6.	HASH ARRAYS

 
Hash arrays provide a mechanism for associating arbitrary lisp objects ("hash keys") with other objects ("hash values"), such that the hash value associated with a particular hash key can be quickly obtained.  A set of associations could be represented as a list or array of pairs, but these schemes are very inefficient when the number of associations is large.  There are functions for creating hash arrays, putting a hash key/value pair in a hash array, and quickly retrieving the hash value associated with a given hash key.
By default, the hash array functions use EQ for comparing hash keys.  This means that if non-atoms are used as hash keys, the exact same object (not a copy) must be used to retrieve the hash value.  However, the user can override this default for any hash array by specifying the functions used to compare hash keys and to "hash" a hash key to a number.  This can be used, for example, to create hash arrays where EQUAL but non-EQ strings will hash to the same value.  Specifying alternative hashing algorithms is described below (page X.XX).
In the description of the functions below, the argument HARRAY should be a value of the function HASHARRAY, which is used to create hash arrays.  For convenience in interactive program development, it may also be NIL, in which case a hash array provided by the system, SYSHASHARRAY, is used; the user must watch out for confusions if this form is used to associate more than one kind of value with the same key.
Note:  For backwards compatibility, the hash array functions will accept a list whose CAR is a hash array, and whose CDR is the "overflow method" for the hash array (see below).  However, hash array functions are guaranteed to perform with maximum efficiency only if a direct value of HASHARRAY is given.
(HASHARRAY MINKEYS OVERFLOW HASHBITSFN EQUIVFN)  	[Function]
Creates a hash array containing at least MINKEYS hash keys, with overflow method OVERFLOW. See discussion of overflow behavior below (page X.XX).
If HASHBITSFN and EQUIVFN are non-NIL, they specify the hashing function and comparison function used to interpret hash keys.  This is described in the section on user-specified hashing functions below (page X.XX).  If HASHBITSFN and EQUIVFN are NIL, the default is to hash EQ hash keys to the same value. 

(HARRAY MINKEYS)  	[Function]
Provided for backward compatibility, this is equivalent to (HASHARRAY MINKEYS 'ERROR).

(HARRAYP X)  	[Function]
Returns X if it is a hash array object; otherwise NIL.
Note that HARRAYP returns NIL if X is a list whose CAR is an HARRAYP, even though this is accepted by the hash array functions.

(HARRAYPROP HARRAY PROP NEWVALUE)  	[NoSpread Function]
Returns the property PROP of HARRAY; PROP can have the system-defined values SIZE (returns the maximum occupancy of HARRAY), NUMKEYS (number of occupied slots), OVERFLOW (overflow method), HASHBITSFN (hashing function) and EQUIVFN (comparison function).  Except for SIZE and NUMKEYS, a new value may be specified as NEWVALUE.
By using other values for PROP, the user may also set and get arbitrary property values, to associate additional information with a hash array.
Note:  The HASHBITSFN or EQUIVFN properties can only be changed if the hash array is empty.

(HARRAYSIZE HARRAY)  	[Function]
Equivalent to (HARRAYPROP HARRAY 'SIZE); returns the number of slots in HARRAY.

(CLRHASH HARRAY)  	[Function]
Clears all hash keys/values from HARRAY.  Returns HARRAY.

(PUTHASH KEY VAL HARRAY)  	[Function]
Associates the hash value VAL with the hash key KEY in HARRAY.  Replaces the previous hash value, if any.  If VAL is NIL, any old association is removed (hence a hash value of NIL is not allowed).  If HARRAY is full when PUTHASH is called with a key not already in the hash array, the function HASHOVERFLOW is called, and the PUTHASH is applied to the value returned (see below).  Returns VAL.

(GETHASH KEY HARRAY)  	[Function]
Returns the hash value associated with the hash key KEY in HARRAY.  Returns NIL, if KEY is not found.

(REHASH OLDHARRAY NEWHARRAY)  	[Function]
Hashes all hash keys and values in OLDHARRAY into NEWHARRAY.  The two hash arrays do not have to be (and usually aren't) the same size.  Returns NEWHARRAY.

(MAPHASH HARRAY MAPHFN)  	[Function]
MAPHFN is a function of two arguments.  For each hash key in HARRAY, MAPHFN will be applied to (1) the hash value, and (2) the hash key.  For example,
[MAPHASH A
   (FUNCTION (LAMBDA (VAL KEY)
                  (if (LISTP KEY) then (PRINT VAL)]
will print the hash value for all hash keys that are lists.  MAPHASH returns HARRAY.

