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let crypto = require('crypto')
let { urlAlphabet } = require('../url-alphabet/index.cjs')
// `crypto.randomFill()` is a little faster than `crypto.randomBytes()`,
// because it is possible to use in combination with `Buffer.allocUnsafe()`.
let random = bytes =>
new Promise((resolve, reject) => {
// `Buffer.allocUnsafe()` is faster because it doesn’t flush the memory.
// Memory flushing is unnecessary since the buffer allocation itself resets
// the memory with the new bytes.
crypto.randomFill(Buffer.allocUnsafe(bytes), (err, buf) => {
if (err) {
reject(err)
} else {
resolve(buf)
}
})
})
let customAlphabet = (alphabet, defaultSize = 21) => {
// First, a bitmask is necessary to generate the ID. The bitmask makes bytes
// values closer to the alphabet size. The bitmask calculates the closest
// `2^31 - 1` number, which exceeds the alphabet size.
// For example, the bitmask for the alphabet size 30 is 31 (00011111).
let mask = (2 << (31 - Math.clz32((alphabet.length - 1) | 1))) - 1
// Though, the bitmask solution is not perfect since the bytes exceeding
// the alphabet size are refused. Therefore, to reliably generate the ID,
// the random bytes redundancy has to be satisfied.
// Note: every hardware random generator call is performance expensive,
// because the system call for entropy collection takes a lot of time.
// So, to avoid additional system calls, extra bytes are requested in advance.
// Next, a step determines how many random bytes to generate.
// The number of random bytes gets decided upon the ID size, mask,
// alphabet size, and magic number 1.6 (using 1.6 peaks at performance
// according to benchmarks).
let step = Math.ceil((1.6 * mask * defaultSize) / alphabet.length)
let tick = (id, size = defaultSize) =>
random(step).then(bytes => {
// A compact alternative for `for (var i = 0; i < step; i++)`.
let i = step
while (i--) {
// Adding `|| ''` refuses a random byte that exceeds the alphabet size.
id += alphabet[bytes[i] & mask] || ''
if (id.length >= size) return id
}
return tick(id, size)
})
return size => tick('', size)
}
let nanoid = (size = 21) =>
random((size |= 0)).then(bytes => {
let id = ''
// A compact alternative for `for (var i = 0; i < step; i++)`.
while (size--) {
// It is incorrect to use bytes exceeding the alphabet size.
// The following mask reduces the random byte in the 0-255 value
// range to the 0-63 value range. Therefore, adding hacks, such
// as empty string fallback or magic numbers, is unneccessary because
// the bitmask trims bytes down to the alphabet size.
id += urlAlphabet[bytes[size] & 63]
}
return id
})
module.exports = { nanoid, customAlphabet, random }