feat: add description-width config option

Descriptions and values are drawn as a single line that never wraps.
Because a diagram is scaled to fit the width it is rendered into, the
longest label is what decides that scale: one long description makes the
whole diagram -- bit cells, indices and every other label with it --
shrink until that line fits. A register whose fields carry a sentence
each is then unreadable at any page width, and since each diagram is
measured on its own, a document full of them renders every one at a
different size.

Setting description-width wraps labels at that width instead, so the
length of a description no longer decides how large the diagram is
drawn. Values wrap at the same right edge as the description they belong
to, and the option composes with left-labels, force-descs-on-side and
ltr-bits.

Wrapping means a label is no longer one line tall, so labels can no
longer be spaced by a fixed line height. Each is now offset by the
height it actually occupies, measured before it is placed, which is why
render gained a context block.

The option defaults to none, which keeps the single-line behaviour on
the original code paths. With it unset every diagram in the gallery
renders byte-identical to before.

CHANGELOG.md

@@ -1,5 +1,8 @@
 # Changelog
 
+## [Unreleased]
+- Added `description-width` config option, wrapping descriptions and values instead of drawing them on a single line
+
 ## [v0.3.1] - 2026-06-14
 - Support for non-consecutive ranges ([#13](https://git.kb28.ch/HEL/rivet-typst/pulls/13))
 - Prevent drawing separators on starting lines of fields ([#17](https://git.kb28.ch/HEL/rivet-typst/pulls/17))

docs/config.typ

@@ -26,6 +26,7 @@
 /// - full-page (bool): If true, the page will be resized to fit the diagram and take the background color
 /// - all-bit-i (bool): If true, all bit indices will be rendered, otherwise, only the ends of each range will be displayed
 /// - ltr-bits (bool): If true, bits are placed with the LSB on the left instead of the right
+/// - description-width (float | none): If set, descriptions and values wrap at this width instead of being drawn on a single line
 /// -> dictionary
 #let config(
   default-font-family: "Ubuntu Mono",
@@ -53,7 +54,8 @@
   height: 800,
   full-page: false,
   all-bit-i: true,
-  ltr-bits: false
+  ltr-bits: false,
+  description-width: none
 ) = {}
 
