZ(); k <= blockposition2.getZ(); ++k) {
                    final BlockPosition blockposition3 = new BlockPosition(i, j, k);
                    final IBlockData iblockdata = this.world.getType(blockposition3);
                    try {
                        iblockdata.getBlock().a(this.world, blockposition3, iblockdata, this);
                    }
                    catch (Throwable throwable) {
                        final CrashReport crashreport = CrashReport.a(throwable, "Colliding entity with block");
                        final CrashReportSystemDetails crashreportsystemdetails = crashreport.a("Block being collided with");
                        CrashReportSystemDetails.a(crashreportsystemdetails, blockposition3, iblockdata);
                        throw new ReportedException(crashreport);
                    }
                }
            }
        }
    }
}

So I need to find the method Block#a-World,BlockPosition,IBlockData,Entity-:

Code:

a(World world, BlockPosition blockposition, IBlockData iblockdata, Entity entity)

I could not find it and the code is hard to understand. Please help.
Here is the formatted Block.java class: net.minecraft.server.v1_8_R3.Block