(() => {
  const tasks = [];
  const MAX_HISTORY = 30;
  let seq = 0;
  let button = null;
  let badge = null;
  let panel = null;
  let list = null;
  let summary = null;
  let clearButton = null;

  function formatBytes(value) {
    const bytes = Math.max(0, Number(value) || 0);
    if (bytes < 1024) return `${bytes} B`;
    if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(1)} KB`;
    if (bytes < 1024 * 1024 * 1024) return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
    return `${(bytes / (1024 * 1024 * 1024)).toFixed(2)} GB`;
  }

  function isActive(task) {
    return ['pending', 'running', 'pausing', 'deleting'].includes(task.status);
  }

  function isUnfinished(task) {
    return isActive(task) || task.status === 'paused';
  }

  function statusText(task) {
    if (task.status === 'pending') return '准备下载…';
    if (task.status === 'pausing') return '正在暂停…';
    if (task.status === 'paused') {
      return task.receivedBytes ? `已暂停 · ${formatBytes(task.receivedBytes)}` : '已暂停';
    }
    if (task.status === 'deleting') return '正在删除…';
    if (task.status === 'failed') return task.error || '下载失败';
    if (task.status === 'canceled') return '已取消';
    if (task.status === 'complete') {
      return task.receivedBytes ? `下载完成 · ${formatBytes(task.receivedBytes)}` : '下载完成';
    }
    if (task.totalBytes && task.percent != null) {
      return `${Math.round(task.percent * 100)}% · ${formatBytes(task.receivedBytes)} / ${formatBytes(task.totalBytes)}`;
    }
    return task.receivedBytes ? `${formatBytes(task.receivedBytes)} 已下载` : '正在连接…';
  }

  function taskHtml(task) {
    const ratio = task.status === 'complete'
      ? 1
      : (task.percent == null ? null : Math.max(0, Math.min(1, task.percent)));
    const progressClass = ratio == null && ['pending', 'running', 'pausing'].includes(task.status)
      ? ' indeterminate'
      : '';
    const width = ratio == null ? 0 : Math.round(ratio * 100);
    const book = task.bookTitle && task.bookTitle !== task.name
      ? `<div class="task-center-book">${escapeHtml(task.bookTitle)}</div>`
      : '';
    let actions = '';
    if (task.status === 'complete') {
      actions = `<div class="task-center-actions">
        <button data-task-action="open">打开</button>
        <button data-task-action="reveal">定位</button>
        <button data-task-action="remove">移除</button>
      </div>`;
    } else if (task.status === 'running' || task.status === 'pending') {
      actions = `<div class="task-center-actions">
        <button data-task-action="pause">暂停</button>
        <button data-task-action="delete">删除</button>
      </div>`;
    } else if (task.status === 'paused') {
      actions = `<div class="task-center-actions">
        <button data-task-action="resume">继续</button>
        <button data-task-action="delete">删除</button>
      </div>`;
    } else if (!isActive(task)) {
      actions = `<div class="task-center-actions"><button data-task-action="remove">移除</button></div>`;
    }
    const state = task.status === 'complete'
      ? '已完成'
      : (task.status === 'failed'
        ? '失败'
        : (task.status === 'canceled'
          ? '已取消'
          : (task.status === 'paused' ? '已暂停' : '下载中')));
    return `
      <article class="task-center-item ${task.status}" data-task-id="${escapeHtml(task.id)}">
        <div class="task-center-item-head">
          <div class="task-center-name" title="${escapeHtml(task.name)}">${escapeHtml(task.name)}</div>
          <span class="task-center-state">${state}</span>
        </div>
        ${book}
        <div class="task-center-progress${progressClass}">
          <div class="task-center-progress-fill" style="width:${width}%"></div>
        </div>
        <div class="task-center-item-foot">
          <span class="task-center-status" title="${escapeHtml(statusText(task))}">${escapeHtml(statusText(task))}</span>
          ${actions}
        </div>
      </article>`;
  }

