这个答案中的代码旨在优化类似于OP示例文档的文档,即包含完全相同、完全嵌入字体的副本。它不会合并仅几乎相同的对象,例如将同一字体的多个子集合并为一个联合子集。
在回答的评论过程中,很明显OP的PDF文件中的重复字体实际上是源字体文件的完全副本。要合并这样的重复对象,必须收集文档的复杂对象(数组、字典、流),将它们彼此比较,然后合并重复项。
由于实际上对文档的所有复杂对象进行成对比较可能需要太长时间(特别是在大型文档的情况下),因此以下代码计算了这些对象的哈希值,并仅比较具有相同哈希值的对象。
为了合并重复项,该代码选择其中一个重复项,并用所选择的引用替换任何其他重复项的引用,从文档对象池中删除其他重复项。为了更有效地执行此操作,该代码最初不仅收集所有复杂对象,而且还收集每个对象的所有引用。
优化代码
这是调用以优化PDDocument
的方法:
public void optimize(PDDocument pdDocument) throws IOException {
Map<COSBase, Collection<Reference>> complexObjects = findComplexObjects(pdDocument);
for (int pass = 0; ; pass++) {
int merges = mergeDuplicates(complexObjects);
if (merges <= 0) {
System.out.printf("Pass %d - No merged objects\n\n", pass);
break;
}
System.out.printf("Pass %d - Merged objects: %d\n\n", pass, merges);
}
}
(正在测试的OptimizeAfterMerge方法)
优化需要多次循环,因为某些对象的相等性只有在它们所引用的重复对象已合并之后才能被识别。
以下辅助方法和类收集PDF的复杂对象及其各自的引用:
Map<COSBase, Collection<Reference>> findComplexObjects(PDDocument pdDocument) {
COSDictionary catalogDictionary = pdDocument.getDocumentCatalog().getCOSObject();
Map<COSBase, Collection<Reference>> incomingReferences = new HashMap<>();
incomingReferences.put(catalogDictionary, new ArrayList<>());
Set<COSBase> lastPass = Collections.<COSBase>singleton(catalogDictionary);
Set<COSBase> thisPass = new HashSet<>();
while(!lastPass.isEmpty()) {
for (COSBase object : lastPass) {
if (object instanceof COSArray) {
COSArray array = (COSArray) object;
for (int i = 0; i < array.size(); i++) {
addTarget(new ArrayReference(array, i), incomingReferences, thisPass);
}
} else if (object instanceof COSDictionary) {
COSDictionary dictionary = (COSDictionary) object;
for (COSName key : dictionary.keySet()) {
addTarget(new DictionaryReference(dictionary, key), incomingReferences, thisPass);
}
}
}
lastPass = thisPass;
thisPass = new HashSet<>();
}
return incomingReferences;
}
void addTarget(Reference reference, Map<COSBase, Collection<Reference>> incomingReferences, Set<COSBase> thisPass) {
COSBase object = reference.getTo();
if (object instanceof COSArray || object instanceof COSDictionary) {
Collection<Reference> incoming = incomingReferences.get(object);
if (incoming == null) {
incoming = new ArrayList<>();
incomingReferences.put(object, incoming);
thisPass.add(object);
}
incoming.add(reference);
}
}
(OptimizeAfterMerge辅助方法findComplexObjects
和addTarget
)
interface Reference {
public COSBase getFrom();
public COSBase getTo();
public void setTo(COSBase to);
}
static class ArrayReference implements Reference {
public ArrayReference(COSArray array, int index) {
this.from = array;
this.index = index;
}
@Override
public COSBase getFrom() {
return from;
}
@Override
public COSBase getTo() {
return resolve(from.get(index));
}
@Override
public void setTo(COSBase to) {
from.set(index, to);
}
final COSArray from;
final int index;
}
static class DictionaryReference implements Reference {
public DictionaryReference(COSDictionary dictionary, COSName key) {
this.from = dictionary;
this.key = key;
}
@Override
public COSBase getFrom() {
return from;
}
@Override
public COSBase getTo() {
return resolve(from.getDictionaryObject(key));
}
@Override
public void setTo(COSBase to) {
from.setItem(key, to);
}
final COSDictionary from;
final COSName key;
}
(OptimizeAfterMerge 辅助接口 Reference
,实现为 ArrayReference
和 DictionaryReference
)
以下辅助方法和类最终确定并合并了重复项:
int mergeDuplicates(Map<COSBase, Collection<Reference>> complexObjects) throws IOException {
List<HashOfCOSBase> hashes = new ArrayList<>(complexObjects.size());
for (COSBase object : complexObjects.keySet()) {
hashes.add(new HashOfCOSBase(object));
}
Collections.sort(hashes);
int removedDuplicates = 0;
if (!hashes.isEmpty()) {
int runStart = 0;
int runHash = hashes.get(0).hash;
for (int i = 1; i < hashes.size(); i++) {
int hash = hashes.get(i).hash;
if (hash != runHash) {
int runSize = i - runStart;
if (runSize != 1) {
System.out.printf("Equal hash %d for %d elements.\n", runHash, runSize);
removedDuplicates += mergeRun(complexObjects, hashes.subList(runStart, i));
}
runHash = hash;
runStart = i;
}
}
int runSize = hashes.size() - runStart;
if (runSize != 1) {
