pdf.js/src/core/struct_tree.js

/* Copyright 2021 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 { isDict, isName, isRef } from "./primitives.js";
import { isString, stringToPDFString, warn } from "../shared/util.js";
import { NumberTree } from "./name_number_tree.js";

const MAX_DEPTH = 40;

const StructElementType = {
  PAGE_CONTENT: "PAGE_CONTENT",
  STREAM_CONTENT: "STREAM_CONTENT",
  OBJECT: "OBJECT",
  ELEMENT: "ELEMENT",
};

class StructTreeRoot {
  constructor(rootDict) {
    this.dict = rootDict;
    this.roleMap = new Map();
  }

  init() {
    this.readRoleMap();
  }

  readRoleMap() {
    const roleMapDict = this.dict.get("RoleMap");
    if (!isDict(roleMapDict)) {
      return;
    }
    roleMapDict.forEach((key, value) => {
      if (!isName(value)) {
        return;
      }
      this.roleMap.set(key, value.name);
    });
  }
}

/**
 * Instead of loading the whole tree we load just the page's relevant structure
 * elements, which means we need a wrapper structure to represent the tree.
 */
class StructElementNode {
  constructor(tree, dict) {
    this.tree = tree;
    this.dict = dict;
    this.kids = [];
    this.parseKids();
  }

  get role() {
    const nameObj = this.dict.get("S");
    const name = isName(nameObj) ? nameObj.name : "";
    const { root } = this.tree;
    if (root.roleMap.has(name)) {
      return root.roleMap.get(name);
    }
    return name;
  }

  parseKids() {
    let pageObjId = null;
    const objRef = this.dict.getRaw("Pg");
    if (isRef(objRef)) {
      pageObjId = objRef.toString();
    }
    const kids = this.dict.get("K");
    if (Array.isArray(kids)) {
      for (const kid of kids) {
        const element = this.parseKid(pageObjId, kid);
        if (element) {
          this.kids.push(element);
        }
      }
    } else {
      const element = this.parseKid(pageObjId, kids);
      if (element) {
        this.kids.push(element);
      }
    }
  }

  parseKid(pageObjId, kid) {
    // A direct link to content, the integer is an mcid.
    if (Number.isInteger(kid)) {
      if (this.tree.pageDict.objId !== pageObjId) {
        return null;
      }

      return new StructElement({
        type: StructElementType.PAGE_CONTENT,
        mcid: kid,
        pageObjId,
      });
    }

    // Find the dictionary for the kid.
    let kidDict = null;
    if (isRef(kid)) {
      kidDict = this.dict.xref.fetch(kid);
    } else if (isDict(kid)) {
      kidDict = kid;
    }
    if (!kidDict) {
      return null;
    }
    const pageRef = kidDict.getRaw("Pg");
    if (isRef(pageRef)) {
      pageObjId = pageRef.toString();
    }

    const type = isName(kidDict.get("Type")) ? kidDict.get("Type").name : null;
    if (type === "MCR") {
      if (this.tree.pageDict.objId !== pageObjId) {
        return null;
      }
      return new StructElement({
        type: StructElementType.STREAM_CONTENT,
        refObjId: isRef(kidDict.getRaw("Stm"))
          ? kidDict.getRaw("Stm").toString()
          : null,
        pageObjId,
        mcid: kidDict.get("MCID"),
      });
    }

    if (type === "OBJR") {
      if (this.tree.pageDict.objId !== pageObjId) {
        return null;
      }
      return new StructElement({
        type: StructElementType.OBJECT,
        refObjId: isRef(kidDict.getRaw("Obj"))
          ? kidDict.getRaw("Obj").toString()
          : null,
        pageObjId,
      });
    }

    return new StructElement({
      type: StructElementType.ELEMENT,
      dict: kidDict,
    });
  }
}

class StructElement {
  constructor({
    type,
    dict = null,
    mcid = null,
    pageObjId = null,
    refObjId = null,
  }) {
    this.type = type;
    this.dict = dict;
    this.mcid = mcid;
    this.pageObjId = pageObjId;
    this.refObjId = refObjId;
    this.parentNode = null;
  }
}