  function render() {
    if (!button) return;
    const active = tasks.filter(isActive).length;
    const paused = tasks.filter((task) => task.status === 'paused').length;
    const unread = tasks.filter((task) => task.unread).length;
    const count = active + paused + unread;
    badge.textContent = String(Math.min(99, count));
    badge.classList.toggle('hidden', count === 0);
    badge.classList.toggle('has-result', active === 0 && unread > 0);
    button.title = active
      ? `任务中心，${active} 个下载中`
      : (paused
        ? `任务中心，${paused} 个已暂停`
        : (unread ? `任务中心，${unread} 个新结果` : '任务中心'));
    button.setAttribute('aria-label', button.title);

    const complete = tasks.filter((task) => task.status === 'complete').length;
    const failed = tasks.filter((task) => task.status === 'failed').length;
    summary.textContent = active
      ? `${active} 个下载中${paused ? `，${paused} 个已暂停` : ''}${complete ? `，${complete} 个已完成` : ''}`
      : (tasks.length
        ? `${paused ? `${paused} 个已暂停，` : ''}${complete} 个已完成${failed ? `，${failed} 个失败` : ''}`
        : '下载任务会显示在这里');
    list.innerHTML = tasks.length
      ? tasks.map(taskHtml).join('')
      : '<div class="task-center-empty">暂无下载任务</div>';
    clearButton.classList.toggle('hidden', !tasks.some((task) => !isUnfinished(task)));
  }

  function setPanelOpen(open) {
    panel.classList.toggle('hidden', !open);
    button.setAttribute('aria-expanded', String(open));
    if (open) {
      tasks.forEach((task) => { task.unread = false; });
      render();
    }
  }

  function pruneHistory() {
    let terminal = tasks.filter((task) => !isUnfinished(task)).length;
    for (let i = tasks.length - 1; i >= 0 && terminal > MAX_HISTORY; i--) {
      if (isUnfinished(tasks[i])) continue;
      tasks.splice(i, 1);
      terminal--;
    }
  }

  function updateProgress(task, data, callback) {
    if (!['pausing', 'deleting'].includes(task.status)) task.status = 'running';
    task.receivedBytes = Math.max(0, Number(data && data.receivedBytes) || 0);
    const total = Number(data && data.totalBytes);
    task.totalBytes = Number.isFinite(total) && total > 0 ? total : null;
    const ratio = Number(data && data.percent);
    task.percent = data && data.percent != null && Number.isFinite(ratio)
      ? Math.max(0, Math.min(1, ratio))
      : null;
    render();
    if (typeof callback === 'function') {
      try { callback(data); } catch (e) { /* 页面内进度异常不影响全局任务 */ }
    }
  }

  async function run(task, input, onProgress) {
    task.status = 'running';
    task.error = '';
    render();
    let result;
    try {
      result = await window.api.downloads.run(
        task.requestId,
        input.url,
        input.suggestName,
        input.entryId,
        input.extraHeaders,
        input.meta,
        (data) => updateProgress(task, data, onProgress)
      );
    } catch (error) {
      result = { ok: false, error: (error && error.message) || String(error) };
    }

    if (result && result.ok && result.data && result.data.canceled) {
      task.status = 'canceled';
    } else if (result && result.ok && result.data && result.data.paused) {
      task.status = 'paused';
      task.receivedBytes = Number(result.data.receivedBytes) || task.receivedBytes;
      task.totalBytes = Number(result.data.totalBytes) || task.totalBytes;
      task.percent = task.totalBytes ? task.receivedBytes / task.totalBytes : task.percent;
      task.promise = null;
      render();
      return result;
    } else if (result && result.ok && result.data && result.data.deleted) {
      const index = tasks.indexOf(task);
      if (index >= 0) tasks.splice(index, 1);
      render();
      return result;
    } else if (!result || !result.ok) {
      task.status = 'failed';
      task.error = (result && result.error) || '下载失败';
    } else {
      task.status = 'complete';
      task.percent = 1;
      task.path = result.data.path || '';
      task.receivedBytes = Math.max(task.receivedBytes, Number(result.data.receivedBytes) || 0);
      if (window.Library) window.Library.markDirty();
    }
    task.key = '';
    task.input = null;
    task.promise = null;
    task.unread = panel.classList.contains('hidden');
    pruneHistory();
    render();
    return result;
  }

