tiny BASIC interpreter C source

 

mybasic.c

비주얼 베이직 코드를 수행하는 인터프리터

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/* 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();

main(argc,argv)
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:
exit(0);
}
} while (tok != FINISHED);
}

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

if(!(fp=fopen(fname,"r"))) return 0;

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

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

/* get the variable a value */
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 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(s)
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(s)
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 */
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 equal sign */
if(*token!='=') {
serror(3);
return;
}

get_exp(&value); /* get initial value */

variables[i.var]=value;

get_token();
if(tok!=TO) serror(9); /* read and discard the TO */

get_exp(&i.target); /* get target value */

/* if loop can execute at least once,push info on stack */
if(value>=variables[i.var]) {
i.loc = prog;
fpush(i);
}
else /* otherwise,skip loop code altogether */
while(tok!=NEXT) get_token();
}

/* Execute a NEXT statement. */
void next()
{
struct for_stack i;

i=fpop(); /* read the info */

variables[i.var]++; /* increment control variable */
if(variables[i.var]>i.target) return; /* all don */
fpush(i); /* otherwise, restore the info */
prog = i.loc; /* loop */
}

/* Push function for the FOR stack */
void fpush(i)
struct for_stack i;
{
if(ftos>FOR_NEST)
serror(10);

fstack[ftos]=i;
ftos++;
}

struct for_stack fpop()
{
ftos--;
if(ftos<0) serror(11);
return(fstack[ftos]);
}

/* Execute a simple from of the BASIC INPUT command */
void input()
{
char str[80],var;
int i;

get_token(); /* see if prompt string is present */
if(token_type==QUOTE) {
printf(token); /* if so,print it and check for comma */
get_token();
if(*token!=',') serror(1);
get_token();
}
else printf("? "); /* otherwise,prompt with / */
var = toupper(*token)-'A'; /* get the input var */

scanf("%d",&i); /*read input */

variables[var] = i; /* store it */
}

/* Execute a GOSUB command. */
void gosub()
{
char *loc;

get_token();
/* find the label to call */
loc = find_label(token);
if(loc=='\0')
serror(7); /* label not defined */
else {
gpush(prog); /* save place to return to */
prog = loc; /* start program running at that loc */
}
}

/* Return from GOSUB */
void greturn()
{
prog = gpop();
}

/* GOSUB stack push function. */
void gpush(s)
char *s;
{
gtos++;

if(gtos==SUB_NEST) {
serror(12);
return;
}

gstack[gtos]=s;
}

/* GOSUB stack pop function. */
char *gpop()
{
if(gtos==0) {
serror(13);
return 0;
}
return(gstack[gtos--]);
}
/* recursive descent parser for integer expressions
which may include variables */

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

#define EOL 9
#define FINISHED 10

void level2(),level3(),level4(),level5();
void level6(),primitive(),arith(),unary();

/* Entry point into parser. */
get_exp(result)
int *result;
{
get_token();
if(! *token) {
serror(2);
return;
}
level2(result);
putback();
}
/* Add or subtract two terms. */
void level2(result)
int *result;
{
register char op;
int hold;

level3(result);
while((op = *token) == '+' || op == '-') {
get_token();
level3(&hold);
arith(op,result,&hold);
}
}

/* Multiply or divide two factors. */
void level3(result)
int *result;
{
register char op;
int hold;

level4(result);
while((op = *token) == '*' || op == '/' || op == '%') {
get_token();
level4(&hold);
arith(op,result,&hold);
}
}

/* Process integer exponent. */
void level4(result)
int *result;
{
int hold;

level5(result);
if(*token == '^') {
get_token();
level4(&hold);
arith('^',result,&hold);
}

}

/* Is a unary + or -. */
void level5(result)
int *result;
{
register char op;

op = 0;
if((token_type==DELIMITER) && *token == '+' || *token == '-') {
op = *token;
get_token();
}
level6(result);
if(op)
unary(op,result);
}

/* Process parenthesized expression. */
void level6(result)
int *result;
{
if((*token == '(') && (token_type == DELIMITER)) {
get_token();
level2(result);
if(*token != ')')
serror(1);
get_token();
}
else
primitive(result);
}

/* Find value of number or variable. */
void primitive(result)
int *result;
{
switch(token_type) {
case VARIABLE:
*result = find_var(token);
get_token();
return;
case NUMBER:
*result = atoi(token);
get_token();
return;
default:
serror(0);
}
}

/* Perform the specified arithmetic. */
void arith(o,r,h)
char o;
int *r, *h;
{
register int t, ex;

switch(o) {
case '-':
*r = *r - *h;
break;
case '+':
*r = *r + *h;
break;
case '*':
*r = *r * *h;
break;
case '/':
*r = (*r) / (*h);
break;
case '%':
t = (*r) / (*h);
*r = *r - (t*(*h));
break;
case '^':
ex = *r;
if(*h == 0) {
*r = 1;
break;
}
for(t = *h-1; t>0; --t) *r= (*r) * ex;
break;
}
}
/* Reverce the sign. */
void unary(o,r)
char o;
int *r;
{
if(o == '-') *r = -(*r);
}

/* Find the value of a variable */
int find_var(s)
char *s;
{
if(!isalpha(*s)) {
serror(4); /* not a variable */
return 0;
}
return variables[toupper(*token) - 'A'];
}

/* Display an error message */
serror(error)
int error;
{
static char *e[] = {
"syntex error",
"unbalenced parentheses",
"no expression present",
"equal sign expected",
"not a variable",
"label table full",
"duplicate label",
"undefined label",
"THEN expected",
"TO expected",
"too many nested FOR loops",
"NEXT without FOR",
"too many nested GOSUBs",
"RETURN without GOSUB"
};
printf("%s\n",e[error]);
longjmp(e_buf,1); /* return to save point */
}

/* Get a token. */
get_token()
{
register char *temp;

token_type=0; tok=0;
temp = token;
if(*prog == '\0') { /* end of file */
*token = 0;
tok = FINISHED;
return(token_type=DELIMITER);
}

while(iswhite(*prog)) ++prog; /* skip over white space */

if(*prog=='\n') { /* crLf */
++prog;
tok = EOL; *token='\n';
token[1]='\0';
return(token_type = DELIMITER);
}

if(strchr("+-*^/%=;(),><",*prog)) { /* delimiter */
*temp = *prog;
prog++; /* advanced to next position */
temp++;
*temp=0;
return(token_type=DELIMITER);
}

if(*prog=='"') { /* quoted string */
prog++;
while(*prog!='"' && *prog!='\n') *temp++ = *prog++;
if(*prog=='\n') serror(1);
prog++; *temp=0;
return(token_type=QUOTE);
}

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=STRING;
}

*temp='\0';

/* see if a string is a command or a variable */
if(token_type==STRING) {
tok=look_up(token); /* convert to internal rep */
if(!tok) token_type=VARIABLE;
else token_type=COMMAND; /* is a command */
}
return token_type;
}

/* Return a token to input stream. */
putback()
{
char *t;

t = token;
for(; *t; t++) prog--;
}

/* Look up a token's internal representation in the
token table */
look_up(s)
char *s;
{
register int i,j;
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 true if c is a delimiter. */
isdelim(c)
char c;
{
if(strchr(" ;,+-<>/*%^=()",c) || c==9 || c=='\n' || c==0)
return 1;
return 0;
}

/* Return 1 if c is space or tab */
iswhite(c)
char c;
{
if(c==' ' || c=='\t') return 1;
else return 0;
}