Add methods for copying form fields

ChangeLog

@@ -1,5 +1,11 @@
 2021-02-22  Jay Berkenbilt  <ejb@ql.org>
 
+	* Add QPDFAcroFormDocumentHelper::copyFieldsFromForeignPage to
+	copy form fields from a foreign page into the current file.
+
+	* Add QPDFFormFieldObjectHelper::getTopLevelField to get the
+	top-level field for a given form field.
+
 	* Update pdf-overlay-page example to include copying of
 	annotations.
 

include/qpdf/QPDFAcroFormDocumentHelper.hh

@@ -140,6 +140,11 @@ class QPDFAcroFormDocumentHelper: public QPDFDocumentHelper
     std::vector<QPDFAnnotationObjectHelper>
     getWidgetAnnotationsForPage(QPDFPageObjectHelper);
 
+    // Return form fields for a page.
+    QPDF_DLL
+    std::vector<QPDFFormFieldObjectHelper>
+    getFormFieldsForPage(QPDFPageObjectHelper);
+
     // Return the terminal field that is associated with this
     // annotation. If the annotation dictionary is merged with the
     // field dictionary, the underlying object will be the same, but
@@ -204,6 +209,13 @@ class QPDFAcroFormDocumentHelper: public QPDFDocumentHelper
         QPDF* from_qpdf = nullptr,
         QPDFAcroFormDocumentHelper* from_afdh = nullptr);
 
+    // Copy form fields from a page in a different QPDF object to this
+    // QPDF.
+    QPDF_DLL
+    void copyFieldsFromForeignPage(
+        QPDFPageObjectHelper foreign_page,
+        QPDFAcroFormDocumentHelper& foreign_afdh);
+
   private:
     void analyze();
     void traverseField(QPDFObjectHandle field,

include/qpdf/QPDFFormFieldObjectHelper.hh

@@ -54,6 +54,13 @@ class QPDFFormFieldObjectHelper: public QPDFObjectHelper
     QPDF_DLL
     QPDFFormFieldObjectHelper getParent();
 
+    // Return the top-level field for this field. Typically this will
+    // be the field itself or its parent. If is_different is provided,
+    // it is set to true if the top-level field is different from the
+    // field itself; otherwise it is set to false.
+    QPDF_DLL
+    QPDFFormFieldObjectHelper getTopLevelField(bool* is_different = nullptr);
+
     // Get a field value, possibly inheriting the value from an
     // ancestor node.
     QPDF_DLL

libqpdf/QPDFAcroFormDocumentHelper.cc

@@ -132,6 +132,23 @@ QPDFAcroFormDocumentHelper::getWidgetAnnotationsForPage(QPDFPageObjectHelper h)
     return h.getAnnotations("/Widget");
 }
 
+std::vector<QPDFFormFieldObjectHelper>
+QPDFAcroFormDocumentHelper::getFormFieldsForPage(QPDFPageObjectHelper ph)
+{
+    std::vector<QPDFFormFieldObjectHelper> result;
+    auto widget_annotations = getWidgetAnnotationsForPage(ph);
+    for (auto annot: widget_annotations)
+    {
+        auto field = getFieldForAnnotation(annot);
+        field = field.getTopLevelField();
+        if (field.getObjectHandle().isDictionary())
+        {
+            result.push_back(field);
+        }
+    }
+    return result;
+}
+
 QPDFFormFieldObjectHelper
 QPDFAcroFormDocumentHelper::getFieldForAnnotation(QPDFAnnotationObjectHelper h)
 {
@@ -501,19 +518,8 @@ QPDFAcroFormDocumentHelper::transformAnnotations(
             // annotation and field separately in this case.
             have_field = true;
             // Find the top-level field. It may be the field itself.
-            top_field = ffield_oh;
+            top_field = ffield.getTopLevelField(
-            std::set<QPDFObjGen> seen;
+                &have_parent).getObjectHandle();
-            while (! top_field.getKey("/Parent").isNull())
-            {
-                top_field = top_field.getKey("/Parent");
-                have_parent = true;
-                auto og = top_field.getObjGen();
-                if (seen.count(og))
-                {
-                    break;
-                }
-                seen.insert(og);
-            }
             if (foreign)
             {
                 // copyForeignObject returns the same value if called
@@ -537,7 +543,7 @@ QPDFAcroFormDocumentHelper::transformAnnotations(
             {
                 queue.push_back(top_field);
             }
-            seen.clear();
+            std::set<QPDFObjGen> seen;
             while (! queue.empty())
             {
                 QPDFObjectHandle obj = queue.front();
@@ -664,3 +670,16 @@ QPDFAcroFormDocumentHelper::transformAnnotations(
             "/Rect", QPDFObjectHandle::newFromRectangle(rect));
     }
 }
+
+void
+QPDFAcroFormDocumentHelper::copyFieldsFromForeignPage(
+    QPDFPageObjectHelper foreign_page,
+    QPDFAcroFormDocumentHelper& foreign_afdh)
+{
+    for (auto field: foreign_afdh.getFormFieldsForPage(foreign_page))
+    {
+        auto new_field = this->qpdf.copyForeignObject(
+            field.getObjectHandle());
+        addFormField(new_field);
+    }
+}

libqpdf/QPDFFormFieldObjectHelper.cc

@@ -39,6 +39,29 @@ QPDFFormFieldObjectHelper::getParent()
     return this->oh.getKey("/Parent"); // may be null
 }
 
+QPDFFormFieldObjectHelper
+QPDFFormFieldObjectHelper::getTopLevelField(bool* is_different)
+{
+    auto top_field = this->oh;
+    std::set<QPDFObjGen> seen;
+    while (top_field.isDictionary() &&
+           (! top_field.getKey("/Parent").isNull()))
+    {
+        top_field = top_field.getKey("/Parent");
+        if (is_different)
+        {
+            *is_different = true;
+        }
+        auto og = top_field.getObjGen();
+        if (seen.count(og))
+        {
+            break;
+        }
+        seen.insert(og);
+    }
+    return QPDFFormFieldObjectHelper(top_field);
+}
+
 QPDFObjectHandle
 QPDFFormFieldObjectHelper::getInheritableFieldValue(std::string const& name)
 {