bas.c.rar_Interpreter_basic_basic interpreter_portable_simple in

I like fooling around with tiny interpreters. A long time ago, I found DDS BASIC by Diomidis Spinellis, a winner in the 1990 IOCCC. I decided a fun project would be to modify DDS BASIC, adding enough features so that it could run (what else) Tiny Star Trek. So, I did the following to the original dds.c: Removed enough obfuscation to allow modifications All BASIC keywords are now lowercase Accepts command line input Added a single integer array: @ Added the statement continuation character: ":" Added 'prompt' on input statement Enhanced print statement Added rnd(), asc() and abs() functions Converted the recursive descent expression parser to precedence climbing Enhanced 'if' to allow statements to follow (instead of just "if expr then line number") In the spirit of the original DDS, no error checking - you've been warned! And here it is (still pretty small at only 318 lines of code): Code: #include <stdio.h> #include <stdlib.h> #include <string.h> #include <ctype.h> enum {false, true}; typedef char* CHARP; int *gosub_stackp,gosub_stack[999],line[999],line_off[999], lim[999],var[999],linenum,i,inquote; int *gosub_foop, gosub_foo[999], at[999]; char buff[999],two[2]; CHARP m[12*999],p,q,s,d; // p used as expr ptr FILE *f; #if !defined(randomize) #define randomize() #endif #if !defined(random) static int random_seed = 0; static int random(int n) { random_seed = random_seed * 1103515245 + 12345; return ((((random_seed / 65536) % 32768) & 0x7fffffff) % n); } #endif int findop(char op) { switch (op) { case '=': return 1; case '#': return 1; // <>, not equal case '<': return 2; case '>': return 2; case '$': return 2; // <= case '!': return 2; // >= case '+': return 3; case '-': return 3; case '*': return 4; case '/': return 4; } return -1; } int evalbinary(char op, int l1, int l2) { switch (op) { case '=': return l1 == l2; case '#': return l1 != l2; // <>, not equal case '<': return l1 < l2; case '>': return l1 > l2; case '$': return l1 <= l2; // <= case '!': return l1 >= l2; // >= case '+': return l1 + l2; case '-': return l1 - l2; case '*': return l1 * l2; case '/': return l1 / l2; } return 0; } int expr(int prec) { int o, this_prec; if (*p=='-') { p++; o = -expr(999); } else if (isdigit(*p)) { o = strtol(p,&p,10); // can't use 0 for base, because of hex pbm. } else if (*p=='(') { ++p; o = expr(0); ++p; } else if (*p == '@') { // @(exp) ++p; o = at[expr(999)]; } else if (memcmp(p, "rnd", 3) == 0) { // rnd(exp) p += 3; o = expr(999); o = random(o) + 1; } else if (memcmp(p, "abs", 3) == 0) { // abs(exp) p += 3; o = abs(expr(999)); } else if (memcmp(p, "asc", 3) == 0) { // asc("x") p += 5; o = *p; p += 3; } else o = var[*p++]; while ((this_prec = findop(*p)) > 0 && this_prec >= prec) { char op = *p++; o = evalbinary(op, o, expr(this_prec + 1)); } return o; } void print_string(void) { int width; ++p; width = strchr(p, '"') - p; printf("%*.*s", width, width, p); p += width + 1; } // 'print' [[#num ',' ] expr { ',' [#num ','] expr }] [','] {':' stmt} eol // expr can also be a literal string void print(void) { int print_nl; print_nl = true; for (;;) { int width = 0; if (*p == ':' || *p == '\0') break; print_nl = true; if (*p == '#') { ++p; width = expr(0); if (*p == ',') ++p; } if (*p == '"') print_string(); else printf("%*d", width, expr(0)); if (*p == ',' || *p == ';') { ++p; print_nl = false; } else break; } if (print_nl) printf("\n"); } void enterline(void) { char *s = buff; while (*s && isspace(*s)) s++; linenum=atoi(s); if (m[linenum]) free(m[linenum]); if ((p=strstr(s, " ")) != NULL) strcpy(m[linenum]=malloc(strlen(p)),p+1); else m[linenum]=0; } void load(char *fn) { f=fopen(fn,"r"); while(fgets(buff,999,f)) (*strstr(buff,"\n")=0,enterline()); fclose(f); } void run(void) { int offset = 0; gosub_stackp=gosub_stack; gosub_foop=gosub_foo; linenum=1; for(i=0; i<999; var[i++]=0) ; // set s to point to first char of line while(linenum) { int if_cont; while((s=m[linenum]) == 0) linenum++; if (!strstr(s,"\"")) { while((p=strstr(s,"<>")) != 0) *p++='#',*p=' '; while((p=strstr(s,"<=")) != 0) *p++='$',*p=' '; while((p=strstr(s,">=")) != 0) *p++='!',*p=' '; } // remove extra spaces, line copied to buff d=buff; while((*two=*s) != '\0') { if(*s=='"') inquote++; if(inquote&1||!strstr(" \t",two)) *d++=*s; s++; } inquote= *d = 0; s = buff; line_processed: p = (s += offset); offset = if_cont = 0; if(s[1] == '=') { // assignment a=exp p=s+2; var[*s]=expr(0); } else if (s[0] == '@') { // assignment: @(exp)=exp int ndx; p = s + 1; ndx = expr(999); // use high prec to force end at ')' ++p; at[ndx] = expr(0); } else switch(*s) { case'e': // end case's': // stop linenum=-1; break; case'r': // rem and return if (s[2]!='m') { linenum=*--gosub_stackp; // return offset=*--gosub_foop; } break; case'i': // input [constant_string,] var and if if (s[1]=='n') { // input int tmp; char in_buff[20]; d = p = &s[5]; if (*p == '"') { print_string(); d = ++p; // skip ',' } tmp = *d; p = fgets(in_buff, sizeof(in_buff) - 2, stdin); var[tmp] = isdigit(*p) ? expr(0) : *p; p = ++d; } else { // if p=s+2; if (expr(0)) { --p; if_cont = true; } else p = 0; } break; case'p': // print string and expr p = &s[5]; print(); break; case'g': // goto, gosub p=s+4; if (s[2]=='s') { // gosub *gosub_stackp++=linenum; p++; } linenum=expr(0)-1; if (s[2] == 's') // gosub *gosub_foop++ = (*p == ':') ? p - buff + 1: 0; p = 0; break; case'f': // for *(q=strstr(s,"to"))=0; p=s+5; var[i=s[3]]=expr(0); p=q+2; lim=expr(0); line=linenum; line_off = (*p == ':') ? p - buff + 1: 0; break; case'n':