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

/* A Little C interpreter. */

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

#define NUM_FUNC   	100
#define NUM_GLOBAL_VARS 100
#define NUM_LOCAL_VARS  200
#define NUM_BLOCK	100
#define ID_LEN      	31 
#define FUNC_CALLS  	31
#define NUM_PARAMS	31
#define PROG_SIZE 	10000
#define LOOP_NEST	31

enum tok_types {DELIMITER, IDENTIFIER, NUMBER, KEYWORD, 
                TEMP, STRING, BLOCK};
/* add additional C keyword tokens here */
enum tokens {ARG, CHAR, INT, IF, ELSE, FOR, DO, WHILE,
             SWITCH, RETURN, EOL, FINISHED, END};
/* add additional double operators here (such as ->) */
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};
char *prog;  /* current location in source code */
char *p_buf; /* points to start of program buffer */
jmp_buf e_buf; /* hold environment for longjmp() */

/* An array of these structures will hold the info
   associated with global variables.
*/
struct var_type {
  char var_name[ID_LEN];
  int var_type;
  int value;
}  global_vars[NUM_GLOBAL_VARS];
struct var_type local_var_stack[NUM_LOCAL_VARS];
struct func_type {
  char func_name[ID_LEN];
  char *loc;  /* location of entry point in file */
} func_table[NUM_FUNC];
int call_stack[NUM_FUNC];
struct commands { /* keyword lookup table */
  char command[20];
  char tok;
} table[] = { /* Commands must be entered lowercase */
  "if", IF, /* in this table. */
  "else", ELSE,
  "for", FOR,
  "do", DO,
  "while", WHILE,
  "char", CHAR,
  "int", INT,
  "return", RETURN,
  "end", END,
  "", END  /* mark end of table */
};
char token[80];
char token_type, tok;
int functos;  /* index to top of function call stack */
int func_index; /* index into function table */
int gvar_index; /* index into global variable table */
int lvartos; /* index into local variable stack */
int ret_value; /* function return value */
void print(void), prescan(void);
void decl_global(void), call(void), putback(void);
void decl_local(void), local_push(struct var_type i);
void eval_exp(int *value), sntx_err(int error);
void exec_if(void), find_eob(void), exec_for(void);
void get_params(void), get_args(void);
void exec_while(void), func_push(int i), exec_do(void);
void assign_var(char *var_name, int value);
int load_program(char *p, char *fname), find_var(char *s);
void interp_block(void), func_ret(void);
int func_pop(void), is_var(char *s), get_token(void);
char *find_func(char *name);

