The hash function has a problem with negative cell index

[lyd405121]

Background

• I am wrtiing a fluid simulation using "Position based dynamic"
• To speed up neighbourhood search, I choose hash grid in tutorial 11
• Here is the result, the fluid explode before it hits the boundary

The problem

• So I dive into my code, and finally found what is happening
• It is the hash function to be blamed
• Here my test code in python

nTableSize = 2000 nX = 1 nY = 1 nZ = 1 h1 = abs((nX 92837111) ^ (nY 689287499) ^ (nZ 283923481)) % nTableSize h2 = (73856093 nX + 19349663 nY + 83492791 nZ) % nTableSize print("----------hash-function for (x=1 y=1 z=1)-----------") print("ten minutes physics:", h1) print("interactive-graphics:",h2) print("")

nX = 1 nY = 1 nZ = -1 h1 = abs((nX 92837111) ^ (nY 689287499) ^ (nZ 283923481)) % nTableSize h2 = (73856093 nX + 19349663 nY + 83492791 nZ) % nTableSize print("----------hash-function for (x=1 y=1 z=-1)-----------") print("ten minutes physics:", h1) print("interactive-graphics:",h2)

- Here is the print result 
```python
----------hash-function for (x=1 y=1 z=1)-----------
ten minutes physics: 1621
interactive-graphics: 547

----------hash-function for (x=1 y=1 z=-1)-----------
ten minutes physics: 1621
interactive-graphics: 965

• So if the cell index is negative, it will get the same hash code as same as it is positive

Solution

• I change another hash funciton which is created by interactive-graphics,who I believe are matthias's old friends
• Things go right just as my python code shows above, and the result for my simulation now fixed:

At last

• In the tutorial， the simulaiton is all above the ground plane, so the z nubmer is always positive
• Is there anything I am not doing right ? I hope someone will help me find out or this solution is also helpful to you