4b - the block walk spells a list from one walk, a list and its item counting two levels in every spelling

AGENTS.md

@@ -284,10 +284,16 @@ someone spells it or pins it.
   where the general form fails on the same node. Refusing there refuses a document the general
   form spells, so a refusal the general form does not share belongs in the general form or
   nowhere — save the nested list a tight spelling would swallow, whose refusal the
-  tight-versus-blank answer owns (`todo.md` 2b).
+  tight-versus-blank answer owns (`todo.md` 2b). A readable spelling that must spell its subtree
+  before it can give way — the list, whose thematic-break first line and blank lines exist only
+  spelled — hands that one walk to the general form instead: giving way after the walk walks
+  again at every level, doubling per level (4b).
 - Nothing recurses unbounded: the guards walk iteratively, and blocks, marks and JSON values — an
   attribute's and a carried node's alike — are all held to 500 levels, so a deep document is a
-  `Result` rather than the stack overflow that waits near 2000.
+  `Result` rather than the stack overflow that waits near 2000. A block's level is its count of
+  block ancestors — a list item's children two below the list — in either direction and whichever
+  spelling holds them, so the guards agree at the boundary and no spelling recurses twice per
+  level it counts once (the maintainer, 2026-09-18).
 - A reader takes the text and an index — a sticky regex whose `lastIndex` the caller sets on the
   line before it reads, `indexOf` — never a fresh slice per character, and a per-character walk
   hoists the scan that does not vary with the character. The pipeline persona feeds documents

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

@@ -440,6 +440,15 @@ test('spells a list item whose marker completes a thematic break as a directive'
   const nested: AdfNode = { content: [item({ content: [item()], type: 'bulletList' })], type: 'bulletList' }
   assert.equal(markdown(adfToMarkdown(document(nested))), '- -\n')
   assert.equal(markdown(adfToMarkdown(document({ content: [item(nested)], type: 'bulletList' }))), '!adf:bulletList\n!adf:listItem\n- -\n!adf:/listItem\n!adf:/bulletList\n')
+  assert.equal(
+    markdown(adfToMarkdown(document({ content: [item(paragraph({ text: 'a', type: 'text' }), nested), item({ type: 'rule' })], type: 'bulletList' }))),
+    '!adf:bulletList\n!adf:listItem\na\n\n- -\n!adf:/listItem\n!adf:listItem\n---\n!adf:/listItem\n!adf:/bulletList\n',
+  )
+  const spaced: AdfNode = { content: [{ text: 'a\n \nb', type: 'text' }], type: 'codeBlock' }
+  assert.equal(
+    markdown(adfToMarkdown(document({ attrs: { order: 3 }, content: [item(spaced)], type: 'orderedList' }))),
+    '!adf:orderedList {order=3}\n!adf:listItem\n```\na\n \nb\n```\n!adf:/listItem\n!adf:/orderedList\n',
+  )
 })
 
 test('refuses the characters CommonMark rewrites', () => {
@@ -500,6 +509,17 @@ test('refuses a document nested deeper than the emitter carries', () => {
   let carried: AdfNode = paragraph({ text: 'x', type: 'text' })
   for (let depth = 0; depth < 500; depth += 1) carried = { content: [carried], type: 'blockquote' }
   assert.ok(adfToMarkdown(document(carried)).ok)
+  const listed = (levels: number, first: readonly AdfNode[]): AdfNode => {
+    let list: AdfNode = { content: [{ content: [...first], type: 'listItem' }], type: 'bulletList' }
+    for (let level = 1; level < levels; level += 1) list = { content: [{ content: [...first, list], type: 'listItem' }], type: 'bulletList' }
+    return list
+  }
+  const lists = largestNesting / 2
+  assert.ok(adfToMarkdown(document(listed(lists, []))).ok)
+  assert.equal(code(adfToMarkdown(document(listed(lists + 1, [])))), 'unsupported-nesting-depth')
+  const deep = document(listed(lists, [{ type: 'rule' }]))
+  assert.deepEqual(markdownToAdf(markdown(adfToMarkdown(deep))), { ok: true, value: deep })
+  assert.equal(code(adfToMarkdown(document(listed(lists + 1, [{ type: 'rule' }])))), 'unsupported-nesting-depth')
 })
 