void main(int argc, char *argv[])
{
  if(argc!=2) {
    printf("usage: c <filename>\n");
    exit(1);
  }
  /* allocate memory for the program */
  if((p_buf=(char *) malloc(PROG_SIZE))==NULL) {
    printf("allocation failure");
    exit(1);
  }
  /* load the program to execute */
  if(!load_program(p_buf, argv[1])) exit(1);
  if(setjmp(e_buf)) exit(1); /* initialize long jump buffer */
  /* set program pointer to start of program buffer */
  prog = p_buf; 
  prescan(); /* find the location of all functions
                   and global variables in the program */
  gvar_index = 0;  /* initialize global variable index */
  lvartos = 0;     /* initialize local variable stack index */
  functos = 0;     /* initialize the CALL stack index */
  /* setup call to main() */
  prog = find_func("main");  /* find program starting point */
  if ( prog == NULL ){
    printf("Function 'main' not found\n");
    exit(4);
  }
  prog--; /* back up to opening ( */
  strcpy(token, "main");
  call();  /* call main() to start interpreting */
}
/* Interpret a single statement or block of code.  When
   interp_block() returns from it's initial call, the final
   brace (or a return) in main() has been encountered.
*/
void interp_block(void)
{
  int value;
  char block = 0;
  do {
    token_type = get_token();
    /* If interpreting single statement, return on
       first semicolon.
    */
    /* see what kind of token is up */
    if(token_type==IDENTIFIER) { 
      /* Not a keyword, so process expression. */
	putback();  /* restore token to input stream for
                       further processing by eval_exp() */
	eval_exp(&value);  /* process the expression */
        if(*token!=';') sntx_err(SEMI_EXPECTED);
    }
    else if(token_type==BLOCK) { /* if block delimiter */
      if(*token=='{') /* is a block */
        block = 1; /* interpreting block, not statement */
      else return; /* is a }, so return */
    }
    else /* is keyword */
      switch(tok) {
        case CHAR:
   	case INT:     /* declare local variables */
          putback();
	  decl_local();
	  break;
	case RETURN:  /* return from function call */
	  func_ret();
	  return; 
	case IF:      /* process an if statement */
	  exec_if();
	  break;
	case ELSE:    /* process an else statement */
	  find_eob(); /* find end of else block 
			 and continue execution */
	  break;      
	case WHILE:   /* process a while loop */
	  exec_while();
	  break;
        case DO:      /* process a do-while loop */
	  exec_do();
	  break;
	case FOR: exec_for();
	  break;
        case END:
	  exit(0);
      }
  } while (tok != FINISHED && block);
}
/* Load a program. */
load_program(char *p, char *fname)
{
  FILE *fp;
  int i=0;
  if((fp=fopen(fname, "r"))==NULL) return 0;
  i = 0;
  do {
    *p = getc(fp);
    p++; i++;
  } while(!feof(fp) && i<PROG_SIZE);
  p--;
  while( *p <= ' ' || *p == '\t' )
	p--;
  *(p+1) = '\0'; /* null terminate the program */
  fclose(fp);
  return 1;
}
/* Find the location of all functions in the program
   and store global variables. */
void prescan(void)
{
  char *p;
  char temp[32];
  int brace = 0;  /* When 0, this var tells us that
		     current source position is outside
	  	     of any function. */
  p = prog;
  func_index = 0;
  do { 
    while(brace) {  /* bypass code inside functions */
      get_token();
      if(*token=='{') brace++;
      if(*token=='}') brace--;
    }
    get_token();
    if(tok==CHAR || tok==INT) { /* is global var */
      putback();
      decl_global();
    }      
    else if(token_type==IDENTIFIER) { 
      strcpy(temp, token);
      get_token();
      if(*token=='(') {  /* must be assume a function */
        func_table[func_index].loc = prog;
        strcpy(func_table[func_index].func_name, temp);
        func_index++;
        while(*prog!=')') prog++;
        prog++;
	/* prog points to opening curly brace of function */
      }
      else putback();
    }
    else if(*token=='{') brace++;
  } while(tok!=FINISHED);
  prog = p;
}
/* Return the entry point of the specified function. 
   Return NULL if not found.
*/
char *find_func(char *name)
{
  register int i;
  for(i=0; i<func_index; i++)
    if(!strcmp(name, func_table[i].func_name)){
      return func_table[i].loc;
    }
  return NULL;
}
/* Declare a global variable. */
void decl_global(void)
{
  get_token();  /* get type */
  global_vars[gvar_index].var_type = tok;
  global_vars[gvar_index].value = 0;  /* init to 0 */
  do { /* process comma-separated list */
    get_token();  /* get name */
    strcpy(global_vars[gvar_index].var_name, token);
    get_token();
    gvar_index++;
  } while(*token==',');
  if(*token!=';') sntx_err(SEMI_EXPECTED);
}
/* Declare a local variable. */
void decl_local(void)
{
  struct var_type i;
  get_token();  /* get type */
  i.var_type = tok;
  i.value = 0;  /* init to 0 */
  do { /* process comma-separated list */
    get_token(); /* get var name */
    strcpy(i.var_name, token);
    local_push(i);
    get_token();
  } while(*token==',');
  if(*token!=';') sntx_err(SEMI_EXPECTED);
}
/* Call a function. */
void call(void)
{
  char *loc, *temp;
  int lvartemp;
  loc = find_func(token); /* find entry point of function */
  if(loc==NULL)
    sntx_err(FUNC_UNDEF); /* function not defined */
  else {
    lvartemp = lvartos;  /* save local var stack index */
    get_args();  /* get function arguments */
    temp = prog; /* save return location */
    func_push(lvartemp);  /* save local var stack index */
    prog = loc;  /* reset prog to start of function */
    get_params(); /* load the function's parameters with 
		     the values of the arguments */
    interp_block(); /* interpret the function */
