/**
 * ResolverPool — main-thread client for the parallel-resolution workers.
 *
 * resolveBatch() splits a rowid-ordered batch into ordered chunks, fans the
 * chunks across the pool, and reassembles the results IN CHUNK ORDER, so the
 * caller's admission (edge inserts, row cleanup, failure parking, deferred
 * post-pass queues) is byte-for-byte the sequence the single-threaded loop
 * would have produced. Any worker failure fails the batch — the caller falls
 * back to the sequential path. Kill switch: CODEGRAPH_NO_PARALLEL_RESOLVE=1.
 */

import { Worker } from 'worker_threads';
import * as fs from 'fs';
import * as path from 'path';
import * as os from 'os';
import type { Edge, UnresolvedReference } from '../types';
import type { ResolvedRef, UnresolvedRef } from './types';
import { memoryBudgetBytes } from './memory-budget';

/** One synthesis pass's output: its edge list + worker-measured wall clock. */
export interface SynthPassResult {
  edges: Edge[];
  ms: number;
}

export interface ChunkResult {
  resolved: ResolvedRef[];
  unresolved: UnresolvedRef[];
  deferredChain: UnresolvedRef[];
  deferredThisMember: UnresolvedRef[];
  byMethod: Record<string, number>;
}

interface PoolWorker {
  worker: Worker;
  ready: Promise<void>;
  busy: number;
}

const MIN_PARALLEL_BATCH = 1000;
const CHUNK_SIZE = 500;

/**
 * Minimum TOTAL pending refs before the pool is created at all. Pool boot
 * (module load + readonly DB open + framework detect + cache warm, times N
 * workers) costs real CPU that CONTENDS with sequential resolution on the
 * same cores — measured on a medium repo (~40k refs, ~1.2s of resolution)
 * the pool made indexing slower. It pays off when resolution runs for tens
 * of seconds to minutes (large JVM/Spring-class repos). Override:
 * CODEGRAPH_PARALLEL_RESOLVE_MIN=<refs> (0 forces the pool on).
 */
export function minRefsForPool(): number {
  const raw = process.env.CODEGRAPH_PARALLEL_RESOLVE_MIN;
  if (raw !== undefined) {
    const parsed = Number.parseInt(raw, 10);
    if (Number.isFinite(parsed) && parsed >= 0) return parsed;
  }
  return 150_000;
}

export class ResolverPool {
  private workers: PoolWorker[] = [];
  private nextId = 0;
  private waiters = new Map<number, { resolve: (r: ChunkResult) => void; reject: (e: Error) => void }>();
  private synthWaiters = new Map<number, { resolve: (r: SynthPassResult) => void; reject: (e: Error) => void }>();
  private recycleWaiters = new Map<number, () => void>();
  private failed: Error | null = null;

  /**
   * Pool size from CPU headroom, memory headroom, and the explicit override.
   * Pure — every input injected — so the whole matrix is unit-testable.
   *
   * CPU term: `availableParallelism` (cpuset/affinity-honest — `os.cpus()`
   * enumerates the host's CPUs and sized SIX workers inside a 2-CPU cpuset,
   * §7a.1's false-premise finding), minus one for the persisting main thread,
   * floored at 2 so a true 2-core box keeps the pool's ~2× on synthesis,
   * capped at the long-standing 6.
   *
   * Memory term: workers hold real heap at scale (~1GB each against a 4.6GB
   * kernel-scale DB — six of them OOM-killed a 7GB container once real
   * 8-core concurrency let them peak simultaneously). Estimate per-worker
   * cost from the DB size, keep 30% of the budget for the main thread, and
   * let the smaller term win. Below 2 workers the pool isn't worth its boot
   * cost — callers get null and stay sequential.
   */
  static resolvePoolSize(opts: {
    explicit?: string;
    availableParallelism: number;
    memoryBudget: number;
    dbSizeBytes: number;
  }): number | null {
    if (opts.explicit !== undefined && opts.explicit !== '') {
      const n = Number.parseInt(opts.explicit, 10);
      if (Number.isFinite(n)) {
        if (n <= 0) return null;
        return Math.min(n, 16);
      }
    }
    // No floor: at ap=2 the pool LOSES to sequential outright — measured on
    // the kernel-scale 2-cpuset envelope: resolution 853s sequential vs
    // 1,150s pooled-6-on-2 (§7a.1), and synthesis is Amdahl-bound by its
    // dominant pass (cFnPtrEdges 306s of 358s) so pooling it bought nothing.
    // ap−1 < 2 ⇒ sequential is the fast path, not a fallback.
    const cpuCap = Math.min(opts.availableParallelism - 1, 6);
    const perWorker = Math.min(Math.max(opts.dbSizeBytes * 0.2, 256 * 1024 * 1024), 1.5 * 1024 * 1024 * 1024);
    const memCap = Math.floor((opts.memoryBudget * 0.7) / perWorker);
    const size = Math.min(cpuCap, memCap);
    return size >= 2 ? size : null;
  }

