Sum posix argv integers

sum-args.c

/*
 *  Manoel Vilela © 2019
 *
 *  Why this? Just because I was bored
 *  Date: Tue 12 Mar 2019 12:21:48 AM -03
 */

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

/*
 * Power calculate the base^n operation
 * @param base value to calculate exponentation
 * @param n the exponent
 */
long power(int base, int n) {
    if (n == 0) {
        return 1;
    }
    return base * power(base, n - 1);
}

/*
 * Calculate the size of a string
 * @param string a \0 terminated string
 */
unsigned int string_len(char *string) {
    int i;
    if (string == NULL) {
        return 0;
    }

    for (i = 0; string[i] != '\0'; i++);
    return i;
}

/*
 * Parse char to integer
 * @param c a valid digit between 0 and 9.
 */
int char_to_int(char c) {
    switch(c) {
    case '0': return 0;
    case '1': return 1;
    case '2': return 2;
    case '3': return 3;
    case '4': return 4;
    case '5': return 5;
    case '6': return 6;
    case '7': return 7;
    case '8': return 8;
    case '9': return 9;
    default:
        printf("Invalid digit! ABORT!\n");
        exit(1);
    }
}

/*
 * Parse string to integer
 * @param string a \0 termined string
 */
long parse_integer(char *string) {
    int i = 0;
    char c = 0;
    long acc = 0;
    int s = string_len(string);
#ifdef DEBUG
    printf("=> Start string parsing: '%s'\n", string);
    printf("Len: '%s' -> %d\n", string, s);
#endif

    if (string == NULL || s == 0) {
        printf("Empty string found! ABORT!\n");
        exit(1);
    }

    do {
        c = string[i++];
        long m = power(10, (s - i));
        acc += m * char_to_int(c);
    } while (i < s);

#ifdef DEBUG
    printf("Parsed: %d\n", acc);
#endif

    return acc;
}

int main(int argc, char **argv) {
    long acc = 0;
#ifdef DEBUG
    printf("ARGC: %d\n", argc);
#endif
    while (argc-- > 1) {
        acc += parse_integer(argv[argc]);
    }
    printf("%li\n", acc);
    return 0;
}