ui/accessibility/ax_position.h

// Copyright 2016 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef UI_ACCESSIBILITY_AX_POSITION_H_
#define UI_ACCESSIBILITY_AX_POSITION_H_

#include <stdint.h>

#include <algorithm>
#include <memory>
#include <stack>
#include <string>
#include <utility>
#include <vector>

#include "base/strings/string16.h"
#include "base/strings/string_number_conversions.h"
#include "base/strings/utf_string_conversions.h"
#include "ui/accessibility/ax_enums.h"

namespace ui {

// Defines the type of position in the accessibility tree.
// A tree position is used when referring to a specific child of a node in the
// accessibility tree.
// A text position is used when referring to a specific character of text inside
// a particular node.
// A null position is used to signify that the provided data is invalid or that
// a boundary has been reached.
enum class AXPositionKind { NULL_POSITION, TREE_POSITION, TEXT_POSITION };

// Forward declarations.
template <class AXPositionType, class AXNodeType>
class AXPosition;
template <class AXPositionType, class AXNodeType>
bool operator==(const AXPosition<AXPositionType, AXNodeType>& first,
                const AXPosition<AXPositionType, AXNodeType>& second);
template <class AXPositionType, class AXNodeType>
bool operator!=(const AXPosition<AXPositionType, AXNodeType>& first,
                const AXPosition<AXPositionType, AXNodeType>& second);

// A position in the accessibility tree.
//
// This class could either represent a tree position or a text position.
// Tree positions point to either a child of a specific node or at the end of a
// node (i.e. an "after children" position).
// Text positions point to either a character offset in the text inside a
// particular node including text from all its children, or to the end of the
// node's text, (i.e. an "after text" position).
// On tree positions that have a leaf node as their anchor, we also need to
// distinguish between "before text" and "after text" positions. To do this, if
// the child index is 0 and the anchor is a leaf node, then it's an "after text"
// position. If the child index is |BEFORE_TEXT| and the anchor is a leaf node,
// then his is a "before text" position.
// It doesn't make sense to have a "before text" position on a text position,
// because it is identical to setting its offset to the first character.
//
// To avoid re-computing either the text offset or the child index when
// converting between the two types of positions, both values are saved after
// the first conversion.
//
// This class template uses static polymorphism in order to allow sub-classes to
// be created from the base class without the base class knowing the type of the
// sub-class in advance.
// The template argument |AXPositionType| should always be set to the type of
// any class that inherits from this template, making this a
// "curiously recursive template".
//
// This class can be copied using the |Clone| method. It is designed to be
// immutable.
template <class AXPositionType, class AXNodeType>
class AXPosition {
 public:
  using AXPositionInstance =
      std::unique_ptr<AXPosition<AXPositionType, AXNodeType>>;

  static const int INVALID_TREE_ID = -1;
  static const int32_t INVALID_ANCHOR_ID = -1;
  static const int BEFORE_TEXT = -1;
  static const int INVALID_INDEX = -2;
  static const int INVALID_OFFSET = -1;

  static AXPositionInstance CreateNullPosition() {
    AXPositionInstance new_position(new AXPositionType());
    new_position->Initialize(AXPositionKind::NULL_POSITION, INVALID_TREE_ID,
                             INVALID_ANCHOR_ID, INVALID_INDEX, INVALID_OFFSET,
                             AX_TEXT_AFFINITY_DOWNSTREAM);
    return new_position;
  }

  static AXPositionInstance CreateTreePosition(int tree_id,
                                               int32_t anchor_id,
                                               int child_index) {
    AXPositionInstance new_position(new AXPositionType());
    new_position->Initialize(AXPositionKind::TREE_POSITION, tree_id, anchor_id,
                             child_index, INVALID_OFFSET,
                             AX_TEXT_AFFINITY_DOWNSTREAM);
    return new_position;
  }

  static AXPositionInstance CreateTextPosition(int tree_id,
                                               int32_t anchor_id,
                                               int text_offset,
                                               AXTextAffinity affinity) {
    AXPositionInstance new_position(new AXPositionType());
    new_position->Initialize(AXPositionKind::TEXT_POSITION, tree_id, anchor_id,
                             INVALID_INDEX, text_offset, affinity);
    return new_position;
  }

  AXPosition() {}
  virtual ~AXPosition() {}

  virtual AXPositionInstance Clone() const = 0;

  std::string ToString() const {
    std::string str;
    switch (kind_) {
      case AXPositionKind::NULL_POSITION:
        return "NullPosition";
      case AXPositionKind::TREE_POSITION: {
        std::string str_child_index;
        if (child_index_ == BEFORE_TEXT) {
          str_child_index = "before_text";
        } else if (child_index_ == INVALID_INDEX) {
          str_child_index = "invalid";
        } else {
          str_child_index = base::IntToString(child_index_);
        }
        str = "TreePosition tree_id=" + base::IntToString(tree_id_) +
              " anchor_id=" + base::IntToString(anchor_id_) + " child_index=" +
              str_child_index;
      }
      case AXPositionKind::TEXT_POSITION: {
        std::string str_text_offset;
        if (text_offset_ == INVALID_OFFSET) {
          str_text_offset = "invalid";
        } else {
          str_text_offset = base::IntToString(text_offset_);
        }
        str = "TextPosition tree_id=" + base::IntToString(tree_id_) +
              " anchor_id=" + base::IntToString(anchor_id_) + " text_offset=" +
              str_text_offset + " affinity=" +
              ui::ToString(static_cast<AXTextAffinity>(affinity_));
      }
    }

    if (!IsTextPosition() || text_offset_ > MaxTextOffset())
      return str;

    std::string text = base::UTF16ToUTF8(GetInnerText());
    DCHECK_GE(text_offset_, 0);
    DCHECK_LE(text_offset_, static_cast<int>(text.length()));
    std::string annotated_text;
    if (text_offset_ == MaxTextOffset()) {
      annotated_text = text + "<>";
    } else {
      annotated_text = text.substr(0, text_offset_) + "<" + text[text_offset_] +
                       ">" + text.substr(text_offset_ + 1);
    }

    return str + " annotated_text=" + annotated_text;
  }

  int tree_id() const { return tree_id_; }
  int32_t anchor_id() const { return anchor_id_; }

