/* Copyright 2017 Mozilla Foundation
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import {
  backtrackBeforeAllVisibleElements, binarySearchFirstItem, EventBus,
  getPageSizeInches, getPDFFileNameFromURL, getVisibleElements,
  isPortraitOrientation, isValidRotation, moveToEndOfArray,
  waitOnEventOrTimeout, WaitOnType
} from '../../web/ui_utils';
import { createObjectURL } from '../../src/shared/util';
import isNodeJS from '../../src/shared/is_node';

describe('ui_utils', function() {
  describe('binary search', function() {
    function isTrue(boolean) {
      return boolean;
    }
    function isGreater3(number) {
      return number > 3;
    }

    it('empty array', function() {
      expect(binarySearchFirstItem([], isTrue)).toEqual(0);
    });
    it('single boolean entry', function() {
      expect(binarySearchFirstItem([false], isTrue)).toEqual(1);
      expect(binarySearchFirstItem([true], isTrue)).toEqual(0);
    });
    it('three boolean entries', function() {
      expect(binarySearchFirstItem([true, true, true], isTrue)).toEqual(0);
      expect(binarySearchFirstItem([false, true, true], isTrue)).toEqual(1);
      expect(binarySearchFirstItem([false, false, true], isTrue)).toEqual(2);
      expect(binarySearchFirstItem([false, false, false], isTrue)).toEqual(3);
    });
    it('three numeric entries', function() {
      expect(binarySearchFirstItem([0, 1, 2], isGreater3)).toEqual(3);
      expect(binarySearchFirstItem([2, 3, 4], isGreater3)).toEqual(2);
      expect(binarySearchFirstItem([4, 5, 6], isGreater3)).toEqual(0);
    });
  });

  describe('getPDFFileNameFromURL', function() {
    it('gets PDF filename', function() {
      // Relative URL
      expect(getPDFFileNameFromURL('/pdfs/file1.pdf')).toEqual('file1.pdf');
      // Absolute URL
      expect(getPDFFileNameFromURL(
        'http://www.example.com/pdfs/file2.pdf')).toEqual('file2.pdf');
    });

    it('gets fallback filename', function() {
      // Relative URL
      expect(getPDFFileNameFromURL('/pdfs/file1.txt')).toEqual('document.pdf');
      // Absolute URL
      expect(getPDFFileNameFromURL(
        'http://www.example.com/pdfs/file2.txt')).toEqual('document.pdf');
    });

    it('gets custom fallback filename', function() {
      // Relative URL
      expect(getPDFFileNameFromURL('/pdfs/file1.txt', 'qwerty1.pdf')).
        toEqual('qwerty1.pdf');
      // Absolute URL
      expect(getPDFFileNameFromURL('http://www.example.com/pdfs/file2.txt',
        'qwerty2.pdf')).toEqual('qwerty2.pdf');

      // An empty string should be a valid custom fallback filename.
      expect(getPDFFileNameFromURL('/pdfs/file3.txt', '')).toEqual('');
    });

    it('gets fallback filename when url is not a string', function() {
      expect(getPDFFileNameFromURL(null)).toEqual('document.pdf');

      expect(getPDFFileNameFromURL(null, 'file.pdf')).toEqual('file.pdf');
    });

    it('gets PDF filename from URL containing leading/trailing whitespace',
        function() {
      // Relative URL
      expect(getPDFFileNameFromURL(
        '   /pdfs/file1.pdf   ')).toEqual('file1.pdf');
      // Absolute URL
      expect(getPDFFileNameFromURL(
        '   http://www.example.com/pdfs/file2.pdf   ')).toEqual('file2.pdf');
    });

    it('gets PDF filename from query string', function() {
      // Relative URL
      expect(getPDFFileNameFromURL(
        '/pdfs/pdfs.html?name=file1.pdf')).toEqual('file1.pdf');
      // Absolute URL
      expect(getPDFFileNameFromURL(
        'http://www.example.com/pdfs/pdf.html?file2.pdf')).toEqual('file2.pdf');
    });

