Task: Negative base numbers
C: #include <stdio.h>

const char DIGITS[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
const int DIGITS_LEN = 64;

void encodeNegativeBase(long n, long base, char *out) {
    char *ptr = out;

    if (base > -1 || base < -62) {
        
        out = "";
        return;
    }
    if (n == 0) {
        
        out = "0";
        return;
    }

    
    while (n != 0) {
        long rem = n % base;
        n = n / base;
        if (rem < 0) {
            n++;
            rem = rem - base;
        }
        *ptr = DIGITS[rem];
        ptr++;
    }
    *ptr = 0;

    
    ptr--;
    while (out < ptr) {
        char t = *out;
        *out = *ptr;
        *ptr = t;
        out++;
        ptr--;
    }
    return;
}

long decodeNegativeBase(const char* ns, long base) {
    long value, bb;
    int i;
    const char *ptr;

    if (base < -62 || base > -1) {
        
        return 0;
    }
    if (ns[0] == 0 || (ns[0] == '0' && ns[1] == 0)) {
        
        return 0;
    }

    
    ptr = ns;
    while (*ptr != 0) {
        ptr++;
    }

    
    value = 0;
    bb = 1;
    ptr--;
    while (ptr >= ns) {
        for (i = 0; i < DIGITS_LEN; i++) {
            if (*ptr == DIGITS[i]) {
                value = value + i * bb;
                bb = bb * base;
                break;
            }
        }
        ptr--;
    }

    return value;
}

void driver(long n, long b) {
    char buf[64];
    long value;

    encodeNegativeBase(n, b, buf);
    printf("%12d encoded in base %3d = %12s\n", n, b, buf);

    value = decodeNegativeBase(buf, b);
    printf("%12s decoded in base %3d = %12d\n", buf, b, value);

    printf("\n");
}

int main() {
    driver(10, -2);
    driver(146, -3);
    driver(15, -10);
    driver(12, -62);

    return 0;
}

Java: import java.util.List;
import java.util.Map;
import java.util.Objects;

public class NegativeBaseNumbers {
    private static final String DIGITS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";

    private static String encodeNegBase(long n, int b) {
        if (b < -62 || b > -1) throw new IllegalArgumentException("Parameter b is out of bounds");
        if (n == 0) return "0";
        StringBuilder out = new StringBuilder();
        long nn = n;
        while (nn != 0) {
            int rem = (int) (nn % b);
            nn /= b;
            if (rem < 0) {
                nn++;
                rem -= b;
            }
            out.append(DIGITS.charAt(rem));
        }
        out.reverse();
        return out.toString();
    }

    private static long decodeNegBase(String ns, int b) {
        if (b < -62 || b > -1) throw new IllegalArgumentException("Parameter b is out of bounds");
        if (Objects.equals(ns, "0")) return 0;
        long total = 0;
        long bb = 1;
        for (int i = ns.length() - 1; i >= 0; i--) {
            char c = ns.charAt(i);
            total += DIGITS.indexOf(c) * bb;
            bb *= b;
        }
        return total;
    }

    public static void main(String[] args) {
        List<Map.Entry<Long, Integer>> nbl = List.of(
                Map.entry(10L, -2),
                Map.entry(146L, -3),
                Map.entry(15L, -10),
                Map.entry(-4393346L, -62)
        );
        for (Map.Entry<Long, Integer> p : nbl) {
            String ns = encodeNegBase(p.getKey(), p.getValue());
            System.out.printf("%12d encoded in base %-3d = %s\n", p.getKey(), p.getValue(), ns);
            long n = decodeNegBase(ns, p.getValue());
            System.out.printf("%12s decoded in base %-3d = %d\n\n", ns, p.getValue(), n);
        }
    }
}