  AXNodeType* GetAnchor() const {
    if (tree_id_ == INVALID_TREE_ID || anchor_id_ == INVALID_ANCHOR_ID)
      return nullptr;
    DCHECK_GE(tree_id_, 0);
    DCHECK_GE(anchor_id_, 0);
    return GetNodeInTree(tree_id_, anchor_id_);
  }

  AXPositionKind kind() const { return kind_; }
  int child_index() const { return child_index_; }
  int text_offset() const { return text_offset_; }
  AXTextAffinity affinity() const { return affinity_; }

  bool IsNullPosition() const {
    return kind_ == AXPositionKind::NULL_POSITION || !GetAnchor();
  }

  bool IsTreePosition() const {
    return GetAnchor() && kind_ == AXPositionKind::TREE_POSITION;
  }

  bool IsTextPosition() const {
    return GetAnchor() && kind_ == AXPositionKind::TEXT_POSITION;
  }

  bool AtStartOfAnchor() const {
    if (!GetAnchor())
      return false;

    switch (kind_) {
      case AXPositionKind::NULL_POSITION:
        return false;
      case AXPositionKind::TREE_POSITION:
        if (AnchorChildCount())
          return child_index_ == 0;
        return child_index_ == BEFORE_TEXT;
      case AXPositionKind::TEXT_POSITION:
        return text_offset_ == 0;
    }

    return false;
  }

  bool AtEndOfAnchor() const {
    if (!GetAnchor())
      return false;

    switch (kind_) {
      case AXPositionKind::NULL_POSITION:
        return false;
      case AXPositionKind::TREE_POSITION:
        return child_index_ == AnchorChildCount();
      case AXPositionKind::TEXT_POSITION:
        return text_offset_ == MaxTextOffset();
    }

    return false;
  }

  // This method returns a position instead of a node because this allows us to
  // return the corresponding text offset or child index in the ancestor that
  // relates to the current position.
  // Also, this method uses position instead of tree logic to traverse the tree,
  // because positions can handle moving across multiple trees, while trees
  // cannot.
  AXPositionInstance LowestCommonAncestor(
      const AXPosition<AXPositionType, AXNodeType>& second) const {
    if (IsNullPosition() || second.IsNullPosition())
      return CreateNullPosition();
    if (GetAnchor() == second.GetAnchor())
      return Clone();

    std::stack<AXPositionInstance> ancestors1;
    ancestors1.push(std::move(Clone()));
    while (!ancestors1.top()->IsNullPosition())
      ancestors1.push(std::move(ancestors1.top()->CreateParentPosition()));

    std::stack<AXPositionInstance> ancestors2;
    ancestors2.push(std::move(second.Clone()));
    while (!ancestors2.top()->IsNullPosition())
      ancestors2.push(std::move(ancestors2.top()->CreateParentPosition()));

    AXPositionInstance common_ancestor;
    while (!ancestors1.empty() && !ancestors2.empty() &&
           ancestors1.top()->GetAnchor() == ancestors2.top()->GetAnchor()) {
      common_ancestor = std::move(ancestors1.top());
      ancestors1.pop();
      ancestors2.pop();
    }
    return common_ancestor;
  }

  AXPositionInstance AsTreePosition() const {
    if (IsNullPosition() || IsTreePosition())
      return Clone();

    AXPositionInstance copy = Clone();
    DCHECK(copy);
    DCHECK_NE(copy->text_offset_, INVALID_OFFSET);
    // If the anchor node has no text inside it then the child index should be
    // set to |BEFORE_TEXT|, hence the check if |MaxTextOffset| is greater than
    // 0.
    if (copy->MaxTextOffset() > 0 &&
        copy->text_offset_ >= copy->MaxTextOffset()) {
      copy->child_index_ = copy->AnchorChildCount();
    } else {
      DCHECK_GE(copy->text_offset_, 0);
      copy->child_index_ = BEFORE_TEXT;

      int current_offset = 0;
      for (int i = 0; i < copy->AnchorChildCount(); ++i) {
        AXPositionInstance child = copy->CreateChildPositionAt(i);
        DCHECK(child);
        int child_length = child->MaxTextOffsetInParent();
        if (copy->text_offset_ >= current_offset &&
            copy->text_offset_ < (current_offset + child_length)) {
          copy->child_index_ = i;
          break;
        }

        current_offset += child_length;
      }
    }

    copy->kind_ = AXPositionKind::TREE_POSITION;
    return copy;
  }

  AXPositionInstance AsTextPosition() const {
    if (IsNullPosition() || IsTextPosition())
      return Clone();

