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Idiom #215 Pad string on the left

Prepend extra character c at the beginning of string s to make sure its length is at least m.
The length is the number of characters, not the number of bytes.

repeat(x,max(n-len(s),0)) // s
import "strings"
import "utf8"
if n := utf8.RuneCountInString(s); n < m {
	s = strings.Repeat(c, m-n) + s
s := UTF8PadLeft(s,m,c);
s = s.rjust(m, c)
s.rjust(m, c)
use unicode_width::{UnicodeWidthChar, UnicodeWidthStr};
if let Some(columns_short) = m.checked_sub(s.width()) {
    let padding_width = c
        .filter(|n| *n > 0)
        .expect("padding character should be visible");
    // Saturate the columns_short
    let padding_needed = columns_short + padding_width - 1 / padding_width;
    let mut t = String::with_capacity(s.len() + padding_needed);
    t.extend((0..padding_needed).map(|_| c)
    s = t;

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