stream brainfuck interpreter (Also supports Alphuck,Ecstatic

bf.c

/*
A stream brainfuck interpreter, the input will be read only once.

Also supports Alphuc (https://esolangs.org/wiki/Alphuck),
FuckbeEs (https://esolangs.org/wiki/FuckbeEs),
ReverseFuck (https://esolangs.org/wiki/ReverseFuck),
and the whole TrivialBrainfuckSubstitution (https://esolangs.org/wiki/TrivialBrainfuckSubstitution) language family.

*/

#include<stdio.h>
#include<stdlib.h>
#include<string.h>

void usage(char *name){
	fprintf(stderr,"USAGE:\n\t%s [-a <alphabet>] [<file>]\n\tOR\n\tbf -?\n\tOR\n\tbf --help"
		"\n\t-? / --help\tA switch to show this help menu. The program will terminate after showing this usage information."
		"\n\t<alphabet>\t[OPTIONAL] A sequence of eight unique characters to be used instead of the default alphabet of \"<>+-,.[]\"."
		"\n\t<file>\t[OPTIONAL] The file to be interpreted. If not supplied, stdin will be used.\n"
		"\n\tThe program will put only the output generated by the Brainfuck program to stdout and any errors to stderr.",name);
}

char alphabet[8];

char *buf=NULL;
unsigned long buf_pos=0;
unsigned long buf_size=0;

unsigned long *loops=NULL;
unsigned long level=0;

// true if currently reading from the buffer instead of stdin
short readback=0;

char memory[30000];
int mem_pos=0;

int skip(){
	int level=1;
	char c;
	do{
		if(readback){
			c=buf[buf_pos++];
		}else{
			c=getchar();
		}

		if(c==']'){
			level--;
		}else if(c=='['){
			level++;
		}
	}while(level>0);
}

int buf_put(char c){
	// keep chars in buffer
	if(buf_size==buf_pos){
		// try to resize the buffer to fit the additional characters
		buf=realloc(buf,(buf_size+=40)*sizeof(char));
		if(buf==NULL){
			fputs("?memory\n",stderr);
			return 1;
		}
	}
	buf[buf_pos++]=c;
}

int check_alph(char* a){
	// check alphabet length
	if(strlen(a)!=8) return 0;

	/*
	check that characters are unique;
	check does not have to be performed for last character,
	since it has been crosschecked by every other character before
	*/
	for(int i=0;i<7;i++)
		if(strchr(a+i+1,a[i])!=NULL) return 0;

	return 1;
}

int main(int argc,char** argv){

	memcpy(alphabet,"<>+-,.[]",8); // setup default alphabet

	for(int i=1;i<argc;i++){
		if(strcmp(argv[i],"-?")==0||strcmp(argv[i],"--help")==0){
			fputs("Brainfuck interpreter\n",stderr);
			usage(argv[0]);
			return 0;
		}else if(strcmp(argv[i],"-a")==0){
			if(check_alph(argv[++i])){
				memcpy(alphabet,argv[i++],8);
			}else{
				fputs("?alphabet",stderr);
				usage(argv[0]);
				return 1;
			}
		}else{
			// just redirect file and read from stdin anyway
			if(freopen(argv[1],"r",stdin)==NULL){
				fputs("file not found\n",stderr);
				usage(argv[0]);
				return 1;
			}
		}
	}

	while(!feof(stdin)){
		char c;
		if(readback){
			c=buf[buf_pos++];
		}else{
			// transformed to alphabet index
			c=getchar();

			if(strchr(alphabet,c)==NULL)
				// this character is not relevant
				continue;

			c=strchr(alphabet,c)-alphabet;
		}

		switch(c){// switch on the index in the alphabet
			case 0:
				if(mem_pos==0)
					mem_pos=30000;
				mem_pos--;
				break;
			case 1:
				mem_pos=(mem_pos+1)%30000;
				break;
			case 2:
				memory[mem_pos]++;
				break;
			case 3:
				memory[mem_pos]--;
				break;
			case 4:
				printf("%d",memory[mem_pos]);
				break;
			case 5:
				printf("%c",memory[mem_pos]);
				break;
			case 6:
				if(memory[mem_pos]==0){
					skip();
				}else if (!readback){
					// if readback is already set, this will have already been saved
					loops=realloc(loops,(level++)*sizeof(unsigned long));
					if(level>1){
						if(loops==NULL){
							fputs("?memory\n",stderr);
							return 1;
						}
						loops[level-2]=buf_pos;
					}
				}
				break;
			case 7:
				if(level==0){
					fputs("?syntax missing opening bracket\n",stderr);
					return 2;
				}
				if(memory[mem_pos]==0){
					level--;
					if(level==0){
						readback=0;
						// reset buffer
						buf=realloc(buf,buf_pos=buf_size=0);
					}
				}else{
					if(!readback){
						buf_put(']');
						// go back to beginning of loop
						readback=1;
					}
					if(level==1){
						buf_pos=0;
					}else{
						buf_pos=loops[level-2];
					}
				}
				break;
		}

		if(level&&!readback&&c!=']'){
			// make sure the caracter is saved if needed
			buf_put(c);
		}
	}

	if(level){
		fputs("?syntax missing closing bracket\n",stderr);
		// free used memory
		free(buf);
		free(loops);
	}
}