basic.c

用C语言写的YABasic解释器

/* A tiny BASIC interpreter */

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

#define NUM_LAB 100
#define LAB_LEN 10 
#define FOR_NEST 25
#define SUB_NEST 25
#define PROG_SIZE 10000

#define DELIMITER  1
#define VARIABLE  2
#define NUMBER    3
#define COMMAND   4
#define STRING   5
#define QUOTE   6

#define PRINT 1
#define INPUT 2
#define IF    3
#define THEN  4
#define FOR   5
#define NEXT  6
#define TO    7
#define GOTO  8
#define EOL   9
#define FINISHED  10
#define GOSUB 11
#define RETURN 12
#define END 13

char *prog;  /* holds expression to be analyzed */
jmp_buf e_buf; /* hold environment for longjmp() */

int variables[26]= {    /* 26 user variables,  A-Z */
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0
};

struct commands { /* keyword lookup table */
  char command[20];
  char tok;
} table[] = { /* Commands must be entered lowercase */
  "print", PRINT, /* in this table. */
  "input", INPUT,
  "if", IF,
  "then", THEN,
  "goto", GOTO,
  "for", FOR,
  "next", NEXT,
  "to", TO,
  "gosub", GOSUB,
  "return", RETURN,
  "end", END,
  "", END  /* mark end of table */
};

char token[80];
char token_type, tok;

struct label {
  char name[LAB_LEN];
  char *p;  /* points to place to go in source file*/
};
struct label label_table[NUM_LAB];

char *find_label(), *gpop();

struct for_stack {
  int var; /* counter variable */
  int target;  /* target value */
  char *loc;
} fstack[FOR_NEST]; /* stack for FOR/NEXT loop */
struct for_stack fpop();

char *gstack[SUB_NEST]; /* stack for gosub */

int ftos;  /* index to top of FOR stack */
int gtos;  /* index to top of GOSUB stack */

void print(), scan_labels(), find_eol(), exec_goto();
void exec_if(), exec_for(), next(), fpush(), input();
void gosub(), greturn(), gpush(), label_init();
void serror(), get_exp(), putback();
void level2(), level3(), level4(), level5(), level6(), primitive();
void unary(), arith();
int load_program(p, fname);

int main(int argc, char *argv[])
{
  char in[80];
  int answer;
  char *p_buf;
  char *t;

  if(argc!=2) {
    printf("usage: run <filename>\n");
    exit(1);
  }

  /* allocate memory for the program */
  if(!(p_buf=(char *) malloc(PROG_SIZE))) {
    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 the long jump buffer */

  prog = p_buf;
  scan_labels(); /* find the labels in the program */
  ftos = 0; /* initialize the FOR stack index */
  gtos = 0; /* initialize the GOSUB stack index */
  do {
    token_type = get_token();
    /* check for assignment statement */
    if(token_type==VARIABLE) {
      putback(); /* return the var to the input stream */
      assignment(); /* must be assignment statement */
    }
    else /* is command */
      switch(tok) {
        case PRINT:
   print();
     break;
        case GOTO:
   exec_goto();
   break;
 case IF:
   exec_if();
   break;
 case FOR:
   exec_for();
   break;
 case NEXT:
   next();
   break;
   case INPUT:
   input();
   break;
        case GOSUB:
   gosub();
   break;
 case RETURN:
   greturn();
   break;
        case END:
			system("pause");
   exit(0);
      }
  } while (tok != FINISHED);
}

/* Load a program. */
load_program(char *p, char *fname)
{
  FILE *fp;
  int i=0;

  if(!(fp=fopen(fname, "rb"))) return 0;
  
  i = 0;
  do {
    *p = getc(fp);
    p++; i++;
  } while(!feof(fp) && i<PROG_SIZE);
  *(p-2) = '\0'; /* null terminate the program */
  fclose(fp);
  return 1;
}

/* Assign a variable a value. */
assignment()
{
  int var, value;

  /* get the variable name */
  get_token();
  if(!isalpha(*token)) {
    serror(4);
    return;
  }

  var = toupper(*token)-'A';
 
  /* get the equals sign */
  get_token();
  if(*token!='=') {
    serror(3);
    return;
  }

  /* get the value to assign to var */
  get_exp(&value);