  function start(input) {
    const key = String(input && input.key || '');
    const existing = key && tasks.find((task) => task.key === key && isUnfinished(task));
    if (existing) {
      if (existing.status === 'paused') {
        existing.input.onProgress = input && input.onProgress;
        existing.promise = run(existing, existing.input, existing.input.onProgress);
      }
      return existing.promise || Promise.resolve({ ok: true, data: { paused: true } });
    }

    const meta = input && input.meta || {};
    const task = {
      id: `download_${Date.now().toString(36)}_${(++seq).toString(36)}`,
      requestId: `task_${Date.now().toString(36)}_${seq.toString(36)}`,
      key,
      name: String(input && input.suggestName || meta.title || '未命名下载'),
      bookTitle: String(meta.title || ''),
      status: 'pending',
      receivedBytes: 0,
      totalBytes: null,
      percent: null,
      error: '',
      path: '',
      unread: false,
      promise: null,
      input: {
        url: String(input && input.url || ''),
        suggestName: String(input && input.suggestName || ''),
        entryId: input && input.entryId,
        extraHeaders: input && input.extraHeaders,
        meta,
        onProgress: input && input.onProgress
      }
    };
    tasks.unshift(task);
    pruneHistory();
    render();
    task.promise = run(task, task.input, task.input.onProgress);
    return task.promise;
  }

  async function taskAction(action, id) {
    const index = tasks.findIndex((task) => task.id === id);
    if (index < 0) return;
    const task = tasks[index];
    if (action === 'open' && task.path) {
      const result = await window.api.openPath(task.path);
      if (!result.ok) await confirmModal('打开失败', result.error || '无法打开该文件');
      return;
    }
    if (action === 'reveal' && task.path) {
      window.api.showItem(task.path);
      return;
    }
    if (action === 'pause' && ['pending', 'running'].includes(task.status)) {
      const previousStatus = task.status;
      task.status = 'pausing';
      render();
      const result = await window.api.downloads.pause(task.requestId);
      if ((!result || !result.ok || !result.data) && task.status === 'pausing') {
        task.status = previousStatus;
        render();
      }
      return;
    }
    if (action === 'resume' && task.status === 'paused' && task.input) {
      task.promise = run(task, task.input, task.input.onProgress);
      return;
    }
    if (action === 'delete' && isUnfinished(task)) {
      const previousStatus = task.status;
      task.status = 'deleting';
      render();
      const result = await window.api.downloads.delete(task.requestId);
      if ((!result || !result.ok || !result.data) && task.status === 'deleting') {
        task.status = previousStatus;
        task.error = (result && result.error) || '';
        render();
        return;
      }
      if (!task.promise) {
        tasks.splice(index, 1);
        render();
      }
      return;
    }
    if (action === 'remove' && !isUnfinished(task)) {
      tasks.splice(index, 1);
      render();
    }
  }

  function init() {
    button = $('taskCenterBtn');
    badge = $('taskCenterBadge');
    panel = $('taskCenterPanel');
    list = $('taskCenterList');
    summary = $('taskCenterSummary');
    clearButton = $('taskCenterClear');

    button.onclick = () => setPanelOpen(panel.classList.contains('hidden'));
    list.onclick = (event) => {
      const actionButton = event.target.closest('[data-task-action]');
      const item = event.target.closest('[data-task-id]');
      if (actionButton && item) taskAction(actionButton.dataset.taskAction, item.dataset.taskId);
    };
    clearButton.onclick = () => {
      for (let i = tasks.length - 1; i >= 0; i--) {
        if (!isUnfinished(tasks[i])) tasks.splice(i, 1);
      }
      render();
    };
    document.addEventListener('pointerdown', (event) => {
      if (!panel.classList.contains('hidden') && !event.target.closest('.task-center-wrap')) {
        setPanelOpen(false);
      }
    });
    document.addEventListener('keydown', (event) => {
      if (event.key === 'Escape' && !panel.classList.contains('hidden')) setPanelOpen(false);
    });
    render();
  }

  window.DownloadCenter = { init, start };
})();