/*
 * Copyright (C) 2013 Google Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 *     * Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 *     * Redistributions in binary form must reproduce the above
 * copyright notice, this list of conditions and the following disclaimer
 * in the documentation and/or other materials provided with the
 * distribution.
 *     * Neither the name of Google Inc. nor the names of its
 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

/* eslint-disable rulesdir/no_underscored_properties */

import * as Components from '../components/components.js';
import * as Platform from '../platform/platform.js';
import * as UI from '../ui/ui.js';

import {ConsoleViewMessage} from './ConsoleViewMessage.js';  // eslint-disable-line no-unused-vars

interface SelectionModel {
  item: number;
  node: Node;
  offset: number;
}

export class ConsoleViewport {
  element: HTMLElement;
  _topGapElement: HTMLElement;
  _topGapElementActive: boolean;
  _contentElement: HTMLElement;
  _bottomGapElement: HTMLElement;
  _bottomGapElementActive: boolean;
  _provider: ConsoleViewportProvider;
  _virtualSelectedIndex: number;
  _firstActiveIndex: number;
  _lastActiveIndex: number;
  _renderedItems: ConsoleViewportElement[];
  _anchorSelection: SelectionModel|null;
  _headSelection: SelectionModel|null;
  _itemCount: number;
  _cumulativeHeights: Int32Array;
  _muteCopyHandler: boolean;
  _observer: MutationObserver;
  _observerConfig: {
    childList: boolean,
    subtree: boolean,
  };
  _stickToBottom: boolean;
  _selectionIsBackward: boolean;
  _lastSelectedElement?: HTMLElement|null;
  _cachedProviderElements?: (ConsoleViewportElement|null)[];

  constructor(provider: ConsoleViewportProvider) {
    this.element = (document.createElement('div') as HTMLElement);
    this.element.style.overflow = 'auto';
    this._topGapElement = this.element.createChild('div');
    this._topGapElement.style.height = '0px';
    this._topGapElement.style.color = 'transparent';
    this._topGapElementActive = false;
    this._contentElement = this.element.createChild('div');
    this._bottomGapElement = this.element.createChild('div');
    this._bottomGapElement.style.height = '0px';
    this._bottomGapElement.style.color = 'transparent';
    this._bottomGapElementActive = false;

    // Text content needed for range intersection checks in _updateSelectionModel.
    // Use Unicode ZERO WIDTH NO-BREAK SPACE, which avoids contributing any height to the element's layout overflow.
    this._topGapElement.textContent = '\uFEFF';
    this._bottomGapElement.textContent = '\uFEFF';

    UI.ARIAUtils.markAsHidden(this._topGapElement);
    UI.ARIAUtils.markAsHidden(this._bottomGapElement);

    this._provider = provider;
    this.element.addEventListener('scroll', this._onScroll.bind(this), false);
    this.element.addEventListener('copy', (this._onCopy.bind(this) as EventListener), false);
    this.element.addEventListener('dragstart', (this._onDragStart.bind(this) as (arg0: Event) => unknown), false);
    this._contentElement.addEventListener('focusin', (this._onFocusIn.bind(this) as EventListener), false);
    this._contentElement.addEventListener('focusout', (this._onFocusOut.bind(this) as EventListener), false);
    this._contentElement.addEventListener('keydown', (this._onKeyDown.bind(this) as EventListener), false);
    this._virtualSelectedIndex = -1;
    this._contentElement.tabIndex = -1;

    this._firstActiveIndex = -1;
    this._lastActiveIndex = -1;
    this._renderedItems = [];
    this._anchorSelection = null;
    this._headSelection = null;
    this._itemCount = 0;
    this._cumulativeHeights = new Int32Array(0);
    this._muteCopyHandler = false;

