59 lines
2.3 KiB
JavaScript
59 lines
2.3 KiB
JavaScript
import { getRandomBytesAsync } from 'expo-random'
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import { urlAlphabet } from '../url-alphabet/index.js'
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let random = getRandomBytesAsync
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let customAlphabet = (alphabet, size) => {
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// First, a bitmask is necessary to generate the ID. The bitmask makes bytes
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// values closer to the alphabet size. The bitmask calculates the closest
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// `2^31 - 1` number, which exceeds the alphabet size.
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// For example, the bitmask for the alphabet size 30 is 31 (00011111).
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let mask = (2 << (31 - Math.clz32((alphabet.length - 1) | 1))) - 1
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// Though, the bitmask solution is not perfect since the bytes exceeding
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// the alphabet size are refused. Therefore, to reliably generate the ID,
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// the random bytes redundancy has to be satisfied.
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// Note: every hardware random generator call is performance expensive,
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// because the system call for entropy collection takes a lot of time.
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// So, to avoid additional system calls, extra bytes are requested in advance.
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// Next, a step determines how many random bytes to generate.
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// The number of random bytes gets decided upon the ID size, mask,
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// alphabet size, and magic number 1.6 (using 1.6 peaks at performance
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// according to benchmarks).
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let step = Math.ceil((1.6 * mask * size) / alphabet.length)
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let tick = id =>
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random(step).then(bytes => {
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// A compact alternative for `for (var i = 0; i < step; i++)`.
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let i = step
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while (i--) {
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// Adding `|| ''` refuses a random byte that exceeds the alphabet size.
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id += alphabet[bytes[i] & mask] || ''
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// `id.length + 1 === size` is a more compact option.
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if (id.length === +size) return id
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}
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return tick(id)
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})
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return () => tick('')
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}
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let nanoid = (size = 21) =>
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random(size).then(bytes => {
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let id = ''
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// A compact alternative for `for (var i = 0; i < step; i++)`.
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while (size--) {
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// It is incorrect to use bytes exceeding the alphabet size.
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// The following mask reduces the random byte in the 0-255 value
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// range to the 0-63 value range. Therefore, adding hacks, such
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// as empty string fallback or magic numbers, is unneccessary because
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// the bitmask trims bytes down to the alphabet size.
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id += urlAlphabet[bytes[size] & 63]
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}
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return id
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})
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export { nanoid, customAlphabet, random }
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