Fossil SCM delta compression algorithm

The cool thing about it is that plain text inputs generate plain text deltas (binary inputs, of course, may generate binary deltas).

• Format
• Algorithm
• Original implementation
• Demo

Installation

$ npm install fossil-delta

Usage

import { createDelta, applyDelta } from 'fossil-delta';

createDelta(source, target)

Returns a delta from source to target.

source and target must be a Uint8Array or an Array of bytes. The same type will be returned.

applyDelta(origin, delta[, opts])

Returns the target by applying the delta to the origin.

origin and delta must be a Uint8Array or an Array of bytes. The same type will be returned.

Throws an error if the delta fails to apply (e.g., if it is corrupted).

Optional argument opts can be:

{
    verifyChecksum: false
}

to disable checksum verification (which is enabled by default.)

getDeltaTargetSize(delta)

Returns the size of the target for this delta.

Throws an error if it cannot read the size from the delta.

Strings

A nice property of the Fossil Delta Encoding is that it produces pure plain text deltas if the source and target are plain text.

To simplify working with plain text strings, the library provides the following convenience functions, which automatically encode and decode strings using TextEncoder and TextDecoder before processing:

• createStringDelta(source: string, target: string): string
• applyStringDelta(origin: string, delta: string, [, opts]): string
• getStringDeltaTargetSize(delta: string): number

Note that getStringDeltaTargetSize() will return the size of the target string in UTF-8 bytes, not characters (that is, it's not always equal to string length of the resulting target).

Development

I develop with Bun.

To build the library, run:

$ bun run build

npm, yarn will also work for building.

To run tests, run:

$ bun test

Migration from v1.x.x

fossil-delta.js is now an ES module and there is no minified version included.

API renames:

• create -> createDelta
• apply -> applyDelta
• outputSize -> getDeltaTargetSize

Important: createDelta and applyDelta now return the same type as the input (Uint8Array if given Uint8Array, Array if given Array), instead of always returning an Array.