Number basis change

basis_change.cpp

string basis_change(string source, int from, int to) {
    assert(from >= 2 && from <= 16);
    assert(to >= 2 && to <= 16);
    
    long long num = 0;
    int power = 0;

    bool negative = false;
    
    for (int i = source.length() - 1; i >= 0; i--) {
        char c = toupper(source[i]);
        int digit = 0;

        if (i == 0 && c == '-') {
            negative = true;
            continue;
        }

        if (c >= '0' && c <= '9') {
            digit = c - '0';
        } else if (c >= 'A' && c <= 'F') {
            digit = c - 'A' + 10;
        }
        
        num += digit * pow(from, power++);
    }
    
    if (num == 0) {
        return "0";
    }

    string result;
    while (num > 0) {
        int remainder = num % to;
        char digit;
        
        if (remainder < 10) {
            digit = '0' + remainder;
        } else {
            digit = 'A' + (remainder - 10);
        }
        
        result.push_back(digit);
        num /= to;
    }

    if (negative) result.push_back('-');

    reverse(result.begin(), result.end());
    return result;
}