124 lines
3.7 KiB
Markdown
124 lines
3.7 KiB
Markdown
---
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title: "Gap Buffer"
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date: 2024-07-06T21:27:19+01:00
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draft: false # I don't care for draft mode, git has branches for that
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description: "As featured in GNU Emacs"
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tags:
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- algorithms
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- data structures
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- python
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categories:
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- programming
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series:
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- Cool algorithms
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favorite: false
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disable_feed: false
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---
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The [_Gap Buffer_][wiki] is a popular data structure for text editors to
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represent files and editable buffers. The most famous of them probably being
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[GNU Emacs][emacs].
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[wiki]: https://en.wikipedia.org/wiki/Gap_buffer
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[emacs]: https://www.gnu.org/software/emacs/manual/html_node/elisp/Buffer-Gap.html
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<!--more-->
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## What does it do?
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A _Gap Buffer_ is simply a list of characters, similar to a normal string, with
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the added twist of splitting it into two side: the prefix and suffix, on either
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side of the cursor. In between them, a gap is left to allow for quick
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insertion at the cursor.
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Moving the cursor moves the gap around the buffer, the prefix and suffix getting
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shorter/longer as required.
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## Implementation
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I'll be writing a sample implementation in Python, as with the rest of the
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[series]({{< ref "/series/cool-algorithms/">}}). I don't think it showcases the
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elegance of the _Gap Buffer_ in action like a C implementation full of
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`memmove`s would, but it does makes it short and sweet.
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### Representation
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We'll be representing the gap buffer as an actual list of characters.
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Given that Python doesn't _have_ characters, let's settle for a list of strings,
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each representing a single character...
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```python
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Char = str
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class GapBuffer:
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# List of characters, contains prefix and suffix of string with gap in the middle
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_buf: list[Char]
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# The gap is contained between [start, end) (i.e: buf[start:end])
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_gap_start: int
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_gap_end: int
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# Visual representation of the gap buffer:
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# This is a very [ ]long string.
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# |<----------------------------------------------->| capacity
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# |<------------>| |<-------->| string
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# |<------------------->| gap
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# |<------------>| prefix
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# |<-------->| suffix
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def __init__(self, initial_capacity: int = 16) -> None:
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assert initial_capacity > 0
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# Initialize an empty gap buffer
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self._buf = [""] * initial_capacity
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self._gap_start = 0
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self._gap_end = initial_capacity
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```
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### Accessors
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I'm mostly adding these for exposition, and making it easier to write `assert`s
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later.
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```python
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@property
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def capacity(self) -> int:
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return len(self._buf)
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@property
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def gap_length(self) -> int:
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return self._gap_end - self._gap_start
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@property
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def string_length(self) -> int:
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return self.capacity - self.gap_length
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@property
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def prefix_length(self) -> int:
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return self._gap_start
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@property
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def suffix_length(self) -> int:
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return self.capacity - self._gap_end
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```
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### Growing the buffer
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I've written this method in a somewhat non-idiomatic manner, to make it closer
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to how it would look in C using `realloc` instead.
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It would be more efficient to use slicing to insert the needed extra capacity
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directly, instead of making a new buffer and copying characters over.
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```python
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def grow(self, capacity: int) -> None:
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assert capacity >= self.capacity
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# Create a new buffer with the new capacity
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new_buf = [""] * capacity
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# Move the prefix/suffix to their place in the new buffer
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added_capacity = capacity - len(self._buf)
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new_buf[: self._gap_start] = self._buf[: self._gap_start]
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new_buf[self._gap_end + added_capacity :] = self._buf[self._gap_end :]
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# Use the new buffer, account for added capacity
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self._buf = new_buf
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self._gap_end += added_capacity
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```
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