/**
 * LeetCode topic-tag slug → the campaign landing routes that teach it, in
 * default preference order. The one answer to "does the campaign cover this
 * tag", read by the sides that must agree:
 *
 * - `components/ProblemBadges.astro` links a covered tag's badge to its
 *   landing page, and every other tag's badge to `/guides/<slug>`;
 * - `components/BacklinksList.astro` lists, under a landing or guide page,
 *   every problem whose badge links there — both via `tagTarget()`;
 * - `apps/cli/sync.ts` creates a placeholder `content/guides/<slug>.mdx` for
 *   every tag a problem page carries that is NOT a key here.
 *
 * Add a key when a campaign topic starts teaching a tag; its guide page is then
 * dead weight, delete it by hand. A tag naming two topics (DFS/BFS: tree and
 * graph; DP: 1-D and grid) lists both; the badge picks per page.
 */
export const TOPIC_LANDINGS: Record<string, readonly string[]> = {
  "hash-table": ["/hash-table"],
  "prefix-sum": ["/prefix-sum"],
  "two-pointers": ["/two-pointers"],
  "sliding-window": ["/sliding-window"],
  "binary-search": ["/binary-search"],
  stack: ["/stack"],
  "monotonic-stack": ["/monotonic-stack"],
  "linked-list": ["/linked-list"],
  matrix: ["/matrix-index-math"],
  "depth-first-search": ["/tree-dfs", "/graph-dfs"],
  "breadth-first-search": ["/tree-bfs", "/graph-bfs"],
  "binary-search-tree": ["/binary-search-tree"],
  "heap-priority-queue": ["/heap-priority-queue"],
  trie: ["/trie"],
  "topological-sort": ["/topological-sort"],
  "union-find": ["/union-find"],
  backtracking: ["/backtracking"],
  "dynamic-programming": ["/1d-dynamic-programming", "/multi-d-grid-dp"],
  greedy: ["/greedy"],
};

/** One of LeetCode's topic tags, as a problem page's `topics:` frontmatter carries it. */
export interface TopicTag {
  name: string;
  slug: string;
}

/**
 * Where a problem page's badge for `tag` points: a root-relative landing or
 * guide route (the caller adds the base), or LeetCode's problem list when the
 * site serves neither, so a badge never links to a 404.
 *
 * A tag naming two landings links to the page's own topic — its route's first
 * segment — when that is one of them; otherwise DFS/BFS prefer the tree topic
 * for a problem LeetCode also tags `Tree` and the graph topic for the rest.
 */
export function tagTarget(
  tag: TopicTag,
  page: { route: string; tags: readonly TopicTag[] },
  served: ReadonlySet<string>,
): { href: string; kind: "landing" | "guide" | "external" } {
  const landings = (TOPIC_LANDINGS[tag.slug] ?? []).filter((path) => served.has(path));
  const own = `/${page.route.split("/")[1]}`;
  const prefer = page.tags.some((t) => t.slug === "tree") ? "/tree-" : "/graph-";
  const landing = landings.includes(own) ? own : (landings.find((p) => p.startsWith(prefer)) ?? landings[0]);
  if (landing) return { href: landing, kind: "landing" };
  const guide = `/guides/${tag.slug}`;
  if (served.has(guide)) return { href: guide, kind: "guide" };
  return { href: `https://leetcode.com/problem-list/${tag.slug}/`, kind: "external" };
}