    AXPositionInstance copy = Clone();
    DCHECK(copy);
    // Check if it is a "before text" position.
    if (copy->child_index_ == BEFORE_TEXT) {
      // "Before text" positions can only appear on leaf nodes.
      DCHECK(!copy->AnchorChildCount());
      // If the current text offset is valid, we don't touch it.
      if (copy->text_offset_ < 0 || copy->text_offset_ >= copy->MaxTextOffset())
        copy->text_offset_ = 0;
    } else if (copy->child_index_ == copy->AnchorChildCount()) {
      copy->text_offset_ = copy->MaxTextOffset();
    } else {
      DCHECK_GE(copy->child_index_, 0);
      DCHECK_LT(copy->child_index_, copy->AnchorChildCount());
      int new_offset = 0;
      for (int i = 0; i <= child_index_; ++i) {
        AXPositionInstance child = copy->CreateChildPositionAt(i);
        DCHECK(child);
        int child_length = child->MaxTextOffsetInParent();

        // If the current text offset is valid, we don't touch it.
        // Otherwise, we reset it to the beginning of the current child node.
        if (i == child_index_ &&
            (copy->text_offset_ < new_offset ||
             copy->text_offset_ > (new_offset + child_length) ||
             // When the text offset is equal to the text's length but this is
             // not an "after text" position.
             (!copy->AtEndOfAnchor() &&
              copy->text_offset_ == (new_offset + child_length)))) {
          copy->text_offset_ = new_offset;
          break;
        }

        new_offset += child_length;
      }
    }

    copy->kind_ = AXPositionKind::TEXT_POSITION;
    return copy;
  }

  AXPositionInstance AsLeafTextPosition() const {
    if (IsNullPosition() || !AnchorChildCount())
      return AsTextPosition();

    AXPositionInstance child_position;
    AXPositionInstance tree_position = AsTreePosition();
    // If we get an "after children" position, we should return an "after
    // children" position on the last child and recurse.
    if (tree_position->AtEndOfAnchor()) {
      child_position =
          tree_position->CreateChildPositionAt(AnchorChildCount() - 1);
      // Affinity needs to be maintained, because we are not moving the position
      // but simply changing the anchor to the deepest leaf.
      child_position->affinity_ = affinity_;
      return child_position->CreatePositionAtEndOfAnchor()
          ->AsLeafTextPosition();
    }

    // Adjust the text offset.
    // No need to check for "before text" positions here because they are only
    // present on leaf anchor nodes.
    int adjusted_offset = AsTextPosition()->text_offset_;
    for (int i = 0; i < tree_position->child_index_; ++i) {
      child_position = tree_position->CreateChildPositionAt(i);
      DCHECK(child_position);
      adjusted_offset -= child_position->MaxTextOffsetInParent();
    }
    DCHECK_GE(adjusted_offset, 0);

    child_position =
        tree_position->CreateChildPositionAt(tree_position->child_index_)
            ->AsTextPosition();
    child_position->text_offset_ = adjusted_offset;
    // Affinity needs to be maintained, because we are not moving the position
    // but simply changing the anchor to the deepest leaf.
    child_position->affinity_ = affinity_;
    return child_position->AsLeafTextPosition();
  }

  AXPositionInstance CreatePositionAtStartOfAnchor() const {
    switch (kind_) {
      case AXPositionKind::NULL_POSITION:
        return CreateNullPosition();
      case AXPositionKind::TREE_POSITION:
        if (!AnchorChildCount())
          return CreateTreePosition(tree_id_, anchor_id_, BEFORE_TEXT);
        return CreateTreePosition(tree_id_, anchor_id_, 0 /* child_index */);
      case AXPositionKind::TEXT_POSITION:
        return CreateTextPosition(tree_id_, anchor_id_, 0 /* text_offset */,
                                  AX_TEXT_AFFINITY_DOWNSTREAM);
    }
    return CreateNullPosition();
  }

  AXPositionInstance CreatePositionAtEndOfAnchor() const {
    switch (kind_) {
      case AXPositionKind::NULL_POSITION:
        return CreateNullPosition();
      case AXPositionKind::TREE_POSITION:
        return CreateTreePosition(tree_id_, anchor_id_, AnchorChildCount());
      case AXPositionKind::TEXT_POSITION:
        return CreateTextPosition(tree_id_, anchor_id_, MaxTextOffset(),
                                  AX_TEXT_AFFINITY_DOWNSTREAM);
    }
    return CreateNullPosition();
  }

  AXPositionInstance CreateChildPositionAt(int child_index) const {
    if (IsNullPosition())
      return CreateNullPosition();

    if (child_index < 0 || child_index >= AnchorChildCount())
      return CreateNullPosition();

    int tree_id = INVALID_TREE_ID;
    int32_t child_id = INVALID_ANCHOR_ID;
    AnchorChild(child_index, &tree_id, &child_id);
    DCHECK_NE(tree_id, INVALID_TREE_ID);
    DCHECK_NE(child_id, INVALID_ANCHOR_ID);
    switch (kind_) {
      case AXPositionKind::NULL_POSITION:
        NOTREACHED();
        return CreateNullPosition();
      case AXPositionKind::TREE_POSITION: {
        AXPositionInstance child_position =
            CreateTreePosition(tree_id, child_id, 0 /* child_index */);
        // If the child's anchor is a leaf node, make this a "before text"
        // position.
        if (!child_position->AnchorChildCount())
          child_position->child_index_ = BEFORE_TEXT;
        return child_position;
      }
      case AXPositionKind::TEXT_POSITION:
        return CreateTextPosition(tree_id, child_id, 0 /* text_offset */,
                                  AX_TEXT_AFFINITY_DOWNSTREAM);
    }

    return CreateNullPosition();
  }

  AXPositionInstance CreateParentPosition() const {
    if (IsNullPosition())
      return CreateNullPosition();

    int tree_id = INVALID_TREE_ID;
    int32_t parent_id = INVALID_ANCHOR_ID;
    AnchorParent(&tree_id, &parent_id);
    if (tree_id == INVALID_TREE_ID || parent_id == INVALID_ANCHOR_ID)
      return CreateNullPosition();

