/**
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* Secure random string generator with custom alphabet.
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*
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* Alphabet must contain 256 symbols or less. Otherwise, the generator
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* will not be secure.
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*
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* @param {asyncGenerator} random The random bytes generator.
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* @param {string} alphabet Symbols to be used in new random string.
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* @param {size} size The number of symbols in new random string.
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*
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* @return {Promise} Promise with random string.
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*
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* @example
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* const formatAsync = require('nanoid/async/format')
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*
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* function random (size) {
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* const result = []
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* for (let i = 0; i < size; i++) {
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* result.push(randomByte())
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* }
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* return Promise.resolve(result)
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* }
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*
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* formatAsync(random, "abcdef", 5).then(id => {
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* model.id = id //=> "fbaef"
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* })
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*
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* @name formatAsync
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* @function
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*/
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module.exports = function (random, alphabet, size) {
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var mask = (2 << Math.log(alphabet.length - 1) / Math.LN2) - 1
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var step = Math.ceil(1.6 * mask * size / alphabet.length)
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function tick (id) {
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return random(step).then(function (bytes) {
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for (var i = 0; i < step; i++) {
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var byte = bytes[i] & mask
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if (alphabet[byte]) {
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id += alphabet[byte]
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if (id.length === size) return id
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}
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}
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return tick(id)
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})
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}
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return tick('')
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}
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/**
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* @callback asyncGenerator
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* @param {number} bytes The number of bytes to generate.
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* @return {Promise} Promise with array of random bytes.
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*/
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