import childProcess from "node:child_process";
import crypto from "node:crypto";
import { mkdir, mkdtemp, readFile, realpath, rename, rm, stat } from "node:fs/promises";
import path from "node:path";
import { promisify } from "node:util";
import type { CapabilityRepositoryResolver } from "./assembly.js";

const execFile = promisify(childProcess.execFile);

const sourceKey = (kind: string, repository: string, commit: string) =>
  `${kind}\0${repository}\0${commit.toLowerCase()}`;

const checkoutName = (kind: string, repository: string, commit: string) => {
  const digest = crypto
    .createHash("sha256")
    .update(sourceKey(kind, repository, commit))
    .digest("hex")
    .slice(0, 24);
  return `${kind}-${digest}`;
};

export const createGitCapabilityResolver = async (options: {
  checkoutRoot: string;
  snapshotMap?: string;
  snapshotOnly?: boolean;
}): Promise<CapabilityRepositoryResolver> => {
  await mkdir(options.checkoutRoot, { recursive: true });
  const checkoutRoot = await realpath(options.checkoutRoot);
  const snapshots = new Map<string, string>();
  if (options.snapshotMap) {
    const snapshotMapPath = await realpath(options.snapshotMap);
    const document = JSON.parse(await readFile(snapshotMapPath, "utf8")) as {
      resources?: Array<{
        kind: string;
        repository: string;
        commit: string;
        directory: string;
      }>;
    };
    for (const entry of document.resources ?? []) {
      if (entry.kind !== "interface" && entry.kind !== "package") {
        throw new Error(`Snapshot map has unsupported resource kind ${entry.kind}`);
      }
      const key = sourceKey(entry.kind, entry.repository, entry.commit);
      if (snapshots.has(key)) throw new Error(`Snapshot map repeats ${key}`);
      const directory = path.resolve(path.dirname(snapshotMapPath), entry.directory);
      snapshots.set(key, await realpath(directory));
    }
  }

  const checkouts = new Map<string, Promise<{ directory: string }>>();
  return async (source, kind) => {
    const key = sourceKey(kind, source.repository, source.commit);
    const snapshot = snapshots.get(key);
    if (snapshot) return { directory: snapshot };
    if (options.snapshotOnly) throw new Error(`No offline snapshot for ${kind} ${source.repository}@${source.commit}`);
    const existing = checkouts.get(key);
    if (existing) return await existing;
    const pending = (async () => {
      const directory = path.join(
        checkoutRoot,
        checkoutName(kind, source.repository, source.commit),
      );
      const verify = async (checkout: string) => {
        const { stdout } = await execFile("git", ["-C", checkout, "rev-parse", "HEAD"]);
        if (stdout.trim().toLowerCase() !== source.commit.toLowerCase()) {
          throw new Error(`Locked commit mismatch for ${source.repository}: wanted ${source.commit}, fetched ${stdout.trim()}`);
        }
        const { stdout: changes } = await execFile("git", ["-C", checkout, "status", "--porcelain", "--untracked-files=all"]);
        if (changes.trim()) throw new Error(`Dependency checkout was modified: ${checkout}`);
      };
      // Only complete, checked clones become visible under the deterministic name.
      // Concurrent resolvers may fetch independently, but cannot observe a partial clone.
      if (await stat(directory).then(() => true, (error: NodeJS.ErrnoException) => {
        if (error.code === "ENOENT") return false;
        throw error;
      })) {
        await verify(directory);
        return { directory };
      }
      const staging = await mkdtemp(path.join(checkoutRoot, ".fetch-"));
      const checkout = path.join(staging, "checkout");
      try {
        await execFile("git", [
        "-c",
        "advice.detachedHead=false",
        "clone",
        "--depth",
        "1",
        "--single-branch",
        "--branch",
        `quixos-reachability/${source.commit.toLowerCase()}`,
        source.repository,
        checkout,
      ]);
        await verify(checkout);
        try { await rename(checkout, directory); }
        catch (error) {
          if (!["EEXIST", "ENOTEMPTY"].includes((error as NodeJS.ErrnoException).code ?? "")) throw error;
          await verify(directory);
        }
      } finally {
        await rm(staging, { recursive: true, force: true });
      }
      return { directory };
    })();
    checkouts.set(key, pending);
    try { return await pending; }
    catch (error) { checkouts.delete(key); throw error; }
  };
};