/**
 * initAttachmentZone — wire up drag-and-drop, clipboard paste, and file-picker
 * for a form that already has a multipart file input.
 *
 * Uses a DataTransfer object as a mutable file store so that
 * fileInput.files always reflects the full set of chosen files and
 * native form submission works unchanged.
 */
function initAttachmentZone({ formEl, fileInput, dropZone, fileListEl, pasteTarget }) {
  const dt = new DataTransfer();
  let dragCounter = 0;

  // ── Helpers ──────────────────────────────────────────────────────────

  function fileKey(f) {
    return f.name + '|' + f.size + '|' + f.lastModified;
  }

  function currentKeys() {
    const keys = new Set();
    for (let i = 0; i < dt.items.length; i++) {
      keys.add(fileKey(dt.files[i]));
    }
    return keys;
  }

  function addFiles(files) {
    const keys = currentKeys();
    for (const f of files) {
      if (!keys.has(fileKey(f))) {
        dt.items.add(f);
        keys.add(fileKey(f));
      }
    }
    sync();
  }

  function removeFile(index) {
    dt.items.remove(index);
    sync();
  }

  function sync() {
    fileInput.files = dt.files;
    renderList();
  }

  // ── File list rendering ─────────────────────────────────────────────

  function formatSize(bytes) {
    if (bytes < 1024) return bytes + ' B';
    if (bytes < 1024 * 1024) return (bytes / 1024).toFixed(1) + ' KB';
    return (bytes / (1024 * 1024)).toFixed(1) + ' MB';
  }

  function renderList() {
    fileListEl.innerHTML = '';
    if (dt.files.length === 0) {
      fileListEl.classList.add('hidden');
      return;
    }
    fileListEl.classList.remove('hidden');

    for (let i = 0; i < dt.files.length; i++) {
      const f = dt.files[i];
      const row = document.createElement('div');
      row.className = 'flex items-center justify-between rounded-md bg-gray-50 px-3 py-1.5 text-sm';

      const info = document.createElement('span');
      info.className = 'text-gray-700 truncate';
      info.textContent = f.name;

      const size = document.createElement('span');
      size.className = 'text-gray-400 ml-2 whitespace-nowrap';
      size.textContent = formatSize(f.size);

      const left = document.createElement('div');
      left.className = 'flex items-center min-w-0';
      left.appendChild(info);
      left.appendChild(size);

      const btn = document.createElement('button');
      btn.type = 'button';
      btn.className = 'ml-2 text-gray-400 hover:text-red-500 text-xs font-bold';
      btn.textContent = '\u00d7';
      btn.setAttribute('aria-label', 'Remove ' + f.name);
      btn.addEventListener('click', function () { removeFile(i); });

      row.appendChild(left);
      row.appendChild(btn);
      fileListEl.appendChild(row);
    }
  }

  // ── Drag-and-drop ───────────────────────────────────────────────────

  function highlight() {
    dropZone.classList.add('ring-blue-400', 'bg-blue-50', 'border-blue-400');
  }

  function unhighlight() {
    dropZone.classList.remove('ring-blue-400', 'bg-blue-50', 'border-blue-400');
  }

  dropZone.addEventListener('dragenter', function (e) {
    e.preventDefault();
    dragCounter++;
    highlight();
  });

  dropZone.addEventListener('dragover', function (e) {
    e.preventDefault();
  });

  dropZone.addEventListener('dragleave', function (e) {
    e.preventDefault();
    dragCounter--;
    if (dragCounter <= 0) {
      dragCounter = 0;
      unhighlight();
    }
  });

  dropZone.addEventListener('drop', function (e) {
    e.preventDefault();
    dragCounter = 0;
    unhighlight();
    if (e.dataTransfer && e.dataTransfer.files.length) {
      addFiles(e.dataTransfer.files);
    }
  });

  // ── File picker (additive) ──────────────────────────────────────────

  fileInput.addEventListener('change', function () {
    addFiles(fileInput.files);
  });

  // ── Clipboard paste ─────────────────────────────────────────────────

  function pasteTimestamp() {
    const d = new Date();
    const pad = function (n) { return n < 10 ? '0' + n : '' + n; };
    return d.getFullYear() +
      pad(d.getMonth() + 1) +
      pad(d.getDate()) + '-' +
      pad(d.getHours()) +
      pad(d.getMinutes()) +
      pad(d.getSeconds());
  }

  if (pasteTarget) {
    pasteTarget.addEventListener('paste', function (e) {
      var items = e.clipboardData && e.clipboardData.items;
      if (!items) return;

      var files = [];
      for (var i = 0; i < items.length; i++) {
        if (items[i].kind === 'file') {
          var f = items[i].getAsFile();
          if (!f) continue;
          // Rename generic pasted names like "image.png"
          if (/^image\.\w+$/.test(f.name)) {
            var ext = f.name.split('.').pop();
            var renamed = new File([f], 'paste-' + pasteTimestamp() + '.' + ext, { type: f.type });
            files.push(renamed);
          } else {
            files.push(f);
          }
        }
      }

      if (files.length > 0) {
        addFiles(files);
        // Don't prevent default — let text pastes through
      }
    });
  }
}