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;
}

Go: package main

import (
    "fmt"
    "log"
    "strings"
)

const digits = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"

func reverse(bs []byte) []byte {
    for i, j := 0, len(bs)-1; i < len(bs)/2; i, j = i+1, j-1 {
        bs[i], bs[j] = bs[j], bs[i]
    }
    return bs
}

func encodeNegBase(n, b int64) (string, error) {
    if b < -62 || b > -1 {
        return "", fmt.Errorf("base must be between -62 and -1 inclusive")
    }
    if n == 0 {
        return "0", nil
    }
    var out []byte
    for n != 0 {
        rem := n % b
        n /= b
        if rem < 0 {
            n++
            rem -= b
        }
        out = append(out, digits[int(rem)])
    }
    reverse(out)
    return string(out), nil
}

func decodeNegBase(ns string, b int64) (int64, error) {
    if b < -62 || b > -1 {
        return 0, fmt.Errorf("base must be between -62 and -1 inclusive")
    }
    if ns == "0" {
        return 0, nil
    }
    total := int64(0)
    bb := int64(1)
    bs := []byte(ns)
    reverse(bs)
    for _, c := range bs {
        total += int64(strings.IndexByte(digits, c)) * bb
        bb *= b
    }
    return total, nil
}

func main() {
    numbers := []int64{10, 146, 15, -942, 1488588316238}
    bases := []int64{-2, -3, -10, -62, -62}
    for i := 0; i < len(numbers); i++ {
        n := numbers[i]
        b := bases[i]
        ns, err := encodeNegBase(n, b)
        if err != nil {
            log.Fatal(err)
        }
        fmt.Printf("%13d encoded in base %-3d = %s\n", n, b, ns)
        n, err = decodeNegBase(ns, b)
        if err != nil {
            log.Fatal(err)
        }
        fmt.Printf("%13s decoded in base %-3d = %d\n\n", ns, b, n)
    }
}