/*
 * BUILDING YOUR OWN C INTERPRETER
 * by Herbert Schildt
 * From Dr. Dobbs Journal - August 1989.
 * Adapted to the Amiga by Jeff Lydiatt.
 */

/* Recursive descent parser for integer expressions
   which may include variables and function calls.  */

#include "setjmp.h"
#include "math.h"
#include "ctype.h"
#include "stdlib.h"
#include "string.h"
#include "stdio.h"

#define NUM_FUNC   	100
#define NUM_GLOBAL_VARS 100
#define NUM_LOCAL_VARS  200
#define ID_LEN      	31 
#define FUNC_CALLS  	31
#define PROG_SIZE 	10000
#define FOR_NEST	31

enum tok_types {DELIMITER, IDENTIFIER, NUMBER, KEYWORD, TEMP,
	        STRING, BLOCK};
enum tokens {ARG, CHAR, INT, IF, ELSE, FOR, DO, WHILE, SWITCH,
             RETURN, EOL, FINISHED, END};
enum double_ops {LT=1, LE, GT, GE, EQ, NE};
/* These are the constants used to call sntx_err() when
   a syntax error occurs.  Add more if you like.
   NOTE: SYNTAX is a generic error message used when
   nothing else seems appropriate.
*/
enum error_msg 
     {SYNTAX, UNBAL_PARENS, NO_EXP, EQUALS_EXPECTED,
      NOT_VAR, PARAM_ERR, SEMI_EXPECTED,
      UNBAL_BRACES, FUNC_UNDEF, TYPE_EXPECTED,
      NEST_FUNC, RET_NOCALL, PAREN_EXPECTED,
      WHILE_EXPECTED, QUOTE_EXPECTED, NOT_TEMP,
      TOO_MANY_LVARS};
extern char *prog;  /* current location in source code */
extern char *p_buf;  /* points to start of program buffer */
extern jmp_buf e_buf; /* hold environment for longjmp() */
/* An array of these structures will hold the info
   associated with global variables.
*/
extern struct var_type {
  char var_name[32];
  int  var_type;
  int value;
}  global_vars[NUM_GLOBAL_VARS];
/*  This is the function call stack. */
extern struct func_type {
  char func_name[32];
  char *loc;  /* location of function entry point in file */
} func_stack[NUM_FUNC];
/* Keyword table */
extern struct commands {
  char command[20];
  char tok;
} table[];
/* "Standard library" functions are declared here so
   they can be put into the internal function table that
   follows.
*/
int call_getche(void), call_putch(void);
int call_puts(void), print(void), getnum(void);