  /**
   * Create a pool when the compiled worker exists (absent when running from
   * source in tests → callers use the sequential path), the kill switch is
   * off, and the machine has the cores AND memory to carry it. Returns null
   * otherwise. `CODEGRAPH_RESOLVE_WORKERS` overrides the computed size
   * (0 disables the pool; values are capped at 16).
   */
  static tryCreate(dbPath: string, projectRoot: string): ResolverPool | null {
    if (process.env.CODEGRAPH_NO_PARALLEL_RESOLVE === '1') return null;
    const workerScript = path.join(__dirname, 'resolver-worker.js');
    if (!fs.existsSync(workerScript)) return null;
    let dbSizeBytes = 0;
    try {
      dbSizeBytes = fs.statSync(dbPath).size;
    } catch { /* fresh/missing file — the 256MB per-worker floor applies */ }
    const ap = os.availableParallelism();
    const budget = memoryBudgetBytes();
    const size = ResolverPool.resolvePoolSize({
      explicit: process.env.CODEGRAPH_RESOLVE_WORKERS,
      availableParallelism: ap,
      memoryBudget: budget,
      dbSizeBytes,
    });
    // Both outcomes log under SYNTH_TIMINGS — a silent null is how §7a.1's
    // diagnostic run hid the memory-term misfire for a whole 25-minute cycle.
    if (process.env.CODEGRAPH_SYNTH_TIMINGS) {
      console.error(
        `[pool-timing] pool ${size === null ? 'disabled' : `size=${size}`} (ap=${ap} budget=${Math.round(budget / 1024 / 1024)}MB db=${Math.round(dbSizeBytes / 1024 / 1024)}MB)`
      );
    }
    if (size === null) return null;
    try {
      return new ResolverPool(workerScript, dbPath, projectRoot, size);
    } catch {
      return null;
    }
  }

  private constructor(workerScript: string, dbPath: string, projectRoot: string, size: number) {
    for (let i = 0; i < size; i++) {
      const worker = new Worker(workerScript);
      let readyResolve!: () => void;
      let readyReject!: (e: Error) => void;
      const ready = new Promise<void>((resolve, reject) => {
        readyResolve = resolve;
        readyReject = reject;
      });
      const pw: PoolWorker = { worker, ready, busy: 0 };
      worker.on('message', (msg: { type: string; id?: number; message?: string; edges?: Edge[]; ms?: number } & Partial<ChunkResult>) => {
        if (msg.type === 'ready') {
          readyResolve();
        } else if (msg.type === 'result' && msg.id !== undefined) {
          pw.busy--;
          const waiter = this.waiters.get(msg.id);
          this.waiters.delete(msg.id);
          waiter?.resolve({
            resolved: msg.resolved!,
            unresolved: msg.unresolved!,
            deferredChain: msg.deferredChain!,
            deferredThisMember: msg.deferredThisMember!,
            byMethod: msg.byMethod!,
          });
        } else if (msg.type === 'synth-result' && msg.id !== undefined) {
          pw.busy--;
          const waiter = this.synthWaiters.get(msg.id);
          this.synthWaiters.delete(msg.id);
          waiter?.resolve({ edges: msg.edges ?? [], ms: msg.ms ?? 0 });
        } else if (msg.type === 'recycled' && msg.id !== undefined) {
          const waiter = this.recycleWaiters.get(msg.id);
          this.recycleWaiters.delete(msg.id);
          waiter?.();
        } else if (msg.type === 'error') {
          pw.busy--;
          const err = new Error(`resolver worker: ${msg.message}`);
          if (msg.id !== undefined && this.waiters.has(msg.id)) {
            const waiter = this.waiters.get(msg.id)!;
            this.waiters.delete(msg.id);
            waiter.reject(err);
          } else if (msg.id !== undefined && this.synthWaiters.has(msg.id)) {
            const waiter = this.synthWaiters.get(msg.id)!;
            this.synthWaiters.delete(msg.id);
            waiter.reject(err);
          } else {
            this.fail(err);
          }
        }
      });
      worker.on('error', (err) => {
        this.fail(err instanceof Error ? err : new Error(String(err)));
        readyReject(this.failed!);
      });
      worker.on('exit', (code) => {
        if (code !== 0) {
          this.fail(new Error(`resolver worker exited with code ${code}`));
          readyReject(this.failed!);
        }
      });
      worker.postMessage({ type: 'open', dbPath, projectRoot });
      this.workers.push(pw);
    }
  }

  private fail(err: Error): void {
    if (!this.failed) this.failed = err;
    for (const [, waiter] of this.waiters) waiter.reject(this.failed);
    this.waiters.clear();
    for (const [, waiter] of this.synthWaiters) waiter.reject(this.failed);
    this.synthWaiters.clear();
    // Pending recycles resolve rather than reject: their per-call timeout
    // owns rejection, and the recycle caller checks this.failed next round.
    for (const [, done] of this.recycleWaiters) done();
    this.recycleWaiters.clear();
  }