    // Listen for any changes to descendants and trigger a refresh. This ensures
    // that items updated asynchronously will not break stick-to-bottom behavior
    // if they change the scroll height.
    this._observer = new MutationObserver(this.refresh.bind(this));
    this._observerConfig = {childList: true, subtree: true};
    this._stickToBottom = false;
    this._selectionIsBackward = false;
  }

  stickToBottom(): boolean {
    return this._stickToBottom;
  }

  setStickToBottom(value: boolean): void {
    this._stickToBottom = value;
    if (this._stickToBottom) {
      this._observer.observe(this._contentElement, this._observerConfig);
    } else {
      this._observer.disconnect();
    }
  }

  hasVirtualSelection(): boolean {
    return this._virtualSelectedIndex !== -1;
  }

  copyWithStyles(): void {
    this._muteCopyHandler = true;
    this.element.ownerDocument.execCommand('copy');
    this._muteCopyHandler = false;
  }

  _onCopy(event: ClipboardEvent): void {
    if (this._muteCopyHandler) {
      return;
    }

    const text = this._selectedText();
    if (!text) {
      return;
    }

    event.preventDefault();

    if (this._selectionContainsTable()) {
      this.copyWithStyles();
    } else if (event.clipboardData) {
      event.clipboardData.setData('text/plain', text);
    }
  }

  _onFocusIn(event: FocusEvent): void {
    const renderedIndex =
        this._renderedItems.findIndex(item => item.element().isSelfOrAncestor((event.target as Node | null)));
    if (renderedIndex !== -1) {
      this._virtualSelectedIndex = this._firstActiveIndex + renderedIndex;
    }
    let focusLastChild = false;
    // Make default selection when moving from external (e.g. prompt) to the container.
    if (this._virtualSelectedIndex === -1 && this._isOutsideViewport((event.relatedTarget as Element | null)) &&
        event.target === this._contentElement && this._itemCount) {
      focusLastChild = true;
      this._virtualSelectedIndex = this._itemCount - 1;

      // Update stick to bottom before scrolling into view.
      this.refresh();
      this.scrollItemIntoView(this._virtualSelectedIndex);
    }
    this._updateFocusedItem(focusLastChild);
  }

  _onFocusOut(event: FocusEvent): void {
    if (this._isOutsideViewport((event.relatedTarget as Element | null))) {
      this._virtualSelectedIndex = -1;
    }
    this._updateFocusedItem();
  }

  _isOutsideViewport(element: Element|null): boolean {
    return element !== null && !element.isSelfOrDescendant(this._contentElement);
  }

  _onDragStart(event: DragEvent): boolean {
    const text = this._selectedText();
    if (!text) {
      return false;
    }
    if (event.dataTransfer) {
      event.dataTransfer.clearData();
      event.dataTransfer.setData('text/plain', text);
      event.dataTransfer.effectAllowed = 'copy';
    }
    return true;
  }

  _onKeyDown(event: KeyboardEvent): void {
    if (UI.UIUtils.isEditing() || !this._itemCount || event.shiftKey) {
      return;
    }
    let isArrowUp = false;
    switch (event.key) {
      case 'ArrowUp':
        if (this._virtualSelectedIndex > 0) {
          isArrowUp = true;
          this._virtualSelectedIndex--;
        } else {
          return;
        }
        break;
      case 'ArrowDown':
        if (this._virtualSelectedIndex < this._itemCount - 1) {
          this._virtualSelectedIndex++;
        } else {
          return;
        }
        break;
      case 'Home':
        this._virtualSelectedIndex = 0;
        break;
      case 'End':
        this._virtualSelectedIndex = this._itemCount - 1;
        break;
      default:
        return;
    }
    event.consume(true);
    this.scrollItemIntoView(this._virtualSelectedIndex);
    this._updateFocusedItem(isArrowUp);
  }