struct intern_func_type {
  char *f_name; /* function name */
  int (* p)();  /* pointer to the function */
} intern_func[] = {
  "getche", call_getche,
  "putch", call_putch,
  "puts", call_puts,
  "print", print,
  "getnum", getnum,
  "", 0  /* null terminate the list */
};
extern char token[80]; /* string representation of token */
extern char token_type; /* contains type of token */
extern char tok; /* internal representation of token */
extern int ret_value; /* function return value */
void eval_exp(int *value), eval_exp1(int *value);
void eval_exp2(int *value);
void eval_exp3(int *value), eval_exp4(int *value);
void eval_exp5(int *value), atom(int *value);
void eval_exp0(int *value);
void sntx_err(int error), putback(void);
void assign_var(char *var_name, int value);
int isdelim(char c), look_up(char *s), iswhite(char c);
int find_var(char *s), get_token(void);
int internal_func(char *s);
int is_var(char *s);
char *find_func(char *name);
void call(void);
/* Entry point into parser. */
void eval_exp(int *value)
{
  get_token(); 
  if(!*token) {
    sntx_err(NO_EXP); 
    return; 
  }
  if(*token==';') {
    *value = 0; /* empty expression */
    return;
  }
  eval_exp0(value); 
  putback(); /* return last token read to input stream */
}
/* Process an assignment expression */
void eval_exp0(int *value)
{ char temp[ID_LEN];  /* holds name of var receiving 
                         the assignment */
  register int temp_tok;
  if(token_type==IDENTIFIER) {
    if(is_var(token)) {  /* if a var, see if assignment */
      strcpy(temp, token);
      temp_tok = token_type;
      get_token();
      if(*token=='=') {  /* is an assignment */
	get_token();
	eval_exp0(value);  /* get value to assign */
	assign_var(temp, *value);  /* assign the value */
    	return;
      }
      else {  /* not an assignment */
	putback();  /* restore original token */
	strcpy(token, temp);
	token_type = temp_tok;
      }
    }
  }
  eval_exp1(value);
}
/* This array is used by eval_exp1().  Because 
   some compilers cannot initialize an array within a 
   function it is defined as a global variable.
*/
char relops[7] = {
  LT, LE, GT, GE, EQ, NE, 0
};
/* Process relational operators. */
void eval_exp1(int *value)
{
  int partial_value, l_value; 
  register char op;
  eval_exp2(value); 
  l_value= *value;
  op = *token;
  if(strchr(relops, op)) {
    get_token(); 
    eval_exp2(&partial_value); 
    switch(op) {  /* perform the relational operation */
      case LT: 
        *value = l_value < partial_value;
        break;
      case LE:
        *value = l_value <= partial_value;
        break;
      case GT:
        *value = l_value > partial_value;
        break;
      case GE:
        *value = l_value >= partial_value;
        break;
      case EQ: 
        *value = l_value == partial_value;
        break;
      case NE:
        *value = l_value != partial_value;
         break;
    }
  }
}
/*  Add or subtract two terms. */
void eval_exp2(int *value)
{
  register char  op; 
  int partial_value; 
  eval_exp3(value); 
  while((op = *token) == '+' || op == '-') {
    get_token(); 
    eval_exp3(&partial_value); 
    switch(op) {  /* add or subtract */
      case '-':
        *value = *value - partial_value; 
        break; 
      case '+':
        *value = *value + partial_value; 
        break; 
    }
  }
}
/* Multiply or divide two factors. */
void eval_exp3(int *value)
{
  register char  op; 
  int partial_value, t; 
  eval_exp4(value); 
  while((op = *token) == '*' || op == '/' || op == '%') {
    get_token(); 
    eval_exp4(&partial_value); 
    switch(op) { /* mul, div, or modulus */
      case '*':  
        *value = *value * partial_value; 
        break; 
      case '/':
        *value = (*value) / partial_value; 
        break; 
      case '%':
        t = (*value) / partial_value; 
        *value = *value-(t * partial_value); 
        break; 
    }
  }
}
/* Is a unary + or -. */
void eval_exp4(int *value)
{
  register char  op; 
  op = '\0'; 
  if(*token=='+' || *token=='-') {
    op = *token; 
    get_token(); 
  }
  eval_exp5(value); 
  if(op)
    if(op=='-') *value = -(*value); 
}
/* Process parenthesized expression. */
void eval_exp5(int *value)
{
  if((*token == '(')) {
    get_token(); 
    eval_exp0(value);   /* get subexpression */
    if(*token != ')') sntx_err(PAREN_EXPECTED); 
    get_token(); 
  }
  else
    atom(value); 
}
/* Find value of number, variable or function. */
void atom(int *value)
{
  int i;
  switch(token_type) {
  case IDENTIFIER:
    i = internal_func(token);
    if(i!= -1) {  /* call "standard library" function */
      *value = (*intern_func[i].p)();
    }
    else
    if(find_func(token)){  /* call user-defined function */
      call();
      *value = ret_value;
    }
    else  *value = find_var(token);  /* get var's value */
    get_token(); 
    return; 
