Commit cf285946 authored by Paul Eggert's avatar Paul Eggert

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.
parent 4a1507b8
......@@ -132,7 +132,7 @@ When you add an association to a hash table and the table is full,
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 @@ to understand how hash tables work, and what a @dfn{hash code} means.
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 @@ The function @var{test-fn} should accept two arguments, two keys, and
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 @@ under the property @code{hash-table-test}; the property value's form is
@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 @@ result reflects identity of @var{obj}, but not its contents.
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 @@ in which case a hash code is generated for the value.
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
......
......@@ -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.
......
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