 /// Dark theme config

manual.typ

@@ -203,6 +203,36 @@ For values depending on other ranges, see #link(<format-dependencies>)[Dependenc
 }
 ```
 
+=== Wrapping descriptions <format-description-width>
+
+Descriptions and values are drawn on a single line. A diagram is scaled to fit
+the width it is given, so a long description makes the whole diagram, bit cells
+and all, shrink until it fits:
+
+#let long-schema = (structures: (main: (bits: 16, ranges: (
+  "15-8": (name: "op", description: "Operation to perform on the two operands"),
+  "7-0": (
+    name: "flags",
+    description: "Flags applied to the operation, see the table below for the meaning of each bit",
+    values: ("00000001": "carry in, added to the result before the flags are evaluated")
+  )
+))))
+#let long-sch = schema.load(long-schema)
+
+#align(center, schema.render(long-sch, width: 75%))
+
+Setting the #doc-ref("config.config") option `description-width` wraps them at
+that width instead, leaving the diagram at its natural size. The width is given
+in the same units as `bit-width`, and values wrap at the same right edge as the
+description they belong to. Each label is offset by the height it actually
+occupies, so wrapping onto several lines does not make labels overlap.
+
+```typ
+schema.render(sch, config: config.config(description-width: 300))
+```
+
+#align(center, schema.render(long-sch, width: 75%, config: lib.config.config(description-width: 300)))
+
 #pagebreak(weak: true)
 
 == Dependencies <format-dependencies>

src/config.typ

@@ -25,6 +25,7 @@
   full-page: false,
   all-bit-i: true,
   ltr-bits: false,
+  description-width: none,
 ) = {
   return (
     default-font-family: default-font-family,
@@ -53,6 +54,7 @@
     full-page: full-page,
     all-bit-i: all-bit-i,
     ltr-bits: ltr-bits,
+    description-width: description-width,
   )
 }
 

src/renderer.typ

@@ -41,6 +41,24 @@
   )
 }
 
+// A wrapped label is built as plain content so that its height can be measured
+// before it is placed, and the label after it offset by what it really takes.
+#let text-block(txt, color, width, font: none, italic: false, size: 1em) = {
+  let text-params = (:)
+  if font != none {
+    text-params.insert("font", font)
+  }
+  if italic {
+    text-params.insert("style", "italic")
+  }
+
+  box(width: width * 1pt, text(txt, fill: color, size: size, ..text-params))
+}
+
+#let draw-block(block, x, y) = {
+  draw.content((x, -y), block, anchor: "north-west", stroke: none)
+}
+
 #let draw-line(color, a, b) = {
   let (x0, y0) = a
   let (x1, y1) = b
@@ -113,15 +131,26 @@
   for (val, desc) in values.pairs().sorted(key: p => p.first()) {
     desc-y += gap
     let txt = val + " = " + desc
-    shapes += draw-text(
-      txt, txt-col, desc-x + bit-w / 2, desc-y,
-      anchor: "north-west",
-      font: config.italic-font-family,
-      italic: true,
-      size: config.italic-font-size
-    )
+    if config.description-width == none {
+      shapes += draw-text(
+        txt, txt-col, desc-x + bit-w / 2, desc-y,
+        anchor: "north-west",
+        font: config.italic-font-family,
+        italic: true,
+        size: config.italic-font-size
+      )
 
-    desc-y += config.italic-font-size / 1.2pt
+      desc-y += config.italic-font-size / 1.2pt
+    } else {
+      let block = text-block(
+        txt, txt-col, config.description-width - bit-w / 2,
+        font: config.italic-font-family,
+        italic: true,
+        size: config.italic-font-size
+      )
+      shapes += draw-block(block, desc-x + bit-w / 2, desc-y)
+      desc-y += measure(block).height.pt()
+    }
   }
 
   return (shapes, desc-x, desc-y)
@@ -149,22 +178,47 @@
   shapes += draw-underbracket(config, start-x, start-x + width, start-y)
 
   let mid-x = start-x + width / 2
-  shapes += draw-link(config, mid-x, start-y, desc-x, desc-y)
+  let link-y = if config.description-width == none {
+    desc-y
+  } else {
+    // wrapped labels hang from desc-y, so aim at the middle of their first line
+    desc-y + config.default-font-size.pt() / 2 - bit-h / 2
+  }
+  shapes += draw-link(config, mid-x, start-y, desc-x, link-y)
 
   let txt-x = desc-x
 
   if config.left-labels {
-    txt-x -= range_.description.len() * config.default-font-size / 2pt
+    // desc-x is the right edge of the label when they are on the left, so shift
+    // to its left edge: exactly the box width when wrapping, an estimate from
+    // the character count otherwise
+    txt-x -= if config.description-width == none {
+      range_.description.len() * config.default-font-size / 2pt
+    } else {
+      config.description-width
+    }
   }
 
-  shapes += draw-text(
-    range_.description,
-    config.text-color,
-    txt-x, desc-y + bit-h / 2,
-    anchor: "west"
-  )
+  if config.description-width == none {
+    shapes += draw-text(
+      range_.description,
+      config.text-color,
+      txt-x, desc-y + bit-h / 2,
+      anchor: "west"
+    )
 
-  desc-y += config.default-font-size / 0.75pt
+    desc-y += config.default-font-size / 0.75pt
+  } else {
+    let block = text-block(
+      range_.description,
+      config.text-color,
+      config.description-width,
+      font: config.default-font-family,
+      size: config.default-font-size
+    )
+    shapes += draw-block(block, txt-x, desc-y)
+    desc-y += measure(block).height.pt()
+  }
 
   if range_.values != none and range_.depends-on == none {
     let shapes_
@@ -519,7 +573,7 @@
   return (shapes, desc-y)
 }
 
-#let render(config, schema, width: 100%) = {
+#let render(config, schema, width: 100%) = context {
   set text(
     font: config.default-font-family,
     size: config.default-font-size