Convert Hexadecimal String to Integer Without strtol or sscanf functions

Code

#include <stdio.h>
#include <stdint.h>

uint16_t hex_to_uint(const char *str) {
    uint16_t decimal = 0;
    int len = 0;

    while(str[len] != '\0'){
        len++;
    }

    int value = 0;
    for (int i = 0; i < len; i++) {
        if (str[i] >= '0' && str[i] <= '9') {
            value = str[i] - '0';
        } else if (str[i] >= 'A' && str[i] <= 'F') {
            value = str[i] - 'A' + 10;
        } else if (str[i] >= 'a' && str[i] <= 'f') {
            value = str[i] - 'a' + 10;
        }
        decimal = decimal * 16 + value;
    }
    return decimal;
}

int main() {
    char hex[10];
    scanf("%s", hex);
    printf("%u", hex_to_uint(hex));
    return 0;
}

Solving Approach