Handling UTF16-BE Strings In Pdfio Dictionaries

by Alex Johnson 48 views

Introduction

This article addresses the challenge of handling UTF16-BE (Big Endian) encoded strings within dictionaries when working with the pdfio library. The user encountered an issue where strings in a PDF dictionary, encoded in UTF16-BE, were not being correctly interpreted by pdfioDictGetString. Instead of displaying the actual characters, the function returned a two-character string representing the hexadecimal values (e.g., "0xFE 0xFF"). This problem originates from the _pdfioValueRead function and pdfioStringCreate function, which are designed to handle ASCII strings, not UTF16-BE.

We'll delve into the details of the problem, analyze the PDF structure involved, and explore potential solutions to correctly handle UTF16-BE encoded strings in pdfio dictionaries.

Understanding the Problem: UTF16-BE Encoding in PDF Dictionaries

To effectively address the issue, it’s essential to understand the context in which UTF16-BE encoding is used within PDF files. PDF documents can embed strings using various encodings, including the standard ASCII and the more comprehensive Unicode encodings like UTF16-BE. UTF16-BE is particularly useful for representing a wide range of characters, including those from non-Latin alphabets.

In the provided example, the PDF object (1 0 obj) contains a dictionary with entries like /Producer, /Title, /Author, and /Creator. The values associated with these keys are strings encoded in UTF16-BE. The encoding is signaled by the byte order mark \376\377 (which is equivalent to hexadecimal FE FF), indicating that the characters are encoded in UTF-16 with big-endian byte order. Each character is represented by two bytes.

For instance, the string \376\377\000P\000D\000F\000C\000r\000e\000a\000t\000o\000r corresponds to "PDFCreator" in UTF16-BE. The issue arises because the pdfio library's string handling functions, specifically _pdfioValueRead and pdfioStringCreate, are primarily designed to process ASCII strings, which use a single byte per character. When encountering a UTF16-BE encoded string, these functions interpret each byte as an individual ASCII character, leading to the incorrect representation.

The core problem lies in the mismatch between the encoding of the string in the PDF and the decoding capabilities of the pdfio library. To resolve this, we need to modify or extend pdfio to correctly interpret UTF16-BE encoded strings.

Analyzing the Code: _pdfioValueRead and pdfioStringCreate

To implement a solution, we need to examine the relevant code sections within the pdfio library. The user has identified _pdfioValueRead and pdfioStringCreate as the key functions involved in the string processing.

_pdfioValueRead is likely responsible for reading the value of a dictionary entry from the PDF data stream. When it encounters a string, it calls pdfioStringCreate to construct a string object from the raw bytes. If _pdfioValueRead does not recognize the UTF16-BE byte order mark (FE FF), it will treat the bytes as individual ASCII characters.

pdfioStringCreate is then responsible for creating a string object from the provided bytes. If this function assumes ASCII encoding, it will not correctly interpret the two-byte characters of UTF16-BE. This results in each byte being treated as a separate character, leading to the "0xFE 0xFF" representation the user observed.

Therefore, the solution requires modifications to either or both of these functions to handle UTF16-BE encoding. This might involve:

  1. Detecting the UTF16-BE byte order mark: Modifying _pdfioValueRead to check for the presence of FE FF at the beginning of a string. If detected, the function should treat the string as UTF16-BE encoded.
  2. Decoding UTF16-BE: Implementing a UTF16-BE decoding routine within pdfioStringCreate or a helper function. This routine would convert the two-byte sequences into Unicode characters.
  3. Creating Unicode strings: Ensuring that the resulting string object can store Unicode characters, rather than being limited to ASCII.

By making these changes, pdfio can correctly interpret and represent UTF16-BE encoded strings from PDF dictionaries.

Potential Solutions and Implementation Guidance

Based on the analysis, here are a few potential solutions to correctly handle UTF16-BE strings in pdfio dictionaries:

1. Modify _pdfioValueRead to Detect UTF16-BE

The first step is to modify the _pdfioValueRead function to recognize the UTF16-BE byte order mark. This involves checking for the presence of the byte sequence 0xFE 0xFF at the beginning of a string value. If this sequence is found, it indicates that the string is encoded in UTF16-BE.

Here’s a conceptual outline of the changes needed:

  1. Read the first two bytes: When _pdfioValueRead encounters a string, it should read the first two bytes of the string value.
  2. Check for the byte order mark: Compare these bytes to 0xFE 0xFF. If they match, the string is UTF16-BE encoded.
  3. Handle UTF16-BE strings differently: If the byte order mark is present, the function should proceed with UTF16-BE decoding. Otherwise, it can continue with the default ASCII handling.

2. Implement UTF16-BE Decoding

Once UTF16-BE is detected, the next step is to decode the string. This involves converting the two-byte sequences into Unicode characters. A new function or a modified pdfioStringCreate can handle this decoding.

