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