// SPDX-License-Identifier: MIT
// Learning project: bounded reporting for ordinary local task files.
import { open } from "node:fs/promises";
import { resolve } from "node:path";
import { pathToFileURL } from "node:url";

export const MAX_BYTES = 65536;
export const MAX_RECORDS = 1000;

export function summarise(records) {
  if (!Array.isArray(records) || records.length > MAX_RECORDS) {
    throw new Error("Expected at most 1000 records");
  }
  const counts = { closed: 0, open: 0 };
  for (const record of records) {
    if (record === null || typeof record !== "object" || Array.isArray(record) ||
        Object.keys(record).sort().join(",") !== "status,title") {
      throw new Error("Expected title and status only");
    }
    const { title, status } = record;
    if (typeof title !== "string" || title.trim().length === 0 || [...title].length > 100) {
      throw new Error("Expected a nonblank title of at most 100 code points");
    }
    if (status !== "open" && status !== "closed") {
      throw new Error("Expected open or closed status");
    }
    counts[status] += 1;
  }
  return { by_status: counts, records: records.length };
}

export async function readRecords(path) {
  const handle = await open(path, "r");
  try {
    if (!(await handle.stat()).isFile()) throw new Error("Expected an ordinary file");
    const buffer = Buffer.alloc(MAX_BYTES + 1);
    let length = 0;
    while (length < buffer.length) {
      const { bytesRead } = await handle.read(buffer, length, buffer.length - length, null);
      if (bytesRead === 0) break;
      length += bytesRead;
    }
    if (length > MAX_BYTES) throw new Error("Input exceeds 65536 bytes");
    const text = new TextDecoder("utf-8", { fatal: true, ignoreBOM: true })
      .decode(buffer.subarray(0, length));
    if (text.startsWith("\uFEFF")) throw new Error("BOM is not allowed");
    return JSON.parse(text);
  } finally {
    await handle.close();
  }
}

export async function main(args = process.argv.slice(2)) {
  if (args.length === 1 && args[0] === "--help") {
    console.log("Usage: node task_report.mjs INPUT.json");
    return 0;
  }
  if (args.length !== 1) {
    console.error("error: expected one input file; use --help");
    return 2;
  }
  let result;
  try {
    result = summarise(await readRecords(args[0]));
  } catch {
    console.error("error: invalid or unreadable task file");
    return 2;
  }
  console.log(JSON.stringify(result));
  return 0;
}

if (process.argv[1] && import.meta.url === pathToFileURL(resolve(process.argv[1])).href) {
  process.exitCode = await main();
}