Created
September 29, 2022 17:31
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'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; | |
} |
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