bf.c

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

// initialize the tape with 30,000 zeroes
unsigned char tape[30000] = {0};

// set the pointer to point at the left-most cell of the tape
unsigned char* ptr = tape;

int main(int argc, char* argv[]) {
    int current_char;
    size_t loop;
    FILE* inp = fopen(argv[1], "r");
    if (inp == NULL) {
        perror(argv[0]);
        return 1;
    }

    setvbuf(stdout, NULL, _IONBF, 0);

    for (;;) {
        current_char = fgetc(inp);
        if (current_char == EOF) {
            if (feof(inp) == 0)
                perror(argv[0]);
            return 0;
        }

        if (current_char == '>') {
            ++ptr;
        } else if (current_char == '<') {
            --ptr;
        } else if (current_char == '+') {
            if (*ptr == 255)
                *ptr = 0;
            else
                ++*ptr
        } else if (current_char == '-') {
            if (*ptr == 0)
                *ptr = 255;
            else
                --*ptr
        } else if (current_char == '.' ) {
            if (putchar(*ptr) == EOF) {
                if (feof(stdout) == 0)
                    perror(argv[0]);
                return 0;
            }
        } else if (current_char == ',') {
            int x = getchar();
            if (x == EOF) {
                if (feof(stdin) == 0)
                    perror(argv[0]);
                return 0;
            }
            *ptr = x;
        } else if (current_char == '[' && *ptr == 0) {
            loop = 1;
            while (loop > 0) {
                current_char = fgetc(inp);
                if (current_char == EOF) {
                    if (feof(inp) == 0)
                        perror(argv[0]);
                    fprintf(stderr, "Unbalanced parens!\n");
                    return 1;
                }
                if (current_char == '[') {
                    loop++;
                } else if (current_char == ']') {
                    loop--;
                }
            }
        } else if (current_char == ']' && *ptr) {
            loop = 1;
            while (loop > 0) {
                if (fseek(inp, -2L, SEEK_CUR) != 0) {
                    perror(argv[0]);
                    return 0;
                }
                current_char = fgetc(inp);
                if (current_char == EOF) {
                    if (feof(inp) == 0)
                        perror(argv[0]);
                    fprintf(stderr, "Unbalanced parens!\n");
                    return 1;
                }
                if (current_char == '[') {
                    loop--;
                } else if (current_char == ']') {
                    loop++;
                }
            }
        }
    }
}

bf.py

from collections import defaultdict

def execute(code, inp = "", steps=10000):
    tape = defaultdict(lambda: 0)
    cur = 0
    loc = 0
    read = 0
    ret = ""

    while steps > 0:
        try:
            c = code[loc]
        except IndexError:
            return ret
        if c == ">":
            cur += 1
        elif c == "<":
            cur -= 1
        elif c == "+":
            if tape[cur] == 255:
                tape[cur] = 0
            else:
                tape[cur] += 1
        elif c == "-":
            if tape[cur] == 0:
                tape[cur] = 255
            else:
                tape[cur] -= 1
        elif c == ".":
            ret += chr(tape[cur])
        elif c == ",":
            try:
                tape[cur] = ord(inp[read])
                read += 1
            except IndexError:
                raise Exception("No data left to read from input")
        elif c == "[" and tape[cur] == 0:
            loop = 1
            loc += 1
            while loop > 0:
                try:
                    c = code[loc]
                except IndexError:
                    raise Exception("Unbalanced parens")
                if c == "[":
                    loop += 1
                elif c == "]":
                    loop -= 1
                loc += 1
        elif c == "]" and tape[cur] != 0:
            loop = 1
            loc -= 1
            while loop > 0:
                try:
                    c = code[loc]
                except IndexError:
                    raise Exception("Unbalanced parens")
                if c == "[":
                    loop -= 1
                elif c == "]":
                    loop += 1
                loc -= 1
        loc += 1
        steps -= 1
    return "You used too many steps!"