Judge a delimiter by the run it sits in, not the segment it came from

src/adf-to-markdown.test.ts

@@ -190,6 +190,10 @@ test('refuses a mark spelling that cannot open or close where it sits', () => {
   assert.equal(emitted({ text: 'un ', type: 'text' }, marked('-real', strong), { text: ' istic', type: 'text' }), 'un **-real** istic\n')
   assert.equal(emitted(marked('a.', strong)), '**a.**\n')
   assert.equal(emitted({ text: 'x', type: 'text' }, marked('a', strong), { text: 'y', type: 'text' }), 'x**a**y\n')
+  const em: AdfMark = { type: 'em' }
+  assert.equal(code(adfToMarkdown(document(paragraph({ text: 'x', type: 'text' }, marked('a.', em), marked('b', strong))))), 'unspellable-mark')
+  assert.equal(emitted({ text: 'x', type: 'text' }, marked('ab', em, strong), { text: 'y', type: 'text' }), 'x***ab***y\n')
+  assert.equal(code(adfToMarkdown(document(paragraph({ marks: [em, em], text: 'x', type: 'text' })))), 'unsupported-node-shape')
 })
 
 test('refuses marks and attributes nested deeper than the emitter carries', () => {

src/markdown-escaping.ts

@@ -2,13 +2,16 @@ import { escapesLineClaim, isUnicodeWhitespace, opensBracketedAutolink, startsEn
 
 export type InlineSegment = {
   kind: 'emphasis-close' | 'emphasis-open' | 'link-text' | 'literal' | 'syntax'
+  mark?: string
   text: string
 }
 
-export type AssembledLine = { line: string; unspellableDelimiter: string | undefined }
+export type AssembledLine = { line: string; unspellableMark: string | undefined }
 
 export type LineContainer = 'heading' | 'paragraph'
 
+type DelimiterRun = { character: string; closes: boolean; end: number; mark: string; opens: boolean; start: number }
+
 const delimiters = ['*', '_', '`', '~']
 
 const asciiPunctuation = /[!"#$%&'()*+,\-./:;<=>?@[\\\]^_`{|}~]/
@@ -67,21 +70,39 @@ function escape(segments: readonly InlineSegment[], container: LineContainer): A
     placements.push(output.length)
     output += scan.charAt(index)
   }
-  return { line: output, unspellableDelimiter: unspellableDelimiter(segments, output, placements) }
+  return { line: output, unspellableMark: unspellableMark(segments, output, placements) }
 }
 
-function unspellableDelimiter(segments: readonly InlineSegment[], output: string, placements: readonly number[]): string | undefined {
+function unspellableMark(segments: readonly InlineSegment[], output: string, placements: readonly number[]): string | undefined {
+  for (const run of delimiterRuns(segments, placements)) {
+    const before = charAt(output, run.start - 1)
+    const after = output.charAt(run.end)
+    if (run.opens && !isLeftFlanking(before, after)) return run.mark
+    if (run.closes && !isRightFlanking(before, after)) return run.mark
+  }
+  return undefined
+}
+
+function delimiterRuns(segments: readonly InlineSegment[], placements: readonly number[]): DelimiterRun[] {
+  const runs: DelimiterRun[] = []
   let cursor = 0
   for (const segment of segments) {
     const start = placements[cursor] ?? 0
     cursor += segment.text.length
     if (segment.kind !== 'emphasis-close' && segment.kind !== 'emphasis-open') continue
-    const before = charAt(output, start - 1)
-    const after = output.charAt(start + segment.text.length)
-    const spellable = segment.kind === 'emphasis-open' ? isLeftFlanking(before, after) : isRightFlanking(before, after)
-    if (!spellable) return segment.text
+    const closes = segment.kind === 'emphasis-close'
+    const end = start + segment.text.length
+    const mark = segment.mark ?? segment.text
+    const previous = runs[runs.length - 1]
+    if (previous !== undefined && previous.end === start && previous.character === segment.text.charAt(0)) {
+      previous.closes = previous.closes || closes
+      previous.end = end
+      previous.opens = previous.opens || !closes
+      continue
+    }
+    runs.push({ character: segment.text.charAt(0), closes, end, mark, opens: !closes, start })
   }
-  return undefined
+  return runs
 }
 
 function mergesWithSyntax(scan: string, kinds: readonly (InlineSegment['kind'] | undefined)[], index: number): boolean {

src/markdown-inline.ts

@@ -21,8 +21,8 @@ export function emitInlineLine(nodes: readonly AdfNode[], container: LineContain
   const segments = emitRun(nodes, 0, 0, { atBlockEnd: true, container, inLinkText: false, path })
   if (!segments.ok) return segments
   const assembled = assembleInlineLine(segments.value, container)
-  if (assembled.unspellableDelimiter !== undefined) {
-    return failure('unspellable-mark', `the ${assembled.unspellableDelimiter} spelling cannot open or close where it sits`, path)
+  if (assembled.unspellableMark !== undefined) {
+    return failure('unspellable-mark', `the ${assembled.unspellableMark} spelling cannot open or close where it sits`, path)
   }
   const line = assembled.line
   for (const [index, single] of line.split('\n').entries()) {
@@ -80,6 +80,8 @@ function emitLeaf(node: AdfNode, context: InlineContext, index: number): Result<
   if (unspelled !== undefined) {
     return failure('unspelled-node-attribute', `the ${node.type} attribute ${unspelled} has no canonical markdown spelling`, path)
   }
+  const types = (node.marks ?? []).map((mark) => mark.type)
+  if (new Set(types).size !== types.length) return failure('unsupported-node-shape', `a ${node.type} node carries one mark type twice`, path)
   if (node.type === 'hardBreak') {
     if (context.container === 'heading' || context.atBlockEnd) return success([{ kind: 'syntax', text: ':hardBreak{}' }])
     return success([{ kind: 'syntax', text: '\\\n' }])
@@ -102,7 +104,7 @@ function emitMarkedRun(nodes: readonly AdfNode[], mark: AdfMark, depth: number,
   if (!inner.ok) return inner
   const text = inner.value.map((segment) => segment.text).join('')
   if (holdsEdgeWhitespace(text)) return failure('unspellable-whitespace', `the ${mark.type} spelling cannot open or close beside whitespace`, path)
-  return success([{ kind: 'emphasis-open', text: spelling }, ...inner.value, { kind: 'emphasis-close', text: spelling }])
+  return success([{ kind: 'emphasis-open', mark: mark.type, text: spelling }, ...inner.value, { kind: 'emphasis-close', mark: mark.type, text: spelling }])
 }
 
 function emitCodeSpan(nodes: readonly AdfNode[], depth: number, path: ConvertErrorPath): Result<InlineSegment[]> {