    DCHECK_NE(tree_id, INVALID_TREE_ID);
    DCHECK_NE(parent_id, INVALID_ANCHOR_ID);
    switch (kind_) {
      case AXPositionKind::NULL_POSITION:
        NOTREACHED();
        return CreateNullPosition();
      case AXPositionKind::TREE_POSITION:
        return CreateTreePosition(tree_id, parent_id, AnchorIndexInParent());
      case AXPositionKind::TEXT_POSITION:
        // Make sure that our affinity is propagated to our parent because by
        // design our parent includes all our text.
        return CreateTextPosition(tree_id, parent_id,
                                  AnchorTextOffsetInParent() + text_offset_,
                                  affinity_);
    }

    return CreateNullPosition();
  }

  // Creates a position using the next text-only node as its anchor.
  // Assumes that text-only nodes are leaf nodes.
  AXPositionInstance CreateNextTextAnchorPosition() const {
    AXPositionInstance next_leaf(CreateNextAnchorPosition());
    while (!next_leaf->IsNullPosition() && next_leaf->AnchorChildCount()) {
      next_leaf = next_leaf->CreateNextAnchorPosition();
    }

    DCHECK(next_leaf);
    return next_leaf->AsTextPosition();
  }

  // Creates a position using the previous text-only node as its anchor.
  // Assumes that text-only nodes are leaf nodes.
  AXPositionInstance CreatePreviousTextAnchorPosition() const {
    AXPositionInstance previous_leaf(CreatePreviousAnchorPosition());
    while (!previous_leaf->IsNullPosition() &&
           previous_leaf->AnchorChildCount()) {
      previous_leaf = previous_leaf->CreatePreviousAnchorPosition();
    }

    DCHECK(previous_leaf);
    return previous_leaf->AsTextPosition();
  }

  // The following methods work across anchors.

  AXPositionInstance CreateNextCharacterPosition() const {
    bool was_tree_position = IsTreePosition();
    AXPositionInstance text_position = AsTextPosition();
    if (text_position->IsNullPosition())
      return text_position;

    if ((text_position->text_offset_ + 1) < text_position->MaxTextOffset()) {
      text_position->text_offset_ += 1;
      // Even if our affinity was upstream, moving to the next character should
      // inevitably reset it to downstream.
      text_position->affinity_ = AX_TEXT_AFFINITY_DOWNSTREAM;
    } else {
      // Moving to the end of the current anchor first is essential. Otherwise
      // |CreateNextAnchorPosition| might return our deepest left-most child
      // because we are using pre-order traversal.
      text_position = text_position->CreatePositionAtEndOfAnchor();
      text_position = text_position->CreateNextTextAnchorPosition();
    }

    if (was_tree_position)
      text_position = text_position->AsTreePosition();
    return text_position;
  }

  AXPositionInstance CreatePreviousCharacterPosition() const {
    bool was_tree_position = IsTreePosition();
    AXPositionInstance text_position = AsTextPosition();
    if (text_position->IsNullPosition())
      return text_position;

    if (text_position->text_offset_ > 0) {
      text_position->text_offset_ -= 1;
      // Even if the new position is at the beginning of the line, the affinity
      // is defaulted to downstream for simplicity.
      text_position->affinity_ = AX_TEXT_AFFINITY_DOWNSTREAM;
    } else {
      text_position = text_position->CreatePreviousTextAnchorPosition();
      text_position = text_position->CreatePositionAtEndOfAnchor();
      if (!text_position->AtStartOfAnchor())
        --text_position->text_offset_;
    }

    if (was_tree_position)
      text_position = text_position->AsTreePosition();
    return text_position;
  }

  AXPositionInstance CreateNextWordStartPosition() const {
    bool was_tree_position = IsTreePosition();
    AXPositionInstance text_position = AsLeafTextPosition();
    if (text_position->IsNullPosition())
      return text_position;

    // Ignore any nodes with no text or no word boundaries.
    while (!text_position->IsNullPosition() &&
           (!text_position->MaxTextOffset() ||
            text_position->GetWordStartOffsets().empty())) {
      text_position = text_position->CreateNextTextAnchorPosition();
    }

    if (text_position->IsNullPosition())
      return text_position;

    const std::vector<int32_t> word_starts =
        text_position->GetWordStartOffsets();
    auto iterator =
        std::upper_bound(word_starts.begin(), word_starts.end(),
                         static_cast<int32_t>(text_position->text_offset_));
    if (iterator == word_starts.end()) {
      do {
        text_position = text_position->CreateNextTextAnchorPosition();
      } while (!text_position->IsNullPosition() &&
               (!text_position->MaxTextOffset() ||
                text_position->GetWordStartOffsets().empty()));
      if (text_position->IsNullPosition())
        return text_position;

      // In case there are some non-word characters in front of the first word
      // in this text node.
      const std::vector<int32_t> word_starts =
          text_position->GetWordStartOffsets();
      text_position->text_offset_ = static_cast<int>(word_starts[0]);
    } else {
      text_position->text_offset_ = static_cast<int>(*iterator);
      text_position->affinity_ = AX_TEXT_AFFINITY_DOWNSTREAM;
    }

    // If the word boundary is in the same subtree, return a position rooted at
    // the current position.
    // This is necessary because we don't want to return any position that might
    // be in the shadow DOM if the original position was not.
    AXPositionInstance common_ancestor =
        text_position->LowestCommonAncestor(*this);
    if (GetAnchor() == common_ancestor->GetAnchor())
      return common_ancestor;

    if (was_tree_position)
      text_position = text_position->AsTreePosition();
    return text_position;
  }

