From: derek <derekwbarnett@gmail.com>
Date: Mon, 3 Oct 2011 20:33:44 +0000 (-0400)
Subject: MultiReader (&MultiMerger) now using Algorithms::Sort objects
X-Git-Url: https://git.donarmstrong.com/?p=bamtools.git;a=commitdiff_plain;h=daf64972794dbbdae4bea6a0495578708c86363b

MultiReader (&MultiMerger) now using Algorithms::Sort objects
---

diff --git a/src/api/BamAlgorithms.h b/src/api/BamAlgorithms.h
index c62e68e..8a8e16b 100644
--- a/src/api/BamAlgorithms.h
+++ b/src/api/BamAlgorithms.h
@@ -12,133 +12,7 @@
 #ifndef BAMALGORITHMS_H
 #define BAMALGORITHMS_H
 
-#include <api/api_global.h>
-#include <api/BamAlignment.h>
-#include <api/BamReader.h>
-#include <api/BamMultiReader.h>
-#include <algorithm>
-#include <functional>
-#include <string>
-#include <vector>
+#include <api/algorithms/Sort.h>
 
-namespace BamTools {
-namespace Algorithms {
-
-// -------------------------------------------------------
-// Built-in function objects for comparing BamAlignments
-
-typedef std::binary_function<BamAlignment, BamAlignment, bool> BamAlignmentComparer;
-
-// Algorithms::SortByName<Compare>
-// compare alignments by name (default comparison is std::less<std::string> )
-template<template <typename> class Compare = std::less>
-struct API_EXPORT SortByName : public BamAlignmentComparer {
-    bool operator()(const BamTools::BamAlignment& lhs,
-                    const BamTools::BamAlignment& rhs)
-    {
-        Compare<std::string> comp;
-        return comp(lhs.Name, rhs.Name);
-    }
-};
-
-// Algorithms::SortByPosition<Compare>
-// compare alignments by position (default comparison is std::less<int>)
-template<template <typename> class Compare = std::less>
-struct API_EXPORT SortByPosition : public BamAlignmentComparer {
-    bool operator()(const BamTools::BamAlignment& lhs,
-                    const BamTools::BamAlignment& rhs)
-    {
-        // force unmapped aligmnents to end
-        if ( lhs.RefID == -1 ) return false;
-        if ( rhs.RefID == -1 ) return true;
-
-        // otherwise compare first on RefID, then position
-        Compare<int32_t> comp;
-        if ( lhs.RefID == rhs.RefID )
-            return comp(lhs.Position, rhs.Position);
-        return comp(lhs.RefID, rhs.RefID);
-    }
-};
-
-// Algorithms::SortByTag<Compare>("XY")
-// compare alignments by tag value (default comparison is std::less<T>)
-// where T is the expected tag type (e.g. RG -> string, NM -> int, etc.)
-template<typename T, template <typename> class Compare = std::less>
-struct API_EXPORT SortByTag : public BamAlignmentComparer {
-
-    // ctor - needed to provide the tag name ("RG", "NM", "Aq", etc)
-    explicit SortByTag(const std::string& tag) : m_tag(tag) { }
-
-    bool operator()(const BamTools::BamAlignment& lhs,
-                    const BamTools::BamAlignment& rhs)
-    {
-        // force alignments without tag to end
-        T lhsTagValue;
-        T rhsTagValue;
-        if ( !lhs.GetTag(m_tag, lhsTagValue) ) return false;
-        if ( !rhs.GetTag(m_tag, rhsTagValue) ) return true;
-
-        // otherwise compare tag values
-        Compare<T> comp;
-        return comp(lhsTagValue, rhsTagValue);
-    }
-
-    private:
-        std::string m_tag;
-};
-
-// Algorithms::Unsorted
-// placeholder comparison object, ignores the alignments' data
-// N.B. - returning false typically retains initial insertion order
-struct API_EXPORT Unsorted : public BamAlignmentComparer {
-    bool operator()(const BamTools::BamAlignment& /*lhs*/,
-                    const BamTools::BamAlignment& /*rhs*/)
-    {
-        return false;
-    }
-};
-
-API_EXPORT template<typename Compare>
-std::vector<BamAlignment> SortReaderRegion(BamReader& reader,
-                                           const BamRegion& region,
-                                           const Compare& comp = Compare())
-{
-    // return empty container if unable to find region
-    if ( !reader.IsOpen() )          return std::vector<BamAlignment>();
-    if ( !reader.SetRegion(region) ) return std::vector<BamAlignment>();
-
-    // iterate through region, grabbing alignments
-    BamAlignment al;
-    std::vector<BamAlignment> results;
-    while ( reader.GetNextAlignment(al) )
-        results.push_back(al);
-
-    // sort & return alignments
-    std::sort(results.begin(), results.end(), comp);
-    return results;
-}
-
-API_EXPORT template<typename Compare>
-std::vector<BamAlignment> SortReaderRegion(BamMultiReader& reader,
-                                           const BamRegion& region,
-                                           const Compare& comp = Compare())
-{
-    // return empty container if unable to find region
-    if ( !reader.HasOpenReaders() )  return std::vector<BamAlignment>();
-    if ( !reader.SetRegion(region) ) return std::vector<BamAlignment>();
-
-    // iterate through region, grabbing alignments
-    BamAlignment al;
-    std::vector<BamAlignment> results;
-    while ( reader.GetNextAlignment(al) )
-        results.push_back(al);
-
-    // sort & return alignments
-    std::sort(results.begin(), results.end(), comp);
-    return results;
-}
-
-} // namespace Algorithms
-} // namespace BamTools
 
