expanddecider.patch

diff --git a/xapian-bindings/xapian-headers.i b/xapian-bindings/xapian-headers.i
index 85ec281..c0ee37a 100644
--- a/xapian-bindings/xapian-headers.i
+++ b/xapian-bindings/xapian-headers.i
@@ -316,6 +316,7 @@ SUBCLASSABLE(Xapian, ExpandDecider)
  * array of strings (or whatever the equivalent is in the target language).
  */
 %ignore Xapian::ExpandDeciderFilterTerms;
+%ignore Xapian::ExpandDeciderFilterPrefix;
 %include <xapian/expanddecider.h>
 
 SUBCLASSABLE(Xapian, KeyMaker)
diff --git a/xapian-core/api/expanddecider.cc b/xapian-core/api/expanddecider.cc
index 35ac754..a8adc26 100644
--- a/xapian-core/api/expanddecider.cc
+++ b/xapian-core/api/expanddecider.cc
@@ -21,6 +21,7 @@
 #include <config.h>
 
 #include <xapian/expanddecider.h>
+#include "stringutils.h"
 
 using namespace std;
 
@@ -45,4 +46,10 @@ ExpandDeciderFilterTerms::operator()(const string &term) const
     return i == rejects.end();
 }
 
+bool    
+ExpandDeciderFilterPrefix::operator()(const string &term) const
+{
+    return startswith(term, prefix);
+}
+
 }
diff --git a/xapian-core/include/xapian/expanddecider.h b/xapian-core/include/xapian/expanddecider.h
index d2d0a19..2ba93f7 100644
--- a/xapian-core/include/xapian/expanddecider.h
+++ b/xapian-core/include/xapian/expanddecider.h
@@ -96,6 +96,24 @@ class XAPIAN_VISIBILITY_DEFAULT ExpandDeciderFilterTerms : public ExpandDecider
     virtual bool operator()(const std::string &term) const;
 };
 
+/** ExpandDecider subclass which restrict terms to a particular prefix
+ *
+ *  ExpandDeciderFilterPrefix provides an easy way to choose terms with a  
+ *  particular prefix when generating an ESet.
+ */
+class XAPIAN_VISIBILITY_DEFAULT ExpandDeciderFilterPrefix : public ExpandDecider {
+    std::string prefix;
+
+  public:
+    /** The parameter specify the prefix of terms to be retained
+     *  @param prefix_   restrict terms to the particular prefix_ 
+     */
+    ExpandDeciderFilterPrefix(const std::string &prefix_)
+	: prefix(prefix_) { }   
+
+    virtual bool operator() (const std::string &term) const;
+};
+
 }
 
 #endif // XAPIAN_INCLUDED_EXPANDDECIDER_H
diff --git a/xapian-core/tests/api_anydb.cc b/xapian-core/tests/api_anydb.cc
index 3e09e13..a93e4c0 100644
--- a/xapian-core/tests/api_anydb.cc
+++ b/xapian-core/tests/api_anydb.cc
@@ -639,6 +639,57 @@ DEFINE_TESTCASE(expandfunctor1, backend) {
     return true;
 }
 
+DEFINE_TESTCASE(expanddeciderfilterprefix2, backend) {
+    Xapian::Enquire enquire(get_database("apitest_simpledata"));
+    enquire.set_query(Xapian::Query("this"));
+
+    Xapian::MSet mymset = enquire.get_mset(0, 10);
+    TEST(mymset.size() >= 2);
+
+    Xapian::RSet myrset;
+    Xapian::MSetIterator i = mymset.begin();
+    myrset.add_document(*i);
+    myrset.add_document(*(++i));
+
+    Xapian::ESet myeset_orig = enquire.get_eset(1000, myrset);
+    unsigned int neweset_size = 0;
+
+    //choose the first char in the first term as prefix 
+    Xapian::ESetIterator j = myeset_orig.begin();
+    TEST(myeset_orig.size() >= 1);
+    string prefix = (*j).substr(0, 1);
+    Xapian::ExpandDeciderFilterPrefix myfunctor(prefix);
+
+    for ( ; j != myeset_orig.end(); ++j) {
+        if (myfunctor(*j)) neweset_size++;
+    }
+    Xapian::ESet myeset = enquire.get_eset(neweset_size, myrset, &myfunctor);
+
+    Xapian::ESetIterator orig = myeset_orig.begin();
+    Xapian::ESetIterator filt = myeset.begin();
+    for (; orig != myeset_orig.end() && filt != myeset.end(); ++orig, ++filt) {
+	// skip over items that shouldn't be in myeset
+	while (orig != myeset_orig.end() && !myfunctor(*orig)) {
+	    ++orig;
+	}
+
+	TEST_AND_EXPLAIN(*orig == *filt &&
+			 orig.get_weight() == filt.get_weight(),
+			 "Mismatch in items " << *orig << " vs. " << *filt
+			 << " after filtering");
+    }
+
+    while (orig != myeset_orig.end() && !myfunctor(*orig)) {
+	++orig;
+    }
+
+    TEST_EQUAL(orig, myeset_orig.end());
+    TEST_AND_EXPLAIN(filt == myeset.end(),
+		     "Extra items in the filtered eset.");
+
+    return true;
+}
+
 // tests the percent cutoff option
 DEFINE_TESTCASE(pctcutoff1, backend) {
     Xapian::Enquire enquire(get_database("apitest_simpledata"));
diff --git a/xapian-core/tests/api_nodb.cc b/xapian-core/tests/api_nodb.cc
index d3f35ba..4a20630 100644
--- a/xapian-core/tests/api_nodb.cc
+++ b/xapian-core/tests/api_nodb.cc
@@ -433,3 +433,28 @@ DEFINE_TESTCASE(emptymset1, !backend) {
 		   emptymset.get_termfreq("foo"));
     return true;
 }
+
+DEFINE_TESTCASE(expanddeciderfilterprefix1, !backend) {
+    vector<string> myvec_orig;
+    myvec_orig.push_back("one");
+    myvec_orig.push_back("two");
+    myvec_orig.push_back("t");
+    myvec_orig.push_back("");
+    myvec_orig.push_back("Two");
+    myvec_orig.push_back("twitter");
+
+    string prefix = "tw";
+    vector<string> myvec;
+    Xapian::ExpandDeciderFilterPrefix decider(prefix);
+    vector<string>::iterator it = myvec_orig.begin();
+    for (; it != myvec_orig.end(); ++it) {
+        if (decider(*it)) myvec.push_back(*it);
+    }
+
+    TEST_EQUAL(myvec.size(), 2);
+    TEST_EQUAL(myvec[0], "two");
+    TEST_EQUAL(myvec[1], "twitter");
+
+    return true;
+}
+