2024-07-07 00:33:47 +02:00
|
|
|
---
|
|
|
|
|
title: "Gap Buffer"
|
|
|
|
|
date: 2024-07-06T21:27:19+01:00
|
|
|
|
|
draft: false # I don't care for draft mode, git has branches for that
|
|
|
|
|
description: "As featured in GNU Emacs"
|
|
|
|
|
tags:
|
|
|
|
|
- algorithms
|
|
|
|
|
- data structures
|
|
|
|
|
- python
|
|
|
|
|
categories:
|
|
|
|
|
- programming
|
|
|
|
|
series:
|
|
|
|
|
- Cool algorithms
|
|
|
|
|
favorite: false
|
|
|
|
|
disable_feed: false
|
|
|
|
|
---
|
|
|
|
|
|
|
|
|
|
The [_Gap Buffer_][wiki] is a popular data structure for text editors to
|
|
|
|
|
represent files and editable buffers. The most famous of them probably being
|
|
|
|
|
[emacs].
|
|
|
|
|
|
|
|
|
|
[wiki]: https://en.wikipedia.org/wiki/Gap_buffer
|
|
|
|
|
[emacs]: https://www.gnu.org/software/emacs/manual/html_node/elisp/Buffer-Gap.html
|
|
|
|
|
|
|
|
|
|
<!--more-->
|
2024-07-07 00:34:49 +02:00
|
|
|
|
|
|
|
|
## What does it do?
|
|
|
|
|
|
|
|
|
|
A _Gap Buffer_ is simply a list of characters, similar to a normal string, but
|
|
|
|
|
with the added twist of splitting it into two side: the prefix and suffix, on
|
|
|
|
|
either side of the cursor. In between them, a gap is left to allow for quick
|
|
|
|
|
insertion at the cursor.
|
|
|
|
|
|
|
|
|
|
Moving the cursor moves the gap around the buffer, the prefix and suffix getting
|
|
|
|
|
shorter/longer as required.
|
2024-07-07 00:35:39 +02:00
|
|
|
|
|
|
|
|
## Implementation
|
|
|
|
|
|
|
|
|
|
I'll be writing a sample implementation in Python, to keep with the rest of the
|
|
|
|
|
series, This does not showcase the elegance of the _Gap Buffer_ in action like a
|
|
|
|
|
C implementation full of `memmove`s would.
|
|
|
|
|
|
|
|
|
|
### Representation
|
|
|
|
|
|
|
|
|
|
We'll be representing the gap buffer as an actual list of characters.
|
|
|
|
|
|
|
|
|
|
Given that Python doesn't _have_ characters, wel'll have to settle for a list of
|
|
|
|
|
strings, each representing a single character...
|
|
|
|
|
|
|
|
|
|
```python
|
|
|
|
|
Char = str
|
|
|
|
|
|
|
|
|
|
class GapBuffer:
|
|
|
|
|
# List of characters, contains prefix and suffix of string with gap in the middle
|
|
|
|
|
_buf: list[Char]
|
|
|
|
|
# The gap is contained between [start, end) (i.e: buf[start:end])
|
|
|
|
|
_gap_start: int
|
|
|
|
|
_gap_end: int
|
|
|
|
|
|
|
|
|
|
# Visual representation of the gap buffer:
|
|
|
|
|
# This is a very [ ]long string.
|
|
|
|
|
# |<----------------------------------------------->| capacity
|
|
|
|
|
# |<------------>| |<-------->| string
|
|
|
|
|
# |<------------------->| gap
|
|
|
|
|
# |<------------>| prefix
|
|
|
|
|
# |<-------->| suffix
|
|
|
|
|
def __init__(self, initial_capacity: int = 16) -> None:
|
|
|
|
|
assert initial_capacity > 0
|
|
|
|
|
# Initialize an empty gap buffer
|
|
|
|
|
self._buf = [""] * initial_capacity
|
|
|
|
|
self._gap_start = 0
|
|
|
|
|
self._gap_end = initial_capacity
|
|
|
|
|
```
|
