Macro: SB-EXT:DEFINE-HASH-TABLE-TEST

Defines NAME as a new kind of hash table test for use with the :TEST argument to MAKE-HASH-TABLE, and associates a default HASH-FUNCTION with it.

NAME must be a symbol naming a global two argument equivalence predicate. Afterwards both ‘NAME and #’NAME can be used with :TEST argument. In both cases HASH-TABLE-TEST will return the symbol NAME.

HASH-FUNCTION must be a symbol naming a global hash function consistent with the predicate, or be a LAMBDA form implementing one in the current lexical environment. The hash function must compute the same hash code for any two objects for which NAME returns true, and subsequent calls with already hashed objects must always return the same hash code.

Note: The :HASH-FUNCTION keyword argument to MAKE-HASH-TABLE can be used to override the specified default hash-function.

Attempting to define NAME in a locked package as hash-table test causes a package lock violation.

Examples:

;;; 1.

;; We want to use objects of type FOO as keys (by their ;; names.) EQUALP would work, but would make the names ;; case-insensitive – which we don’t want. (defstruct foo (name nil :type (or null string)))

;; Define an equivalence test function and a hash function. (defun foo-name= (f1 f2) (equal (foo-name f1) (foo-name f2))) (defun sxhash-foo-name (f) (sxhash (foo-name f)))

(define-hash-table-test foo-name= sxhash-foo-name)

;; #’foo-name would work too. (defun make-foo-table () (make-hash-table :test ‘foo-name=))

;;; 2.

(defun == (x y) (= x y))

(define-hash-table-test == (lambda (x) ;; Hash codes must be consistent with test, so ;; not (SXHASH X), since ;; (= 1 1.0) => T ;; (= (SXHASH 1) (SXHASH 1.0)) => NIL ;; Note: this doesn’t deal with complex numbers or ;; bignums too large to represent as double floats. (sxhash (coerce x ‘double-float))))

;; #’== would work too (defun make-number-table () (make-hash-table :test ‘==))

Lambda list

(sb-impl::name sb-impl::hash-function)