  _updateFocusedItem(focusLastChild?: boolean): void {
    const selectedElement = this.renderedElementAt(this._virtualSelectedIndex);
    const changed = this._lastSelectedElement !== selectedElement;
    const containerHasFocus = this._contentElement === this.element.ownerDocument.deepActiveElement();
    if (this._lastSelectedElement && changed) {
      this._lastSelectedElement.classList.remove('console-selected');
    }
    if (selectedElement && (focusLastChild || changed || containerHasFocus) && this.element.hasFocus()) {
      selectedElement.classList.add('console-selected');
      // Do not focus the message if something within holds focus (e.g. object).
      if (focusLastChild) {
        this.setStickToBottom(false);
        this._renderedItems[this._virtualSelectedIndex - this._firstActiveIndex].focusLastChildOrSelf();
      } else if (!selectedElement.hasFocus()) {
        selectedElement.focus({preventScroll: true});
      }
    }
    if (this._itemCount && !this._contentElement.hasFocus()) {
      this._contentElement.tabIndex = 0;
    } else {
      this._contentElement.tabIndex = -1;
    }
    this._lastSelectedElement = selectedElement;
  }

  contentElement(): Element {
    return this._contentElement;
  }

  invalidate(): void {
    delete this._cachedProviderElements;
    this._itemCount = this._provider.itemCount();
    if (this._virtualSelectedIndex > this._itemCount - 1) {
      this._virtualSelectedIndex = this._itemCount - 1;
    }
    this._rebuildCumulativeHeights();
    this.refresh();
  }

  _providerElement(index: number): ConsoleViewportElement|null {
    if (!this._cachedProviderElements) {
      this._cachedProviderElements = new Array(this._itemCount);
    }
    let element = this._cachedProviderElements[index];
    if (!element) {
      element = this._provider.itemElement(index);
      this._cachedProviderElements[index] = element;
    }
    return element;
  }

  _rebuildCumulativeHeights(): void {
    const firstActiveIndex = this._firstActiveIndex;
    const lastActiveIndex = this._lastActiveIndex;
    let height = 0;
    this._cumulativeHeights = new Int32Array(this._itemCount);
    for (let i = 0; i < this._itemCount; ++i) {
      if (firstActiveIndex <= i && i - firstActiveIndex < this._renderedItems.length && i <= lastActiveIndex) {
        height += this._renderedItems[i - firstActiveIndex].element().offsetHeight;
      } else {
        height += this._provider.fastHeight(i);
      }
      this._cumulativeHeights[i] = height;
    }
  }

  _rebuildCumulativeHeightsIfNeeded(): void {
    let totalCachedHeight = 0;
    let totalMeasuredHeight = 0;
    // Check whether current items in DOM have changed heights. Tolerate 1-pixel
    // error due to double-to-integer rounding errors.
    for (let i = 0; i < this._renderedItems.length; ++i) {
      const cachedItemHeight = this._cachedItemHeight(this._firstActiveIndex + i);
      const measuredHeight = this._renderedItems[i].element().offsetHeight;
      if (Math.abs(cachedItemHeight - measuredHeight) > 1) {
        this._rebuildCumulativeHeights();
        return;
      }
      totalMeasuredHeight += measuredHeight;
      totalCachedHeight += cachedItemHeight;
      if (Math.abs(totalCachedHeight - totalMeasuredHeight) > 1) {
        this._rebuildCumulativeHeights();
        return;
      }
    }
  }

  _cachedItemHeight(index: number): number {
    return index === 0 ? this._cumulativeHeights[0] :
                         this._cumulativeHeights[index] - this._cumulativeHeights[index - 1];
  }

  _isSelectionBackwards(selection: Selection|null): boolean {
    if (!selection || !selection.rangeCount || !selection.anchorNode || !selection.focusNode) {
      return false;
    }
    const range = document.createRange();
    range.setStart(selection.anchorNode, selection.anchorOffset);
    range.setEnd(selection.focusNode, selection.focusOffset);
    return range.collapsed;
  }

  _createSelectionModel(itemIndex: number, node: Node, offset: number): {
    item: number,
    node: Node,
    offset: number,
  } {
    return {item: itemIndex, node: node, offset: offset};
  }

