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exprpars.c

/* ---------------------------------------------------------------------------

   exprpars.c
   
   Notes:

      This file contains routines for parsing 'if{n}def'-style expressions.

   Revision history:

      4.10.0
            B.Marr            2006-07-19
            
            Reformatted comments and code to match my standards.
            
            B.Marr            2006-07-12
            
            Provide explicit casting in several spots to avoid warnings in
            a "gcc 3.4.2 on Solaris 8" environment, based on a report from
            David Mathog <mathog at mendel.bio.caltech.edu>.
            
            Get rid of all the '#ifdef PROTOS' checks, which are pretty
            much obsolete these days and just needlessly clutter up the
            code.
            
      4.8.0 B.Marr            2004-11-15
            
            Remove Ctl-L (page eject) characters from source file.
            
            Rename 'find_sym()' routine to 'find_sym_name()' for
            unambiguous searches now that we have new 'find_sym_val()'
            routine.
            
      4.7   AWR   02/24/1998  add prototypes (using PROTO macro)
                              to function pointer declarations

                  12/21/1997  clean up gcc warnings in -Wall mode

                  07/27/1997  delete obsolete FPR and PRT macros

      4.6   AWR   12/05/1995  delete vestiges of never-pursued idea
                              from next_token(): permit "if -<flag>"
                              syntax, which would have determined
                              whether or not "-<flag>" had been set

      4.5   AWR   04/28/1993  restructure function definitions so
                              function name appears in first column
                              (to facilitate searching for definition
                              by name)

      4.0   AWR   02/06/1991  Author

*/

/* ---------------------------------------------------------------------------

   Header Files

*/

#include <ctype.h>
#include <string.h>
#include <stdio.h>

#include "pcaldefs.h"
#include "protos.h"

/* ---------------------------------------------------------------------------

   Type, Struct, & Enum Declarations

*/

typedef short OPERAND;   /* types for operand and operator stacks */
typedef short OPERATOR;

typedef struct {
   char *name;   /* token spelling */
   short type;   /* token type code */
   short value;   /* associated value */
} TOKEN;

typedef struct {
   short prec;   /* precedence */
   short type;   /* token type (TK_UNARYOP or TK_BINARYOP) */
   OPERAND (*pfcn) (OPERAND *);   /* dispatch function */
} OPR;

/* ---------------------------------------------------------------------------

   Constant Declarations

*/

/*
 * token type code definitions:
 */
#define TK_UNKNOWN       0              /* codes returned by next_token() */
#define TK_IDENT         1
#define TK_LPAREN        2
#define TK_RPAREN        3
#define TK_UNARYOP       4
#define TK_BINARYOP      5
#define TK_ENDINPUT      6
#define TK_STARTINPUT    7              /* special code for start symbol */

/* bit position for token type codes (cf. where_ok[] below) */
#define ID_OK           (1 << TK_IDENT)
#define LP_OK           (1 << TK_LPAREN)
#define RP_OK           (1 << TK_RPAREN)
#define UO_OK           (1 << TK_UNARYOP)
#define BO_OK           (1 << TK_BINARYOP)
#define ST_OK           (1 << TK_STARTINPUT)
#define NEVER_OK        0

/*
 * operator-related definitions:
 */

#define OP_AND          0       /* operator subcodes */
#define OP_OR           1
#define OP_XOR          2
#define OP_NEGATE       3

#define ENDINPUT_PREC   -1      /* arbitrary number < lowest op. prec  */
#define OR_PREC          1      /* operator precedence levels */
#define XOR_PREC         2
#define AND_PREC         3
#define NEGATE_PREC      4
#define PAREN_PREC       8      /* arbitrary number > highest op. prec */

/* lower bits of operator stack entry are code; higher are precedence */
#define OPR_BITS        4
#define OPR_MASK        ((1 << OPR_BITS) - 1)
#define PREC(op)        ((op) >> OPR_BITS)
#define OPCODE(op)      ((op) & OPR_MASK)
#define MAKE_OPR(p, o)  (((p) << OPR_BITS) | (o))

/* size of operand and operator stacks */
#define MAX_OP   20

/* ---------------------------------------------------------------------------

   Macro Definitions

*/

/* is token "curr" legal after "prev"? (cf. where_ok[] below) */
#define IS_LEGAL(curr, prev)   (where_ok[curr] & (1 << (prev)))