  /* assign the value */
  variables[var] = value;
}

/* Execute a simple version of the BASIC PRINT statement */
void print()
{
  int answer;
  int len=0, spaces;
  char last_delim;

  do {
    get_token(); /* get next list item */
    if(tok==EOL || tok==FINISHED) break;
    if(token_type==QUOTE) { /* is string */
      printf(token);
      len += strlen(token);
      get_token();
    }
    else { /* is expression */
      putback();
      get_exp(&answer);
      get_token();
      len += printf("%d", answer);
    }
    last_delim = *token; 

    if(*token==';') {
      /* compute number of spaces to move to next tab */
      spaces = 8 - (len % 8); 
      len += spaces; /* add in the tabbing position */
      while(spaces) { 
 printf(" ");
        spaces--;
      }
    }
    else if(*token==',') /* do nothing */;
    else if(tok!=EOL && tok!=FINISHED) serror(0); 
  } while (*token==';' || *token==',');

  if(tok==EOL || tok==FINISHED) {
    if(last_delim != ';' && last_delim!=',') printf("\n");
  }
  else serror(0); /* error is not , or ; */

}

/* Find all labels. */
void scan_labels()
{
  int addr;
  char *temp;

  label_init();  /* zero all labels */
  temp = prog;   /* save pointer to top of program */

  /* if the first token in the file is a label */
  get_token();
  if(token_type==NUMBER) {
    strcpy(label_table[0].name,token);
    label_table[0].p=prog;
  }

  find_eol();
  do {     
    get_token();
    if(token_type==NUMBER) {
      addr = get_next_label(token);
      if(addr==-1 || addr==-2) {
          (addr==-1) ?serror(5):serror(6);
      }
      strcpy(label_table[addr].name, token);
      label_table[addr].p = prog;  /* current point in program */
    }
    /* if not on a blank line, find next line */
    if(tok!=EOL) find_eol();
  } while(tok!=FINISHED);
  prog = temp;  /* restore to original */
}

/* Find the start of the next line. */
void find_eol()
{
  while(*prog!='\n'  && *prog!='\0') ++prog;
  if(*prog) prog++;
}

/* Return index of next free position in label array. 
   A -1 is returned if the array is full.
   A -2 is returned when duplicate label is found.
*/
get_next_label(char *s)
{
  register int t;

  for(t=0;t<NUM_LAB;++t) {
    if(label_table[t].name[0]==0) return t;
    if(!strcmp(label_table[t].name,s)) return -2; /* dup */
  }

  return -1;
}

/* Find location of given label.  A null is returned if
   label is not found; otherwise a pointer to the position
   of the label is returned.
*/
char *find_label(char *s)
{
  register int t;

  for(t=0; t<NUM_LAB; ++t) 
    if(!strcmp(label_table[t].name,s)) return label_table[t].p;
  return '\0'; /* error condition */
}

/* Execute a GOTO statement. */
void exec_goto()
{

  char *loc;

  get_token(); /* get label to go to */
  /* find the location of the label */
  loc = find_label(token);
  if(loc=='\0')
    serror(7); /* label not defined */

  else prog=loc;  /* start program running at that loc */
}

/* Initialize the array that holds the labels. 
   By convention, a null label name indicates that
   array position is unused.
*/
void label_init()
{
  register int t;

  for(t=0; t<NUM_LAB; ++t) label_table[t].name[0]='\0';
}

/* Execute an IF statement. */
void exec_if()
{
  int x , y, cond;
  char op;

  get_exp(&x); /* get left expression */

  get_token(); /* get the operator */
  if(!strchr("=<>", *token)) {
    serror(0); /* not a legal operator */
    return;
  }
  op=*token;

  get_exp(&y); /* get right expression */

  /* determine the outcome */
  cond = 0;
  switch(op) {
    case '<':
      if(x<y) cond=1;
      break;
    case '>':
      if(x>y) cond=1;
      break;
    case '=':
      if(x==y) cond=1;
      break;
  }
  if(cond) { /* is true so process target of IF */
    get_token();
    if(tok!=THEN) {
      serror(8);
      return;
    }/* else program execution starts on next line */
  }
  else find_eol(); /* find start of next line */
}

/* Execute a FOR loop. */
void exec_for()
{
  struct for_stack i;
  int value;

  get_token(); /* read the control variable */
  if(!isalpha(*token)) {
    serror(4);
    return;
  }

  i.var=toupper(*token)-'A'; /* save its index */

  get_token(); /* read the equals sign */
  if(*token!='=') {
    serror(3);