 #endif // BAM_ALGORITHMS_H
diff --git a/src/api/BamMultiReader.cpp b/src/api/BamMultiReader.cpp
index 06055df..35e6758 100644
--- a/src/api/BamMultiReader.cpp
+++ b/src/api/BamMultiReader.cpp
@@ -3,13 +3,13 @@
 // Marth Lab, Department of Biology, Boston College
 // All rights reserved.
 // ---------------------------------------------------------------------------
-// Last modified: 15 March 2011 (DB)
+// Last modified: 1 October 2011 (DB)
 // ---------------------------------------------------------------------------
 // Convenience class for reading multiple BAM files.
 //
 // This functionality allows applications to work on very large sets of files
 // without requiring intermediate merge, sort, and index steps for each file
-// subset.  It also improves the performance of our merge system as it
+// subset. It also improves the performance of our merge system as it
 // precludes the need to sort merged files.
 // ***************************************************************************
 
@@ -193,19 +193,6 @@ bool BamMultiReader::HasOpenReaders(void) const {
     return d->HasOpenReaders();
 }
 
-/*! \fn bool BamMultiReader::IsIndexLoaded(void) const
-    \brief Returns \c true if all BAM files have index data available.
-
-    \deprecated Instead use HasIndexes()
-    \cond
-    See explanation in BamReader.cpp for more details on the deprecation decision.
-    \endcond
-*/
-
-bool BamMultiReader::IsIndexLoaded(void) const {
-    return d->HasIndexes();
-}
-
 /*! \fn bool BamMultiReader::Jump(int refID, int position)
     \brief Performs a random-access jump within current BAM files.
 
@@ -305,16 +292,6 @@ bool BamMultiReader::OpenIndexes(const std::vector<std::string>& indexFilenames)
     return d->OpenIndexes(indexFilenames);
 }
 
-/*! \fn void BamMultiReader::PrintFilenames(void) const
-    \brief Convenience method for printing filenames to stdout.
-    \deprecated Doesn't really belong as an API function. Clients should
-                determine how the data is reported.
-    \sa Filenames(), BamReader::GetFilename()
-*/
-void BamMultiReader::PrintFilenames(void) const {
-    d->PrintFilenames();
-}
-
 /*! \fn bool BamMultiReader::Rewind(void)
     \brief Returns the internal file pointers to the beginning of alignment records.
 