  AXPositionInstance CreatePreviousWordStartPosition() const {
    bool was_tree_position = IsTreePosition();
    AXPositionInstance text_position = AsLeafTextPosition();
    if (text_position->IsNullPosition())
      return text_position;

    if (text_position->AtStartOfAnchor()) {
      text_position = text_position->CreatePreviousTextAnchorPosition();
      text_position = text_position->CreatePositionAtEndOfAnchor();
    }

    // Ignore any nodes with no text or no word boundaries.
    while (!text_position->IsNullPosition() &&
           (!text_position->MaxTextOffset() ||
            text_position->GetWordStartOffsets().empty())) {
      text_position = text_position->CreatePreviousTextAnchorPosition();
      text_position = text_position->CreatePositionAtEndOfAnchor();
    }

    if (text_position->IsNullPosition())
      return text_position;

    const std::vector<int32_t> word_starts =
        text_position->GetWordStartOffsets();
    auto iterator =
        std::lower_bound(word_starts.begin(), word_starts.end(),
                         static_cast<int32_t>(text_position->text_offset_));
    if (iterator == word_starts.begin()) {
      // There must be some non-word characters in front of the first word in
      // this text node.
      do {
        text_position = text_position->CreatePreviousTextAnchorPosition();
      } while (!text_position->IsNullPosition() &&
               (!text_position->MaxTextOffset() ||
                text_position->GetWordStartOffsets().empty()));
      if (text_position->IsNullPosition())
        return text_position;

      const std::vector<int32_t> word_starts =
          text_position->GetWordStartOffsets();
      text_position->text_offset_ = static_cast<int>(*(word_starts.end() - 1));
    } else {
      text_position->text_offset_ = static_cast<int>(*(--iterator));
      text_position->affinity_ = AX_TEXT_AFFINITY_DOWNSTREAM;
    }

    // If the word boundary is in the same subtree, return a position rooted at
    // the current position.
    // This is necessary because we don't want to return any position that might
    // be in the shadow DOM if the original position was not.
    AXPositionInstance common_ancestor =
        text_position->LowestCommonAncestor(*this);
    if (GetAnchor() == common_ancestor->GetAnchor())
      return common_ancestor;

    if (was_tree_position)
      text_position = text_position->AsTreePosition();
    return text_position;
  }

  // Word end positions are one past the last character of the word.
  AXPositionInstance CreateNextWordEndPosition() const {
    bool was_tree_position = IsTreePosition();
    AXPositionInstance text_position = AsLeafTextPosition();
    if (text_position->IsNullPosition())
      return text_position;

    if (text_position->AtEndOfAnchor())
      text_position = text_position->CreateNextTextAnchorPosition();

    // Ignore any nodes with no text or no word boundaries.
    while (!text_position->IsNullPosition() &&
           (!text_position->MaxTextOffset() ||
            text_position->GetWordEndOffsets().empty())) {
      text_position = text_position->CreateNextTextAnchorPosition();
    }

    if (text_position->IsNullPosition())
      return text_position;

    const std::vector<int> word_ends = text_position->GetWordEndOffsets();
    auto iterator = std::upper_bound(word_ends.begin(), word_ends.end(),
                                     text_position->text_offset_);
    if (iterator == word_ends.end()) {
      // We should be in the last word of this text node.
      do {
        text_position = text_position->CreateNextTextAnchorPosition();
      } while (!text_position->IsNullPosition() &&
               (!text_position->MaxTextOffset() ||
                text_position->GetWordEndOffsets().empty()));
      if (text_position->IsNullPosition())
        return text_position;

      const std::vector<int> word_ends = text_position->GetWordEndOffsets();
      text_position->text_offset_ = word_ends[0];
    } else {
      text_position->text_offset_ = *iterator;
      text_position->affinity_ = AX_TEXT_AFFINITY_DOWNSTREAM;
    }

    // If the word boundary is in the same subtree, return a position rooted at
    // the current position.
    // This is necessary because we don't want to return any position that might
    // be in the shadow DOM if the original position was not.
    AXPositionInstance common_ancestor =
        text_position->LowestCommonAncestor(*this);
    if (GetAnchor() == common_ancestor->GetAnchor())
      return common_ancestor;

    if (was_tree_position)
      text_position = text_position->AsTreePosition();
    return text_position;
  }

  // Word end positions are one past the last character of the word.
  AXPositionInstance CreatePreviousWordEndPosition() const {
    bool was_tree_position = IsTreePosition();
    AXPositionInstance text_position = AsLeafTextPosition();
    if (text_position->IsNullPosition())
      return text_position;

    if (text_position->AtStartOfAnchor()) {
      text_position = text_position->CreatePreviousTextAnchorPosition();
      text_position = text_position->CreatePositionAtEndOfAnchor();
    }

    // Ignore any nodes with no text or no word boundaries.
    while (!text_position->IsNullPosition() &&
           (!text_position->MaxTextOffset() ||
            text_position->GetWordStartOffsets().empty())) {
      text_position = text_position->CreatePreviousTextAnchorPosition();
    }

    if (text_position->IsNullPosition())
      return text_position;

    const std::vector<int32_t> word_ends = text_position->GetWordEndOffsets();
    auto iterator =
        std::lower_bound(word_ends.begin(), word_ends.end(),
                         static_cast<int32_t>(text_position->text_offset_));
    if (iterator == word_ends.begin()) {
      // We must be anywhere up to and including the end of the first word in
      // this node.
      do {
        text_position = text_position->CreatePreviousTextAnchorPosition();
      } while (!text_position->IsNullPosition() &&
               (!text_position->MaxTextOffset() ||
                text_position->GetWordStartOffsets().empty()));
      if (text_position->IsNullPosition())
        return text_position;

      const std::vector<int32_t> word_ends = text_position->GetWordEndOffsets();
      text_position->text_offset_ = static_cast<int>(*(word_ends.end() - 1));
    } else {
      text_position->text_offset_ = static_cast<int>(*(--iterator));
      text_position->affinity_ = AX_TEXT_AFFINITY_DOWNSTREAM;
    }