    it('gets PDF filename from hash string', function() {
      // Relative URL
      expect(getPDFFileNameFromURL(
        '/pdfs/pdfs.html#name=file1.pdf')).toEqual('file1.pdf');
      // Absolute URL
      expect(getPDFFileNameFromURL(
        'http://www.example.com/pdfs/pdf.html#file2.pdf')).toEqual('file2.pdf');
    });

    it('gets correct PDF filename when multiple ones are present', function() {
      // Relative URL
      expect(getPDFFileNameFromURL(
        '/pdfs/file1.pdf?name=file.pdf')).toEqual('file1.pdf');
      // Absolute URL
      expect(getPDFFileNameFromURL(
        'http://www.example.com/pdfs/file2.pdf#file.pdf')).toEqual('file2.pdf');
    });

    it('gets PDF filename from URI-encoded data', function() {
      var encodedUrl = encodeURIComponent(
        'http://www.example.com/pdfs/file1.pdf');
      expect(getPDFFileNameFromURL(encodedUrl)).toEqual('file1.pdf');

      var encodedUrlWithQuery = encodeURIComponent(
        'http://www.example.com/pdfs/file.txt?file2.pdf');
      expect(getPDFFileNameFromURL(encodedUrlWithQuery)).toEqual('file2.pdf');
    });

    it('gets PDF filename from data mistaken for URI-encoded', function() {
      expect(getPDFFileNameFromURL('/pdfs/%AA.pdf')).toEqual('%AA.pdf');

      expect(getPDFFileNameFromURL('/pdfs/%2F.pdf')).toEqual('%2F.pdf');
    });

    it('gets PDF filename from (some) standard protocols', function() {
      // HTTP
      expect(getPDFFileNameFromURL('http://www.example.com/file1.pdf')).
        toEqual('file1.pdf');
      // HTTPS
      expect(getPDFFileNameFromURL('https://www.example.com/file2.pdf')).
        toEqual('file2.pdf');
      // File
      expect(getPDFFileNameFromURL('file:///path/to/files/file3.pdf')).
        toEqual('file3.pdf');
      // FTP
      expect(getPDFFileNameFromURL('ftp://www.example.com/file4.pdf')).
        toEqual('file4.pdf');
    });

    it('gets PDF filename from query string appended to "blob:" URL',
        function() {
      if (isNodeJS()) {
        pending('Blob in not supported in Node.js.');
      }
      var typedArray = new Uint8Array([1, 2, 3, 4, 5]);
      var blobUrl = createObjectURL(typedArray, 'application/pdf');
      // Sanity check to ensure that a "blob:" URL was returned.
      expect(blobUrl.startsWith('blob:')).toEqual(true);

      expect(getPDFFileNameFromURL(blobUrl + '?file.pdf')).toEqual('file.pdf');
    });

    it('gets fallback filename from query string appended to "data:" URL',
        function() {
      var typedArray = new Uint8Array([1, 2, 3, 4, 5]);
      var dataUrl = createObjectURL(typedArray, 'application/pdf',
                                    /* forceDataSchema = */ true);
      // Sanity check to ensure that a "data:" URL was returned.
      expect(dataUrl.startsWith('data:')).toEqual(true);

      expect(getPDFFileNameFromURL(dataUrl + '?file1.pdf')).
        toEqual('document.pdf');

      // Should correctly detect a "data:" URL with leading whitespace.
      expect(getPDFFileNameFromURL('     ' + dataUrl + '?file2.pdf')).
        toEqual('document.pdf');
    });
  });

