I'm writing a prop:equal+hash for a recursive data structure, and I noticed
that it takes several comparisons of pairs of values which are eq? before
equal? realises that there is a cycle, and stops.
Below is the code for the trivial case, where the implementation of equality
just calls itself on the same values.
#lang racket
(struct s ()
#:property prop:equal+hash
(list (λ (a b r) (display ".") (r a b))
(λ (a r) (display "!") (r a))
(λ (a r) (display "!") (r a))))
(equal? (s) (s)) ;; => prints 26 "."
(equal-hash-code (s)) ;; => prints 128 "!"
(equal-secondary-hash-code (s)) ;; => prints 128 "!"
Why doesn't it stop immediately after the first cycle?
Also, where do these magical numbers (26 and 128) come from? I grepped the C
source, but couldn't find anything relevant. The numbers 26 and 128 seem to be
independent of the equality function, so it does not sound like a change of
behaviour once the stack overflows, for example.
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