    // If the word boundary is in the same subtree, return a position rooted at
    // the current position.
    // This is necessary because we don't want to return any position that might
    // be in the shadow DOM if the original position was not.
    AXPositionInstance common_ancestor =
        text_position->LowestCommonAncestor(*this);
    if (GetAnchor() == common_ancestor->GetAnchor())
      return common_ancestor;

    if (was_tree_position)
      text_position = text_position->AsTreePosition();
    return text_position;
  }

  AXPositionInstance CreateNextLineStartPosition() const {
    bool was_tree_position = IsTreePosition();
    AXPositionInstance text_position = AsLeafTextPosition();
    if (text_position->IsNullPosition())
      return text_position;

    // Find the next line break.
    int32_t next_on_line_id = text_position->anchor_id_;
    while (GetNextOnLineID(next_on_line_id) != INVALID_ANCHOR_ID)
      next_on_line_id = GetNextOnLineID(next_on_line_id);
    text_position =
        CreateTextPosition(tree_id_, next_on_line_id, 0 /* text_offset */,
                           AX_TEXT_AFFINITY_DOWNSTREAM);
    text_position =
        text_position->AsLeafTextPosition()->CreateNextTextAnchorPosition();
    if (text_position->IsNullPosition())
      return text_position;

    // If the line boundary is in the same subtree, return a position rooted at
    // the current position.
    // This is necessary because we don't want to return any position that might
    // be in the shadow DOM if the original position was not.
    AXPositionInstance common_ancestor =
        text_position->LowestCommonAncestor(*this);
    if (GetAnchor() == common_ancestor->GetAnchor())
      return common_ancestor;

    if (was_tree_position)
      text_position = text_position->AsTreePosition();
    return text_position;
  }

  AXPositionInstance CreatePreviousLineStartPosition() const {
    bool was_tree_position = IsTreePosition();
    AXPositionInstance text_position = AsLeafTextPosition();
    if (text_position->AtStartOfAnchor())
      text_position = text_position->CreatePreviousTextAnchorPosition();
    if (text_position->IsNullPosition())
      return text_position;

    int32_t previous_on_line_id = text_position->anchor_id_;
    while (GetPreviousOnLineID(previous_on_line_id) != INVALID_ANCHOR_ID)
      previous_on_line_id = GetPreviousOnLineID(previous_on_line_id);
    text_position =
        CreateTextPosition(tree_id_, previous_on_line_id, 0 /* text_offset */,
                           AX_TEXT_AFFINITY_DOWNSTREAM);
    text_position = text_position->AsLeafTextPosition();
    if (text_position->IsNullPosition())
      return text_position;

    // If the line boundary is in the same subtree, return a position rooted at
    // the current position.
    // This is necessary because we don't want to return any position that might
    // be in the shadow DOM if the original position was not.
    AXPositionInstance common_ancestor =
        text_position->LowestCommonAncestor(*this);
    if (GetAnchor() == common_ancestor->GetAnchor())
      return common_ancestor;

    if (was_tree_position)
      text_position = text_position->AsTreePosition();
    return text_position;
  }

  // Line end positions are one past the last character of the line, excluding
  // any newline characters.
  AXPositionInstance CreateNextLineEndPosition() const {
    bool was_tree_position = IsTreePosition();
    AXPositionInstance text_position = AsLeafTextPosition();
    // Skip forward to the next line if we are at the end of one.
    // Note that not all lines end with a hard line break.
    while (text_position->IsInLineBreak() || text_position->AtEndOfAnchor())
      text_position = text_position->CreateNextTextAnchorPosition();
    if (text_position->IsNullPosition())
      return text_position;

    // Find the next line break.
    int32_t next_on_line_id = text_position->anchor_id_;
    while (GetNextOnLineID(next_on_line_id) != INVALID_ANCHOR_ID)
      next_on_line_id = GetNextOnLineID(next_on_line_id);
    text_position =
        CreateTextPosition(tree_id_, next_on_line_id, 0 /* text_offset */,
                           AX_TEXT_AFFINITY_DOWNSTREAM);
    text_position = text_position->AsLeafTextPosition();
    while (text_position->IsInLineBreak()) {
      text_position = text_position->CreatePreviousTextAnchorPosition();
    }
    if (text_position->IsNullPosition())
      return text_position;
    text_position = text_position->CreatePositionAtEndOfAnchor();

    // If the line boundary is in the same subtree, return a position rooted at
    // the current position.
    // This is necessary because we don't want to return any position that might
    // be in the shadow DOM if the original position was not.
    AXPositionInstance common_ancestor =
        text_position->LowestCommonAncestor(*this);
    if (GetAnchor() == common_ancestor->GetAnchor())
      return common_ancestor;

    if (was_tree_position)
      text_position = text_position->AsTreePosition();
    return text_position;
  }

