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.

Install

npm i fractional-indexing-base-26

This package is available on npm as fractional-indexing-base-26.

API

generateKeyBetween

Generate a single key in between two points.

generateKeyBetween(
  a: string | null, // start
  b: string | null, // end
): string;

import { generateKeyBetween } from 'fractional-indexing-base-26';

const first = generateKeyBetween(null, null); // "na"

// Insert after 1st
const second = generateKeyBetween(first, null); // "nb"

// Insert after 2nd
const third = generateKeyBetween(second, null); // "nc"

// Insert before 1st
const zeroth = generateKeyBetween(null, first); // "mz"

// Insert in between 2nd and 3rd (midpoint)
const secondAndHalf = generateKeyBetween(second, third); // "nbn"

generateKeysBetween

Use this when generating multiple keys at some known position, as it spaces out indexes more evenly and leads to shorter keys.

generateKeysBetween(
  a: string | null | undefined, // start
  b: string | null | undefined, // end
  n: number, // number of keys to generate between start and end
): string[];

import { generateKeysBetween } from 'fractional-indexing-base-26';

const first = generateKeysBetween(null, null, 2); // ['na', 'nb']

// Insert two keys after 2nd
generateKeysBetween('na', null, 2); // ['nb', 'nc']

// Insert three keys before 1st
generateKeysBetween(null, 'na', 3); // ['mx', 'my', 'mz']

// Insert two keys in between two keys
generateKeysBetween('na', 'nb', 2); // ['nah', 'nan']