(DMPHASH HARRAY1 HARRAY2 ... HARRAYN)  	[NLambda NoSpread Function]
Prints on the primary output file LOADable forms which will restore the hash-arrays contained as the values of the atoms HARRAY1, HARRAY2, ... HARRAYN.  Example: (DMPHASH SYSHASHARRAY) will dump the system hash-array.
Note: all EQ identities except atoms and small integers are lost by dumping and loading because READ will create new structure for each item.  Thus if two lists contain an EQ substructure, when they are dumped and loaded back in, the corresponding substructures while EQUAL are no longer EQ.  The HORRIBLEVARS file package command (page X.XX) provides a way of dumping hash tables such that these identities are preserved.


6.1  	Hash Overflow

When a hash array becomes full, attempting to add another hash key will cause the function HASHOVERFLOW to be called.  This will either automatically enlarge the hash array, or cause the error HASH TABLE FULL.  How hash overflow is handled is determined by the value of the OVERFLOW property of the hash array (which can be accessed by HARRAYPROP).  The possibilities for the overflow method are:
	the litatom ERROR	The error HASH ARRAY FULL is generated when the hash array overflows.  This is the default overflow behavior for hash arrays returned by HARRAY.
	NIL	The array is automatically enlarged by 1.5. This is the default overflow behavior for hash arrays returned by HASHARRAY.
	a positive integer N	The array is enlarged to include N more slots than it currently has.
	a floating point number F	The array is changed to include F times the number of current slots.
	a function or lambda expression FN	Upon hash overflow, FN is called with the hash array as its argument.  If FN returns a number, that will become the size of the array.  Otherwise, the new size defaults to 1.5 times its previous size.  FN could be used to print a message, or perform some monitor function.
Note:  For backwards compatibility, the hash array functions accept a list whose CAR is the hash array, and whose CDR is the overflow method.  In this case, the overflow method specified in the list overrides the overflow method set in the hash array.  Note that hash array functions are guaranteed to perform with maximum efficiency only if a direct value of HASHARRAY is given.

6.2  	User-Specified Hashing Functions

In general terms, when a key is looked up in a hash array, it is converted to an integer, which is used to index into a linear array.  If the key is not the same as the one found at that index, other indices are tried until it the desired key is found.  The value stored with that key is then returned (from GETHASH) or replaced (from PUTHASH).
The important features of this algorithm, for purposes of customizing hash arrays, are (1) the "hashing function" used to convert a key to an integer; and (2) the comparison function used to compare the key found in the array with the key being looked up.  In order for hash arrays to work correctly, any two objects which are equal according to the comparison function must "hash" to equal integers.
By default, the Interlisp hash array functions use a hashing function that computes an integer from the internal address of a key, and use EQ for comparing keys.  This means that if non-atoms are used as hash keys, the exact same object (not a copy) must be used to retrieve the hash value.
There are some applications for which the EQ constraint is too restrictive.  For example, it may be useful to use strings as hash keys, without the restriction that EQUAL but not EQ strings are considered to be different hash keys.
The user can override this default behavior for any hash array by specifying the functions used to compare keys and to "hash" a key to a number.  This can be done by giving the HASHBITSFN and EQUIVFN arguments to HASHARRAY (page X.XX).
The EQUIVFN argument is a function of two arguments that returns non-NIL when its arguments are considered equal.  The HASHBITSFN argument is a function of one argument that produces a positive small integer (in the range [0..2^16-1]) with the property that objects that are considered equal by the EQUIVFN produce the same hash bits.
For an existing hash array, the function HARRAYPROP (page X.XX) can be used to examine the hashing and equivalence functions as the HASHBITSFN and EQUIVFN hash array properties.  These properties are read-only for non-empty hash arrays, as it makes no sense to change the equivalence relationship once some keys have been hashed.
The following function is useful for creating hash arrays that take strings as hash keys:
(STRINGHASHBITS STRING)  	[Function]
Hashes the string STRING into an integer that can be used as a HASHBITSFN for a hash array.  Strings which are STREQUAL hash to the same integer.

Example:
(HASHARRAY MINKEYS OVERFLOW 'STRINGHASHBITS 'STREQUAL)
creates a hash array where you can use strings as hash keys.

Copyright (c) 1985 Xerox Corporation.  All rights reserved.