501b19b6e0
TLDR; this commit vastly improves how whitespaces are handled when converting from HTML to Markdown. It also adds support for converting HTML <tables> to markdown tables. The previous 'remove_whitespaces!' method was traversing the whole HTML tree and used a heuristic to remove leading and trailing whitespaces whenever it was appropriate (ie. mostly before and after HTML block elements) It was a good idea, but it was very limited and leaded to bad conversion when the html had leading whitespaces on several lines for example. One such example can be found [here](https://meta.discourse.org/t/86782). For various reasons, most of the whitespaces in a HTML file is ignored when the page is being displayed in a browser. The rules that the browsers follow are the [CSS' White Space Processing Rules](https://www.w3.org/TR/css-text-3/#white-space-rules). They can be quite complicated when you take into account RTL languages and other various tidbits but they boils down to the following: - Collapse whitespaces down to one space (0x20) inside an inline context (ie. nodes/tags that are being displaying on the same line) - Remove any leading/trailing whitespaces inside an inline context One quick & dirty way of getting this 90% solved would be to do 'HTML.gsub!(/[[:space:]]+/, " ")'. We would also need to hoist <pre> elements in order to not mess with their whitespaces. Unfortunately, this solution let some whitespaces creep around HTML tags which leads to more '.strip!' calls than I can bear. I decided to "emulate" the browser's handling of whitespaces and came up with a solution in 4 parts 1. remove_not_allowed! The HtmlToMarkdown library is recursively "visiting" all the nodes in the HTML in order to convert them to Markdown. All the nodes that aren't handled by the library (eg. <script>, <style> or any non-textual HTML tags) are "swallowed". In order to reduce the number of nodes visited, the method 'remove_not_allowed!' will automatically delete all the nodes that have no "visitor" (eg. a 'visit_<tag>' method) defined. 2. remove_hidden! Similar purpose as the previous method (eg. reducing number of nodes visited), there's no point trying to convert something that is hidden. The 'remove_hidden!' method removes any nodes that was hidden using the "hidden" HTML attribute, some CSS or with a width or height equal to 0. 3. hoist_line_breaks! The 'hoist_line_breaks!' method is there to handle <br> tags. I know those tiny <br> don't do much but they can be quite annoying. The <br> tags are inline elements but they visually work like a block element (ie. they create a new line). If you have the following HTML "<i>Foo<br>Bar</i>", it ends up visually similar to "<i>Foo</i><br><i>Bar</i>". The latter being much more easy to process than the former, so that's what this method is doing. The "hoist_line_breaks" will hoist <br> tags out of inline tags until their parent is a block element. 4. remove_whitespaces! The "remove_whitespaces!" is where all the whitespace removal is happening. It's broken down into 4 methods as well - remove_whitespaces! - is_inline? - collapse_spaces! - remove_trailing_space! The 'remove_whitespace!' method is recursively walking the HTML tree (skipping <pre> tags). If a node has any children, they will be chunked into groups of inline elements vs block elements. For each chunks of inline elements, it will call the "collapse_space!" and "remove_trailing_space!" methods. For each chunks of block elements, it will call "remote_whitespace!" to keep walking the HTML tree recursively. The "is_inline?" method determines whether a node is part of a inline context. A node is inline iif it's a text node or it's an inline tag, but not <br>, and all its children are also inline. The "collapse_spaces!" method will collapse any kind of (white) space into a single space (" ") character, even accros tags. For example, if we have " Foo \n<i> Bar </i>\t42", it will return "Foo <i>Bar </i>42". Finally, the "remove_trailing_space!" method is there to remove any trailing space that might creep in at the end of the inline chunk. This solution is not 100% bullet-proof. It does not support RTL languages at all and has some caveats that I felt were not worth the work to get properly fixed. FIX: better detection of hidden elements when converting HTML to Markdown FIX: take into account the 'allowed_href_schemes' site setting when converting HTML <a> to Markdown FIX: added support for 'mailto:' scheme when converting <a> from HTML to Markdown FIX: added support for <img> dimensions when converting from HTML to Markdown FIX: added support for <dl>, <dd> and <dt> when converting from HTML to Markdown FIX: added support for multilines emphases, strongs and strikes when converting from HTML to Markdown FIX: added support for <acronym> when converting from HTML to Markdown DEV: remove unused 'sanitize' gem Wow, did you just read all that?! Congratz, here's a cookie: 🍪. |
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Discourse is the 100% open source discussion platform built for the next decade of the Internet. Use it as a:
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