>From cf285946bee56912286f75e4d1215214bc7c5b4b Mon Sep 17 00:00:00 2001 From: Paul Eggert Date: Sun, 21 Jul 2019 11:20:07 -0700 Subject: [PATCH 2/3] Improve doc for hash tables * doc/lispref/hash.texi (Creating Hash, Defining Hash): * src/fns.c (Fsxhash_eq, Fsxhash_eql, Fsxhash_equal): Say that hashes are fixnums. (Fmake_hash_table): Say that that an integer rehash-size should be a fixnum. * doc/lispref/hash.texi (Defining Hash): Say that hash and comparison functions should be consistent and pure, and should return quickly. --- doc/lispref/hash.texi | 27 ++++++++++++++++----------- src/fns.c | 8 ++++---- 2 files changed, 20 insertions(+), 15 deletions(-) diff --git a/doc/lispref/hash.texi b/doc/lispref/hash.texi index 9b900e6309..051531491c 100644 --- a/doc/lispref/hash.texi +++ b/doc/lispref/hash.texi @@ -132,7 +132,7 @@ Creating Hash it grows automatically. This value specifies how to make the hash table larger, at that time. -If @var{rehash-size} is an integer, it should be positive, and the hash +If @var{rehash-size} is a fixnum, it should be positive and the hash table grows by adding approximately that much to the nominal size. If @var{rehash-size} is floating point, it had better be greater than 1, and the hash table grows by multiplying the old size by @@ -239,14 +239,19 @@ Defining Hash You can think of a hash table conceptually as a large array of many slots, each capable of holding one association. To look up a key, -@code{gethash} first computes an integer, the hash code, from the key. -It reduces this integer modulo the length of the array, to produce an +@code{gethash} first computes a fixnum, the hash code, from the key. +It reduces this fixnum modulo the length of the array, to produce an index in the array. Then it looks in that slot, and if necessary in other nearby slots, to see if it has found the key being sought. Thus, to define a new method of key lookup, you need to specify both a function to compute the hash code from a key, and a function to compare -two keys directly. +two keys directly. The two functions should be consistent with each +other: that is, two keys' hash codes should be the same if the keys +compare as equal. Also, since the two functions can be called at any +time (such as by the garbage collector), the functions should be free +of side effects and should return quickly, and their behavior should +depend on only on properties of the keys that do not change. @defun define-hash-table-test name test-fn hash-fn This function defines a new hash table test, named @var{name}. @@ -260,9 +265,9 @@ Defining Hash return non-@code{nil} if they are considered the same. The function @var{hash-fn} should accept one argument, a key, and return -an integer that is the hash code of that key. For good results, the -function should use the whole range of integers for hash codes, -including negative integers. +a fixnum that is the hash code of that key. For good results, the +function should use the whole range of fixnums for hash codes, +including negative fixnums. The specified functions are stored in the property list of @var{name} under the property @code{hash-table-test}; the property value's form is @@ -271,12 +276,12 @@ Defining Hash @defun sxhash-equal obj This function returns a hash code for Lisp object @var{obj}. -This is an integer which reflects the contents of @var{obj} +This is a fixnum that reflects the contents of @var{obj} and the other Lisp objects it points to. If two objects @var{obj1} and @var{obj2} are @code{equal}, then @code{(sxhash-equal @var{obj1})} and @code{(sxhash-equal @var{obj2})} -are the same integer. +are the same fixnum. If the two objects are not @code{equal}, the values returned by @code{sxhash-equal} are usually different, but not always; once in a @@ -294,7 +299,7 @@ Defining Hash If two objects @var{obj1} and @var{obj2} are @code{eq}, then @code{(sxhash-eq @var{obj1})} and @code{(sxhash-eq @var{obj2})} are -the same integer. +the same fixnum. @end defun @defun sxhash-eql obj @@ -305,7 +310,7 @@ Defining Hash If two objects @var{obj1} and @var{obj2} are @code{eql}, then @code{(sxhash-eql @var{obj1})} and @code{(sxhash-eql @var{obj2})} are -the same integer. +the same fixnum. @end defun This example creates a hash table whose keys are strings that are diff --git a/src/fns.c b/src/fns.c index d7e123122d..819eaec7c7 100644 --- a/src/fns.c +++ b/src/fns.c @@ -4700,7 +4700,7 @@ sxhash (Lisp_Object obj, int depth) ***********************************************************************/ DEFUN ("sxhash-eq", Fsxhash_eq, Ssxhash_eq, 1, 1, 0, - doc: /* Return an integer hash code for OBJ suitable for `eq'. + doc: /* Return a fixnum hash code for OBJ suitable for `eq'. If (eq A B), then (= (sxhash-eq A) (sxhash-eq B)). Hash codes are not guaranteed to be preserved across Emacs sessions. */) @@ -4710,7 +4710,7 @@ Hash codes are not guaranteed to be preserved across Emacs sessions. */) } DEFUN ("sxhash-eql", Fsxhash_eql, Ssxhash_eql, 1, 1, 0, - doc: /* Return an integer hash code for OBJ suitable for `eql'. + doc: /* Return a fixnum hash code for OBJ suitable for `eql'. If (eql A B), then (= (sxhash-eql A) (sxhash-eql B)). Hash codes are not guaranteed to be preserved across Emacs sessions. */) @@ -4720,7 +4720,7 @@ Hash codes are not guaranteed to be preserved across Emacs sessions. */) } DEFUN ("sxhash-equal", Fsxhash_equal, Ssxhash_equal, 1, 1, 0, - doc: /* Return an integer hash code for OBJ suitable for `equal'. + doc: /* Return a fixnum hash code for OBJ suitable for `equal'. If (equal A B), then (= (sxhash-equal A) (sxhash-equal B)). Hash codes are not guaranteed to be preserved across Emacs sessions. */) @@ -4744,7 +4744,7 @@ keys. Default is `eql'. Predefined are the tests `eq', `eql', and Default is 65. :rehash-size REHASH-SIZE - Indicates how to expand the table when it -fills up. If REHASH-SIZE is an integer, increase the size by that +fills up. If REHASH-SIZE is a fixnum, increase the size by that amount. If it is a float, it must be > 1.0, and the new size is the old size multiplied by that factor. Default is 1.5. -- 2.17.1