  /** Whether this batch is worth fanning out. */
  static worthParallel(batchLength: number): boolean {
    return batchLength >= MIN_PARALLEL_BATCH;
  }

  async ready(): Promise<void> {
    await Promise.all(this.workers.map((w) => w.ready));
  }

  /**
   * Resolve `refs` across the pool. Chunks preserve input order; the returned
   * arrays are the in-order concatenation of the chunk results.
   */
  async resolveBatch(refs: UnresolvedReference[]): Promise<ChunkResult> {
    if (this.failed) throw this.failed;
    const chunkPromises: Promise<ChunkResult>[] = [];
    for (let i = 0; i < refs.length; i += CHUNK_SIZE) {
      const chunk = refs.slice(i, i + CHUNK_SIZE);
      const id = this.nextId++;
      // Least-busy dispatch keeps workers evenly loaded regardless of chunk
      // cost variance; result order is fixed by the promise array, not by
      // completion order.
      const pw = this.workers.reduce((a, b) => (b.busy < a.busy ? b : a));
      pw.busy++;
      chunkPromises.push(
        new Promise<ChunkResult>((resolve, reject) => {
          this.waiters.set(id, { resolve, reject });
          pw.worker.postMessage({ type: 'resolve', id, refs: chunk });
        })
      );
    }
    const chunks = await Promise.all(chunkPromises);
    const out: ChunkResult = { resolved: [], unresolved: [], deferredChain: [], deferredThisMember: [], byMethod: {} };
    for (const c of chunks) {
      out.resolved.push(...c.resolved);
      out.unresolved.push(...c.unresolved);
      out.deferredChain.push(...c.deferredChain);
      out.deferredThisMember.push(...c.deferredThisMember);
      for (const [k, v] of Object.entries(c.byMethod)) out.byMethod[k] = (out.byMethod[k] || 0) + v;
    }
    return out;
  }

  /**
   * Run one synthesis pass (by SYNTH_PASSES name) on the least-busy worker.
   * The worker reads the committed graph on its own connection and returns
   * the pass's edge list; the caller merges in canonical order. Rejects on
   * worker failure — the caller retries the pass on the main thread.
   */
  async runSynthPass(passName: string): Promise<SynthPassResult> {
    if (this.failed) throw this.failed;
    const id = this.nextId++;
    const pw = this.workers.reduce((a, b) => (b.busy < a.busy ? b : a));
    pw.busy++;
    return new Promise<SynthPassResult>((resolve, reject) => {
      this.synthWaiters.set(id, { resolve, reject });
      pw.worker.postMessage({ type: 'synth', id, pass: passName });
    });
  }

  /**
   * Ask every worker to close and reopen its read-only connection, and wait
   * for all acks. MUST be called only at the pool-idle boundary (all fanned
   * chunks settled, next batch not yet dispatched) — the workers close their
   * connections in place. Why: a long-lived reader pins WAL checkpoint
   * progress, and the deep WAL behind it taxes every main-thread B-tree
   * page operation (writes-under-readers, plan §7a.6 — deletes 42.6→118.8s
   * from 0 to 4 attached readers). Releasing the read marks periodically
   * lets the existing checkpoints advance, keeping the WAL shallow WITHOUT
   * the full-park folds an aggressive valve pays (+129s measured at 64MB).
   * A recycle failure fails the pool — the caller's sequential fallback
   * covers the rest of the run.
   */
  async recycleWorkers(): Promise<void> {
    if (this.failed) throw this.failed;
    await Promise.all(
      this.workers.map(
        (pw) =>
          new Promise<void>((resolve, reject) => {
            const id = this.nextId++;
            const t = setTimeout(() => {
              if (this.recycleWaiters.delete(id)) {
                const err = new Error('resolver worker recycle timed out');
                this.fail(err);
                reject(err);
              }
            }, 10_000);
            this.recycleWaiters.set(id, () => {
              clearTimeout(t);
              resolve();
            });
            pw.worker.postMessage({ type: 'recycle', id });
          })
      )
    );
  }

  async destroy(): Promise<void> {
    await Promise.all(
      this.workers.map(
        (pw) =>
          new Promise<void>((resolve) => {
            const t = setTimeout(() => {
              void pw.worker.terminate().then(() => resolve());
            }, 5000);
            pw.worker.once('exit', () => {
              clearTimeout(t);
              resolve();
            });
            pw.worker.postMessage({ type: 'close' });
          })
      )
    );
  }
}