#define NT_RETURN(p, t, v)    do { *ptype = t; *pvalue = v; return p; } while (0)

/* ---------------------------------------------------------------------------

   Data Declarations (including externals)

*/

/* prototypes for dispatch functions, declared early for use in operator
   table... */
static OPERAND do_and(OPERAND *);
static OPERAND do_or(OPERAND *);
static OPERAND do_xor(OPERAND *);
static OPERAND do_negate(OPERAND *);

/* operator table - entries must be in same order as 'OP_XXX' constants */
OPR opr_tbl[] = {
   { AND_PREC,    TK_BINARYOP, do_and },
   { OR_PREC,     TK_BINARYOP, do_or },
   { XOR_PREC,    TK_BINARYOP, do_xor },
   { NEGATE_PREC, TK_UNARYOP,  do_negate }
};


/* set of tokens which each token may legally follow (in TK_XXX order) */
int where_ok[] = {
   NEVER_OK                                      ,  /* TK_UNKNOWN  */
   ST_OK         | LP_OK         | UO_OK | BO_OK ,  /* TK_IDENT    */
   ST_OK         | LP_OK         | UO_OK | BO_OK ,  /* TK_LPAREN   */
   ID_OK | LP_OK | RP_OK                         ,  /* TK_RPAREN   */
   ST_OK         | LP_OK                 | BO_OK ,  /* TK_UNARYOP  */
   ID_OK         | RP_OK                         ,  /* TK_BINARYOP */
   ST_OK | ID_OK         | RP_OK                    /* TK_ENDINPUT */
};

/* token table - note that substrings must follow longer strings */
TOKEN token_tbl[] = {
        { "&&", TK_BINARYOP,    OP_AND },       /* synonym for "&" */
        { "&",  TK_BINARYOP,    OP_AND },
        { "||", TK_BINARYOP,    OP_OR },        /* synonym for "|" */
        { "|",  TK_BINARYOP,    OP_OR },
        { "!",  TK_UNARYOP,     OP_NEGATE },
        { "^",  TK_BINARYOP,    OP_XOR },
        { "(",  TK_LPAREN,      0 },
        { ")",  TK_RPAREN,      0 },
        { NULL, TK_UNKNOWN,     0 }             /* must be last entry */
};

/* ---------------------------------------------------------------------------

   External Routine References & Function Prototypes

*/

/* ---------------------------------------------------------------------------

   do_and

   Notes:

      This routine is a dispatch function for a specific operator.

*/
static OPERAND do_and (OPERAND *ptop)
{
   return ptop[0] && ptop[-1];
}

/* ---------------------------------------------------------------------------

   do_and

   Notes:

      This routine is a dispatch function for a specific operator.

*/
static OPERAND do_or (OPERAND *ptop)
{
   return ptop[0] || ptop[-1];
}

/* ---------------------------------------------------------------------------

   do_and

   Notes:

      This routine is a dispatch function for a specific operator.

*/
static OPERAND do_xor (OPERAND *ptop)
{
   return (ptop[0] ^ ptop[-1]) != 0;
}

/* ---------------------------------------------------------------------------

   do_and

   Notes:

      This routine is a dispatch function for a specific operator.

*/
static OPERAND do_negate (OPERAND *ptop)
{
   return ! ptop[0];
}

/* ---------------------------------------------------------------------------

   lookup_token

   Notes:

      This routine looks up a token in the table.  It returns a pointer to the
      table entry.
      
*/
static TOKEN * lookup_token (char *p)
{
   TOKEN *ptok;
   
   for (ptok = token_tbl;
        ptok->name && strncmp(p, ptok->name, strlen(ptok->name));
        ptok++)
      ;
   
   return ptok;
}

/* ---------------------------------------------------------------------------

   next_token

   Notes:

      This routine fetchs the next token from the input string.

      It fills in its type and value and returns a pointer to the following
      character.

