blog/content/posts/2024-07-06-gap-buffer/index.md

title	date	draft	description	tags	categories	series	favorite	disable_feed
Gap Buffer	2024-07-06T21:27:19+01:00	false	As featured in GNU Emacs	algorithms data structures python	programming	Cool algorithms	false	false

The Gap Buffer is a popular data structure for text editors to represent files and editable buffers. The most famous of them probably being GNU Emacs.

What does it do?

A Gap Buffer is simply a list of characters, similar to a normal string, 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.

Implementation

I'll be writing a sample implementation in Python, as with the rest of the [series]({{< ref "/series/cool-algorithms/">}}). I don't think it showcases the elegance of the Gap Buffer in action like a C implementation full of memmoves would, but it does makes it short and sweet.

Representation

We'll be representing the gap buffer as an actual list of characters.

Given that Python doesn't have characters, let's settle for a list of strings, each representing a single character...

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

Accessors

I'm mostly adding these for exposition, and making it easier to write asserts later.

@property
def capacity(self) -> int:
  return len(self._buf)

@property
def gap_length(self) -> int:
  return self._gap_end - self._gap_start

@property
def string_length(self) -> int:
  return self.capacity - self.gap_length

@property
def prefix_length(self) -> int:
  return self._gap_start

@property
def suffix_length(self) -> int:
  return self.capacity - self._gap_end