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46 lines
1.0 KiB
46 lines
1.0 KiB
8 years ago
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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 `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 `string`
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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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