 test('emits an empty list item without trailing whitespace', () => {

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

@@ -1,7 +1,7 @@
 import type { AdfDocument, AdfNode } from '../../adf/document.ts'
 import type { BlockDirective } from '../../adf/block-directives.ts'
 import { adfDocumentFault, carriesOnly, nodeAttrs, nodeContent, nodeMarks } from '../../adf/document.ts'
-import { blockDirective } from '../../adf/block-directives.ts'
+import { blockDirective, blockDirectives } from '../../adf/block-directives.ts'
 import { blockDirectiveForm } from '../block-directive-forms.ts'
 import { carriedBlock } from '../opaque-carry.ts'
 import { emitInlineLine } from './inline-line.ts'
@@ -12,7 +12,7 @@ import { languageSlot } from '../code-language.ts'
 import { largestNesting } from '../../nesting.ts'
 import { listBreakSpelling } from '../list-break.ts'
 import { spellBlockDirectiveOpener } from './block-directive-spelling.ts'
-import { spellDirectiveCloser } from '../directive-syntax.ts'
+import { spellDirectiveCloser, spellDirectiveOpener } from '../directive-syntax.ts'
 import { tryImage } from './image.ts'
 import { tryPipeTable } from './pipe-table.ts'
 
@@ -20,8 +20,10 @@ type BlockContainer = 'directive' | 'document' | 'list-item'
 type BlockSpelling = 'commonmark' | 'directive' | 'list'
 type EmittedBlock = { spelling: BlockSpelling; text: string }
 type PlacedBlock = EmittedBlock & { node: AdfNode }
+type WalkedItem = { blocks: readonly PlacedBlock[]; node: AdfNode }
 
 const largestListMarker = 999999999
+const listItemOpener = spellDirectiveOpener('listItem', undefined, '')
 
 export function adfToMarkdown(document: AdfDocument): Result<string> {
   const fault = adfDocumentFault(document)
@@ -33,6 +35,12 @@ export function adfToMarkdown(document: AdfDocument): Result<string> {
 }
 
 function emitBlocks(nodes: readonly AdfNode[], container: BlockContainer, path: ConvertErrorPath, depth: number): Result<string> {
+  const blocks = walkBlocks(nodes, path, depth)
+  if (!blocks.ok) return blocks
+  return success(joinBlocks(blocks.value, container))
+}
+
+function walkBlocks(nodes: readonly AdfNode[], path: ConvertErrorPath, depth: number): Result<PlacedBlock[]> {
   if (depth > largestNesting) return failure('unsupported-nesting-depth', `the document nests deeper than the ${largestNesting} levels the emitter carries`, path)
   const blocks: PlacedBlock[] = []
   for (const [index, node] of nodes.entries()) {
@@ -40,13 +48,17 @@ function emitBlocks(nodes: readonly AdfNode[], container: BlockContainer, path:
     if (!block.ok) return block
     blocks.push({ ...block.value, node })
   }
+  return success(blocks)
+}
+
+function joinBlocks(blocks: readonly PlacedBlock[], container: BlockContainer): string {
   let text = ''
   for (const [index, block] of blocks.entries()) {
     const previous = blocks[index - 1]
     if (previous !== undefined) text += separationBetween(previous, block, container)
     text += block.text
   }
-  return success(text)
+  return text
 }
 
 function separationBetween(previous: PlacedBlock, next: PlacedBlock, container: BlockContainer): string {
@@ -73,13 +85,14 @@ function emitBlock(node: AdfNode, path: ConvertErrorPath, depth: number): Result
   if (directive === undefined) return commonMarkLine(carriedBlock(node, path, depth))
   const readable = readableBlock(node, path, depth)
   if (readable !== undefined) return readable
-  return emitDirectiveBlock(node, directive, path, depth)
+  return emitDirectiveBlock(node, directive, path, depth, () => walkBlocks(nodeContent(node), path, depth + 1))
 }
 
 export function commonMarkSpelling(node: AdfNode, path: ConvertErrorPath, depth: number): Result<null> | undefined {
   const readable = readableBlock(node, path, depth)
   if (readable === undefined) return undefined
-  return readable.ok ? success(null) : readable
+  if (!readable.ok) return readable
+  return readable.value.spelling === 'directive' ? undefined : success(null)
 }
 
 function readableBlock(node: AdfNode, path: ConvertErrorPath, depth: number): Result<EmittedBlock> | undefined {
@@ -111,21 +124,25 @@ function directivePair(node: AdfNode, opener: string, body: string): EmittedBloc
   return { spelling: 'directive', text: `${opener}\n${body === '' ? '' : `${body}\n`}${spellDirectiveCloser(node.type)}` }
 }
 
