import { eq, and, isNotNull, sql } from 'drizzle-orm';
import type { LibSQLDatabase } from 'drizzle-orm/libsql';
import type * as schema from '../db/schema/index';
import { contentItems } from '../db/schema/index';
import { systemConfig } from '../db/schema/index';
import { access } from 'node:fs/promises';

// ── Types ──

export interface ScanResult {
  checked: number;
  missing: number;
  duration: number; // milliseconds
}

interface DownloadedRow {
  id: number;
  filePath: string;
}

type Db = LibSQLDatabase<typeof schema>;

// ── Constants ──

const BATCH_SIZE = 100;
const SCAN_LAST_RUN_KEY = 'missing_file_scan_last_run';
const SCAN_LAST_RESULT_KEY = 'missing_file_scan_last_result';

// ── Scanner ──

export class MissingFileScanner {
  constructor(private readonly db: Db) {}

  /**
   * Scan all content items with status='downloaded' and a non-null filePath.
   * For each, check if the file exists on disk. If not, update status to 'missing'.
   * Works in batches of BATCH_SIZE to bound memory usage on large libraries.
   */
  async scanAll(): Promise<ScanResult> {
    const start = Date.now();
    let checked = 0;
    let missing = 0;
    let lastId = 0;

    console.log('[missing-file-scanner] Scan started');

    // Cursor-based pagination: since we mutate status from 'downloaded' to 'missing'
    // during iteration, offset-based pagination would skip rows. Using id > lastId
    // ensures we always pick up the next unconsumed batch.
    while (true) {
      const batch = await this.db
        .select({ id: contentItems.id, filePath: contentItems.filePath })
        .from(contentItems)
        .where(
          and(
            eq(contentItems.status, 'downloaded'),
            isNotNull(contentItems.filePath),
            sql`${contentItems.filePath} != ''`,
            sql`${contentItems.id} > ${lastId}`
          )
        )
        .orderBy(contentItems.id)
        .limit(BATCH_SIZE);

      if (batch.length === 0) break;

      const missingIds: number[] = [];

      for (const row of batch as DownloadedRow[]) {
        checked++;
        const exists = await fileExists(row.filePath);
        if (!exists) {
          missingIds.push(row.id);
          missing++;
          console.log(`[missing-file-scanner] File missing: id=${row.id} path=${row.filePath}`);
        }
      }

      // Batch-update missing items
      if (missingIds.length > 0) {
        await this.markMissing(missingIds);
      }

      // Advance cursor to the last processed id
      lastId = batch[batch.length - 1].id as number;

      // If batch was smaller than BATCH_SIZE, we've exhausted the result set
      if (batch.length < BATCH_SIZE) break;
    }

    const duration = Date.now() - start;
    const result: ScanResult = { checked, missing, duration };

    console.log(`[missing-file-scanner] Scan completed: checked=${checked} missing=${missing} duration=${duration}ms`);

    // Persist scan metadata
    await this.persistScanResult(result);

    return result;
  }

  /**
   * Get the last scan result from system_config.
   * Returns null if no scan has been run yet.
   */
  async getLastScanResult(): Promise<{ lastRun: string; result: ScanResult } | null> {
    const rows = await this.db
      .select({ key: systemConfig.key, value: systemConfig.value })
      .from(systemConfig)
      .where(eq(systemConfig.key, SCAN_LAST_RUN_KEY));

    if (rows.length === 0) return null;

    const resultRows = await this.db
      .select({ value: systemConfig.value })
      .from(systemConfig)
      .where(eq(systemConfig.key, SCAN_LAST_RESULT_KEY));

    return {
      lastRun: rows[0].value,
      result: resultRows.length > 0 ? JSON.parse(resultRows[0].value) : { checked: 0, missing: 0, duration: 0 },
    };
  }

  // ── Private ──

  private async markMissing(ids: number[]): Promise<void> {
    // SQLite has a variable limit; chunk if needed, but BATCH_SIZE=100 is well within limits
    await this.db
      .update(contentItems)
      .set({
        status: 'missing',
        updatedAt: sql`(datetime('now'))`,
      })
      .where(sql`${contentItems.id} IN (${sql.join(ids.map(id => sql`${id}`), sql`, `)})`);
  }

  private async persistScanResult(result: ScanResult): Promise<void> {
    const now = new Date().toISOString();
    const resultJson = JSON.stringify(result);

    // Upsert last run timestamp
    await this.db
      .insert(systemConfig)
      .values({ key: SCAN_LAST_RUN_KEY, value: now })
      .onConflictDoUpdate({
        target: systemConfig.key,
        set: { value: now, updatedAt: sql`(datetime('now'))` },
      });

    // Upsert last result
    await this.db
      .insert(systemConfig)
      .values({ key: SCAN_LAST_RESULT_KEY, value: resultJson })
      .onConflictDoUpdate({
        target: systemConfig.key,
        set: { value: resultJson, updatedAt: sql`(datetime('now'))` },
      });
  }
}

// ── Helpers ──

async function fileExists(filePath: string): Promise<boolean> {
  try {
    await access(filePath);
    return true;
  } catch {
    return false;
  }
}