  _updateSelectionModel(selection: Selection|null): boolean {
    const range = selection && selection.rangeCount ? selection.getRangeAt(0) : null;
    if (!range || (!selection || selection.isCollapsed) || !this.element.hasSelection()) {
      this._headSelection = null;
      this._anchorSelection = null;
      return false;
    }

    let firstSelectedIndex: number = Number.MAX_VALUE;
    let lastSelectedIndex = -1;

    let hasVisibleSelection = false;
    for (let i = 0; i < this._renderedItems.length; ++i) {
      if (range.intersectsNode(this._renderedItems[i].element())) {
        const index = i + this._firstActiveIndex;
        firstSelectedIndex = Math.min(firstSelectedIndex, index);
        lastSelectedIndex = Math.max(lastSelectedIndex, index);
        hasVisibleSelection = true;
      }
    }
    const topOverlap = range.intersectsNode(this._topGapElement) && this._topGapElementActive;
    const bottomOverlap = range.intersectsNode(this._bottomGapElement) && this._bottomGapElementActive;
    if (!topOverlap && !bottomOverlap && !hasVisibleSelection) {
      this._headSelection = null;
      this._anchorSelection = null;
      return false;
    }

    if (!this._anchorSelection || !this._headSelection) {
      this._anchorSelection = this._createSelectionModel(0, this.element, 0);
      this._headSelection = this._createSelectionModel(this._itemCount - 1, this.element, this.element.children.length);
      this._selectionIsBackward = false;
    }

    const isBackward = this._isSelectionBackwards(selection);
    const startSelection = this._selectionIsBackward ? this._headSelection : this._anchorSelection;
    const endSelection = this._selectionIsBackward ? this._anchorSelection : this._headSelection;
    let firstSelected: SelectionModel|null = null;
    let lastSelected: SelectionModel|null = null;
    if (hasVisibleSelection) {
      firstSelected = this._createSelectionModel(firstSelectedIndex, (range.startContainer as Node), range.startOffset);
      lastSelected = this._createSelectionModel(lastSelectedIndex, (range.endContainer as Node), range.endOffset);
    }
    if (topOverlap && bottomOverlap && hasVisibleSelection) {
      firstSelected = (firstSelected && firstSelected.item < startSelection.item) ? firstSelected : startSelection;
      lastSelected = (lastSelected && lastSelected.item > endSelection.item) ? lastSelected : endSelection;
    } else if (!hasVisibleSelection) {
      firstSelected = startSelection;
      lastSelected = endSelection;
    } else if (topOverlap) {
      firstSelected = isBackward ? this._headSelection : this._anchorSelection;
    } else if (bottomOverlap) {
      lastSelected = isBackward ? this._anchorSelection : this._headSelection;
    }

    if (isBackward) {
      this._anchorSelection = lastSelected;
      this._headSelection = firstSelected;
    } else {
      this._anchorSelection = firstSelected;
      this._headSelection = lastSelected;
    }
    this._selectionIsBackward = isBackward;
    return true;
  }

  _restoreSelection(selection: Selection|null): void {
    if (!selection || !this._anchorSelection || !this._headSelection) {
      return;
    }

    const clampSelection = (selection: SelectionModel, isSelectionBackwards: boolean): {
      element: Node,
      offset: number,
    } => {
      if (this._firstActiveIndex <= selection.item && selection.item <= this._lastActiveIndex) {
        return {element: selection.node, offset: selection.offset};
      }

      const element = selection.item < this._firstActiveIndex ? this._topGapElement : this._bottomGapElement;
      return {element, offset: isSelectionBackwards ? 1 : 0};
    };

    const {element: anchorElement, offset: anchorOffset} =
        clampSelection(this._anchorSelection, Boolean(this._selectionIsBackward));
    const {element: headElement, offset: headOffset} = clampSelection(this._headSelection, !this._selectionIsBackward);
    selection.setBaseAndExtent(anchorElement, anchorOffset, headElement, headOffset);
  }

