// A per-channel content signature for incremental export builds.
//
// The archive + compose stages redo work per channel every build. To skip a
// channel whose content hasn't changed we need a stable answer to "did channel
// X change since we last built its output?". There is no per-channel fingerprint
// in the index, but build-index DOES persist per-VIDEO freshness in the `mtimes`
// LMDB sub-DB (key [channelSlug, videoDir] -> {metaMs, transcriptMs, subsMs,…}).
// Folding a channel's mtime records into one hash reuses the exact freshness
// truth the rest of the build already trusts — so "unchanged" here means the
// same thing build-index means, and it sidesteps the mtime-drift-across-shards
// hazard that a raw fs.stat would reintroduce (the LMDB mtimes are the
// reconciled source, captured once at index time).
//
// The signature is over INPUTS, never the produced bytes: archive zips embed a
// `generatedAt` timestamp (archiveTranscripts.ts channelManifest) so they are
// not byte-reproducible; only an input signature is stable across builds.

import { createHash } from "node:crypto";
import { existsSync } from "node:fs";
import { open } from "lmdb";
import type { Paths } from "./paths";

// Structural subset of buildIndex.ts's MtimeRecord — only the fields that feed
// the signature. `digestMs` is the newest mtime across BOTH digest sidecars
// (machine + human overrides); without it in the hash below, a digest-only
// change would leave every archive believing the channel was unchanged.
type MtimeRecord = {
  metaMs: number;
  transcriptMs: number | null;
  subsMs: number | null;
  digestMs: number | null;
};
type PathKey = [string, string];

export type ChannelSigner = {
  // A hex signature for the channel's on-disk content, or null when it can't be
  // determined (no index yet) — callers should treat null as "cannot cache,
  // rebuild unconditionally". `extra` folds in caller-specific inputs that also
  // affect the output (e.g. archive build options, channel config).
  signature(slug: string, extra?: string): string | null;
  close(): Promise<void>;
};

// Open a read-only view of the index's per-video mtimes for signature lookups.
// Returns a no-op signer (signature() -> null) when the index doesn't exist yet
// or can't be opened, so a fresh checkout simply builds everything.
export function openChannelSigner(paths: Paths): ChannelSigner {
  if (!existsSync(paths.lmdbPath)) {
    return { signature: () => null, close: async () => {} };
  }
  let root: ReturnType<typeof open>;
  try {
    root = open({ path: paths.lmdbPath, readOnly: true, maxDbs: 14 });
  } catch {
    return { signature: () => null, close: async () => {} };
  }
  const mtimes = root.openDB<MtimeRecord, PathKey>({
    name: "mtimes",
    encoding: "msgpack",
  });
  const meta = root.openDB<unknown, string>({ name: "meta", encoding: "msgpack" });
  // Fold the index schema version in so a schema bump (which rewrites the whole
  // index) invalidates every cached signature. Deliberately NOT the `generation`
  // counter — that bumps whenever ANY channel changes, which would needlessly
  // invalidate every OTHER channel's signature.
  const schema = String((meta.get("schema") as number | undefined) ?? "none");

  return {
    signature(slug: string, extra = ""): string | null {
      const h = createHash("sha1");
      h.update(`schema:${schema}\n`);
      if (extra) h.update(`extra:${extra}\n`);
      let sawAny = false;
      for (const { key, value } of mtimes.getRange({
        start: [slug],
        end: [slug, "￿"],
      })) {
        const k = key as PathKey;
        if (k[0] !== slug) break;
        sawAny = true;
        h.update(
          `${k[1]}\t${value.metaMs}\t${value.transcriptMs ?? ""}\t${value.subsMs ?? ""}\t${value.digestMs ?? ""}\n`,
        );
      }
      // A channel with zero indexed videos still gets a stable signature (schema
      // + extra), so an empty channel can be cached like any other.
      void sawAny;
      return h.digest("hex");
    },
    async close() {
      await root.close();
    },
  };
}