Brainf**k

main.c

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

static char buf[30000];
static char input[30000];
static int idx = 0;
static char *ptr = buf;

void interpreter(int start);

/**
 * Brainf**k interpreter implemented in C.
 * ref. https://ja.wikipedia.org/wiki/Brainfuck
 */
int main(void)
{
    char c;
    int i = 0;
    for (i = 0;; i++) {
        c = getchar();
        input[i] = c;
        if (c == EOF) {
            break;
        }
    }
    interpreter(-1);
    return 0;
}

void interpreter(int start)
{
    char c;

    idx++;
    for (;;) {
        // printf("%d", idx);
        c = input[idx];
        if (c == EOF) {
            break;
        }
        // printf("%c\n", c);

        if (c == '>') {
            ptr++;
        } else if (c == '<') {
            ptr--;
        } else if (c == '+') {
            (*ptr)++;
        } else if (c == '-') {
            (*ptr)--;
        } else if (c == '.') {
            putchar(*ptr);
        } else if (c == ',') {
            *ptr = getchar();
        } else if (c == '[') {
            if (*ptr) {
                interpreter(idx-1);
            } else {
                for (;;) {
                    c = input[idx];
                    // printf("%c\n", c);
                    if (c == ']') {
                        break;
                    }
                    idx++;
                }
            }
        } else if (c == ']') {
            idx = start;
            return;
        } else {
            ; // skip
        }
        idx++;
    }
    return;
}