  _selectionContainsTable(): boolean {
    if (!this._anchorSelection || !this._headSelection) {
      return false;
    }

    const start = this._selectionIsBackward ? this._headSelection.item : this._anchorSelection.item;
    const end = this._selectionIsBackward ? this._anchorSelection.item : this._headSelection.item;

    for (let i = start; i <= end; i++) {
      const element = (this._providerElement(i) as ConsoleViewMessage);
      if (element && element.consoleMessage().type === 'table') {
        return true;
      }
    }

    return false;
  }

  refresh(): void {
    this._observer.disconnect();
    this._innerRefresh();
    if (this._stickToBottom) {
      this._observer.observe(this._contentElement, this._observerConfig);
    }
  }

  _innerRefresh(): void {
    if (!this._visibleHeight()) {
      return;
    }  // Do nothing for invisible controls.

    if (!this._itemCount) {
      for (let i = 0; i < this._renderedItems.length; ++i) {
        this._renderedItems[i].willHide();
      }
      this._renderedItems = [];
      this._contentElement.removeChildren();
      this._topGapElement.style.height = '0px';
      this._bottomGapElement.style.height = '0px';
      this._firstActiveIndex = -1;
      this._lastActiveIndex = -1;
      this._updateFocusedItem();
      return;
    }

    const selection = this.element.getComponentSelection();
    const shouldRestoreSelection = this._updateSelectionModel(selection);

    const visibleFrom = this.element.scrollTop;
    const visibleHeight = this._visibleHeight();
    const activeHeight = visibleHeight * 2;
    this._rebuildCumulativeHeightsIfNeeded();

    // When the viewport is scrolled to the bottom, using the cumulative heights estimate is not
    // precise enough to determine next visible indices. This stickToBottom check avoids extra
    // calls to refresh in those cases.
    if (this._stickToBottom) {
      this._firstActiveIndex =
          Math.max(this._itemCount - Math.ceil(activeHeight / this._provider.minimumRowHeight()), 0);
      this._lastActiveIndex = this._itemCount - 1;
    } else {
      this._firstActiveIndex = Math.max(
          Platform.ArrayUtilities.lowerBound(
              this._cumulativeHeights, visibleFrom + 1 - (activeHeight - visibleHeight) / 2,
              Platform.ArrayUtilities.DEFAULT_COMPARATOR),
          0);
      // Proactively render more rows in case some of them will be collapsed without triggering refresh. @see crbug.com/390169
      this._lastActiveIndex = this._firstActiveIndex + Math.ceil(activeHeight / this._provider.minimumRowHeight()) - 1;
      this._lastActiveIndex = Math.min(this._lastActiveIndex, this._itemCount - 1);
    }

    const topGapHeight = this._cumulativeHeights[this._firstActiveIndex - 1] || 0;
    const bottomGapHeight =
        this._cumulativeHeights[this._cumulativeHeights.length - 1] - this._cumulativeHeights[this._lastActiveIndex];

    function prepare(this: ConsoleViewport): void {
      this._topGapElement.style.height = topGapHeight + 'px';
      this._bottomGapElement.style.height = bottomGapHeight + 'px';
      this._topGapElementActive = Boolean(topGapHeight);
      this._bottomGapElementActive = Boolean(bottomGapHeight);
      this._contentElement.style.setProperty('height', '10000000px');
    }

    this._partialViewportUpdate(prepare.bind(this));
    this._contentElement.style.removeProperty('height');
    // Should be the last call in the method as it might force layout.
    if (shouldRestoreSelection) {
      this._restoreSelection(selection);
    }
    if (this._stickToBottom) {
      this.element.scrollTop = 10000000;
    }
  }

  _partialViewportUpdate(prepare: () => void): void {
    const itemsToRender = new Set<ConsoleViewportElement>();
    for (let i = this._firstActiveIndex; i <= this._lastActiveIndex; ++i) {
      const providerElement = this._providerElement(i);
      console.assert(Boolean(providerElement), 'Expected provider element to be defined');
      if (providerElement) {
        itemsToRender.add(providerElement);
      }
    }
    const willBeHidden = this._renderedItems.filter(item => !itemsToRender.has(item));
    for (let i = 0; i < willBeHidden.length; ++i) {
      willBeHidden[i].willHide();
    }
    prepare();
    let hadFocus = false;
    for (let i = 0; i < willBeHidden.length; ++i) {
      hadFocus = hadFocus || willBeHidden[i].element().hasFocus();
      willBeHidden[i].element().remove();
    }

