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45 lines
1.0 KiB
45 lines
1.0 KiB
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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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