-function emitDirectiveBlock(node: AdfNode, directive: BlockDirective, path: ConvertErrorPath, depth: number): Result<EmittedBlock> {
+function emitDirectiveBlock(node: AdfNode, directive: BlockDirective, path: ConvertErrorPath, depth: number, walkBody: () => Result<PlacedBlock[]>): Result<EmittedBlock> {
   if (node.text !== undefined) return failure('unsupported-node-shape', `a ${node.type} carries no text: this one holds text`, path)
   if (blockDirectiveForm(node.type) === 'leaf' && nodeContent(node).length > 0) return failure('unsupported-node-shape', `a ${node.type} holds no content: this one holds some`, path)
   if (directive.contentModel === 'code') return emitCodeDirective(node, directive, path, depth)
   const opener = spellBlockDirectiveOpener(node, directive)
   if (opener === undefined) return commonMarkLine(carriedBlock(node, path, depth))
-  return emitDirectiveBody(node, directive, opener, path, depth)
+  return emitDirectiveBody(node, directive, opener, path, walkBody)
 }
 
-function emitDirectiveBody(node: AdfNode, directive: BlockDirective, opener: string, path: ConvertErrorPath, depth: number): Result<EmittedBlock> {
+function emitDirectiveBody(node: AdfNode, directive: BlockDirective, opener: string, path: ConvertErrorPath, walkBody: () => Result<PlacedBlock[]>): Result<EmittedBlock> {
   if (blockDirectiveForm(node.type) === 'leaf') return success({ spelling: 'directive', text: opener })
-  const content = nodeContent(node)
-  const body = directive.contentModel === 'inline' ? emitInlineLine(content, 'paragraph', path) : emitBlocks(content, 'directive', path, depth + 1)
-  if (!body.ok) return body
-  return success(directivePair(node, opener, body.value))
+  if (directive.contentModel === 'inline') {
+    const line = emitInlineLine(nodeContent(node), 'paragraph', path)
+    if (!line.ok) return line
+    return success(directivePair(node, opener, line.value))
+  }
+  const blocks = walkBody()
+  if (!blocks.ok) return blocks
+  return success(directivePair(node, opener, joinBlocks(blocks.value, 'directive')))
 }
 
 function emitBlockquote(node: AdfNode, path: ConvertErrorPath, depth: number): Result<EmittedBlock> | undefined {
@@ -197,16 +214,25 @@ function emitList(node: AdfNode, path: ConvertErrorPath, depth: number): Result<
   const start = listStart(node, items.length)
   if (start === undefined || items.length === 0) return undefined
   if (items.some((item) => item.type !== 'listItem' || !carriesOnly(item, []))) return undefined
-  const lines: string[] = []
+  const walked: WalkedItem[] = []
   for (const [offset, item] of items.entries()) {
-    const emitted = emitListItem(item, ordered ? `${start + offset}. ` : '- ', [...path, 'content', offset], depth)
-    if (emitted === undefined) return undefined
-    if (!emitted.ok) return emitted
-    lines.push(emitted.value)
+    const blocks = walkBlocks(nodeContent(item), [...path, 'content', offset], depth + 2)
+    if (!blocks.ok) return blocks
+    walked.push({ blocks: blocks.value, node: item })
+  }
+  const lines: string[] = []
+  for (const [offset, item] of walked.entries()) {
+    const line = listItemLines(item.blocks, ordered ? `${start + offset}. ` : '- ')
+    if (line === undefined) return emitDirectiveBlock(node, ordered ? blockDirectives.orderedList : blockDirectives.bulletList, path, depth, () => success(directiveItems(walked)))
+    lines.push(line)
   }
   return success({ spelling: 'list', text: lines.join('\n') })
 }
 
+function directiveItems(items: readonly WalkedItem[]): PlacedBlock[] {
+  return items.map((item) => ({ ...directivePair(item.node, listItemOpener, joinBlocks(item.blocks, 'directive')), node: item.node }))
+}
+
 function listStart(node: AdfNode, items: number): number | undefined {
   if (node.type !== 'orderedList') return 0
   const start = nodeAttrs(node)['order']
@@ -214,16 +240,15 @@ function listStart(node: AdfNode, items: number): number | undefined {
   return start + items - 1 > largestListMarker ? undefined : start
 }
 
