format.browser.ts

// Borrowed from https://github.com/ai/nanoid/blob/master/format.browser.js

export default function (random: (bytes: number) => Uint8Array, alphabet: string, size: number) {
    // 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.
    const 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
    const step = -~(1.6 * mask * size / alphabet.length)
    let id = ""

    while (true) {
        const bytes = random(step)
        // Compact alternative for `for (var i = 0; i < step; i++)`
        let 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
        }
    }
}