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PDF logical structure reader in Obj-C

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Sample Obj-C code for using PDFTron SDK to explore the logical structure and content of a tagged PDF file, then dumps the information to the console window. In tagged PDF files, StructTree acts as a central repository for information related to a PDF document's logical structure. The tree consists of StructElement-s and ContentItem-s which are leaf nodes of the structure tree. Learn more about our Obj-C PDF Library and PDF Parsing & Content Extraction Library.

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//---------------------------------------------------------------------------------------
// Copyright (c) 2001-2021 by PDFTron Systems Inc. All Rights Reserved.
// Consult legal.txt regarding legal and license information.
//---------------------------------------------------------------------------------------

#import <OBJC/PDFNetOBJC.h>
#import <Foundation/Foundation.h>

//---------------------------------------------------------------------------------------
// This sample explores the structure and content of a tagged PDF document and dumps 
// the structure information to the console window.
//
// In tagged PDF documents StructTree acts as a central repository for information 
// related to a PDF document's logical structure. The tree consists of StructElement-s
// and ContentItem-s which are leaf nodes of the structure tree.
//
// The sample can be extended to access and extract the marked-content elements such 
// as text and images.
//---------------------------------------------------------------------------------------


NSString *PrintIdent(int ident) { 
    int i;
    NSString * str= @"\n";
    for (i=0; i<ident; ++i) {
        str = [str stringByAppendingString: @"  "]; 
    }
    return str;
}

// Used in code snippet 1.
NSString* ProcessStructElement(PTSElement *element, int ident)
{
    if (![element IsValid]) {
        return @"";
    }

    NSString *result = @"";
    // Print out the type and title info, if any.
    result = [result stringByAppendingFormat: @"%@Type: %@", PrintIdent(ident++), [element GetType]];
    if ([element HasTitle]) {
        result = [result stringByAppendingFormat: @". Title: %@", [element GetTitle]];
    }

    int num = [element GetNumKids];
    int i;
    for (i=0; i<num; ++i) 
    {
        // Check is the kid is a leaf node (i.e. it is a ContentItem).
        if ([element IsContentItem: i]) { 
            PTContentItem *cont = [element GetAsContentItem: i]; 
            PTContentItemType type = [cont GetType];

            PTPage *page = [cont GetPage];

            result = [result stringByAppendingFormat: @"%@Content Item. Part of page #%d%@", PrintIdent(ident), [page GetIndex], PrintIdent(ident)];
            
            switch (type) {
                case e_ptMCID:
                case e_ptMCR:
                    result = [result stringByAppendingFormat: @"MCID: %d", [cont GetMCID]];
                    break;
                case e_ptOBJR:
                    {
                        result = [result stringByAppendingString: @"OBJR "];
                        PTObj *ref_obj;
                        if ((ref_obj = [cont GetRefObj]) != NULL)
                            result = [result stringByAppendingFormat: @"- Referenced Object#: %u", [ref_obj GetObjNum]];
                    }
                    break;
                default: 
                    break;
            }
        }
        else {  // the kid is another StructElement node.
            result = [result stringByAppendingString: ProcessStructElement([element GetAsStructElem: i], ident)];
        }
    }
    return result;
}

// Used in code snippet 2.
NSString* ProcessLogicalStructureTestElements(PTElementReader *reader)
{
    PTElement *element;
    NSString *result = @"";
    while ((element = [reader Next]) != NULL)     // Read page contents
    {
        // In this sample we process only paths & text, but the code can be 
        // extended to handle any element type.
        PTElementType type = [element GetType];
        if (type == e_ptpath || type == e_pttext_obj || type == e_ptpath) 
        {   
            switch (type)    {
            case e_ptpath:                // Process path ...
                result = [result stringByAppendingString: @"\nPATH: "];
                break; 
            case e_pttext_obj:                 // Process text ...
                result = [result stringByAppendingFormat: @"\nTEXT: %@\n", [element GetTextString]];
                break;
            case e_ptform:                // Process form XObjects
                result = [result stringByAppendingString: @"\nFORM XObject: "];
                //reader.FormBegin(); 
                //ProcessLogicalStructureTestElements(reader);
                //reader.End(); 
                break;
            default:
                break;
            }

            // Check if the element is associated with any structural element.
            // Content items are leaf nodes of the structure tree.
            PTSElement *struct_parent = [element GetParentStructElement];
            if ([struct_parent IsValid]) {
                // Print out the parent structural element's type, title, and object number.
                result = [result stringByAppendingFormat: @" Type: %@, MCID: %d", [struct_parent GetType], [element GetStructMCID]];
                if ([struct_parent HasTitle]) {
                    result = [result stringByAppendingFormat: @". Title: %@", [struct_parent GetTitle]];
                }
                result = [result stringByAppendingFormat: @", Obj#: %u", [[struct_parent GetSDFObj] GetObjNum]];
            }
        }
    }
    return result;
}

// Used in code snippet 3.
//typedef map<int, string> MCIDPageMap;
NSMutableDictionary *MCIDPageMap;
NSMutableDictionary *MCIDDocMap;
//typedef map<int, MCIDPageMap> MCIDDocMap;