    const wasShown = [];
    let anchor: (ChildNode|null) = this._contentElement.firstChild;
    for (const viewportElement of itemsToRender) {
      const element = viewportElement.element();
      if (element !== anchor) {
        const shouldCallWasShown = !element.parentElement;
        if (shouldCallWasShown) {
          wasShown.push(viewportElement);
        }
        this._contentElement.insertBefore(element, anchor);
      } else {
        anchor = anchor.nextSibling;
      }
    }
    for (let i = 0; i < wasShown.length; ++i) {
      wasShown[i].wasShown();
    }
    this._renderedItems = Array.from(itemsToRender);

    if (hadFocus) {
      this._contentElement.focus();
    }
    this._updateFocusedItem();
  }

  _selectedText(): string|null {
    this._updateSelectionModel(this.element.getComponentSelection());
    if (!this._headSelection || !this._anchorSelection) {
      return null;
    }

    let startSelection: SelectionModel|null = null;
    let endSelection: SelectionModel|null = null;
    if (this._selectionIsBackward) {
      startSelection = this._headSelection;
      endSelection = this._anchorSelection;
    } else {
      startSelection = this._anchorSelection;
      endSelection = this._headSelection;
    }

    const textLines = [];
    for (let i = startSelection.item; i <= endSelection.item; ++i) {
      const providerElement = this._providerElement(i);
      console.assert(Boolean(providerElement));
      if (!providerElement) {
        continue;
      }
      const element = providerElement.element();
      const lineContent = element.childTextNodes().map(Components.Linkifier.Linkifier.untruncatedNodeText).join('');
      textLines.push(lineContent);
    }

    const endProviderElement = this._providerElement(endSelection.item);
    const endSelectionElement = endProviderElement && endProviderElement.element();
    if (endSelectionElement && endSelection.node && endSelection.node.isSelfOrDescendant(endSelectionElement)) {
      const itemTextOffset = this._textOffsetInNode(endSelectionElement, endSelection.node, endSelection.offset);
      if (textLines.length > 0) {
        textLines[textLines.length - 1] = textLines[textLines.length - 1].substring(0, itemTextOffset);
      }
    }

    const startProviderElement = this._providerElement(startSelection.item);
    const startSelectionElement = startProviderElement && startProviderElement.element();
    if (startSelectionElement && startSelection.node && startSelection.node.isSelfOrDescendant(startSelectionElement)) {
      const itemTextOffset = this._textOffsetInNode(startSelectionElement, startSelection.node, startSelection.offset);
      textLines[0] = textLines[0].substring(itemTextOffset);
    }

    return textLines.join('\n');
  }

  _textOffsetInNode(itemElement: Element, selectionNode: Node, offset: number): number {
    // If the selectionNode is not a TextNode, we may need to convert a child offset into a character offset.
    const textContentLength = selectionNode.textContent ? selectionNode.textContent.length : 0;
    if (selectionNode.nodeType !== Node.TEXT_NODE) {
      if (offset < selectionNode.childNodes.length) {
        selectionNode = (selectionNode.childNodes.item(offset) as Node);
        offset = 0;
      } else {
        offset = textContentLength;
      }
    }

    let chars = 0;
    let node: Node|null = itemElement;
    while ((node = node.traverseNextNode(itemElement)) && node !== selectionNode) {
      if (node.nodeType !== Node.TEXT_NODE ||
          (node.parentElement &&
           (node.parentElement.nodeName === 'STYLE' || node.parentElement.nodeName === 'SCRIPT'))) {
        continue;
      }
      chars += Components.Linkifier.Linkifier.untruncatedNodeText(node).length;
    }
    // If the selected node text was truncated, treat any non-zero offset as the full length.
    const untruncatedContainerLength = Components.Linkifier.Linkifier.untruncatedNodeText(selectionNode).length;
    if (offset > 0 && untruncatedContainerLength !== textContentLength) {
      offset = untruncatedContainerLength;
    }
    return chars + offset;
  }

