CLHS: Function SXHASH

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Function SXHASH

Syntax:

sxhash object => hash-code

Arguments and Values:

object---an object.

hash-code---a non-negative fixnum.

Description:

sxhash returns a hash code for object.

The manner in which the hash code is computed is implementation-dependent, but subject to certain constraints:

1. `(equal` x`` y`)` implies `(= (sxhash` x`) (sxhash` y`))`.

2. For any two objects, x and y, both of which are bit vectors, characters, conses, numbers, pathnames, strings, or symbols, and which are similar, `(sxhash` x`)` and `(sxhash` y`)` yield the same mathematical value even if x and y exist in different Lisp images of the same implementation. See Section 3.2.4 (Literal Objects in Compiled Files).

3. The hash-code for an object is always the same within a single session provided that the object is not visibly modified with regard to the equivalence test equal. See Section 18.1.2 (Modifying Hash Table Keys).

4. The hash-code is intended for hashing. This places no verifiable constraint on a conforming implementation, but the intent is that an implementation should make a good-faith effort to produce hash-codes that are well distributed within the range of non-negative fixnums.

5. Computation of the hash-code must terminate, even if the object contains circularities.

Examples:

     (= (sxhash (list 'list "ab")) (sxhash (list 'list "ab"))) =>  true
     (= (sxhash "a") (sxhash (make-string 1 :initial-element #\a))) =>  true
     (let ((r (make-random-state)))
       (= (sxhash r) (sxhash (make-random-state r))))
    =>  implementation-dependent

Side Effects: None.

Affected By:

The implementation.

Exceptional Situations: None.

See Also: None.

Notes:

Many common hashing needs are satisfied by make-hash-table and the related functions on hash tables. sxhash is intended for use where the pre-defined abstractions are insufficient. Its main intent is to allow the user a convenient means of implementing more complicated hashing paradigms than are provided through hash tables.

The hash codes returned by sxhash are not necessarily related to any hashing strategy used by any other function in Common Lisp.

For objects of types that equal compares with eq, item 3 requires that the hash-code be based on some immutable quality of the identity of the object. Another legitimate implementation technique would be to have sxhash assign (and cache) a random hash code for these objects, since there is no requirement that similar but non-eq objects have the same hash code.

Although similarity is defined for symbols in terms of both the symbol's name and the packages in which the symbol is accessible, item 3 disallows using package information to compute the hash code, since changes to the package status of a symbol are not visible to equal.

The following X3J13 cleanup issue, not part of the specification, applies to this section:

- SXHASH-DEFINITION:SIMILAR-FOR-SXHASH

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