// Used in code snippet 3.
void ProcessLogicalStructureTestElements2(PTElementReader *reader, NSMutableDictionary *mcid_page_map)
{
    PTElement *element;
    while ((element = [reader Next]) != NULL) // Read page contents
    {
        // In this sample we process only text, but the code can be extended 
        // to handle paths, images, or any other Element type.
        int mcid = [element GetStructMCID];
        if (mcid>= 0 && [element GetType] == e_pttext_obj) {
            NSString *val = [element GetTextString];
            id key = @(mcid);
            BOOL exist = [mcid_page_map.allKeys containsObject: key];
            if (exist) {
                NSString *str = mcid_page_map[key];
                mcid_page_map[key] = [str stringByAppendingString: val];
            } 
            else {
                mcid_page_map[key] = val;
            }
        }
    }
}

// Used in code snippet 3.
NSString* ProcessStructElement2(PTSElement *element, NSMutableDictionary *mcid_doc_map, int ident) 
{
    if (![element IsValid]) {
        return @"";
    }
    NSString *result = @"";
    // Print out the type and title info, if any.
    result = [result stringByAppendingString: PrintIdent(ident)];
    result = [result stringByAppendingFormat: @"<%@", [element GetType]];
    if ([element HasTitle]) {
        result = [result stringByAppendingFormat: @" title=\"%@\"", [element GetTitle]];
    }
    result = [result stringByAppendingString: @">"];

    int num = [element GetNumKids];
    int i;
    for (i=0; i<num; ++i) 
    {        
        if ([element IsContentItem: i]) { 
            PTContentItem *cont = [element GetAsContentItem: i]; 
            if ([cont GetType] == e_ptMCID) {
                int page_num = [[cont GetPage] GetIndex];
                id key = @(page_num);
                BOOL exist = [mcid_doc_map.allKeys containsObject: key];
                
                if (exist) {
                    NSMutableDictionary *mcid_page_map = mcid_doc_map[key];
                    id key2 = @([cont GetMCID]);
                    BOOL exist2 = [mcid_page_map.allKeys containsObject: key2];
                    if (exist2) {
                        NSString *str = mcid_page_map[key2];
                        result = [result stringByAppendingString: str]; 
                    }                    
                }
            }
        }
        else {  // the kid is another StructElement node.
            result = [result stringByAppendingString: ProcessStructElement2([element GetAsStructElem :i], mcid_doc_map, ident+1)];
        }
    }

    result = [result stringByAppendingString: PrintIdent(ident)];
    result = [result stringByAppendingFormat: @"</%@>", [element GetType]];
    return result;
}


int main(int argc, char *argv[])
{
    @autoreleasepool {
        int ret = 0;
        [PTPDFNet Initialize: 0];

        @try    // Extract logical structure from a PDF document
        {
            PTPDFDoc *doc = [[PTPDFDoc alloc] initWithFilepath: @"../../TestFiles/tagged.pdf"];
            [doc InitSecurityHandler];

            NSLog(@"____________________________________________________________");
            NSLog(@"Sample 1 - Traverse logical structure tree...");
            {
                PTSTree *tree = [doc GetStructTree];
                if ([tree IsValid]) {
                    NSLog(@"Document has a StructTree root.");

                int i;
                    for (i=0; i<[tree GetNumKids]; ++i) {
                        // Recursively get structure info for all child elements.
                        NSLog(@"%@", ProcessStructElement([tree GetKid: i], 0));
                    }
                }
                else {
                    NSLog(@"This document does not contain any logical structure.");
                }
            }
            NSLog(@"Done 1.");

            NSLog(@"____________________________________________________________");
            NSLog(@"Sample 2 - Get parent logical structure elements from");
            NSLog(@"layout elements.");
            {
                PTElementReader *reader = [[PTElementReader alloc] init];
            PTPageIterator *itr;
                for (itr = [doc GetPageIterator: 1]; [itr HasNext]; [itr Next]) {
                    [reader Begin: [itr Current]];
                    NSLog(@"%@", ProcessLogicalStructureTestElements(reader));
                    [reader End];
                }
            }
            NSLog(@"Done 2.");

            NSLog(@"____________________________________________________________");
            NSLog(@"Sample 3 - 'XML style' extraction of PDF logical structure and page content.");
            {
                NSMutableDictionary *mcid_doc_map = [[NSMutableDictionary alloc] init];
                PTElementReader *reader = [[PTElementReader alloc] init];
                PTPageIterator *itr;
                for (itr = [doc GetPageIterator: 1]; [itr HasNext]; [itr Next]) {
                    [reader Begin: [itr Current]];
                    NSMutableDictionary *arr = [[NSMutableDictionary alloc] init];
                id key = @([[itr Current] GetIndex]);
                mcid_doc_map[key] = arr;
                    ProcessLogicalStructureTestElements2(reader, mcid_doc_map[key]);
                    [reader End];
                }

                PTSTree *tree = [doc GetStructTree];
                if ([tree IsValid]) {
                    int i;
                    for (i=0; i<[tree GetNumKids]; ++i) {
                        NSLog(@"%@", ProcessStructElement2([tree GetKid: i], mcid_doc_map, 0));
                    }
                }
            }
            NSLog(@"Done 3.");
            [doc SaveToFile: @"../../TestFiles/Output/LogicalStructure.pdf" flags: e_ptlinearized];
        }
        @catch(NSException *e) 
        {
            NSLog(@"%@", e.reason);
            ret = 1;
        }
        [PTPDFNet Terminate: 0];
        return ret;
    }
}

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