41 lines
1.8 KiB
JavaScript
41 lines
1.8 KiB
JavaScript
// This file replaces `format.js` in bundlers like webpack or Rollup,
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// according to `browser` config in `package.json`.
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module.exports = function (random, alphabet, size) {
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// We can’t use bytes bigger than the alphabet. To make bytes values closer
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// to the alphabet, we apply bitmask on them. We look for the closest
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// `2 ** x - 1` number, which will be bigger than alphabet size. If we have
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// 30 symbols in the alphabet, we will take 31 (00011111).
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// We do not use faster Math.clz32, because it is not available in browsers.
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var mask = (2 << Math.log(alphabet.length - 1) / Math.LN2) - 1
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// Bitmask is not a perfect solution (in our example it will pass 31 bytes,
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// which is bigger than the alphabet). As a result, we will need more bytes,
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// than ID size, because we will refuse bytes bigger than the alphabet.
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// Every hardware random generator call is costly,
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// because we need to wait for entropy collection. This is why often it will
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// be faster to ask for few extra bytes in advance, to avoid additional calls.
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// Here we calculate how many random bytes should we call in advance.
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// It depends on ID length, mask / alphabet size and magic number 1.6
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// (which was selected according benchmarks).
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// -~f => Math.ceil(f) if n is float number
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// -~i => i + 1 if n is integer number
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var step = -~(1.6 * mask * size / alphabet.length)
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var id = ''
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while (true) {
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var bytes = random(step)
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// Compact alternative for `for (var i = 0; i < step; i++)`
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var i = step
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while (i--) {
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// If random byte is bigger than alphabet even after bitmask,
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// we refuse it by `|| ''`.
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id += alphabet[bytes[i] & mask] || ''
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// More compact than `id.length + 1 === size`
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if (id.length === +size) return id
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}
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}
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}
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