4b - review nits: the list's headroom folds into its item loop, emitBlocks inlined, panel-wrapped boundary pinned, 4c note says cubic

src/markdown/emit/adf-to-markdown.test.ts

@@ -520,6 +520,7 @@ test('refuses a document nested deeper than the emitter carries', () => {
   const deep = document(listed(directiveLists, [{ type: 'rule' }]))
   assert.deepEqual(markdownToAdf(markdown(adfToMarkdown(deep))), { ok: true, value: deep })
   assert.equal(code(adfToMarkdown(document(listed(directiveLists + 1, [{ type: 'rule' }])))), 'unsupported-nesting-depth')
+  assert.equal(code(adfToMarkdown(document({ content: [listed(directiveLists, [{ type: 'rule' }])], type: 'panel' }))), 'unsupported-nesting-depth')
   const chain = (levels: number): AdfNode => {
     let card: AdfNode = { type: 'blockCard' }
     for (let level = 0; level < levels; level += 1) card = { content: [card], type: 'blockCard' }

src/markdown/emit/adf-to-markdown.ts

@@ -31,18 +31,13 @@ export function adfToMarkdown(document: AdfDocument): Result<string> {
   const fault = adfDocumentFault(document)
   if (fault !== undefined) return faulted(fault, [])
   if (document.version !== 1) return failure('unsupported-document-version', `no markdown spelling carries ADF version ${document.version}`, [])
-  const blocks = emitBlocks(nodeContent(document), 'document', [], 0)
-  if (!blocks.ok) return blocks
-  return success(blocks.value === '' ? '' : `${blocks.value}\n`)
-}
-
-function emitBlocks(nodes: readonly AdfNode[], container: BlockContainer, path: ConvertErrorPath, depth: number): Result<string> {
-  const walk = walkBlocks(nodes, path, depth)
+  const walk = walkBlocks(nodeContent(document), [], 0)
   if (!walk.ok) return walk
-  return success(joinBlocks(walk.value.blocks, container))
+  const text = joinBlocks(walk.value.blocks, 'document')
+  return success(text === '' ? '' : `${text}\n`)
 }
 
-// The walk's headroom is the least slack any depth guard below it has, so a spelling that sinks the walked blocks a level can refuse rather than walk again.
+// headroom: the least slack any depth guard below the walk has.
 function walkBlocks(nodes: readonly AdfNode[], path: ConvertErrorPath, depth: number): Result<Walk> {
   let headroom = largestNesting - depth
   if (headroom < 0) return tooDeep(path)
@@ -224,25 +219,28 @@ function emitList(node: AdfNode, path: ConvertErrorPath, depth: number): Result<
   if (start === undefined || items.length === 0) return undefined
   if (items.some((item) => item.type !== 'listItem' || !carriesOnly(item, []))) return undefined
   const walked: WalkedItem[] = []
+  let headroom = Number.POSITIVE_INFINITY
   for (const [offset, item] of items.entries()) {
     const walk = walkBlocks(nodeContent(item), [...path, 'content', offset], depth + 1)
     if (!walk.ok) return walk
+    headroom = Math.min(headroom, walk.value.headroom)
     walked.push({ node: item, walk: walk.value })
   }
-  const headroom = Math.min(...walked.map((item) => item.walk.headroom))
   const lines: string[] = []
   for (const [offset, item] of walked.entries()) {
     const line = listItemLines(item.walk.blocks, ordered ? `${start + offset}. ` : '- ')
-    if (line === undefined) return headroom < 1 ? tooDeep(path) : emitDirectiveBlock(node, ordered ? blockDirectives.orderedList : blockDirectives.bulletList, path, depth, () => success(directiveItems(walked)))
+    if (line === undefined) {
+      if (headroom < 1) return tooDeep(path)
+      return emitDirectiveBlock(node, ordered ? blockDirectives.orderedList : blockDirectives.bulletList, path, depth, () => success({ blocks: directiveItems(walked), headroom: headroom - 1 }))
+    }
     lines.push(line)
   }
   return success({ headroom, spelling: 'list', text: lines.join('\n') })
 }
 
 // The directive form sinks each item's blocks a level below where the walk read them.
-function directiveItems(items: readonly WalkedItem[]): Walk {
-  const blocks = items.map((item) => ({ ...directivePair(item.node, listItemOpener, joinBlocks(item.walk.blocks, 'directive'), item.walk.headroom - 1), node: item.node }))
-  return { blocks, headroom: Math.min(...blocks.map((block) => block.headroom)) }
+function directiveItems(items: readonly WalkedItem[]): PlacedBlock[] {
+  return items.map((item) => ({ ...directivePair(item.node, listItemOpener, joinBlocks(item.walk.blocks, 'directive'), item.walk.headroom - 1), node: item.node }))
 }
 
 function listStart(node: AdfNode, items: number): number | undefined {

todo.md

@@ -64,11 +64,9 @@ proves 12, 13 spells 11's gaps in 12's grammar, and 12 rewrites code 4b and 4c c
       export persona runs in bulk walks the document twice. Both walks are linear, so this is a
       constant factor rather than 4b's class change, and the parting is what gives depth its own
       code (§8) — measure before joining them back.
-      **Settled** (the maintainer, 2026-09-18): the limit stays 500 readable lists. The one walk
-      counts items one below the list, as the readable form does, and reports its headroom; a
-      list falling back to the directive form refuses when that form's extra level no longer
-      fits. Counting every list twice was rejected for halving the limit, counting the directive
-      form once for doubling the parser's frames per level.
+      **Settled** (the maintainer, 2026-09-18): the limit stays 500 readable lists, the walk
+      reporting its headroom (§11). Counting every list twice was rejected for halving the limit,
+      counting the directive form once for doubling the parser's frames per level.
       **Measured** (2026-09-18): `adfDocumentFault` walks a 9 MB document in 52 ms against 314 ms
       for the emit, so its two walks stay parted.
 - [ ] **4c — The scanning rule's remaining sites (`0.2.0`).** A trailing-anchored regex re-walks
@@ -90,9 +88,10 @@ proves 12, 13 spells 11's gaps in 12's grammar, and 12 rewrites code 4b and 4c c
       `readDirectiveContent`'s scan splits into named steps with that fix rather than keeping its
       complexity (the maintainer, 2026-09-16). A sixth 4b leaves behind: the parser asks
       `commonMarkSpelling` at every directive-spelled list it reads, and the answer spells the whole
-      subtree below, so nested directive lists cost O(depth × subtree) — 250 rule-first levels parse
-      in 1.3 s at 16.5 kB, 1.5 MB of that shape at 250 levels in 0.6 s — bounded by the depth guard like
-      `readNestedDirective` (the maintainer, 2026-09-18).
+      subtree below, itself quadratic in the depth left, so nested directive lists cost about the
+      cube of their depth — 250 rule-first levels parse in 1.3 s at 16.5 kB, 1.5 MB of that shape
+      at 250 levels in 0.6 s — bounded by the depth guard like `readNestedDirective` (the
+      maintainer, 2026-09-18).
 - [ ] **4d — What the gate says while it runs (`0.2.1`).** `ci.sh` runs nine legs and announces
       none of them, so five minutes of a Gitea run read as silence and a hang cannot be told from
       a slow pull — the maintainer hit exactly this on the `0.1.0` release. Three causes, each its