    prog = temp; /* reset the program pointer */
    lvartos = func_pop(); /* reset the local var stack */
  }
}
/* Push the arguments to a function onto the local
   variable stack. */
void get_args(void)
{
  int value, count, temp[NUM_PARAMS];
  struct var_type i;
  count = 0;
  get_token();
  if(*token!='(') sntx_err(PAREN_EXPECTED);
  /* process a comma-separated list of values */
  do {
    eval_exp(&value);
    temp[count] = value;  /* save temporarily */
    get_token();
    count++;
  }while(*token==',');
  count--;
  /* now, push on local_var_stack in reverse order */
  for(; count>=0; count--) {
    i.value = temp[count];
    i.var_type = ARG;
    local_push(i);
  }
}
/* Get function parameters. */
void get_params(void)
{
  struct var_type *p;
  int i;
  i = lvartos-1;
  do { /* process comma-separated list of parameters */
    get_token();  
    p = &local_var_stack[i];
    if(*token!=')') {
      if(tok!=INT && tok!=CHAR) sntx_err(TYPE_EXPECTED);
      p->var_type = token_type;
      get_token();
      /* link parameter name with argument already on 
	 local var stack */
      strcpy(p->var_name, token);
      get_token();
      i--;
    }
    else break;
  } while(*token==',');
  if(*token!=')') sntx_err(PAREN_EXPECTED); 
}
/* Return from a function. */
void func_ret(void)
{
  int value;
  value = 0;
  /* get return value, if any */
  eval_exp(&value);
  ret_value = value;
}
/* Push local variable */
void local_push(struct var_type i)
{
  if(lvartos>NUM_LOCAL_VARS)
   sntx_err(TOO_MANY_LVARS);
  local_var_stack[lvartos] = i;
  lvartos++;
}
/* Pop index into local variable stack. */
func_pop(void)
{
  functos--;
  if(functos<0) sntx_err(RET_NOCALL);
  return(call_stack[functos]);
}
/* Push index of local variable stack. */
void func_push(int i)
{
  if(functos>NUM_FUNC)
   sntx_err(NEST_FUNC);
  call_stack[functos] = i;
  functos++;
}
/* Assign a value to a variable. */
void assign_var(char *var_name, int value)
{
  register int i;
  /* first, see if it's a local variable */
  for(i=lvartos-1; i>=call_stack[functos-1]; i--)  {
    if(!strcmp(local_var_stack[i].var_name, var_name)) {
      local_var_stack[i].value = value;
      return;
    }
  }
  if(i < call_stack[functos-1]) 
  /* if not local, try global var table */
    for(i=0; i<NUM_GLOBAL_VARS; i++)
      if(!strcmp(global_vars[i].var_name, var_name)) {
        global_vars[i].value = value;
        return;
      }
  sntx_err(NOT_VAR); /* variable not found */
}
/* Find the value of a variable. */
int find_var(char *s)
{
  register int i;
  /* first, see if it's a local variable */
  for(i=lvartos-1; i>=call_stack[functos-1]; i--) 
    if(!strcmp(local_var_stack[i].var_name, token))
      return local_var_stack[i].value;
  /* otherwise, try global vars */
  for(i=0; i<NUM_GLOBAL_VARS; i++)
    if(!strcmp(global_vars[i].var_name, s))
      return global_vars[i].value;
  sntx_err(NOT_VAR); /* variable not found */
}
/* Determine if an identifier is a variable. Return
   1 if variable is found; 0 otherwise.
*/
int is_var(char *s)
{
  register int i;
  /* first, see if it's a local variable */
  for(i=lvartos-1; i>=call_stack[functos-1]; i--) 
    if(!strcmp(local_var_stack[i].var_name, token))
      return 1;
  /* otherwise, try global vars */
  for(i=0; i<NUM_GLOBAL_VARS; i++)
    if(!strcmp(global_vars[i].var_name, s))
      return 1;
  return 0; 
}
/* Execute an IF statement. */
void exec_if(void)
{
  int cond;
  eval_exp(&cond); /* get left expression */
  if(cond) { /* is true so process target of IF */
    interp_block();
  }
  else { /* otherwise skip around IF block and
	    process the ELSE, if present */
    find_eob(); /* find start of next line */
    get_token();
    if(tok!=ELSE) {
      putback();  /* restore token if
                     no ELSE is present */
      return;
    }
    interp_block();
  }
}
/* Execute a while loop. */
void exec_while(void)
{
  int cond;
  char *temp;
  putback();
  temp = prog;  /* save location of top of while loop */
  get_token();
  eval_exp(&cond);  /* check the conditional expression */
  if(cond) interp_block();  /* if true, interpret */
  else {  /* otherwise, skip around loop */
    find_eob();
    return;
  }
  prog = temp;  /* loop back to top */
}
/*Execute a do loop. */
void exec_do(void)
{
  int cond;
  char *temp;
  putback();
  temp = prog;  /* save location of top of do loop */
  get_token(); /* get start of loop */
  interp_block(); /* interpret loop */
  get_token(); 
  if(tok!=WHILE) sntx_err(WHILE_EXPECTED);
  eval_exp(&cond); /* check the loop condition */
  if(cond) prog = temp; /* if true loop; otherwise, 
                           continue on */
}
/* Find the end of a block. */
void find_eob(void)
{
  int brace;
  get_token();
  brace = 1;
  do {
    get_token();
    if(*token=='{') brace++;
    else if(*token=='}') brace--;
  } while(brace);
}
/* Execute a while loop. */
void exec_for(void)
{
  int cond;
  char *temp, *temp2;
  int brace ;
  get_token();
  eval_exp(&cond);  /*initialization expression */
  if(*token!=';') sntx_err(SEMI_EXPECTED);
  prog++; /* get past the ; */
  temp = prog;
  for(;;) {
    eval_exp(&cond);  /* check the condition */    
    if(*token!=';') sntx_err(SEMI_EXPECTED);
    prog++; /* get past the ; */
    temp2 = prog;
    /* find the start of the for block */
    brace = 1;
    while(brace) {
      get_token();
      if(*token=='(') brace++;
      if(*token==')') brace--;
    }
    if(cond) interp_block();  /* if true, interpret */
    else {  /* otherwise, skip around loop */
      find_eob();
      return;
    }
    prog = temp2;
    eval_exp(&cond); /* do the increment */
    prog = temp;  /* loop back to top */
  } 
}