A brainfuck interpreter in C with dynamic memory allocation

bf_redux.c

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


char* add_cell(char* tape, size_t *tape_size) {
    char* tmp = (char*)realloc(tape, (++*tape_size) * sizeof(char));

    if (tmp == NULL) {
        printf("FATAL: Could not reallocate tape memory!");
        exit(1);
    }

    if (!tape[*tape_size-1]) {
        tape[*tape_size-1] = 0;
    }

    return tmp;
}


char* remove_cell(char* tape, size_t *tape_size, char* ptr) {
    char tail = tape[*tape_size-1];

    // if the cell is zeroed and not pointed to currently
    if (tail == 0 && ptr != &tape[*tape_size-1] && *tape_size > 0) {
        char* tmp = (char*)realloc(tape, (--*tape_size) * sizeof(char));
        if (tmp == NULL) {
            printf("FATAL: Could not reallocate tape memory!");
            exit(1);
        }
        return tmp;
    }
    return tape;
}


void interpret(char* input) {
    size_t tape_size = 1;
    char* tape;
    char* ptr;

    char current_char;
    size_t i;
    size_t loop;

    tape = (char*)calloc(tape_size, sizeof(char));
    ptr = tape;

    for (i = 0; input[i] != 0; i++) {
        current_char = input[i];
        if (current_char == '>') {
            tape = add_cell(tape, &tape_size);
            ++ptr;
        } else if (current_char == '<') {
            --ptr;
        } else if (current_char == '+') {
            ++*ptr;
        } else if (current_char == '-') {
            --*ptr;
        } else if (current_char == '.' ) {
            putchar(*ptr);
        } else if (current_char == ',') {
            *ptr = getchar();
        } else if (current_char == ']' && *ptr) {
            loop = 1;
            while (loop > 0) {
                current_char = input[--i];
                if (current_char == '[') {
                    loop--;
                } else if (current_char == ']') {
                    loop++;
                }
            }
        }
        // garbage collect
        tape = remove_cell(tape, &tape_size, ptr);
    }
    free(tape);
}


int main() {
    interpret(",[.[-],]");  // outputs input
    return 0;
}

Missing the '[' case.