Flexichain An editable sequence and its gap-buffer implementation - - PowerPoint PPT Presentation

Flexichain An editable sequence and its gap-buffer implementation

Robert Strandh, Tim Moore, Matthieu Villeneuve

strandh@labri.fr, moore@labri.fr, matthieu.villeneuve@free.fr

Laboratoire Bordelais de Recherche en Informatique (LaBRI)

Flexichain – Lisp and Scheme Conference, Oslo 2004 – p.1/19

API Model

An editable sequence of objects containing an arbitrary number of cursors.

Arbitrary Objects Cursors

Flexichain – Lisp and Scheme Conference, Oslo 2004 – p.2/19

Uses of API

• Emacs-like text editors for the entire buffer
• Emacs-like text editors for one line of a buffer
• Emacs-like text editors for the sequence of

lines

• Gsharp (at 4 different levels)

Flexichain – Lisp and Scheme Conference, Oslo 2004 – p.3/19

Possible representations

• Doubly linked list
• Gap buffer

Flexichain – Lisp and Scheme Conference, Oslo 2004 – p.4/19

Doubly linked list

Advantages: fast, easy to implement Inconveniences: no direct access, high memory

• verhead

Flexichain – Lisp and Scheme Conference, Oslo 2004 – p.5/19

Gap buffer

Advantages: direct access, low memory overhead Inconveniences: slow worst-case, hard to implement

Flexichain – Lisp and Scheme Conference, Oslo 2004 – p.6/19

Previous work

• Multics Emacs: a doubly-linked list of lines,

each line a vector (special instructions)

• GNU Emacs: entire buffer is a gap buffer
• Hemlock: doubly-linked list of lines, the open

line is a gap buffer

• Goatee (McCLIM): doubly-linked list of lines,

each line is a gap buffer

• Gsharp: currently uses singly-linked lists

Flexichain – Lisp and Scheme Conference, Oslo 2004 – p.7/19

Purpose of Flexichain

In order of priorities:

• 1. Replace specialized, slow code in Gsharp
• 2. Replace line implementation in Goatee
• 3. Replace list of lines in Goatee
• 4. Perhaps use in portable Hemlock

Flexichain – Lisp and Scheme Conference, Oslo 2004 – p.8/19

Two layers: Flexichain and cursorchain

The Flexichain layer uses positions to access, insert, and delete elements. The Cursorchain layer uses cursors. The two are compatible.

Flexichain – Lisp and Scheme Conference, Oslo 2004 – p.9/19

The Flexichain layer

insert<* chain element position insert>* chain element position delete* chain position element* chain position (setf element*) position element chain

Flexichain – Lisp and Scheme Conference, Oslo 2004 – p.10/19

Stack and queue operations

push-start chain element push-end chain element pop-start chain pop-end chain rotate chain &optional (n 1)

Flexichain – Lisp and Scheme Conference, Oslo 2004 – p.11/19

Implementation of Flexichain layer

• Use a gap buffer
• Consider the buffer as circular (avoids bad

worst cases)

• Expand and shrink factors
• Moving the gap is messy

Flexichain – Lisp and Scheme Conference, Oslo 2004 – p.12/19

Moving the gap

Three situations exist: Move left or right according to the number of elements that need to be moved in each case.

Flexichain – Lisp and Scheme Conference, Oslo 2004 – p.13/19

Moving the gap

Flexichain – Lisp and Scheme Conference, Oslo 2004 – p.14/19

The Cursorchain layer

at-beginning-p cursor at-end-p cursor move< cursor move> cursor insert< object cursor insert> object cursor delete< cursor &optional (n 1) delete> cursor &optional (n 1) element< cursor element> cursor

Flexichain – Lisp and Scheme Conference, Oslo 2004 – p.15/19

The Cursorchain layer

• cursorchain is a subclass of flexichain
• cursors store physical position rather than

logical positions in order to avoid updating all cursors at every operation

• to avoid memory leaks, we use weak

references to store cursors

Flexichain – Lisp and Scheme Conference, Oslo 2004 – p.16/19

The Cursorchain layer

• we use an internal protocol for resizing and for

moving the gap

• cursor update are done by :before and

:after methods of internal protocol

Flexichain – Lisp and Scheme Conference, Oslo 2004 – p.17/19

State of implementation

Flexichain is finished. Cursorchain will be finished this summer (we all have daytime jobs).

Flexichain – Lisp and Scheme Conference, Oslo 2004 – p.18/19

That’s all

Thank You! Questions?

Flexichain – Lisp and Scheme Conference, Oslo 2004 – p.19/19