  describe('EventBus', function () {
    it('dispatch event', function () {
      var eventBus = new EventBus();
      var count = 0;
      eventBus.on('test', function(evt) {
        expect(evt).toEqual(undefined);
        count++;
      });
      eventBus.dispatch('test');
      expect(count).toEqual(1);
    });
    it('dispatch event with arguments', function() {
      const eventBus = new EventBus();
      let count = 0;
      eventBus.on('test', function(evt) {
        expect(evt).toEqual({ abc: 123, });
        count++;
      });
      eventBus.dispatch('test', {
        abc: 123,
      });
      expect(count).toEqual(1);
    });
    it('dispatch different event', function () {
      var eventBus = new EventBus();
      var count = 0;
      eventBus.on('test', function () {
        count++;
      });
      eventBus.dispatch('nottest');
      expect(count).toEqual(0);
    });
    it('dispatch event multiple times', function () {
      var eventBus = new EventBus();
      var count = 0;
      eventBus.dispatch('test');
      eventBus.on('test', function () {
        count++;
      });
      eventBus.dispatch('test');
      eventBus.dispatch('test');
      expect(count).toEqual(2);
    });
    it('dispatch event to multiple handlers', function () {
      var eventBus = new EventBus();
      var count = 0;
      eventBus.on('test', function () {
        count++;
      });
      eventBus.on('test', function () {
        count++;
      });
      eventBus.dispatch('test');
      expect(count).toEqual(2);
    });
    it('dispatch to detached', function () {
      var eventBus = new EventBus();
      var count = 0;
      var listener = function () {
        count++;
      };
      eventBus.on('test', listener);
      eventBus.dispatch('test');
      eventBus.off('test', listener);
      eventBus.dispatch('test');
      expect(count).toEqual(1);
    });
    it('dispatch to wrong detached', function () {
      var eventBus = new EventBus();
      var count = 0;
      eventBus.on('test', function () {
        count++;
      });
      eventBus.dispatch('test');
      eventBus.off('test', function () {
        count++;
      });
      eventBus.dispatch('test');
      expect(count).toEqual(2);
    });
    it('dispatch to detached during handling', function () {
      var eventBus = new EventBus();
      var count = 0;
      var listener1 = function () {
        eventBus.off('test', listener2);
        count++;
      };
      var listener2 = function () {
        eventBus.off('test', listener1);
        count++;
      };
      eventBus.on('test', listener1);
      eventBus.on('test', listener2);
      eventBus.dispatch('test');
      eventBus.dispatch('test');
      expect(count).toEqual(2);
    });

    it('should not, by default, re-dispatch to DOM', function(done) {
      if (isNodeJS()) {
        pending('Document in not supported in Node.js.');
      }
      const eventBus = new EventBus();
      let count = 0;
      eventBus.on('test', function(evt) {
        expect(evt).toEqual(undefined);
        count++;
      });
      function domEventListener() {
        done.fail('shall not dispatch DOM event.');
      }
      document.addEventListener('test', domEventListener);

      eventBus.dispatch('test');

      Promise.resolve().then(() => {
        expect(count).toEqual(1);

        document.removeEventListener('test', domEventListener);
        done();
      });
    });
    it('should re-dispatch to DOM', function(done) {
      if (isNodeJS()) {
        pending('Document in not supported in Node.js.');
      }
      const eventBus = new EventBus({ dispatchToDOM: true, });
      let count = 0;
      eventBus.on('test', function(evt) {
        expect(evt).toEqual(undefined);
        count++;
      });
      function domEventListener(evt) {
        expect(evt.detail).toEqual({});
        count++;
      }
      document.addEventListener('test', domEventListener);

      eventBus.dispatch('test');

      Promise.resolve().then(() => {
        expect(count).toEqual(2);

        document.removeEventListener('test', domEventListener);
        done();
      });
    });
    it('should re-dispatch to DOM, with arguments (without internal listeners)',
        function(done) {
      if (isNodeJS()) {
        pending('Document in not supported in Node.js.');
      }
      const eventBus = new EventBus({ dispatchToDOM: true, });
      let count = 0;
      function domEventListener(evt) {
        expect(evt.detail).toEqual({ abc: 123, });
        count++;
      }
      document.addEventListener('test', domEventListener);

      eventBus.dispatch('test', {
        abc: 123,
      });

      Promise.resolve().then(() => {
        expect(count).toEqual(1);

        document.removeEventListener('test', domEventListener);
        done();
      });
    });
  });

  describe('isValidRotation', function() {
    it('should reject non-integer angles', function() {
      expect(isValidRotation()).toEqual(false);
      expect(isValidRotation(null)).toEqual(false);
      expect(isValidRotation(NaN)).toEqual(false);
      expect(isValidRotation([90])).toEqual(false);
      expect(isValidRotation('90')).toEqual(false);
      expect(isValidRotation(90.5)).toEqual(false);
    });

