mirror of https://github.com/denoland/deno.git
53 lines
1.4 KiB
JavaScript
53 lines
1.4 KiB
JavaScript
/* This program is free software. It comes without any warranty, to
|
|
* the extent permitted by applicable law. You can redistribute it
|
|
* and/or modify it under the terms of the Do What The Fuck You Want
|
|
* To Public License, Version 2, as published by Sam Hocevar. See
|
|
* http://www.wtfpl.net/ for more details. */
|
|
'use strict';
|
|
module.exports = leftPad;
|
|
|
|
var cache = [
|
|
'',
|
|
' ',
|
|
' ',
|
|
' ',
|
|
' ',
|
|
' ',
|
|
' ',
|
|
' ',
|
|
' ',
|
|
' '
|
|
];
|
|
|
|
function leftPad (str, len, ch) {
|
|
// convert `str` to a `string`
|
|
str = str + '';
|
|
// `len` is the `pad`'s length now
|
|
len = len - str.length;
|
|
// doesn't need to pad
|
|
if (len <= 0) return str;
|
|
// `ch` defaults to `' '`
|
|
if (!ch && ch !== 0) ch = ' ';
|
|
// convert `ch` to a `string` cuz it could be a number
|
|
ch = ch + '';
|
|
// cache common use cases
|
|
if (ch === ' ' && len < 10) return cache[len] + str;
|
|
// `pad` starts with an empty string
|
|
var pad = '';
|
|
// loop
|
|
while (true) {
|
|
// add `ch` to `pad` if `len` is odd
|
|
if (len & 1) pad += ch;
|
|
// divide `len` by 2, ditch the remainder
|
|
len >>= 1;
|
|
// "double" the `ch` so this operation count grows logarithmically on `len`
|
|
// each time `ch` is "doubled", the `len` would need to be "doubled" too
|
|
// similar to finding a value in binary search tree, hence O(log(n))
|
|
if (len) ch += ch;
|
|
// `len` is 0, exit the loop
|
|
else break;
|
|
}
|
|
// pad `str`!
|
|
return pad + str;
|
|
}
|