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From: edw@ius2.cs.cmu.edu (Eddie Wyatt)
Newsgroups: comp.lang.c
Subject: Re: enum - enum ?
Message-ID: <1209@ius2.cs.cmu.edu>
Date: Fri, 19-Jun-87 12:52:56 EDT
Article-I.D.: ius2.1209
Posted: Fri Jun 19 12:52:56 1987
Date-Received: Tue, 23-Jun-87 01:11:43 EDT
References: <7906@brl-adm.ARPA>
Organization: Carnegie-Mellon University, CS/RI
Lines: 106



  Example of the pathological case for enums array indexes.  (An implementors
perspective)


	typedef enum	{W = -10, X = -100, Y = 10, P = 0, Z = 0} PATHO ;


	int sumation(x)
    	    int x[PATHO];
    	    {
    	    register PATH i;
    	    register int sum;
	
    	    for (i = W; i <= Z; i = successor(i))
		    sum += x[i];

    	    return(sum);
    	    }

  The task here is to sum the over the first 5 indexes of  array x.

  Lets consider the code that needs to be generated for the statements:

    		for (i = W; i <= Z; i = successor(i))
	    	    sum += x[i];

    	first try

		for (i = -10, i <= 0; i = enum_get_next(i))
	    	    sum += *(x + lookup_hashtable(i));

	execute the loop and we get :

		i = -10, -100, 10 - exit loop.

  	Nope that can't be right.


  Assume we can fix the above problem somehow and lets consider the code
generated for, lets say:

		sum += x[P];

	try

		sum += *(x + lookup_hashtable(0));

	Hmmm does lookup_hashtable return 3 or 4???


   Lets consider an alternate solution enums as array indexes.  Allow each
enumeration element  have two values associated with it - the place it
holds in the enumeration (for W that would be the value 0) and the
assigned value (for W that would be the value -10).

  From the context, one MIGHT be able to determine which is the correct
value to use.  In the above case.


	for (i = W; i <= Z; i = successor(i))
	    	    sum += x[i];

yields :

	for (i = 0; i <= 4; i++)
		sum += *(x+i);

    The above context prescribes that W,Z and i be intepreted as "place"
values, not the "assigned" values.  Look here too, no hashing function
either - efficient code.

   The major question here is whether the correct interpretation can
always be choosen given the context the variable is being used in. 


Example :

	typdef enum {RED = 30, GREEN = 40, BLUE = 50} COLORTABLE;
	......
	colortable[RED] = 64;
	.....

Are we indexing colortable[0] or colortable[30]??

We might say that if colortable is defined as

	int colortable[COLORTABLE] then the actual index is 0.

If colortable is defined as

	int colartable[] or colortable[256] 

then we are refering to index 30.

  There are more things to be thought out, but this is a posible
implementation alternative for enums in C.


-- 
					Eddie Wyatt

e-mail: edw@ius2.cs.cmu.edu

terrorist, cryptography, DES, drugs, cipher, secret, decode, NSA, CIA, NRO.