    it('should reject non-multiple of 90 degree angles', function() {
      expect(isValidRotation(45)).toEqual(false);
      expect(isValidRotation(-123)).toEqual(false);
    });

    it('should accept valid angles', function() {
      expect(isValidRotation(0)).toEqual(true);
      expect(isValidRotation(90)).toEqual(true);
      expect(isValidRotation(-270)).toEqual(true);
      expect(isValidRotation(540)).toEqual(true);
    });
  });

  describe('isPortraitOrientation', function() {
    it('should be portrait orientation', function() {
      expect(isPortraitOrientation({
        width: 200,
        height: 400,
      })).toEqual(true);

      expect(isPortraitOrientation({
        width: 500,
        height: 500,
      })).toEqual(true);
    });

    it('should be landscape orientation', function() {
      expect(isPortraitOrientation({
        width: 600,
        height: 300,
      })).toEqual(false);
    });
  });

  describe('waitOnEventOrTimeout', function() {
    let eventBus;

    beforeAll(function(done) {
      eventBus = new EventBus();
      done();
    });

    afterAll(function() {
      eventBus = null;
    });

    it('should reject invalid parameters', function(done) {
      let invalidTarget = waitOnEventOrTimeout({
        target: 'window',
        name: 'DOMContentLoaded',
      }).then(function() {
        throw new Error('Should reject invalid parameters.');
      }, function(reason) {
        expect(reason instanceof Error).toEqual(true);
      });

      let invalidName = waitOnEventOrTimeout({
        target: eventBus,
        name: '',
      }).then(function() {
        throw new Error('Should reject invalid parameters.');
      }, function(reason) {
        expect(reason instanceof Error).toEqual(true);
      });

      let invalidDelay = waitOnEventOrTimeout({
        target: eventBus,
        name: 'pagerendered',
        delay: -1000,
      }).then(function() {
        throw new Error('Should reject invalid parameters.');
      }, function(reason) {
        expect(reason instanceof Error).toEqual(true);
      });

      Promise.all([invalidTarget, invalidName, invalidDelay]).then(done,
                                                                   done.fail);
    });

    it('should resolve on event, using the DOM', function(done) {
      if (isNodeJS()) {
        pending('Document in not supported in Node.js.');
      }
      let button = document.createElement('button');

      let buttonClicked = waitOnEventOrTimeout({
        target: button,
        name: 'click',
        delay: 10000,
      });
      // Immediately dispatch the expected event.
      button.click();

      buttonClicked.then(function(type) {
        expect(type).toEqual(WaitOnType.EVENT);
        done();
      }, done.fail);
    });

    it('should resolve on timeout, using the DOM', function(done) {
      if (isNodeJS()) {
        pending('Document in not supported in Node.js.');
      }
      let button = document.createElement('button');

      let buttonClicked = waitOnEventOrTimeout({
        target: button,
        name: 'click',
        delay: 10,
      });
      // Do *not* dispatch the event, and wait for the timeout.

      buttonClicked.then(function(type) {
        expect(type).toEqual(WaitOnType.TIMEOUT);
        done();
      }, done.fail);
    });

    it('should resolve on event, using the EventBus', function(done) {
      let pageRendered = waitOnEventOrTimeout({
        target: eventBus,
        name: 'pagerendered',
        delay: 10000,
      });
      // Immediately dispatch the expected event.
      eventBus.dispatch('pagerendered');

      pageRendered.then(function(type) {
        expect(type).toEqual(WaitOnType.EVENT);
        done();
      }, done.fail);
    });

    it('should resolve on timeout, using the EventBus', function(done) {
      let pageRendered = waitOnEventOrTimeout({
        target: eventBus,
        name: 'pagerendered',
        delay: 10,
      });
      // Do *not* dispatch the event, and wait for the timeout.

