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Idiom #216 Pad a string on both sides

Add the extra character c at the beginning and ending of string s to make sure its length is at least m.
After the padding the original content of s should be at the center of the result.
The length is the number of characters, not the number of bytes.

E.g. with s="abcd", m=10 and c="X" the result should be "XXXabcdXXX".

import "encoding/utf8"
import "strings"
n := utf8.RuneCountInString(s)
if n < m {
	nleft := (m - n) / 2
	nright := (m - n) - nleft
	left, right := strings.Repeat(string(c), nleft), strings.Repeat(string(c), nright)
	s = left + s + right
}

c is a rune
  i = (m-len(s))/2
  j = (m - len(s)) - (m-len(s))/2
  s = repeat(c,i) // s // repeat(c,j)
New implementation...
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Bart 
n := utf8.RuneCountInString(s)
if n < m {
	nleft := (m - n) / 2
	nright := (m - n) - nleft
	left, right := strings.Repeat(string(c), nleft), strings.Repeat(string(c), nright)
	s = left + s + right
}
  i = (m-len(s))/2
  j = (m - len(s)) - (m-len(s))/2
  s = repeat(c,i) // s // repeat(c,j)
while (s.length() < m) s = c + s + c;
s := UTF8PadCenter(s,m,c);
my $total_padding = $m-length($s);
if( $total_padding ) {
   my $l = int($total_padding/2);
   my $r = $total_padding-$l;
   $s = join "", ($c x $l), $s, ($c x $r);
}
sub center {
    my ($s, $m, $c) = @_;
    my $slen = length $s;
    return $s if $slen > $m;
    $c //= ' ';
    my $r = $c x $m;
    my $p = int($m/2 - $slen/2);
    substr($r, $p, $slen, $s);
    return $r;    
}

print center("abcd",10,"X");

s = s.center(m, c)
s = f'{s:{c}^{m}}'
s = s.center(m, c)

let n = (m + s.len())/2;
s = format!("{s:X>n$}");
s = format!("{out:X<m$}");
s = s.pad(m, c, Alignment::Middle, true);