Make markdownToAdf(adfToMarkdown(doc)) deep-equal doc: textBreak, empty keys, -0, typed carry fences, code blocks of several text nodes

src/markdown/empty-keys.ts

@@ -0,0 +1,30 @@
+import type { DirectiveAttributes, DirectiveValue, Read } from './directive-syntax.ts'
+import type { EmptyKey } from '../adf/document.ts'
+import { emptyKeys } from '../adf/document.ts'
+import { unsupportedNodeShape } from './directive-syntax.ts'
+
+type EmptyKeysRead = { empty: ReadonlySet<EmptyKey>; rest: Map<string, DirectiveValue> }
+
+const emptyValue = 'empty'
+
+// spec/flavour.md, Attributes: no attribute value spells an empty object or array.
+export function spellEmptyKeys(held: Parameters<typeof emptyKeys>[0]): [string, string][] {
+  return emptyKeys(held).map((key) => [key, emptyValue])
+}
+
+export function spellsEmpty(value: DirectiveValue | undefined): boolean {
+  return value?.spelling === emptyValue
+}
+
+export function readEmptyKeys(attributes: DirectiveAttributes, keys: readonly EmptyKey[]): Read<EmptyKeysRead> {
+  const rest = new Map(attributes)
+  const empty = new Set<EmptyKey>()
+  for (const key of keys) {
+    const spelled = rest.get(key)
+    if (spelled === undefined) continue
+    if (!spellsEmpty(spelled)) return { fault: unsupportedNodeShape(`the reserved key ${key} reads ${key}=${emptyValue} alone: this one spells ${key}=${spelled.spelling}`) }
+    empty.add(key)
+    rest.delete(key)
+  }
+  return { value: { empty, rest } }
+}