      pageRendered.then(function(type) {
        expect(type).toEqual(WaitOnType.TIMEOUT);
        done();
      }, done.fail);
    });
  });

  describe('getPageSizeInches', function () {
    it('gets page size (in inches)', function() {
      const page = {
        view: [0, 0, 595.28, 841.89],
        userUnit: 1.0,
        rotate: 0,
      };
      const { width, height, } = getPageSizeInches(page);

      expect(+width.toPrecision(3)).toEqual(8.27);
      expect(+height.toPrecision(4)).toEqual(11.69);
    });

    it('gets page size (in inches), for non-default /Rotate entry', function() {
      const pdfPage1 = { view: [0, 0, 612, 792], userUnit: 1, rotate: 0, };
      const { width: width1, height: height1, } = getPageSizeInches(pdfPage1);

      expect(width1).toEqual(8.5);
      expect(height1).toEqual(11);

      const pdfPage2 = { view: [0, 0, 612, 792], userUnit: 1, rotate: 90, };
      const { width: width2, height: height2, } = getPageSizeInches(pdfPage2);

      expect(width2).toEqual(11);
      expect(height2).toEqual(8.5);
    });
  });

  describe('getVisibleElements', function() {
    // These values are based on margin/border values in the CSS, but there
    // isn't any real need for them to be; they just need to take *some* value.
    const BORDER_WIDTH = 9;
    const SPACING = 2 * BORDER_WIDTH - 7;

    // This is a helper function for assembling an array of view stubs from an
    // array of arrays of [width, height] pairs, which represents wrapped lines
    // of pages. It uses the above constants to add realistic spacing between
    // the pages and the lines.
    //
    // If you're reading a test that calls makePages, you should think of the
    // inputs to makePages as boxes with no borders, being laid out in a
    // container that has no margins, so that the top of the tallest page in
    // the first row will be at y = 0, and the left of the first page in
    // the first row will be at x = 0. The spacing between pages in a row, and
    // the spacing between rows, is SPACING. If you wanted to construct an
    // actual HTML document with the same layout, you should give each page
    // element a margin-right and margin-bottom of SPACING, and add no other
    // margins, borders, or padding.
    //
    // If you're reading makePages itself, you'll see a somewhat more
    // complicated picture because this suite of tests is exercising
    // getVisibleElements' ability to account for the borders that real page
    // elements have. makePages tests this by subtracting a BORDER_WIDTH from
    // offsetLeft/Top and adding it to clientLeft/Top. So the element stubs that
    // getVisibleElements sees may, for example, actually have an offsetTop of
    // -9. If everything is working correctly, this detail won't leak out into
    // the tests themselves, and so the tests shouldn't use the value of
    // BORDER_WIDTH at all.
    function makePages(lines) {
      const result = [];
      let lineTop = 0, id = 0;
      for (const line of lines) {
        const lineHeight = line.reduce(function(maxHeight, pair) {
          return Math.max(maxHeight, pair[1]);
        }, 0);
        let offsetLeft = -BORDER_WIDTH;
        for (const [clientWidth, clientHeight] of line) {
          const offsetTop =
            lineTop + (lineHeight - clientHeight) / 2 - BORDER_WIDTH;
          const div = {
            offsetLeft, offsetTop, clientWidth, clientHeight,
            clientLeft: BORDER_WIDTH, clientTop: BORDER_WIDTH,
          };
          result.push({ id, div, });
          ++id;
          offsetLeft += clientWidth + SPACING;
        }
        lineTop += lineHeight + SPACING;
      }
      return result;
    }

    // This is a reimplementation of getVisibleElements without the
    // optimizations.
    function slowGetVisibleElements(scroll, pages) {
      const views = [];
      const { scrollLeft, scrollTop, } = scroll;
      const scrollRight = scrollLeft + scroll.clientWidth;
      const scrollBottom = scrollTop + scroll.clientHeight;
      for (const view of pages) {
        const { div, } = view;
        const viewLeft = div.offsetLeft + div.clientLeft;
        const viewRight = viewLeft + div.clientWidth;
        const viewTop = div.offsetTop + div.clientTop;
        const viewBottom = viewTop + div.clientHeight;

        if (viewLeft < scrollRight && viewRight > scrollLeft &&
            viewTop < scrollBottom && viewBottom > scrollTop) {
          const hiddenHeight = Math.max(0, scrollTop - viewTop) +
                               Math.max(0, viewBottom - scrollBottom);
          const hiddenWidth = Math.max(0, scrollLeft - viewLeft) +
                              Math.max(0, viewRight - scrollRight);
          const visibleArea = (div.clientHeight - hiddenHeight) *
                              (div.clientWidth - hiddenWidth);
          const percent =
            (visibleArea * 100 / div.clientHeight / div.clientWidth) | 0;
          views.push({ id: view.id, x: viewLeft, y: viewTop, view, percent, });
        }
      }
      return { first: views[0], last: views[views.length - 1], views, };
    }

