LL(1) Calculator Written in C

calculator.c

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

#define BUFFER_SIZE 256

int value();
int integer();
int factor();
int term();

const char *expr = NULL;
int at = 0;

int main() {
    puts("Please enter an arithmetic expression:");
    char buf[BUFFER_SIZE];
    memset(buf, BUFFER_SIZE, 0);
    gets(buf);
    expr = buf;
    printf("The result is %d.\n", factor());
    return 0;
}

int value() {
    if (expr[at] == '(') {
        at++;
        int v = factor();
        if (expr[at] == ')') {
            at++;
            return v;
        } else {
            printf("Expected a bracket at %d instead of %c.\n", at, expr[at]);
            exit(1);
        }
    } else {
        return integer();
    }
}

int integer() {
    int v = 0;
    int negative = expr[at] == '-';
    if (negative) {
        at++;
    }
    if (isdigit(expr[at])) {
        do {
            v *= 10;
            v += expr[at++] - '0';
        } while(isdigit(expr[at]));
    } else {
        printf("Expected a integer at position %d.\n", at);
        exit(1);
    }
    return negative ? -v : v;
}

int factor() {
    int v = term();
    while (expr[at] == '+' || expr[at] == '-') {
        if (expr[at++] == '+') {
            v += term();
        } else {
            v -= term();
        }
    }
    return v;
}

int term() {
    int v = value();
    while (expr[at] == '*' || expr[at] == '/') {
        if (expr[at++] == '*') {
            v *= value();
        } else {
            v /= value();
        }
    }
    return v;
}