System.out.printf("Equal hash %d for %d elements.\n", runHash, runSize);
removedDuplicates += mergeRun(complexObjects, hashes.subList(runStart, hashes.size()));
}
}
return removedDuplicates;
}
int mergeRun(Map<COSBase, Collection<Reference>> complexObjects, List<HashOfCOSBase> run) {
int removedDuplicates = 0;
List<List<COSBase>> duplicateSets = new ArrayList<>();
for (HashOfCOSBase entry : run) {
COSBase element = entry.object;
for (List<COSBase> duplicateSet : duplicateSets) {
if (equals(element, duplicateSet.get(0))) {
duplicateSet.add(element);
element = null;
break;
}
}
if (element != null) {
List<COSBase> duplicateSet = new ArrayList<>();
duplicateSet.add(element);
duplicateSets.add(duplicateSet);
}
}
System.out.printf("Identified %d set(s) of identical objects in run.\n", duplicateSets.size());
for (List<COSBase> duplicateSet : duplicateSets) {
if (duplicateSet.size() > 1) {
COSBase surviver = duplicateSet.remove(0);
Collection<Reference> surviverReferences = complexObjects.get(surviver);
for (COSBase object : duplicateSet) {
Collection<Reference> references = complexObjects.get(object);
for (Reference reference : references) {
reference.setTo(surviver);
surviverReferences.add(reference);
}
complexObjects.remove(object);
removedDuplicates++;
}
surviver.setDirect(false);
}
}
return removedDuplicates;
}
boolean equals(COSBase a, COSBase b) {
if (a instanceof COSArray) {
if (b instanceof COSArray) {
COSArray aArray = (COSArray) a;
COSArray bArray = (COSArray) b;
if (aArray.size() == bArray.size()) {
for (int i=0; i < aArray.size(); i++) {
if (!resolve(aArray.get(i)).equals(resolve(bArray.get(i))))
return false;
}
return true;
}
}
} else if (a instanceof COSDictionary) {
if (b instanceof COSDictionary) {
COSDictionary aDict = (COSDictionary) a;
COSDictionary bDict = (COSDictionary) b;
Set<COSName> keys = aDict.keySet();
if (keys.equals(bDict.keySet())) {
for (COSName key : keys) {
if (!resolve(aDict.getItem(key)).equals(bDict.getItem(key)))
return false;
}
return true;
}
}
}
return false;
}
static COSBase resolve(COSBase object) {
while (object instanceof COSObject)
object = ((COSObject)object).getObject();
return object;
}
(OptimizeAfterMerge 辅助方法包括 mergeDuplicates
、mergeRun
、equals
和 resolve
)
static class HashOfCOSBase implements Comparable<HashOfCOSBase> {
public HashOfCOSBase(COSBase object) throws IOException {
this.object = object;
this.hash = calculateHash(object);
}
int calculateHash(COSBase object) throws IOException {
if (object instanceof COSArray) {
int result = 1;
for (COSBase member : (COSArray)object)
result = 31 * result + member.hashCode();
return result;
} else if (object instanceof COSDictionary) {
int result = 3;
for (Map.Entry<COSName, COSBase> entry : ((COSDictionary)object).entrySet())
result += entry.hashCode();
if (object instanceof COSStream) {
try ( InputStream data = ((COSStream)object).createRawInputStream() ) {
MessageDigest md = MessageDigest.getInstance("MD5");
byte[] buffer = new byte[8192];
int bytesRead = 0;
while((bytesRead = data.read(buffer)) >= 0)
md.update(buffer, 0, bytesRead);
result = 31 * result + Arrays.hashCode(md.digest());
} catch (NoSuchAlgorithmException e) {
throw new IOException(e);
}
}
return result;
} else {
throw new IllegalArgumentException(String.format("Unknown complex COSBase type %s", object.getClass().getName()));
}
}
final COSBase object;
final int hash;
@Override
public int compareTo(HashOfCOSBase o) {
int result = Integer.compare(hash, o.hash);
if (result == 0)
result = Integer.compare(hashCode(), o.hashCode());
return result;
}
}
(OptimizeAfterMerge类中的HashOfCOSBase
辅助类)
将代码应用于原帖示例文档
原帖示例文档大小约为6.5MB。可以按照以下方式应用上述代码:
PDDocument pdDocument = PDDocument.load(SOURCE);
optimize(pdDocument);
pdDocument.save(RESULT);
结果是一个小于 700 KB 的 PDF 文档且看起来是完整的。(如果有什么遗漏,请告诉我,我会尝试修复。)
警告
一方面,该优化程序将无法识别所有相同的重复项。特别是在出现循环引用的情况下,对象的重复圆圈不会被识别,因为代码仅在它们的内容相同的情况下才会识别重复项,而这通常在重复对象的圆圈中不会发生。
另一方面,在某些情况下,该优化程序可能已经过于热心,因为某些重复项可能需要作为单独的对象存在,以便 PDF 查看器将每个实例视为单独的实体。
此外,该程序会触及文件中的各种对象,甚至包括定义 PDF 内部结构的那些对象,但它不会尝试更新任何管理此结构的 PDFBox 类(PDDocument
、PDDocumentCatalog
、PDAcroForm
等)。因此,请仅将此程序应用于刚加载的未修改的 PDDocument
实例,并立即保存文档,以免出现任何未完成的更改损坏整个文档。