@@ -380,17 +357,3 @@ bool BamMultiReader::SetRegion(const int& leftRefID,
     BamRegion region(leftRefID, leftPosition, rightRefID, rightPosition);
     return d->SetRegion(region);
 }
-
-/*! \fn void BamMultiReader::SetSortOrder(const SortOrder& order)
-    \brief Sets the expected sorting order for reading across multiple BAM files.
-
-    Default is BamMultiReader::SortedByPosition.
-
-    The SortOrder determines how the reader determines which alignment is "next"
-    from among its open readers.
-
-    \param order expected sort order
-*/
-void BamMultiReader::SetSortOrder(const SortOrder& order) {
-    d->SetSortOrder(order);
-}
diff --git a/src/api/BamMultiReader.h b/src/api/BamMultiReader.h
index cc49ec8..43ef56a 100644
--- a/src/api/BamMultiReader.h
+++ b/src/api/BamMultiReader.h
@@ -3,7 +3,7 @@
 // Marth Lab, Department of Biology, Boston College
 // All rights reserved.
 // ---------------------------------------------------------------------------
-// Last modified: 15 March 2011 (DB)
+// Last modified: 1 October 2011 (DB)
 // ---------------------------------------------------------------------------
 // Convenience class for reading multiple BAM files.
 // ***************************************************************************
@@ -26,12 +26,6 @@ namespace Internal {
 
 class API_EXPORT BamMultiReader {
 
-    public:
-        enum SortOrder { SortedByPosition = 0
-                       , SortedByReadName
-                       , Unsorted
-                       };
-
     // constructor / destructor
     public:
         BamMultiReader(void);
@@ -74,12 +68,9 @@ class API_EXPORT BamMultiReader {
 
         // retrieves next available alignment
         bool GetNextAlignment(BamAlignment& alignment);
-        // retrieves next available alignmnet (without populating the alignment's string data fields)
+        // retrieves next available alignment (without populating the alignment's string data fields)
         bool GetNextAlignmentCore(BamAlignment& alignment);
 
-        // sets the expected sorting order for reading across multiple BAM files
-        void SetSortOrder(const SortOrder& order);
-
         // ----------------------
         // access auxiliary data
         // ----------------------
@@ -110,13 +101,6 @@ class API_EXPORT BamMultiReader {
         // changes the caching behavior of the index data
         void SetIndexCacheMode(const BamIndex::IndexCacheMode& mode);
 
-    // deprecated methods
-    public:
-        // returns \c true if all BAM files have index data available.
-        bool IsIndexLoaded(void) const;
-        // convenience method for printing filenames to stdout
-        void PrintFilenames(void) const;
-
     // private implementation
     private:
         Internal::BamMultiReaderPrivate* d;
diff --git a/src/api/CMakeLists.txt b/src/api/CMakeLists.txt
index e25c24b..7306f56 100644
--- a/src/api/CMakeLists.txt
+++ b/src/api/CMakeLists.txt
@@ -61,7 +61,7 @@ install( TARGETS BamTools-static ARCHIVE DESTINATION "lib/bamtools")
 include(../ExportHeader.cmake)
 set(ApiIncludeDir "api")
 ExportHeader(APIHeaders api_global.h             ${ApiIncludeDir})
-ExportHeader(APIHeaders BamAlgorithm.h           ${ApiIncludeDir})
+ExportHeader(APIHeaders BamAlgorithms.h           ${ApiIncludeDir})
 ExportHeader(APIHeaders BamAlignment.h           ${ApiIncludeDir})
 ExportHeader(APIHeaders BamAux.h                 ${ApiIncludeDir})
 ExportHeader(APIHeaders BamConstants.h           ${ApiIncludeDir})
@@ -77,3 +77,6 @@ ExportHeader(APIHeaders SamReadGroup.h           ${ApiIncludeDir})
 ExportHeader(APIHeaders SamReadGroupDictionary.h ${ApiIncludeDir})
 ExportHeader(APIHeaders SamSequence.h            ${ApiIncludeDir})
 ExportHeader(APIHeaders SamSequenceDictionary.h  ${ApiIncludeDir})
+
+set(AlgorithmsIncludeDir "api/algorithms")
+ExportHeader(AlgorithmsHeaders algorithms/Sort.h ${AlgorithmsIncludeDir})
diff --git a/src/api/SamReadGroupDictionary.cpp b/src/api/SamReadGroupDictionary.cpp
index 69903ff..af4e341 100644
--- a/src/api/SamReadGroupDictionary.cpp
+++ b/src/api/SamReadGroupDictionary.cpp
@@ -3,7 +3,7 @@
 // Marth Lab, Department of Biology, Boston College
 // All rights reserved.
 // ---------------------------------------------------------------------------
-// Last modified: 18 April 2011 (DB)
+// Last modified: 1 October 2011 (DB)
 // ---------------------------------------------------------------------------
 // Provides methods for operating on a collection of SamReadGroup entries.
 // ***************************************************************************
@@ -65,6 +65,13 @@ void SamReadGroupDictionary::Add(const std::string& readGroupId) {
     Add( SamReadGroup(readGroupId) );
 }
 
