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