-function emitListItem(item: AdfNode, marker: string, path: ConvertErrorPath, depth: number): Result<string> | undefined {
-  const inner = emitBlocks(nodeContent(item), 'list-item', path, depth + 1)
-  if (!inner.ok) return inner
-  if (inner.value === '') return success(marker.trimEnd())
-  const body = inner.value.split('\n')
+function listItemLines(blocks: readonly PlacedBlock[], marker: string): string | undefined {
+  const inner = joinBlocks(blocks, 'list-item')
+  if (inner === '') return marker.trimEnd()
+  const body = inner.split('\n')
   if (body.some((line) => line !== '' && isBlankLine(line))) return undefined
   const indent = ' '.repeat(marker.length)
   const lines = body.map((line, index) => (index === 0 ? `${marker}${line}` : line === '' ? '' : `${indent}${line}`))
   if (isThematicBreak(lines[0] ?? '')) return undefined
-  return success(lines.join('\n'))
+  return lines.join('\n')
 }
 
 function emitParagraph(node: AdfNode, path: ConvertErrorPath): Result<EmittedBlock> | undefined {

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

@@ -675,6 +675,12 @@ test('refuses input nested deeper than the parser carries', () => {
   assert.ok(markdownToAdf(nest(repeated('panel'), '!adf:paragraph {localId=a-1}\nPart.\n!adf:/paragraph\n')).ok)
   assert.deepEqual(position(markdownToAdf(nest(['expand', ...repeated('panel'), 'expand'], 'Part.\n'))), { line: 501, offset: 5501 })
   assert.equal(code(markdownToAdf(nest(['panel', ...repeated('expand'), 'panel'], 'Part.\n'))), 'unsupported-nesting-depth')
+  const listed = (levels: number): string => `${'!adf:bulletList\n!adf:listItem\n---\n'.repeat(levels)}${'!adf:/listItem\n!adf:/bulletList\n'.repeat(levels)}`
+  const lists = largestNesting / 2
+  assert.ok(markdownToAdf(listed(lists)).ok)
+  assert.equal(code(markdownToAdf(listed(lists + 1))), 'unsupported-nesting-depth')
+  assert.ok(markdownToAdf(`${'- '.repeat(lists)}a\n`).ok)
+  assert.equal(code(markdownToAdf(`${'- '.repeat(lists + 1)}a\n`)), 'unsupported-nesting-depth')
 })
 
 test('decodes the backslash escapes CommonMark spells, and keeps the rest literal', () => {

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

@@ -150,7 +150,7 @@ function withContent(node: AdfNode, content: readonly AdfNode[]): AdfNode {
 function listNode(node: AdfNode, items: readonly Block[][], definitions: LinkDefinitions, path: ConvertErrorPath, depth: number): Result<AdfNode> {
   const content: AdfNode[] = []
   for (const [index, blocks] of items.entries()) {
-    const item = containerNode({ type: 'listItem' }, blocks, definitions, [...path, 'content', index], depth)
+    const item = containerNode({ type: 'listItem' }, blocks, definitions, [...path, 'content', index], depth + 1)
     if (!item.ok) return item
     content.push(item.value)
   }

src/nesting.ts

@@ -1,2 +1,2 @@
-// One level per block-list recursion in either direction — a list and its items count once — or the two guards disagree.
+// A block's depth is its count of block ancestors, a list item's children two below the list, in either direction and whichever spelling holds them — or the two guards disagree.
 export const largestNesting = 500

todo.md

@@ -64,6 +64,12 @@ 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 one walk cannot know the spelling it will take,
+      so a list and its item count two levels in every spelling and both directions — the readable
+      list counted one and the directive form two, and letting the directive form count one put
+      twice the frames on the stack at 500 — leaving 250 nested lists the limit.
+      **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
       its run from every start position, so an interior whitespace run costs quadratic time rather
       than linear — 3h measured 80k spaces inside an ATX heading at 11.3s, and 3ms once the walk
@@ -81,7 +87,11 @@ proves 12, 13 spells 11's gaps in 12's grammar, and 12 rewrites code 4b and 4c c
       (the stability-reviewer, 2026-09-16). §11's scanning rule is
       the whole argument; the pipeline persona feeds documents nobody typed.
       `readDirectiveContent`'s scan splits into named steps with that fix rather than keeping its
-      complexity (the maintainer, 2026-09-16).
+      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 the same shape 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