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spaceship-mode and tabble-mode: text/code alignment for variable-width fonts

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# spaceship-mode
spaceship-mode and tabble-mode: text/code alignment for variable-width fonts

## Purpose

Two minor modes for Emacs, `spaceship-mode` and `tabble-mode`, automatically alter the width of certain spaces and/or tabs in the buffer to align buffer text.  The two modes operate independently and can be used separately or together.  They serve two purposes, broadly speaking:

1. With a variable-width font, these modes provide some of the alignment functionality that you would otherwise have gotten for free by counting characters in a fixed-width font.  This is the main reason I wrote them.
	* Using `spaceship-mode`, the initial spaces on a line will have their width adjusted to match the same number of characters on previous lines, so that the left edge of the text/code will be aligned as it would be in a fixed-width font.
	* Using `tabble-mode`, you can use tabs to align text in other places than the left edge.  `tabble-mode` is essentially an implementation of [elastic tabstops](https://nickgravgaard.com/elastic-tabstops/).
2. Certain features may be useful even with fixed-width fonts:
	* Using `tabble-mode` you can quickly create automatically-aligned tables (“tabbles”) in text or code using tab characters as the column separator.
	* Using `spaceship-mode` with `spaceship-auto-preserve` (considered experimental and disabled by default), aligned code blocks will have their alignment preserved in certain situations when the code they are aligned to changes position during editing.

## Caveats

* **Non-standard space/tab conventions.** You must use spaces and tabs consistently in a way that supports the mode.  In particular, with `spaceship-mode` you must strictly adhere to the convention of using tabs for indentation and spaces for alignment; otherwise it won't work.
* **No user interface.** This code is provided as a backend only, and relies on conventions that are contradictory to the assumptions of stock Emacs. It will take some effort to integrate it into your workflow.  See example setup below for some clues on how to get started.
* **No promises.** At time of release this code is untested with most major modes and popular emacs extensions.  Issue reports are welcome, but please read the next section carefully first to be sure you understand the expected behavior.  Not all programming languages have syntax compatible with these conventions.

## What It Does, Precisely

### spaceship-mode

Certain spaces have their widths adjusted to match that of another character on the previous line; generally speaking the nth initial space will match the nth character on the previous line, which might itself be an initial space that matches something further back.  To be precise, a character matches another character exactly when:

* All previous characters on the same line match something, and
* There is a corresponding character on the previous line and it is eligible to be matched by this character:
	* A tab can match only another tab.
	* A space can match any character *except* a tab.
	* Other characters do not match anything.

Each matching space has its width modified to be the same as the character it matches.  This applies recursively: if the matched character's width is itself modified, the modified width is matched.

The rules produce good results when tabs are used for indentation, meaning a standard horizontal offset, and spaces are used for alignment.  The rules allow relative indentation: if a line has spaces and then tabs, the tabs cause indentation by a standard amount relative to the alignment point indicated by the spaces.

All tabs which are part of the leading space of a line (the spaces and tabs occuring before any printing character) have their width standardized by `spaceship-mode`.  The variable `spaceship-tab-pixel-width` determines the width of these tabs.  The usual Emacs variables controlling the display of tabs are ignored (but they still apply for other tabs, unless `tabble-mode` is also used).

### tabble-mode

Let's call tabs which are not part of the leading space *tabble tabs*.  A maximal sequence of consecutive lines containing tabble tabs and with the same leading-space form a *tabble*.  `tabble-mode` adjusts the width of all tabble tabs so that the columns of each tabble are left-aligned.  Note that the first cell of each row cannot be empty since otherwise there would be no tabble tabs in that line and the tabble would be ended.

### spaceship-auto-preserve

If the variable `spaceship-auto-preserve` is set to `t`, then `spaceship-mode` will also try to modify the leading space of aligned blocks of code to preserve the alignment whenever the alignment point shifts position due to editing.  This feature is considered experimental; see the code for details.

## Example Setup

1. Clone the repo or otherwise acquire the files.  Here I assume you put them in `~/spaceship-mode/`.
2. Add the following code to your init file:
```
(push "~/spaceship-mode/" load-path)

(require 'spaceship-mode)
(require 'tabble-mode)

;; (setq spaceship-auto-preserve t) ; uncomment this if you dare

;; always use tabble-mode with spaceship-mode
(add-hook 'spaceship-mode-hook '(lambda () (tabble-mode 1)))

;; define a variable-width font face to use with spaceship-mode.  Replace the
;; "Sans Serif" with any nice Unicode font you have on your system, such as
;; "DejaVu Serif" or "STIX Two Math".
(defface spaceship-face
  '((t :family "Sans Serif"))
  "sans serif (should be variable-width)")
	
;; we will make text-mode always use spaceship-mode, with some tweaks to prevent
;; emacs from clobbering the space/tabs conventions
(add-hook
 'text-mode-hook
 '(lambda ()
    (face-remap-add-relative 'default 'spaceship-face)
    (spaceship-mode 1)
    ;; make auto-indent just copy previous line's leading space
    (setq-local indent-line-function 'spaceship-simple-indent-line-function)
    ;; auto-indent only new line, not previous
    (setq electric-indent-inhibit t)
    ;; use C-backspace to delete all line indentation before point
    (local-set-key [C-backspace] 'spaceship-delete-indentation-or-word)
    ;; make tab just insert tab
    (local-set-key [tab] '(lambda () (interactive) (insert "\t")))))
```
3. Make a copy of `~/spaceship-mode/test.txt` and give it a whirl.

## Known Issues

* These modes do not work correctly with `line-number-mode`.
* Changing the font can mess up the alignment, or anything else that affects display widths without runnning `after-change-functions` or `text-scale-mode-hook`.  You can manually run the commands `spaceship-do-buffer` and/or `tabble-do-buffer` to fix this when it happens.

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