  case NUMBER: /* is numeric constant */
    *value = atoi(token); 
    get_token();   
    return; 
  case DELIMITER: /* see if character constant */
    if(*token=='\'') {
      *value = *prog;
      prog++;
      if(*prog!='\'') sntx_err(QUOTE_EXPECTED);
      prog++;
      get_token();
    }
    return;
  default:
    if(*token==')') return; /* process empty expression */
    else sntx_err(SYNTAX); /* syntax error */
  }
}
/* Display an error message. */
void sntx_err(int error)
{
  char *p, *temp;
  int linecount = 0;
  register int i;
  static char *e[]= {   
    "syntax error", 
    "unbalanced parentheses", 
    "no expression present",
    "equals sign expected",
    "not a variable",
    "parameter error",
    "semicolon expected",
    "unbalanced braces",
    "function undefined",
    "type specifier expected",
    "too many nested function calls",
    "return without call",
    "parentheses expected",
    "while expected",
    "closing quote expected",
    "not a string",
    "too many local variables"
  }; 
  printf("%s", e[error]); 
  p = p_buf;
  while(p != prog) {  /* find line number of error */
    p++;
    if(*p == '\n') {
      linecount++;
    }
  }
  printf(" in line %d\n", linecount);
  temp = p;
  for(i=0; i<20 && p>p_buf && *p!='\n'; i++, p--);
  for(i=0; i<30 && p<=temp; i++, p++) printf("%c", *p);
  longjmp(e_buf, 1); /* return to save point */
}
/* Get a token. */
get_token(void)
{
  register char *temp;
  token_type = 0; tok = 0;
  temp = token;
  *temp = '\0';
 /* skip over white space */
  while(iswhite(*prog) && *prog) ++prog; 
  if(*prog=='\n') { 
    ++prog; 
    /* skip over white space */
    while(iswhite(*prog) && *prog) ++prog;
  }
  if(*prog=='\0') { /* end of file */
    *token = '\0';
    tok = FINISHED;
    return(token_type=DELIMITER);
  }
  if(strchr("{}", *prog)) { /* block delimiters */
    *temp = *prog;
    temp++;
    *temp = '\0';
    prog++;
    return (token_type = BLOCK);
  }
  /* look for comments */
  if(*prog=='/')
    if(*(prog+1)=='*') { /* is a comment */
      prog += 2;
      do { /* find end of comment */
        while(*prog!='*') prog++;
        prog++;
      } while (*prog!='/');
      prog++;
    }
  if(strchr("!<>=", *prog)) { /* is or might be
			      a relation operator */
    switch(*prog) {
      case '=': if(*(prog+1)=='=') {
	  prog++; prog++;
          *temp = EQ;
          temp++; *temp = EQ; temp++;
	  *temp = '\0';
  	}
	break;
      case '!': if(*(prog+1)=='=') {
	  prog++; prog++;
          *temp = NE;
          temp++; *temp = NE; temp++;
	  *temp = '\0';
	}
	break;
      case '<': if(*(prog+1)=='=') {
	  prog++; prog++;
          *temp = LE; temp++; *temp = LE;
	}
	else {
	  prog++;
	  *temp = LT;
	}
        temp++;
	*temp = '\0';
	break;
      case '>': if(*(prog+1)=='=') {
	  prog++; prog++;
          *temp = GE; temp++; *temp = GE;
	} 
	else {
	  prog++;
	  *temp = GT;
	}
        temp++;
	*temp = '\0';
	break;
    }
    if(*token) return(token_type = DELIMITER);
  }
  if(strchr("+-*^/%=;(),'", *prog)){ /* delimiter */
    *temp = *prog;
    prog++; /* advance to next position */
    temp++;
    *temp = '\0'; 
    return (token_type=DELIMITER);
  }
  if(*prog=='"') { /* quoted string */
    prog++;
    while(*prog!='"'&& *prog!='\n') *temp++ = *prog++;
    if(*prog=='\n') sntx_err(SYNTAX);
    prog++; *temp = '\0';
    return(token_type=STRING);
  }
  if(isdigit(*prog)) { /* number */
    while(!isdelim(*prog)) *temp++ = *prog++;
    *temp = '\0';
    return(token_type = NUMBER);
  }
  if(isalpha(*prog)) { /* var or command */
    while(!isdelim(*prog)) *temp++ = *prog++;
    token_type=TEMP;
  }
  *temp = '\0';
  /* see if a string is a command or a variable */
  if(token_type==TEMP) {
    tok = look_up(token); /* convert to internal rep */
    if(tok) token_type = KEYWORD; /* is a keyword */
    else token_type = IDENTIFIER;
  }
  return token_type;
}
/* Return a token to input stream. */
void putback(void) 
{
  char *t; 
  t = token; 
  for(; *t; t++) prog--; 
}
/* Look up a token's internal representation in the
   token table.
*/
look_up(char *s)
{
  register int i;
  char *p;
  /* convert to lowercase */
  p = s;
  while(*p){ *p = tolower(*p); p++; }
  /* see if token is in table */
  for(i=0; *table[i].command; i++)
      if(!strcmp(table[i].command, s)) return table[i].tok;
  return 0; /* unknown command */
}
/* Return index of internal library function or -1 if
   not found.
*/
internal_func(char *s)
{
  int i;
  for(i=0; intern_func[i].f_name[0]; i++) {
    if(!strcmp(intern_func[i].f_name, s))  return i;
  }
  return -1;
}
/* Return true if c is a delimiter. */
isdelim(char c)
{
  if(strchr(" !;,+-<>'/*%^=()", c) || c==9 ||
     c=='\n' || c==0) return 1;  
  return 0; 
}
/* Return 1 if c is space or tab. */
iswhite(char c)
{
  if(c==' ' || c=='\t') return 1;
  else return 0;
}