    // This function takes a fixed layout of pages and compares the system under
    // test to the slower implementation above, for a range of scroll viewport
    // sizes and positions.
    function scrollOverDocument(pages, horizontally = false) {
      const size = pages.reduce(function(max, { div, }) {
        return Math.max(
          max,
          horizontally ?
            div.offsetLeft + div.clientLeft + div.clientWidth :
            div.offsetTop + div.clientTop + div.clientHeight);
      }, 0);
      // The numbers (7 and 5) are mostly arbitrary, not magic: increase them to
      // make scrollOverDocument tests faster, decrease them to make the tests
      // more scrupulous, and keep them coprime to reduce the chance of missing
      // weird edge case bugs.
      for (let i = 0; i < size; i += 7) {
        // The screen height (or width) here (j - i) doubles on each inner loop
        // iteration; again, this is just to test an interesting range of cases
        // without slowing the tests down to check every possible case.
        for (let j = i + 5; j < size; j += (j - i)) {
          const scroll = horizontally ? {
            scrollTop: 0,
            scrollLeft: i,
            clientHeight: 10000,
            clientWidth: j - i,
          } : {
            scrollTop: i,
            scrollLeft: 0,
            clientHeight: j - i,
            clientWidth: 10000,
          };
          expect(getVisibleElements(scroll, pages, false, horizontally))
            .toEqual(slowGetVisibleElements(scroll, pages));
        }
      }
    }

    it('with pages of varying height', function() {
      const pages = makePages([
        [[50, 20], [20, 50]],
        [[30, 12], [12, 30]],
        [[20, 50], [50, 20]],
        [[50, 20], [20, 50]],
      ]);
      scrollOverDocument(pages);
    });

    it('widescreen challenge', function() {
      const pages = makePages([
        [[10, 50], [10, 60], [10, 70], [10, 80], [10, 90]],
        [[10, 90], [10, 80], [10, 70], [10, 60], [10, 50]],
        [[10, 50], [10, 60], [10, 70], [10, 80], [10, 90]],
      ]);
      scrollOverDocument(pages);
    });

    it('works with horizontal scrolling', function() {
      const pages = makePages([
        [[10, 50], [20, 20], [30, 10]],
      ]);
      scrollOverDocument(pages, true);
    });

    it('handles `sortByVisibility` correctly', function() {
      const scrollEl = {
        scrollTop: 75,
        scrollLeft: 0,
        clientHeight: 750,
        clientWidth: 1500,
      };
      const views = makePages([
        [[100, 150]],
        [[100, 150]],
        [[100, 150]],
      ]);

      const visible = getVisibleElements(scrollEl, views);
      const visibleSorted = getVisibleElements(scrollEl, views,
                                               /* sortByVisibility = */ true);

      const viewsOrder = [], viewsSortedOrder = [];
      for (const view of visible.views) {
        viewsOrder.push(view.id);
      }
      for (const view of visibleSorted.views) {
        viewsSortedOrder.push(view.id);
      }
      expect(viewsOrder).toEqual([0, 1, 2]);
      expect(viewsSortedOrder).toEqual([1, 2, 0]);
    });

    it('handles views being empty', function() {
      const scrollEl = {
        scrollTop: 10,
        scrollLeft: 0,
        clientHeight: 750,
        clientWidth: 1500,
      };
      const views = [];

      expect(getVisibleElements(scrollEl, views)).toEqual({
        first: undefined, last: undefined, views: [],
      });
    });

    it('handles all views being hidden (without errors)', function() {
      const scrollEl = {
        scrollTop: 100000,
        scrollLeft: 0,
        clientHeight: 750,
        clientWidth: 1500,
      };
      const views = makePages([
        [[100, 150]],
        [[100, 150]],
        [[100, 150]],
      ]);

      expect(getVisibleElements(scrollEl, views)).toEqual({
        first: undefined, last: undefined, views: [],
      });
    });

