Fractional Indexing
This is based on Implementing Fractional Indexing by David Greenspan .
Fractional indexing is a technique to create an ordering that can be used for Realtime Editing of Ordered Sequences.
This implementation includes variable-length integers, and the prepend/append optimization described in David's article.
API
generateKeyBetween
Generate a single key in between two points.
generateKeyBetween(
a: string | null | undefined, // start
b: string | null | undefined, // end
digits?: string | undefined = BASE_62_DIGITS, // optional character encoding
): string;import { generateKeyBetween } from 'fractional-indexing';
const first = generateKeyBetween(null, null); // "a0"
// Insert after 1st
const second = generateKeyBetween(first, null); // "a1"
// Insert after 2nd
const third = generateKeyBetween(second, null); // "a2"
// Insert before 1st
const zeroth = generateKeyBetween(null, first); // "Zz"
// Insert in between 2nd and 3rd (midpoint)
const secondAndHalf = generateKeyBetween(second, third); // "a1V"generateNKeysBetween
Use this when generating multiple keys at some known position, as it spaces out indexes more evenly and leads to shorter keys.
generateNKeysBetween(
a: string | null | undefined, // start
b: string | null | undefined, // end
n: number // number of keys to generate evenly between start and end
digits?: string | undefined = BASE_62_DIGITS, // optional character encoding
): string[];import { generateNKeysBetween } from 'fractional-indexing';
const first = generateNKeysBetween(null, null, 2); // ['a0', 'a1']
// Insert two keys after 2nd
generateNKeysBetween(first[1], null, 2); // ['a2', 'a3']
// Insert two keys before 1st
generateNKeysBetween(null, first[0], 2); // ['Zy', 'Zz']
// Insert two keys in between 1st and 2nd (midpoints)
generateNKeysBetween(second, third, 2); // ['a0G', 'a0V']Other Languages
These should be byte-for-byte compatible.
| Language | Repo |
|---|---|
| Go | https://github.com/rocicorp/fracdex |
| Python | https://github.com/httpie/fractional-indexing-python |
| Kotlin | https://github.com/darvelo/fractional-indexing-kotlin |