First commit

frontend/app/utils/routes.ts

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+/* Route-shape helpers for the reader experience.
+
+   An article route is a page of the open book drawn by ArticleBookStack —
+   pages laid out side by side, older ones parked with their spines showing —
+   while the article edit page and the new-article page remain plain,
+   full-height pages. An article lives at /wiki/<slug>; one of its language
+   versions at /wiki/<lang>/<slug>, e.g. /wiki/fr/bayes-theorem. */
+
+const READER_PATH = /^\/wiki\/([^/]+?)\/?$/
+const VERSION_READER_PATH = /^\/wiki\/([a-z]{2})\/([^/]+?)\/?$/
+const WIKI_SUB_PATH = /^\/wiki\/[^/]+\/[^/]+/
+
+/** What a reader address names: the article's identity, and which language
+ *  version the address asks for (null for the article's own version). */
+export interface ReaderLocation {
+  identity: string
+  lang: string | null
+}
+
+export function parseReaderPath(path: string): ReaderLocation | null {
+  const version = VERSION_READER_PATH.exec(path)
+  // /wiki/<x>/edit is the editor whatever <x> is — never a version address
+  if (version && version[2]!.toLowerCase() !== 'edit') {
+    return { identity: decodeURIComponent(version[2]!), lang: version[1]! }
+  }
+  const plain = READER_PATH.exec(path)
+  if (plain) return { identity: decodeURIComponent(plain[1]!), lang: null }
+  return null
+}
+
+export function isReaderPath(path: string): boolean {
+  return parseReaderPath(path) !== null
+}
+
+export function wikiSlugFromPath(path: string): string | null {
+  return parseReaderPath(path)?.identity ?? null
+}
+
+/**
+ * The address of one language version: the canonical document lives at the
+ * article's own URL, translations at /wiki/<lang>/<identity>.
+ */
+export function versionHref(identity: string, version: { language?: string; slug: string }): string {
+  if (version.slug === identity) return `/wiki/${encodeURIComponent(identity)}`
+  return `/wiki/${encodeURIComponent(version.language ?? 'en')}/${encodeURIComponent(identity)}`
+}
+
+/**
+ * The "reading surface": the reader itself plus the edit/new-article pages —
+ * the pages that belong to the open book. The stack of open pages survives the
+ * whole session either way; this marks the pages outside it (the list, the
+ * graph, home), which are the places worth returning to once the last page is
+ * closed.
+ */
+export function isReadingSurface(path: string): boolean {
+  return isReaderPath(path) || WIKI_SUB_PATH.test(path) || path === '/new'
+}