  // Line end positions are one past the last character of the line, excluding
  // any newline characters.
  AXPositionInstance CreatePreviousLineEndPosition() const {
    bool was_tree_position = IsTreePosition();
    AXPositionInstance text_position = AsLeafTextPosition();
    if (text_position->IsNullPosition())
      return text_position;

    int32_t previous_on_line_id = text_position->anchor_id_;
    while (GetPreviousOnLineID(previous_on_line_id) != INVALID_ANCHOR_ID)
      previous_on_line_id = GetPreviousOnLineID(previous_on_line_id);
    text_position =
        CreateTextPosition(tree_id_, previous_on_line_id, 0 /* text_offset */,
                           AX_TEXT_AFFINITY_DOWNSTREAM);
    text_position =
        text_position->AsLeafTextPosition()->CreatePreviousTextAnchorPosition();
    while (text_position->IsInLineBreak()) {
      text_position = text_position->CreatePreviousTextAnchorPosition();
    }
    if (text_position->IsNullPosition())
      return text_position;
    text_position = text_position->CreatePositionAtEndOfAnchor();

    // If the line boundary is in the same subtree, return a position rooted at
    // the current position.
    // This is necessary because we don't want to return any position that might
    // be in the shadow DOM if the original position was not.
    AXPositionInstance common_ancestor =
        text_position->LowestCommonAncestor(*this);
    if (GetAnchor() == common_ancestor->GetAnchor())
      return common_ancestor;

    if (was_tree_position)
      text_position = text_position->AsTreePosition();
    return text_position;
  }

  // TODO(nektar): Add sentence and paragraph navigation methods.

  // Abstract methods.

  // Returns the text that is present inside the anchor node, including any text
  // found in descendant nodes.
  virtual base::string16 GetInnerText() const = 0;

 protected:
  AXPosition(const AXPosition<AXPositionType, AXNodeType>& other) = default;
  virtual AXPosition<AXPositionType, AXNodeType>& operator=(
      const AXPosition<AXPositionType, AXNodeType>& other) = default;

  virtual void Initialize(AXPositionKind kind,
                          int tree_id,
                          int32_t anchor_id,
                          int child_index,
                          int text_offset,
                          AXTextAffinity affinity) {
    kind_ = kind;
    tree_id_ = tree_id;
    anchor_id_ = anchor_id;
    child_index_ = child_index;
    text_offset_ = text_offset;
    affinity_ = affinity;

    if (!GetAnchor() || kind_ == AXPositionKind::NULL_POSITION ||
        (kind_ == AXPositionKind::TREE_POSITION &&
         (child_index_ != BEFORE_TEXT &&
          (child_index_ < 0 || child_index_ > AnchorChildCount()))) ||
        (kind_ == AXPositionKind::TEXT_POSITION &&
         (text_offset_ < 0 || text_offset_ > MaxTextOffset()))) {
      // Reset to the null position.
      kind_ = AXPositionKind::NULL_POSITION;
      tree_id_ = INVALID_TREE_ID;
      anchor_id_ = INVALID_ANCHOR_ID;
      child_index_ = INVALID_INDEX;
      text_offset_ = INVALID_OFFSET;
      affinity_ = AX_TEXT_AFFINITY_DOWNSTREAM;
    }
  }

  // Uses depth-first pre-order traversal.
  AXPositionInstance CreateNextAnchorPosition() const {
    if (IsNullPosition())
      return CreateNullPosition();

    if (AnchorChildCount()) {
      if (IsTreePosition()) {
        return CreateChildPositionAt(child_index_);
      } else {
        // We have to find the child node that encompasses the current text
        // offset.
        AXPositionInstance tree_position = AsTreePosition();
        DCHECK(tree_position);
        return tree_position->CreateChildPositionAt(
            tree_position->child_index_);
      }
    }

    AXPositionInstance current_position = Clone();
    AXPositionInstance parent_position = CreateParentPosition();
    while (!parent_position->IsNullPosition()) {
      // Get the next sibling if it exists, otherwise move up to the parent's
      // next sibling.
      int index_in_parent = current_position->AnchorIndexInParent();
      if (index_in_parent < parent_position->AnchorChildCount() - 1) {
        AXPositionInstance next_sibling =
            parent_position->CreateChildPositionAt(index_in_parent + 1);
        DCHECK(next_sibling && !next_sibling->IsNullPosition());
        return next_sibling;
      }

      current_position = std::move(parent_position);
      parent_position = current_position->CreateParentPosition();
    }

    return CreateNullPosition();
  }

  // Uses depth-first pre-order traversal.
  AXPositionInstance CreatePreviousAnchorPosition() const {
    if (IsNullPosition())
      return CreateNullPosition();

    AXPositionInstance parent_position = CreateParentPosition();
    if (parent_position->IsNullPosition())
      return CreateNullPosition();

    // Get the previous sibling's deepest first child if a previous sibling
    // exists, otherwise move up to the parent.
    int index_in_parent = AnchorIndexInParent();
    if (index_in_parent <= 0)
      return parent_position;

    AXPositionInstance leaf =
        parent_position->CreateChildPositionAt(index_in_parent - 1);
    while (!leaf->IsNullPosition() && leaf->AnchorChildCount())
      leaf = leaf->CreateChildPositionAt(0);

    return leaf;
  }

  // Returns the character offset inside our anchor's parent at which our text
  // starts.
  int AnchorTextOffsetInParent() const {
    if (IsNullPosition())
      return INVALID_OFFSET;

    // Calculate how much text there is to the left of this anchor.
    AXPositionInstance tree_position = AsTreePosition();
    DCHECK(tree_position);
    AXPositionInstance parent_position = tree_position->CreateParentPosition();
    DCHECK(parent_position);
    if (parent_position->IsNullPosition())
      return 0;

    int offset_in_parent = 0;
    for (int i = 0; i < parent_position->child_index(); ++i) {
      AXPositionInstance child = parent_position->CreateChildPositionAt(i);
      DCHECK(child);
      offset_in_parent += child->MaxTextOffsetInParent();
    }
    return offset_in_parent;
  }

