/**
 * Disk ring-buffer writer.
 *
 * Appends chunks to a per-session file, enforcing:
 *  - Per-session cap: 100 MB (configurable)
 *  - Global cap: 1 GB across all sessions (configurable)
 *  - Free-space watchdog: refuse writes if free disk < 1 GB
 *  - Idle cleanup: sessions inactive for > 30 days are deleted
 *
 * File format (binary, append-only):
 *   Each record: [seq: 8 bytes BE uint64] [length: 4 bytes BE uint32] [data: N bytes]
 *
 * Risk R1 mitigation: all writes serialised through a per-session async queue.
 * Global cap protected by a module-level mutex (simple flag since JS is single-threaded).
 *
 * Owner: T-1.2
 */

import fs from "node:fs/promises";
import os from "node:os";
import path from "node:path";
import type { SeqNum } from "../sequence.js";

// ---------------------------------------------------------------------------
// Config defaults (can be overridden; T-1.7 will wire these from config.toml)
// ---------------------------------------------------------------------------

export interface BufferConfig {
  stateDir: string;
  perSessionMb: number; // default 100
  globalGb: number; // default 1
  freeMinGb: number; // default 1
  idleDays: number; // default 30
}

function defaultConfig(): BufferConfig {
  return {
    stateDir: path.join(os.homedir(), ".local", "share", "pi-remote"),
    perSessionMb: 100,
    globalGb: 1,
    freeMinGb: 1,
    idleDays: 30,
  };
}

let _config: BufferConfig = defaultConfig();

export function configureBuffer(cfg: Partial<BufferConfig>): void {
  _config = { ..._config, ...cfg };
}

// ---------------------------------------------------------------------------
// Global cap mutex (JS single-threaded so a flag suffices)
// ---------------------------------------------------------------------------

let _globalBusy = false;
let _globalBytes = 0; // tracked in-memory; recalculated on startup

// ---------------------------------------------------------------------------
// Per-session writer
// ---------------------------------------------------------------------------

export class BufferWriter {
  readonly session: string;
  private filePath: string;
  private queue: Promise<void> = Promise.resolve();
  private sessionBytes = 0;
  private lastWriteAt = Date.now();

  constructor(session: string) {
    this.session = session;
    this.filePath = path.join(_config.stateDir, "buffers", `${session}.buf`);
  }

  async open(): Promise<void> {
    await fs.mkdir(path.dirname(this.filePath), { recursive: true });
    // Load existing size for cap tracking
    try {
      const stat = await fs.stat(this.filePath);
      this.sessionBytes = stat.size;
      _globalBytes += stat.size;
    } catch {
      this.sessionBytes = 0;
    }
  }

  /**
   * Enqueue a chunk write. Writes are serialised per session.
   */
  write(seq: SeqNum, data: Buffer): void {
    this.queue = this.queue.then(() => this._write(seq, data));
  }

  private async _write(seq: SeqNum, data: Buffer): Promise<void> {
    const perSessionCap = _config.perSessionMb * 1024 * 1024;
    const globalCap = _config.globalGb * 1024 * 1024 * 1024;

    // Free-space watchdog
    try {
      const { available } = await checkFreeSpace(path.dirname(this.filePath));
      const freeMin = _config.freeMinGb * 1024 * 1024 * 1024;
      if (available < freeMin) return; // silently drop; could emit a warning
    } catch {
      // If we can't check, don't block writes
    }

    // Cap enforcement
    const recordSize = 8 + 4 + data.length;
    if (
      _globalBusy ||
      this.sessionBytes + recordSize > perSessionCap ||
      _globalBytes + recordSize > globalCap
    ) {
      return; // drop oldest strategy: just don't write (ring via truncation not implemented yet)
    }

    _globalBusy = true;
    try {
      const header = Buffer.allocUnsafe(12);
      header.writeBigUInt64BE(BigInt(seq), 0);
      header.writeUInt32BE(data.length, 8);

      await fs.appendFile(this.filePath, Buffer.concat([header, data]));
      this.sessionBytes += recordSize;
      _globalBytes += recordSize;
      this.lastWriteAt = Date.now();
    } finally {
      _globalBusy = false;
    }
  }

  async close(): Promise<void> {
    await this.queue; // drain
  }

  /** Delete the buffer file and reclaim global tracking bytes. */
  async delete(): Promise<void> {
    await this.queue;
    try {
      await fs.unlink(this.filePath);
      _globalBytes = Math.max(0, _globalBytes - this.sessionBytes);
      this.sessionBytes = 0;
    } catch {
      // already gone
    }
  }

  get idleMs(): number {
    return Date.now() - this.lastWriteAt;
  }
}

// ---------------------------------------------------------------------------
// Idle cleanup
// ---------------------------------------------------------------------------

/**
 * Delete buffer files for sessions idle longer than idleDays.
 * Safe to call periodically (e.g. on startup or daily timer).
 */
export async function cleanupIdleBuffers(
  cfg: BufferConfig = _config,
): Promise<string[]> {
  const dir = path.join(cfg.stateDir, "buffers");
  const maxIdleMs = cfg.idleDays * 24 * 60 * 60 * 1000;
  const deleted: string[] = [];

  let entries: fs.Dirent[] = [];
  try {
    entries = await fs.readdir(dir, { withFileTypes: true });
  } catch {
    return deleted;
  }

  for (const entry of entries) {
    if (!entry.name.endsWith(".buf")) continue;
    const fp = path.join(dir, entry.name);
    try {
      const stat = await fs.stat(fp);
      if (Date.now() - stat.mtimeMs > maxIdleMs) {
        await fs.unlink(fp);
        deleted.push(entry.name.replace(/\.buf$/, ""));
      }
    } catch {
      // skip
    }
  }
  return deleted;
}

// ---------------------------------------------------------------------------
// Helpers
// ---------------------------------------------------------------------------

/** Approximate free disk space on the filesystem containing `dir`. */
async function checkFreeSpace(dir: string): Promise<{ available: number }> {
  // Node doesn't expose statvfs directly; use df -k as a fallback.
  // If it fails, caller ignores the error.
  const { execFile } = await import("node:child_process");
  const { promisify } = await import("node:util");
  const exec = promisify(execFile);
  const { stdout } = await exec("df", ["-k", dir]);
  const lines = stdout.trim().split("\n");
  const last = lines[lines.length - 1];
  const parts = last.split(/\s+/);
  // df -k columns: Filesystem 1K-blocks Used Available Use% Mounted
  const availKb = parseInt(parts[3], 10);
  return { available: availKb * 1024 };
}