safe_atoi.c

#include <stdio.h>

const int is_digit(const char c) {
    return c >= '0' && c <= '9';
}

const int is_space(const char c) {
    return c >= 8 && c <= 32;
}

/*
** 'tr_' stands for 'tail recursive'
*/
const int tr_pow(const int res, const int n, const int e) {
    return e < 1 ? res : tr_pow(res * n, n, e - 1);
}

const char *tr_dropwhile(const char *s, const int (*predicate)(const char)) {
    return s && predicate(*s) ? tr_dropwhile(s + 1, predicate) : s;
}

const int tr_count_while(const char *s, const char i, const int (*predicate)(const char)) {
    return s && !predicate(*s) ? i : tr_count_while(s + 1, i + 1, predicate);
}

const int tr_compute(const int res, const char *num, const int dec) {
    return num && !is_digit(*num) ? res : tr_compute(res + (*num - '0') * tr_pow(1, 10, dec), num + 1, dec - 1);
}

const int safe_atoi(const char *s) {
    const char *str = str ? tr_dropwhile(s, &is_space) : s;
    const int  sign = (str && *str == '-') ? -1 : 1;
    const char *num = (str && (sign == -1 || *str == '+')) ? ++str : str;

    return !num ? 0 : sign * tr_compute(0, num, tr_count_while(num, 0, &is_digit) - 1);
}


//////////////////////////////////////////////////
//                                              //
// TESTS                                        //
// run with : gcc safe_atoi.c && ./a.out        //
//                                              //
//////////////////////////////////////////////////

int ft_atoi(const char *s) {
    return (safe_atoi(s));
}

int main(void)
{
    const char *values[] = { "", "0", "-0", "ft", "+2", "--2", "++2", "42", "-42", " 87654321", "-12345678", "12345678987654321", "-12345678987654321", "3.14", "42.ft", "42ft", "42 ft", "ft 42", 0 };

    int i = 0;
    while (values[i]) {
        const int n1 = ft_atoi(values[i]);
        const int n2 = atoi(values[i]);

        printf("%sft_atoi(\"%s\") = %d\n   atoi(\"%s\") = %d\033[0m\n\n", (n1 == n2) ? "\033[92m" : "\033[91m", values[i], n1, values[i], n2);
        ++i;
    }

    // adding a check -> in case of NULL string (no segfault should occur)
    ft_atoi(0);
    return 0;
}