A quite functional implementation of the atoi function in C

ft_atoi.c

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

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

const char *tr_trimhead(const char *s) {
  return s && is_space(*s) ? tr_trimhead(s+1) : s;
}

int tr_pow(const int n, const int e, const int res) {
  return e < 1 ? res : tr_pow(n, e-1, n*res);
}

int ft_pow(const int n, const int e) {
  return tr_pow(n, e, 1);
}

int tr_count_while(const char *s, const char count, int (*check)(char)) {
  return !check(*s) ? count : tr_count_while(s+1, count+1, check);
}

int raise(const char digit, const int exp) {
  return (digit - '0') * tr_pow(10, exp, 1);
}

int tr_atoi(const char *n, const int exp, const int res, int (*check)(char)) {
  return !check(*n) ? res : tr_atoi(n+1, exp-1, res+raise(*n, exp), check);
}

int ft_atoi(const char *s) {
  const char *num     = tr_trimhead(s);
  const int   sign    = num && '-' == *num ? -1 : 1;
  const char *digits  = num && ('-' == *num || '+' == *num) ? ++num : num;
  const int   max_exp = tr_count_while(digits, 0, &is_digit) - 1;

  return !num ? 0 : sign * tr_atoi(digits, max_exp, 0, &is_digit);
}