Task: Negative base numbers
C#: using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace NegativeBaseNumbers {
    class Program {
        const string DIGITS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";

        static string EncodeNegativeBase(long n, int b) {
            if (b < -62 || b > -1) {
                throw new ArgumentOutOfRangeException("b");
            }
            if (n == 0) {
                return "0";
            }
            StringBuilder output = new StringBuilder();
            long nn = n;
            while (nn != 0) {
                int rem = (int)(nn % b);
                nn /= b;
                if (rem < 0) {
                    nn++;
                    rem -= b;
                }
                output.Append(DIGITS[rem]);
            }
            return new string(output.ToString().Reverse().ToArray());
        }

        static long DecodeNegativeBase(string ns, int b) {
            if (b < -62 || b > -1) {
                throw new ArgumentOutOfRangeException("b");
            }
            if (ns == "0") {
                return 0;
            }
            long total = 0;
            long bb = 1;
            for (int i = ns.Length - 1; i >= 0; i--) {
                char c = ns[i];
                total += DIGITS.IndexOf(c) * bb;
                bb *= b;
            }
            return total;
        }

        static void Main(string[] args) {
            List<Tuple<long, int>> nbl = new List<Tuple<long, int>>() {
                new Tuple<long, int>(10,-2),
                new Tuple<long, int>(146,-3),
                new Tuple<long, int>(15,-10),
                new Tuple<long, int>(-34025238427,-62),
            };
            foreach (var p in nbl) {
                string ns = EncodeNegativeBase(p.Item1, p.Item2);
                Console.WriteLine("{0,12} encoded in base {1,-3} = {2}", p.Item1, p.Item2, ns);
                long n = DecodeNegativeBase(ns, p.Item2);
                Console.WriteLine("{0,12} decoded in base {1,-3} = {2}", ns, p.Item2, n);
                Console.WriteLine();
            }
        }
    }
}

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)
    }
}