Module lines 

Line layout: text in, broken lines out.

Knuth-Plass total fit. A paragraph is flattened into style runs, shaped, and modelled as boxes, glue and penalties; the breaker picks the set of breaks with the fewest demerits over the whole paragraph rather than the most text on each line. The break source is UAX #14 with word boundaries from UAX #29, plus optional hyphenation, which enters as a flagged penalty.

Justified text has its glue stretched or shrunk to the measure here. The adjustment lands on the glyphs’ own advances, so a painter positions what it is given and never re-derives spacing. A Line is shaped runs: measurement happened here.

Units: advances come out of the shaper in font units; the measure arrives in points and converts once, via units_per_em * size.

The stages have a file each: flatten makes one string of styled spans, shape turns spans into glyphs, opportunity says where a line may end, breaker picks the breaks, justify moves the glue on a line that was set to its measure, and line is what comes out.

Structs

Broken

One paragraph broken into lines: the lines themselves, and where each of them ended.

Face

The face ::first-line asks its runs for: a family, a slope and a weight, each one set only where the rule set it.

FirstLine

What ::first-line changes about the paragraph it opens.

Generated

Text the sheet generates around one inline element: what ::before and ::after hold, each with the style it is set in.

HangingPunctuation

Which marks may hang past the measure, from hanging-punctuation.

Inherited

Every inline takes the style of the block around it.

InlineBox

What one inline element paints around its runs, and what that takes on each edge.

InlineFragment

One inline element’s box on one line: where it sits, and which of its edges it paints there.

Line

One typeset line: shaped runs plus its width in font units.

LineBreakOptions

How a paragraph is broken and filled. The defaults are ragged right, no hyphenation, and no mark hanging past the measure.

LineLayout

The layout pass: one paragraph → lines that fit the measure.

LineSpan

One span of a line: the runs set in it, and where they go.

Measure

The spans a paragraph breaks to, in reading order.

Opening

What sets a paragraph’s opening apart from the rest of it.

ParagraphStyle

Everything one paragraph’s layout depends on, and the colour its runs are painted in. The style tree compiles down to this.

Patterns

The syllable patterns hyphens: auto breaks words by.

Shaped

A paragraph shaped once, and everything about it the measure does not decide: the flattened text, the runs it shaped into, and the style and options it was set with.

ShapedRun

A run of glyphs sharing one font and size — the paintable unit.

Span

One place a line may start and how wide it may run.

Enums

HangEnd

How far hanging-punctuation goes at the end of a line.

Spans

The spans of one line, read as a slice either way. A band set undivided holds its span inline: the ordinary line of a book does not pay for an allocation.

Traits

InlineStyles

Where line layout gets the style of one inline node.