Task: First perfect square in base n with n unique digits
C++: #include <string>
#include <iostream> 
#include <cstdlib>
#include <math.h>
#include <chrono>
#include <iomanip>

using namespace std;

const int maxBase = 16;  
int base, bmo, tc; 
const string chars = "0123456789ABCDEF"; 
unsigned long long full; 


string toStr(const unsigned long long ull) {
	unsigned long long u = ull; string res = ""; while (u > 0) {
		lldiv_t result1 = lldiv(u, base); res = chars[(int)result1.rem] + res;
		u = (unsigned long long)result1.quot;
	} return res;
}


unsigned long long to10(string s) {
	unsigned long long res = 0; for (char i : s) res = res * base + chars.find(i); return res;
}


bool allIn(const unsigned long long ull) {
	unsigned long long u, found; u = ull; found = 0; while (u > 0) {
		lldiv_t result1 = lldiv(u, base); found |= (unsigned long long)1 << result1.rem;
		u = result1.quot;
	} return found == full;
}


string fixup(int n) {
	string res = chars.substr(0, base); if (n > 0) res = res.insert(n, chars.substr(n, 1));
	return "10" + res.substr(2);
}


void doOne() {
	bmo = base - 1; tc = 0; unsigned long long sq, rt, dn, d;
	int id = 0, dr = (base & 1) == 1 ? base >> 1 : 0, inc = 1, sdr[maxBase] = { 0 };
	full = ((unsigned long long)1 << base) - 1;
	int rc = 0; for (int i = 0; i < bmo; i++) {
		sdr[i] = (i * i) % bmo; if (sdr[i] == dr) rc++; if (sdr[i] == 0) sdr[i] += bmo;
	}
	if (dr > 0) {
		id = base; for (int i = 1; i <= dr; i++)
			if (sdr[i] >= dr) if (id > sdr[i]) id = sdr[i]; id -= dr;
	}
	sq = to10(fixup(id)); rt = (unsigned long long)sqrt(sq) + 1; sq = rt * rt;
	dn = (rt << 1) + 1; d = 1; if (base > 3 && rc > 0) {
		while (sq % bmo != dr) { rt += 1; sq += dn; dn += 2; } 
		inc = bmo / rc; if (inc > 1) { dn += rt * (inc - 2) - 1; d = inc * inc; }
		dn += dn + d;
	} d <<= 1;
	do { if (allIn(sq)) break; sq += dn; dn += d; tc++; } while (true);
	rt += tc * inc;
	cout << setw(4) << base << setw(3) << inc << "  " << setw(2)
		<< (id > 0 ? chars.substr(id, 1) : " ") << setw(10) << toStr(rt) << "  "
		<< setw(20) << left << toStr(sq) << right << setw(12) << tc << endl;
}

int main() {
	cout << "base inc id      root  sqr                   test count" << endl;
	auto st = chrono::system_clock::now();
	for (base = 2; base <= maxBase; base++) doOne();
	chrono::duration<double> et = chrono::system_clock::now() - st;
	cout << "\nComputation time was " << et.count() * 1000 << " milliseconds" << endl;
	return 0;
}

Go: package main

import (
    "fmt"
    "math/big"
    "strconv"
    "time"
)

const maxBase = 27
const minSq36 = "1023456789abcdefghijklmnopqrstuvwxyz"
const minSq36x = "10123456789abcdefghijklmnopqrstuvwxyz"

var bigZero = new(big.Int)
var bigOne = new(big.Int).SetUint64(1)

func containsAll(sq string, base int) bool {
    var found [maxBase]byte
    le := len(sq)
    reps := 0
    for _, r := range sq {
        d := r - 48
        if d > 38 {
            d -= 39
        }
        found[d]++
        if found[d] > 1 {
            reps++
            if le-reps < base {
                return false
            }
        }
    }
    return true
}

func sumDigits(n, base *big.Int) *big.Int {
    q := new(big.Int).Set(n)
    r := new(big.Int)
    sum := new(big.Int).Set(bigZero)
    for q.Cmp(bigZero) == 1 {
        q.QuoRem(q, base, r)
        sum.Add(sum, r)
    }
    return sum
}

func digitalRoot(n *big.Int, base int) int {
    root := new(big.Int)
    b := big.NewInt(int64(base))
    for i := new(big.Int).Set(n); i.Cmp(b) >= 0; i.Set(root) {
        root.Set(sumDigits(i, b))
    }
    return int(root.Int64())
}

func minStart(base int) (string, uint64, int) {
    nn := new(big.Int)
    ms := minSq36[:base]
    nn.SetString(ms, base)
    bdr := digitalRoot(nn, base)
    var drs []int
    var ixs []uint64
    for n := uint64(1); n < uint64(2*base); n++ {
        nn.SetUint64(n * n)
        dr := digitalRoot(nn, base)
        if dr == 0 {
            dr = int(n * n)
        }
        if dr == bdr {
            ixs = append(ixs, n)
        }
        if n < uint64(base) && dr >= bdr {
            drs = append(drs, dr)
        }
    }
    inc := uint64(1)
    if len(ixs) >= 2 && base != 3 {
        inc = ixs[1] - ixs[0]
    }
    if len(drs) == 0 {
        return ms, inc, bdr
    }
    min := drs[0]
    for _, dr := range drs[1:] {
        if dr < min {
            min = dr
        }
    }
    rd := min - bdr
    if rd == 0 {
        return ms, inc, bdr
    }
    if rd == 1 {
        return minSq36x[:base+1], 1, bdr
    }
    ins := string(minSq36[rd])
    return (minSq36[:rd] + ins + minSq36[rd:])[:base+1], inc, bdr
}

func main() {
    start := time.Now()
    var nb, nn big.Int
    for n, k, base := uint64(2), uint64(1), 2; ; n += k {
        if base > 2 && n%uint64(base) == 0 {
            continue
        } 
        nb.SetUint64(n)
        sq := nb.Mul(&nb, &nb).Text(base)
        if !containsAll(sq, base) {
            continue
        }
        ns := strconv.FormatUint(n, base)
        tt := time.Since(start).Seconds()
        fmt.Printf("Base %2d:%15s² = %-27s in %8.3fs\n", base, ns, sq, tt)
        if base == maxBase {
            break
        }
        base++
        ms, inc, bdr := minStart(base)
        k = inc
        nn.SetString(ms, base)
        nb.Sqrt(&nn)
        if nb.Uint64() < n+1 {
            nb.SetUint64(n + 1)
        }
        if k != 1 {
            for {
                nn.Mul(&nb, &nb)
                dr := digitalRoot(&nn, base)
                if dr == bdr {
                    n = nb.Uint64() - k
                    break
                }
                nb.Add(&nb, bigOne)
            }
        } else {
            n = nb.Uint64() - k
        }
    }
}