The previous paragraph layout algorithm had a couple of flaws:
- It always produced line break opportunities between runs although on
the textual level there might have been none.
- It didn't handle trailing spacing correctly in some cases.
- It wouldn't have been easily adaptable to Knuth-Plass style optimal
line breaking because it was fundamentally structured first-fit
run-by-run.
The new paragraph layout algorithm fixes these flaws. It proceeds
roughly in the following stages:
1. Collect all text in the paragraph.
2. Compute BiDi embedding levels.
3. Shape all runs, layout all children and store the resulting items in
a reusable (possibly even cacheable) `ParLayout`.
3. Iterate over all line breaks in the concatenated text.
4. Construct lightweight `LineLayout` objects for full lines instead of
runs. These mostly borrow from the `ParLayout` and only reshape the
first and last run if necessary. The design allows to use Harfbuzz's
UNSAFE_TO_BREAK mechanism to make reshaping more efficient. The size
of a `LineLayout` can be measured without building the line's frame.
5. Build only the selected line's frames and stack them.
This creates a smaller state machine helper type for softness coalescing, which does not own the resulting nodes. While this creates a bit more duplication in stack and par builder, it makes it a lot easier to integrate additional logic into the paragraph builder.
Furthermore:
- Line breaks are now "hard", that is, not coalesced with each other.
- Text nodes with equal style are now merged allowing for example `f{}i` to form a ligature.
This adds overridable functions that markup desugars into. Specifically:
- \ desugars into linebreak
- Two newlines desugar into parbreak
- * desugars into strong
- _ desugars into emph
- = .. desugars into heading
- `..` desugars into raw
- New naming scheme
- TextNode instead of NodeText
- CallExpr instead of ExprCall
- ...
- Less glob imports
- Removes Value::Args variant
- Removes prelude
- Renames Layouted to Fragment
- Moves font into env
- Moves shaping into layout
- Moves frame into separate module
Adds top-edge and bottom-edge parameters to the font function. These define how
the box around a word is computed. The possible values are:
- ascender
- cap-height (default top edge)
- x-height
- baseline (default bottom edge)
- descender
The defaults are chosen so that it's easy to create good-looking designs with
vertical alignment. Since they are much tighter than what most other software
uses by default, the default leading had to be increased to 50% of the font size
and paragraph spacing to 100% of the font size.
The values cap-height and x-height fall back to ascender in case they are zero
because this value may occur in fonts that don't have glyphs with cap- or
x-height (like Twitter Color Emoji). Since cap-height is the default top edge,
doing no fallback would break things badly.
Removes softness in favor of a simple boolean for pages and a more finegread u8
for spacing. This is needed to make paragraph spacing consume line spacing
created by hard line breaks.
- The execution context is a lot more structured: Instead of a magic stack of arbitrary objects there are static objects for pages, stacks and paragraphs
- Page softness/keeping mechanic is now a lot simpler than before
This makes expansion behaviour inheritable by placing it into the area and passing it down during layouting instead of computing some approximation of what we want during execution.