    // This sub-suite is for a notionally internal helper function for
    // getVisibleElements.
    describe('backtrackBeforeAllVisibleElements', function() {
      // Layout elements common to all tests
      const tallPage = [10, 50];
      const shortPage = [10, 10];

      // A scroll position that ensures that only the tall pages in the second
      // row are visible
      const top1 =
        20 + SPACING + // height of the first row
        40; // a value between 30 (so the short pages on the second row are
            // hidden) and 50 (so the tall pages are visible)

      // A scroll position that ensures that all of the pages in the second row
      // are visible, but the tall ones are a tiny bit cut off
      const top2 = 20 + SPACING + // height of the first row
        10; // a value greater than 0 but less than 30

      // These tests refer to cases enumerated in the comments of
      // backtrackBeforeAllVisibleElements.
      it('handles case 1', function() {
        const pages = makePages([
          [[10, 20], [10, 20], [10, 20], [10, 20]],
          [tallPage, shortPage, tallPage, shortPage],
          [[10, 50], [10, 50], [10, 50], [10, 50]],
          [[10, 20], [10, 20], [10, 20], [10, 20]],
          [[10, 20]],
        ]);
        // binary search would land on the second row, first page
        const bsResult = 4;
        expect(backtrackBeforeAllVisibleElements(bsResult, pages, top1))
          .toEqual(4);
      });

      it('handles case 2', function() {
        const pages = makePages([
          [[10, 20], [10, 20], [10, 20], [10, 20]],
          [tallPage, shortPage, tallPage, tallPage],
          [[10, 50], [10, 50], [10, 50], [10, 50]],
          [[10, 20], [10, 20], [10, 20], [10, 20]],
        ]);
        // binary search would land on the second row, third page
        const bsResult = 6;
        expect(backtrackBeforeAllVisibleElements(bsResult, pages, top1))
          .toEqual(4);
      });

      it('handles case 3', function() {
        const pages = makePages([
          [[10, 20], [10, 20], [10, 20], [10, 20]],
          [tallPage, shortPage, tallPage, shortPage],
          [[10, 50], [10, 50], [10, 50], [10, 50]],
          [[10, 20], [10, 20], [10, 20], [10, 20]],
        ]);
        // binary search would land on the third row, first page
        const bsResult = 8;
        expect(backtrackBeforeAllVisibleElements(bsResult, pages, top1))
          .toEqual(4);
      });

      it('handles case 4', function() {
        const pages = makePages([
          [[10, 20], [10, 20], [10, 20], [10, 20]],
          [tallPage, shortPage, tallPage, shortPage],
          [[10, 50], [10, 50], [10, 50], [10, 50]],
          [[10, 20], [10, 20], [10, 20], [10, 20]],
        ]);
        // binary search would land on the second row, first page
        const bsResult = 4;
        expect(backtrackBeforeAllVisibleElements(bsResult, pages, top2))
          .toEqual(4);
      });
    });
  });

  describe('moveToEndOfArray', function() {
    it('works on empty arrays', function() {
      const data = [];
      moveToEndOfArray(data, function() {});
      expect(data).toEqual([]);
    });

    it('works when moving everything', function() {
      const data = [1, 2, 3, 4, 5];
      moveToEndOfArray(data, function() {
        return true;
      });
      expect(data).toEqual([1, 2, 3, 4, 5]);
    });

    it('works when moving some things', function() {
      const data = [1, 2, 3, 4, 5];
      moveToEndOfArray(data, function(x) {
        return x % 2 === 0;
      });
      expect(data).toEqual([1, 3, 5, 2, 4]);
    });

    it('works when moving one thing', function() {
      const data = [1, 2, 3, 4, 5];
      moveToEndOfArray(data, function(x) {
        return x === 1;
      });
      expect(data).toEqual([2, 3, 4, 5, 1]);
    });

    it('works when moving nothing', function() {
      const data = [1, 2, 3, 4, 5];
      moveToEndOfArray(data, function(x) {
        return x === 0;
      });
      expect(data).toEqual([1, 2, 3, 4, 5]);
    });
  });
});