Fix emitting :: for centered column in table.

[Ethiraric]

Emitting :: in a table to indicate alignment is Center seems to break some parsers (at least mdbook's). Emitting :-: appears to work fine.

This commit also removes an allocation when encountering an empty header cell. Rather than allocating a non-empty string, let the string be empty and add a minimum width for emitting alignments.

Also, I replaced what I think is a mismatch between characters and bytes in:

let last_minus_one = name.chars().count().saturating_sub(1);
for c in 0..name.len() {

last_minus_one counts in chars, while c counts in bytes.

This might fix #70.

[Ethiraric]

Looking more into #70, this definitely fixes it.

The issue is that the column has a single-character heading. The unwrap_or(" ".into) is not executed, and the width of the column is set to 1. A width of 1 is too small to express centering (whether it be :-: or ::) and the emitter emits a single : for the alignment.

[Byron]

This is wonderful, thanks so much!

I love the extra test as well, even though I admit that I am not very familiar with the table syntax and trust that it will render fine as well.

[Byron]

let last_minus_one = name.chars().count().saturating_sub(1);
for c in 0..name.len() {

I forgot to comment on this: That's a great catch, and averts a disaster that was just waiting to happen.

And here is the new release: https://github.com/Byron/pulldown-cmark-to-cmark/releases/tag/v15.0.1 .

[Ethiraric]

Not a lot has changed in the output. In any case, this fixes my output for centered columns.

The only change I made to your test is to remove a - in the yo column. This means alignment is written as a single - as opposed to 2.

With empty columns, the emitter did always output 2 characters. This is what is needed for left and right alignment, one more than needed for default alignment (but it causes no error), but one too few for the centered alignment.

A side effect of my change, which fixed #70, is that the minimum width requirement is independent from the length of the header column. If you had the following:

|y|
|:-:|

It would have output

|y|
|:|

[Ethiraric]

And here is the new release: https://github.com/Byron/pulldown-cmark-to-cmark/releases/tag/v15.0.1 .

Wow, I did not expect to have a release so soon! Thanks! You save me from having a temporary workaround <3