*/
static char *next_token (char *p, int *ptype, int *pvalue)
{
   TOKEN *ptok;
   char tokbuf[STRSIZ], *pb;
   
   while (*p && isspace((int)*p)) p++;   /* skip whitespace */

   if (*p == '\0') {   /* end of input? */
      NT_RETURN(p, TK_ENDINPUT, 0);
   }
   
   if (isalpha((int)*p)) {   /* identifier */

      pb = tokbuf;   /* make local copy and look up */
      while (*p && (isalpha((int)*p) || isdigit((int)*p) || *p == '_')) {
         *pb++ = *p++;
      }
      *pb = '\0';
      
      NT_RETURN(p, TK_IDENT, find_sym_name(tokbuf));
   }

   ptok = lookup_token(p);   /* other token */
   NT_RETURN(p + (ptok->name ? strlen(ptok->name) : 1), ptok->type, ptok->value);
}

/* ---------------------------------------------------------------------------

   parse_expr

   Notes:

      This routine parses an expression consisting of identifiers and logical
      operators.

      It returns 'TRUE' if the expression is true (identifier defined =>
      true), 'FALSE' if false, and 'EXPR_ERR' if there's a syntax error in the
      expression.

*/
int parse_expr (char *pbuf)
{
   OPERAND opd_stack[MAX_OP];   /* operand stack - TRUE/FALSE values */
   OPERATOR opr_stack[MAX_OP];   /* operator stack - precedence | op */
   int value, token, plevel, prec, result, npop, opr, opd, prev_token, op;
   
   plevel = 0;   /* paren nesting level */
   opd = opr = -1;   /* indices of stack tops */
   prev_token = TK_STARTINPUT;   /* to detect null expressions */
   
   if (DEBUG(DEBUG_PP)) fprintf(stderr, "evaluating expression '%s'\n", pbuf);
   do {
      pbuf = next_token(pbuf, &token, &value);
      
      /* check that the current token may follow the previous one */
      if (! IS_LEGAL(token, prev_token)) return EXPR_ERR;

      switch(token) {
         
      case TK_IDENT:   /* identifier => 1 if def, 0 if not */
         opd_stack[++opd] = value != PP_SYM_UNDEF;
         break;
         
      case TK_LPAREN:   /* left paren - bump nesting level */
         ++plevel;
         break;
         
      case TK_RPAREN:   /* right paren - decrement nesting */
         if (--plevel < 0) return EXPR_ERR;
         break;
         
      case TK_ENDINPUT:   /* end-of-input - treat as operator */
         if (prev_token == TK_STARTINPUT) return FALSE;   /* null expr => FALSE */
         /* fall through */

      case TK_UNARYOP:
      case TK_BINARYOP:

         /* get precedence of operator, adjusting for paren
          * nesting (TK_ENDINPUT has the lowest precedence
          * of all, to unwind operand/operator stacks at end)
          */
         prec = token == TK_ENDINPUT ? ENDINPUT_PREC : (plevel * PAREN_PREC) + opr_tbl[value].prec;
         
         /* pop (and perform) any equal- or higher-precedence
          * operators on operator stack: extract operator,
          * check for operand stack underflow, execute
          * operator, adjust operand stack height and place
          * result of operator on top
          */
         
         for ( ;
               opr >= 0 && PREC(opr_stack[opr]) >= prec;
               opr--) {
            op = OPCODE(opr_stack[opr]);
            npop = opr_tbl[op].type == TK_UNARYOP ? 0 : 1;
            if (opd < npop) return EXPR_ERR;
            result = (*opr_tbl[op].pfcn)(opd_stack + opd);
            opd_stack[opd -= npop] = result;
         }

         /* push operator (if any) onto stack */
         
         if (token != TK_ENDINPUT) opr_stack[++opr] = MAKE_OPR(prec, value);

         break;
         
      default:   /* should never get here */
         return EXPR_ERR;
         break;
      }
      
      prev_token = token;
      
   } while (token != TK_ENDINPUT);
   
   /* done - check for dangling parens, and leftover operand/operators */
   
   if (DEBUG(DEBUG_PP)) {
      fprintf(stderr, "evaluated to %s\n", opd_stack[0] ? "TRUE" : "FALSE");
   }

   return plevel != 0 || opd != 0 || opr != -1 ? 
      EXPR_ERR :   /* leftover junk - return error */
      opd_stack[0];   /* all OK - return final value */
}

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