[R] hash code for arbitrary object

[Adrian Baddeley]

Can anyone suggest a simple way to calculate a 'hash code'
from an arbitrary R object?

hash(x) should return an integer or string 
with the property that 
     if hash(x) != hash(y) then x and y are not identical
and the time to compute hash(x) should be quite short.

Any suggestions welcome
thanks
Adrian Baddeley