class StructTreePage {
  constructor(structTreeRoot, pageDict) {
    this.root = structTreeRoot;
    this.rootDict = structTreeRoot ? structTreeRoot.dict : null;
    this.pageDict = pageDict;
    this.nodes = [];
  }

  parse() {
    if (!this.root || !this.rootDict) {
      return;
    }

    const parentTree = this.rootDict.get("ParentTree");
    if (!parentTree) {
      return;
    }
    const id = this.pageDict.get("StructParents");
    if (!Number.isInteger(id)) {
      return;
    }
    const numberTree = new NumberTree(parentTree, this.rootDict.xref);
    const parentArray = numberTree.get(id);
    if (!Array.isArray(parentArray)) {
      return;
    }
    const map = new Map();
    for (const ref of parentArray) {
      if (isRef(ref)) {
        this.addNode(this.rootDict.xref.fetch(ref), map);
      }
    }
  }

  addNode(dict, map, level = 0) {
    if (level > MAX_DEPTH) {
      warn("StructTree MAX_DEPTH reached.");
      return null;
    }

    if (map.has(dict)) {
      return map.get(dict);
    }

    const element = new StructElementNode(this, dict);
    map.set(dict, element);

    const parent = dict.get("P");

    if (!parent || isName(parent.get("Type"), "StructTreeRoot")) {
      if (!this.addTopLevelNode(dict, element)) {
        map.delete(dict);
      }
      return element;
    }

    const parentNode = this.addNode(parent, map, level + 1);
    if (!parentNode) {
      return element;
    }
    let save = false;
    for (const kid of parentNode.kids) {
      if (kid.type === StructElementType.ELEMENT && kid.dict === dict) {
        kid.parentNode = element;
        save = true;
      }
    }
    if (!save) {
      map.delete(dict);
    }
    return element;
  }

  addTopLevelNode(dict, element) {
    const obj = this.rootDict.get("K");
    if (!obj) {
      return false;
    }

    if (isDict(obj)) {
      if (obj.objId !== dict.objId) {
        return false;
      }
      this.nodes[0] = element;
      return true;
    }

    if (!Array.isArray(obj)) {
      return true;
    }
    let save = false;
    for (let i = 0; i < obj.length; i++) {
      const kidRef = obj[i];
      if (kidRef && kidRef.toString() === dict.objId) {
        this.nodes[i] = element;
        save = true;
      }
    }
    return save;
  }

  /**
   * Convert the tree structure into a simplifed object literal that can
   * be sent to the main thread.
   * @returns {Object}
   */
  get serializable() {
    function nodeToSerializable(node, parent, level = 0) {
      if (level > MAX_DEPTH) {
        warn("StructTree too deep to be fully serialized.");
        return;
      }
      const obj = Object.create(null);
      obj.role = node.role;
      obj.children = [];
      parent.children.push(obj);
      const alt = node.dict.get("Alt");
      if (isString(alt)) {
        obj.alt = stringToPDFString(alt);
      }

      for (const kid of node.kids) {
        const kidElement =
          kid.type === StructElementType.ELEMENT ? kid.parentNode : null;
        if (kidElement) {
          nodeToSerializable(kidElement, obj, level + 1);
          continue;
        } else if (
          kid.type === StructElementType.PAGE_CONTENT ||
          kid.type === StructElementType.STREAM_CONTENT
        ) {
          obj.children.push({
            type: "content",
            id: `page${kid.pageObjId}_mcid${kid.mcid}`,
          });
        } else if (kid.type === StructElementType.OBJECT) {
          obj.children.push({
            type: "object",
            id: kid.refObjId,
          });
        }
      }
    }

    const root = Object.create(null);
    root.children = [];
    root.role = "Root";
    for (const child of this.nodes) {
      if (!child) {
        continue;
      }
      nodeToSerializable(child, root);
    }
    return root;
  }
}

export { StructTreePage, StructTreeRoot };