// A directory's cheap content signature — moved here, unchanged, from
// compose-site.ts (release 18): the publish stages' `inputSig` is computed
// with the SAME function compose uses to decide what to skip, so "a site is
// fresh" and "compose would skip everything" can never disagree.

import path from "node:path";
import { readdir, stat } from "node:fs/promises";
import { createHash } from "node:crypto";
import type { Dirent } from "node:fs";

// A cheap content signature for a directory: relative path + size + mtime of
// every file within, hashed. It reflects exactly what a recursive copy would
// move, so an unchanged source (build:index skipped it incrementally) yields the
// same signature and the copy is skipped. The shared transcript/subs trees hold
// only a bounded handful of paginated page files per channel, so this is far
// cheaper than the copy it guards. Returns "" when the dir is absent/empty.
//
// `ignoreBasename` excludes files by name from the signature. The shared
// per-channel trees carry a manifest.json that build:index rewrites with a fresh
// `generatedAt` for EVERY channel whenever ANY channel mutates (the shared
// page-writer loop runs over all channels). Its pageCount/slugToPage only change
// when the channel's pages change — which the page files already capture — so
// excluding it lets an unchanged channel stay skipped on a partial-change build
// instead of re-copying all 50-odd channels. The tiny stale manifest left behind
// on a skip is structurally identical (only its timestamp differs).
export async function dirSignature(
  dir: string,
  ignoreBasename?: string,
): Promise<string> {
  const h = createHash("sha1");
  let any = false;
  const walk = async (rel: string): Promise<void> => {
    let ents: Dirent[];
    try {
      ents = await readdir(path.join(dir, rel), { withFileTypes: true });
    } catch {
      return;
    }
    ents.sort((a, b) => (a.name < b.name ? -1 : a.name > b.name ? 1 : 0));
    for (const e of ents) {
      if (!e.isDirectory() && e.name === ignoreBasename) continue;
      const childRel = rel ? `${rel}/${e.name}` : e.name;
      if (e.isDirectory()) {
        await walk(childRel);
      } else {
        any = true;
        const s = await stat(path.join(dir, childRel));
        h.update(`${childRel}\t${s.size}\t${s.mtimeMs}\n`);
      }
    }
  };
  await walk("");
  return any ? h.digest("hex") : "";
}