Here’s a conceptual outline of the UTF16-BE decoding process:

  1. Read two bytes at a time: Read the string two bytes at a time, as each character is represented by two bytes in UTF16-BE.
  2. Convert to Unicode code point: Combine the two bytes into a single 16-bit value. This value represents the Unicode code point of the character.
  3. Append to Unicode string: Append the Unicode character to a string buffer. This buffer should be capable of storing Unicode characters (e.g., using wchar_t in C/C++ or a similar Unicode string type in other languages).
  4. Repeat: Continue this process until the entire UTF16-BE string has been processed.

3. Integrate Decoding into pdfioStringCreate

To keep the code organized, the UTF16-BE decoding logic can be integrated into pdfioStringCreate. This function is already responsible for creating string objects, so it’s a natural place to handle different string encodings.

Here’s how pdfioStringCreate can be modified:

  1. Check for UTF16-BE flag: Add a parameter or a flag to pdfioStringCreate to indicate whether the string is UTF16-BE encoded.
  2. Decode if necessary: If the flag is set, call the UTF16-BE decoding routine. Otherwise, proceed with the default ASCII string creation.

4. Example Code Snippet (Conceptual)

Here’s a conceptual code snippet illustrating how the UTF16-BE detection and decoding might look in C:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// Conceptual function to decode UTF16-BE string
void decodeUTF16BE(const unsigned char *utf16be_str, size_t utf16be_len, wchar_t *unicode_str, size_t unicode_max_len) {
    if (utf16be_len < 2 || unicode_max_len == 0) {
        return;
    }

    size_t unicode_idx = 0;
    for (size_t i = 0; i < utf16be_len - 1 && unicode_idx < unicode_max_len; i += 2) {
        // Combine two bytes into a 16-bit value (Unicode code point)
        wchar_t code_point = (utf16be_str[i] << 8) | utf16be_str[i + 1];
        unicode_str[unicode_idx++] = code_point;
    }

    unicode_str[unicode_idx] = L'\0'; // Null-terminate the Unicode string
}

int main() {
    unsigned char utf16be_bytes[] = {0x00, 0x50, 0x00, 0x44, 0x00, 0x46}; // UTF16-BE for "PDF"
    size_t utf16be_len = sizeof(utf16be_bytes);
    wchar_t unicode_string[100]; // Buffer to hold the decoded Unicode string

    decodeUTF16BE(utf16be_bytes, utf16be_len, unicode_string, 100);

    wprintf(L"Decoded Unicode string: %ls\n", unicode_string); // Output the Unicode string

    return 0;
}

This code snippet demonstrates the basic idea of UTF16-BE decoding. It reads two bytes at a time, combines them into a Unicode code point, and appends the code point to a Unicode string. This is a simplified example and would need to be adapted to fit within the pdfio library's structure and conventions.

5. Handling Byte Order Mark

When processing UTF16-BE strings, it’s important to handle the byte order mark (BOM) correctly. The BOM (0xFE 0xFF) indicates the encoding and byte order of the string. While the BOM itself is not part of the actual string content, it’s used to ensure that the string is decoded correctly.

Here’s how to handle the BOM:

  1. Detect the BOM: As mentioned earlier, _pdfioValueRead should detect the BOM at the beginning of the string.
  2. Skip the BOM: After detecting the BOM, the decoding process should skip these two bytes and start decoding from the third byte onwards. The BOM is not part of the string content and should not be included in the decoded string.

By correctly handling the BOM, pdfio can accurately decode UTF16-BE strings without including the BOM characters in the output.

6. Testing and Validation

After implementing the changes, it’s essential to test and validate the solution. This involves creating PDF files with UTF16-BE encoded strings in the dictionary and verifying that pdfio correctly decodes and displays these strings.

Here are some testing strategies:

  1. Create test PDFs: Generate PDF files with UTF16-BE encoded strings in various dictionary entries (e.g., Title, Author, Subject). Use different characters and languages to ensure comprehensive coverage.
  2. Run tests: Write test cases that use pdfio to read the dictionary entries from the test PDFs and verify that the decoded strings match the expected values.
  3. Check edge cases: Test edge cases, such as empty strings, strings with only the BOM, and strings with invalid UTF16-BE sequences. Ensure that pdfio handles these cases gracefully.

By thoroughly testing the solution, you can ensure that pdfio correctly handles UTF16-BE strings in all scenarios.

Conclusion

Handling UTF16-BE encoded strings in pdfio dictionaries requires careful modifications to the library’s string processing functions. By detecting the UTF16-BE byte order mark, implementing a UTF16-BE decoding routine, and integrating it into pdfioStringCreate, the library can correctly interpret and represent these strings. Remember to handle the byte order mark appropriately and thoroughly test the solution to ensure its robustness.

By following these guidelines, you can enhance pdfio to handle a wider range of PDF documents and string encodings, making it a more versatile tool for PDF processing.

For further information on PDF standards and character encoding, you can refer to the Adobe PDF Specification. This document provides detailed information on the PDF file format and its various features.