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.

Installation

$ pip install fractional-indexing

Usage

from fractional_indexing import generate_key_between


first = generate_key_between(None, None)
assert first == 'a0'

# Insert after 1st
second = generate_key_between(first, None)
assert second == 'a1'

# Insert after 2nd
third = generate_key_between(second, None)
assert third == 'a2'

# Insert before 1st
zeroth = generate_key_between(None, first)
assert zeroth == 'Zz'

# Insert in between 2nd and 3rd. Midpoint
second_and_half = generate_key_between(second, third)
assert second_and_half == 'a1V'

Other Languages

This is a Python port of the original JavaScript implementation by @rocicorp. That means that this implementation is byte-for-byte compatible with:

Language	Repo
JavaScript	https://github.com/rocicorp/fractional-indexing
Go	https://github.com/rocicorp/fracdex