  _onScroll(_event: Event): void {
    this.refresh();
  }

  firstVisibleIndex(): number {
    if (!this._cumulativeHeights.length) {
      return -1;
    }
    this._rebuildCumulativeHeightsIfNeeded();
    return Platform.ArrayUtilities.lowerBound(
        this._cumulativeHeights, this.element.scrollTop + 1, Platform.ArrayUtilities.DEFAULT_COMPARATOR);
  }

  lastVisibleIndex(): number {
    if (!this._cumulativeHeights.length) {
      return -1;
    }
    this._rebuildCumulativeHeightsIfNeeded();
    const scrollBottom = this.element.scrollTop + this.element.clientHeight;
    const right = this._itemCount - 1;
    return Platform.ArrayUtilities.lowerBound(
        this._cumulativeHeights, scrollBottom, Platform.ArrayUtilities.DEFAULT_COMPARATOR, undefined, right);
  }

  renderedElementAt(index: number): HTMLElement|null {
    if (index === -1 || index < this._firstActiveIndex || index > this._lastActiveIndex) {
      return null;
    }
    return this._renderedItems[index - this._firstActiveIndex].element();
  }

  scrollItemIntoView(index: number, makeLast?: boolean): void {
    const firstVisibleIndex = this.firstVisibleIndex();
    const lastVisibleIndex = this.lastVisibleIndex();
    if (index > firstVisibleIndex && index < lastVisibleIndex) {
      return;
    }
    // If the prompt is visible, then the last item must be fully on screen.
    if (index === lastVisibleIndex &&
        this._cumulativeHeights[index] <= this.element.scrollTop + this._visibleHeight()) {
      return;
    }
    if (makeLast) {
      this.forceScrollItemToBeLast(index);
    } else if (index <= firstVisibleIndex) {
      this.forceScrollItemToBeFirst(index);
    } else if (index >= lastVisibleIndex) {
      this.forceScrollItemToBeLast(index);
    }
  }

  forceScrollItemToBeFirst(index: number): void {
    console.assert(index >= 0 && index < this._itemCount, 'Cannot scroll item at invalid index');
    this.setStickToBottom(false);
    this._rebuildCumulativeHeightsIfNeeded();
    this.element.scrollTop = index > 0 ? this._cumulativeHeights[index - 1] : 0;
    if (UI.UIUtils.isScrolledToBottom(this.element)) {
      this.setStickToBottom(true);
    }
    this.refresh();
    // After refresh, the item is in DOM, but may not be visible (items above were larger than expected).
    const renderedElement = this.renderedElementAt(index);
    if (renderedElement) {
      renderedElement.scrollIntoView(true /* alignTop */);
    }
  }

  forceScrollItemToBeLast(index: number): void {
    console.assert(index >= 0 && index < this._itemCount, 'Cannot scroll item at invalid index');
    this.setStickToBottom(false);
    this._rebuildCumulativeHeightsIfNeeded();
    this.element.scrollTop = this._cumulativeHeights[index] - this._visibleHeight();
    if (UI.UIUtils.isScrolledToBottom(this.element)) {
      this.setStickToBottom(true);
    }
    this.refresh();
    // After refresh, the item is in DOM, but may not be visible (items above were larger than expected).
    const renderedElement = this.renderedElementAt(index);
    if (renderedElement) {
      renderedElement.scrollIntoView(false /* alignTop */);
    }
  }

  _visibleHeight(): number {
    // Use offsetHeight instead of clientHeight to avoid being affected by horizontal scroll.
    return this.element.offsetHeight;
  }
}

export interface ConsoleViewportProvider {
  fastHeight(index: number): number;
  itemCount(): number;
  minimumRowHeight(): number;
  itemElement(index: number): ConsoleViewportElement|null;
}

export interface ConsoleViewportElement {
  willHide(): void;
  wasShown(): void;
  element(): HTMLElement;
  focusLastChildOrSelf(): void;
}