function createFindWork({ fetchJson, onItems, onPagination, onStatus, onSelection = () => {} }) {
  let available = [];
  let pagination = { page: 1, total: 0, has_more: false };
  let loadRequest = null;
  let claimRequest = null;
  const previewed = new Set();
  let selecting = false;
  const selected = new Map();

  function itemKey(item) {
    return String(item?.repository || '') + '#' + String(item?.number || '');
  }

  function emitSelection() {
    const state = { active: selecting, count: selected.size, ids: Array.from(selected.keys()) };
    onSelection(state);
    return state;
  }

  function apply(result, append) {
    const incoming = Array.isArray(result?.items) ? result.items : [];
    if (append) {
      const byId = new Map(available.map(item => [item.id, item]));
      incoming.forEach(item => {
        if (item && !byId.has(item.id)) byId.set(item.id, { ...item });
      });
      available = Array.from(byId.values());
    } else {
      available = incoming.map(item => ({ ...item }));
    }
    const availableKeys = new Set(available.map(itemKey));
    previewed.forEach(key => {
      if (!availableKeys.has(key)) previewed.delete(key);
    });
    pagination = {
      page: Number(result?.page) || 1,
      total: Number(result?.total) || available.length,
      has_more: result?.has_more === true,
    };
    onItems(available.slice());
    onPagination({ ...pagination });
  }

  function loadPage(page, append) {
    if (loadRequest) return loadRequest;
    loadRequest = fetchJson('api/v1/available-issues?page=' + page, {
      headers: { Accept: 'application/json' },
    }).then(result => {
      apply(result, append);
      if (result?.refresh_failed === true) {
        onStatus('Showing saved available work. Catalog refresh failed; retrying shortly.');
      } else if (result?.revalidating === true) {
        onStatus('Showing saved available work while the catalog refreshes…');
      }
      return result;
    }).finally(() => { loadRequest = null; });
    return loadRequest;
  }

  return {
    reset(result) {
      apply(result, false);
    },
    load() {
      return loadPage(1, false);
    },
    loadMore() {
      if (!pagination.has_more) return Promise.resolve(false);
      return loadPage(pagination.page + 1, true);
    },
    items() {
      return available.slice();
    },
    startSelection() {
      selecting = true;
      selected.clear();
      return emitSelection();
    },
    cancelSelection() {
      selecting = false;
      selected.clear();
      return emitSelection();
    },
    toggleSelection(item) {
      if (!selecting || !item) return emitSelection();
      const key = itemKey(item);
      if (selected.has(key)) selected.delete(key);
      else selected.set(key, item);
      return emitSelection();
    },
    isSelected(item) {
      return selected.has(itemKey(item));
    },
    selectedItems() {
      return Array.from(selected.values());
    },
    selection() {
      return { active: selecting, count: selected.size, ids: Array.from(selected.keys()) };
    },
    togglePreview(item) {
      const key = itemKey(item);
      if (previewed.has(key)) previewed.delete(key);
      else previewed.add(key);
      return previewed.has(key);
    },
    isPreviewed(item) {
      return previewed.has(itemKey(item));
    },
    claim(item) {
      if (claimRequest) return claimRequest;
      const key = item.repository + '#' + item.number;
      onStatus('Assigning ' + key + '…');
      const path = 'api/v1/repos/' + item.repository.split('/').map(encodeURIComponent).join('/') +
        '/issues/' + encodeURIComponent(item.number) + '/claim';
      claimRequest = fetchJson(path, {
        method: 'PATCH',
        headers: { Accept: 'application/json' },
      }).then(confirmed => {
        if (!Array.isArray(confirmed?.assignees) || !confirmed.assignees.length) {
          throw new Error('Issue assignment was not confirmed.');
        }
        available = available.filter(candidate =>
          candidate.repository !== item.repository || candidate.number !== item.number
        );
        pagination.total = Math.max(available.length, pagination.total - 1);
        pagination.has_more = available.length < pagination.total;
        previewed.delete(itemKey(item));
        selected.delete(itemKey(item));
        emitSelection();
        onItems(available.slice());
        onPagination({ ...pagination });
        onStatus('Assigned ' + key + ' to you.');
        return confirmed;
      }).finally(() => { claimRequest = null; });
      return claimRequest;
    },
  };
}

if (typeof module !== 'undefined' && module.exports) module.exports = createFindWork;