  // Abstract methods.
  virtual void AnchorChild(int child_index,
                           int* tree_id,
                           int32_t* child_id) const = 0;
  virtual int AnchorChildCount() const = 0;
  virtual int AnchorIndexInParent() const = 0;
  virtual void AnchorParent(int* tree_id, int32_t* parent_id) const = 0;
  virtual AXNodeType* GetNodeInTree(int tree_id, int32_t node_id) const = 0;
  // Returns the length of the text that is present inside the anchor node,
  // including any text found in descendant text nodes.
  virtual int MaxTextOffset() const = 0;
  // Returns the length of text that this anchor node takes up in its parent.
  // On some platforms, embedded objects are represented in their parent with a
  // single embedded object character.
  virtual int MaxTextOffsetInParent() const { return MaxTextOffset(); }
  virtual bool IsInLineBreak() const = 0;
  virtual std::vector<int32_t> GetWordStartOffsets() const = 0;
  virtual std::vector<int32_t> GetWordEndOffsets() const = 0;
  virtual int32_t GetNextOnLineID(int32_t node_id) const = 0;
  virtual int32_t GetPreviousOnLineID(int32_t node_id) const = 0;

 private:
  AXPositionKind kind_;
  int tree_id_;
  int32_t anchor_id_;

  // For text positions, |child_index_| is initially set to |-1| and only
  // computed on demand. The same with tree positions and |text_offset_|.
  int child_index_;
  int text_offset_;

  // TODO(nektar): Get rid of affinity and make Blink handle affinity
  // internally since inline text objects don't span lines.
  AXTextAffinity affinity_;
};

template <class AXPositionType, class AXNodeType>
const int AXPosition<AXPositionType, AXNodeType>::INVALID_TREE_ID;
template <class AXPositionType, class AXNodeType>
const int32_t AXPosition<AXPositionType, AXNodeType>::INVALID_ANCHOR_ID;
template <class AXPositionType, class AXNodeType>
const int AXPosition<AXPositionType, AXNodeType>::BEFORE_TEXT;
template <class AXPositionType, class AXNodeType>
const int AXPosition<AXPositionType, AXNodeType>::INVALID_INDEX;
template <class AXPositionType, class AXNodeType>
const int AXPosition<AXPositionType, AXNodeType>::INVALID_OFFSET;

template <class AXPositionType, class AXNodeType>
bool operator==(const AXPosition<AXPositionType, AXNodeType>& first,
                const AXPosition<AXPositionType, AXNodeType>& second) {
  if (first.IsNullPosition() && second.IsNullPosition())
    return true;
  return first.tree_id() == second.tree_id() &&
         first.anchor_id() == second.anchor_id() &&
         first.child_index() == second.child_index() &&
         first.text_offset() == second.text_offset() &&
         first.affinity() == second.affinity();
}

template <class AXPositionType, class AXNodeType>
bool operator!=(const AXPosition<AXPositionType, AXNodeType>& first,
                const AXPosition<AXPositionType, AXNodeType>& second) {
  return !(first == second);
}

template <class AXPositionType, class AXNodeType>
bool operator<(const AXPosition<AXPositionType, AXNodeType>& first,
               const AXPosition<AXPositionType, AXNodeType>& second) {
  if (first.IsNullPosition() || second.IsNullPosition())
    return false;

  std::unique_ptr<AXPosition<AXPositionType, AXNodeType>> first_ancestor =
      first.LowestCommonAncestor(second)->AsTreePosition();
  std::unique_ptr<AXPosition<AXPositionType, AXNodeType>> second_ancestor =
      second.LowestCommonAncestor(first)->AsTreePosition();
  DCHECK_EQ(first_ancestor->GetAnchor(), second_ancestor->GetAnchor());
  return !first_ancestor->IsNullPosition() &&
         first_ancestor->AsTextPosition()->text_offset() <
             second_ancestor->AsTextPosition()->text_offset();
}

template <class AXPositionType, class AXNodeType>
bool operator<=(const AXPosition<AXPositionType, AXNodeType>& first,
                const AXPosition<AXPositionType, AXNodeType>& second) {
  return first == second || first < second;
}

template <class AXPositionType, class AXNodeType>
bool operator>(const AXPosition<AXPositionType, AXNodeType>& first,
               const AXPosition<AXPositionType, AXNodeType>& second) {
  if (first.IsNullPosition() || second.IsNullPosition())
    return false;

  std::unique_ptr<AXPosition<AXPositionType, AXNodeType>> first_ancestor =
      first.LowestCommonAncestor(second)->AsTreePosition();
  std::unique_ptr<AXPosition<AXPositionType, AXNodeType>> second_ancestor =
      second.LowestCommonAncestor(first)->AsTreePosition();
  DCHECK_EQ(first_ancestor->GetAnchor(), second_ancestor->GetAnchor());
  return !first_ancestor->IsNullPosition() &&
         first_ancestor->AsTextPosition()->text_offset() >
             second_ancestor->AsTextPosition()->text_offset();
}

template <class AXPositionType, class AXNodeType>
bool operator>=(const AXPosition<AXPositionType, AXNodeType>& first,
                const AXPosition<AXPositionType, AXNodeType>& second) {
  return first == second || first > second;
}

}  // namespace ui

#endif  // UI_ACCESSIBILITY_AX_POSITION_H_