Comment #16 on issue 1973 by [email protected]: hash(Integer) should
return the int
http://code.google.com/p/sympy/issues/detail?id=1973
The hash theory is really simple: if hash(a) != hash(b), then a != b.
If hash(a) == hash(b), then we need to compare a and b directly to
determine whether a == b or a != b.
As a valid (but useless) example of a hash function, consider hash(x) = 1
for all x.
From this it should be now obvious, that while we can choose any function
for hash(x), in order to be efficient, we need to choose such a function
that assigns a different value to different "x" for as many frequent "x"
(in SymPy) as possible. It is clear that some "x" will collide. One can
debate whether the choice of hash(-1) == hash(-2) is less effective than
the choice hash(-1) != hash(-2) given that -1 and -2 are quite frequent
numbers in SymPy. No doubt it is, but at the same time it is not a big deal.
So we just need to fix all hash() usage to use the above "theory".
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