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The procedures described in this section may be used to make very
efficient key-hashing procedures for arbitrary objects. All of these
procedures are based on *address hashing*, which uses the address of
an object as its hash number. The great advantage of address hashing is
that converting an arbitrary object to a hash number is extremely fast
and takes the same amount of time for any object.

The disadvantage of address hashing is that the garbage collector
changes the addresses of most objects. The hash-table implementation
compensates for this disadvantage by automatically rehashing tables
that use address hashing when garbage collections occur. Thus, in
order to use these procedures for key hashing, it is necessary to tell
the hash-table implementation (by means of the `rehash-after-gc?`
argument to the hash-table type constructors) that the hash numbers
computed by your key-hashing procedure must be recomputed after a
garbage collection.

- procedure:
**eq-hash***object*¶ - procedure:
**eqv-hash***object*¶ - procedure:
**equal-hash***object*¶ These procedures return a hash number for

`object`. The result is always a non-negative integer, and in the case of`eq-hash`

, a non-negative fixnum. Two objects that are equivalent according to`eq?`

,`eqv?`

, or`equal?`

, respectively, will produce the same hash number when passed as arguments to these procedures, provided that the garbage collector does not run during or between the two calls.

- procedure:
**hash-by-identity***key [modulus]*¶ This SRFI 69 procedure returns the same value as

`eq-hash`

, optionally limited by`modulus`.

- procedure:
**hash***key [modulus]*¶ This SRFI 69 procedure returns the same value as

`equal-hash`

, optionally limited by`modulus`.

- obsolete procedure:
**hash-by-eqv***key [modulus]*¶ This procedure returns the same value as

`eqv-hash`

, optionally limited by`modulus`.

- obsolete procedure:
**eq-hash-mod***object modulus*¶ This procedure is the key-hashing procedure used by

`make-strong-eq-hash-table`

.

- obsolete procedure:
**eqv-hash-mod***object modulus*¶ This procedure is the key-hashing procedure used by

`make-strong-eqv-hash-table`

.

- obsolete procedure:
**equal-hash-mod***object modulus*¶ This procedure is the key-hashing procedure used by

`make-equal-hash-table`

.

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