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let crypto = require('crypto')
let { urlAlphabet } = require('./url-alphabet/index.cjs')
// It is best to make fewer, larger requests to the crypto module to
// avoid system call overhead. So, random numbers are generated in a
// pool. The pool is a Buffer that is larger than the initial random
// request size by this multiplier. The pool is enlarged if subsequent
// requests exceed the maximum buffer size.
const POOL_SIZE_MULTIPLIER = 128
let pool, poolOffset
let fillPool = bytes => {
if (!pool || pool.length < bytes) {
pool = Buffer.allocUnsafe(bytes * POOL_SIZE_MULTIPLIER)
crypto.randomFillSync(pool)
poolOffset = 0
} else if (poolOffset + bytes > pool.length) {
crypto.randomFillSync(pool)
poolOffset = 0
}
poolOffset += bytes
}
let random = bytes => {
// `|=` convert `bytes` to number to prevent `valueOf` abusing and pool pollution
fillPool((bytes |= 0))
return pool.subarray(poolOffset - bytes, poolOffset)
}
let customRandom = (alphabet, defaultSize, getRandom) => {
// 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)
return (size = defaultSize) => {
let id = ''
while (true) {
let bytes = getRandom(step)
// A compact alternative for `for (let 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
}
}
}
}
let customAlphabet = (alphabet, size = 21) =>
customRandom(alphabet, size, random)
let nanoid = (size = 21) => {
// `|=` convert `size` to number to prevent `valueOf` abusing and pool pollution
fillPool((size |= 0))
let id = ''
// We are reading directly from the random pool to avoid creating new array
for (let i = poolOffset - size; i < poolOffset; i++) {
// 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[pool[i] & 63]
}
return id
}
module.exports = { nanoid, customAlphabet, customRandom, urlAlphabet, random }