+void SamReadGroupDictionary::Add(const SamReadGroupDictionary& readGroups) {
+    SamReadGroupConstIterator rgIter = readGroups.ConstBegin();
+    SamReadGroupConstIterator rgEnd  = readGroups.ConstEnd();
+    for ( ; rgIter != rgEnd; ++rgIter )
+        Add(*rgIter);
+}
+
 /*! \fn void SamReadGroupDictionary::Add(const std::vector<SamReadGroup>& readGroups)
     \brief Adds multiple read groups to the dictionary.
 
diff --git a/src/api/SamReadGroupDictionary.h b/src/api/SamReadGroupDictionary.h
index 8ec40e2..3d5ff2d 100644
--- a/src/api/SamReadGroupDictionary.h
+++ b/src/api/SamReadGroupDictionary.h
@@ -3,7 +3,7 @@
 // Marth Lab, Department of Biology, Boston College
 // All rights reserved.
 // ---------------------------------------------------------------------------
-// Last modified: 18 April 2011 (DB)
+// Last modified: 1 October 2011 (DB)
 // ---------------------------------------------------------------------------
 // Provides methods for operating on a collection of SamReadGroup entries.
 // ***************************************************************************
@@ -37,6 +37,7 @@ class API_EXPORT SamReadGroupDictionary {
         void Add(const std::string& readGroupId);
 
         // adds multiple read groups
+        void Add(const SamReadGroupDictionary& readGroups);
         void Add(const std::vector<SamReadGroup>& readGroups);
         void Add(const std::vector<std::string>& readGroupIds);
 
diff --git a/src/api/SamSequenceDictionary.cpp b/src/api/SamSequenceDictionary.cpp
index 3e5240d..0c92581 100644
--- a/src/api/SamSequenceDictionary.cpp
+++ b/src/api/SamSequenceDictionary.cpp
@@ -3,7 +3,7 @@
 // Marth Lab, Department of Biology, Boston College
 // All rights reserved.
 // ---------------------------------------------------------------------------
-// Last modified: 18 April 2011 (DB)
+// Last modified: 1 October 2011 (DB)
 // ---------------------------------------------------------------------------
 // Provides methods for operating on a collection of SamSequence entries.
 // *************************************************************************
@@ -65,6 +65,13 @@ void SamSequenceDictionary::Add(const std::string& name, const int& length) {
     Add( SamSequence(name, length) );
 }
 
+void SamSequenceDictionary::Add(const SamSequenceDictionary& sequences) {
+    SamSequenceConstIterator seqIter = sequences.ConstBegin();
+    SamSequenceConstIterator seqEnd  = sequences.ConstEnd();
+    for ( ; seqIter != seqEnd; ++seqIter )
+        Add(*seqIter);
+}
+
 /*! \fn void SamSequenceDictionary::Add(const std::vector<SamSequence>& sequences)
     \brief Adds multiple sequences to the dictionary.
 
diff --git a/src/api/SamSequenceDictionary.h b/src/api/SamSequenceDictionary.h
index 1ac7326..d488e14 100644
--- a/src/api/SamSequenceDictionary.h
+++ b/src/api/SamSequenceDictionary.h
@@ -3,7 +3,7 @@
 // Marth Lab, Department of Biology, Boston College
 // All rights reserved.
 // ---------------------------------------------------------------------------
-// Last modified: 18 April 2011
+// Last modified: 1 October 2011
 // ---------------------------------------------------------------------------
 // Provides methods for operating on a collection of SamSequence entries.
 // ***************************************************************************
@@ -38,6 +38,7 @@ class API_EXPORT SamSequenceDictionary {
         void Add(const std::string& name, const int& length);
 
         // adds multiple sequences
+        void Add(const SamSequenceDictionary& sequences);
         void Add(const std::vector<SamSequence>& sequences);
         void Add(const std::map<std::string, int>& sequenceMap);
 
diff --git a/src/api/algorithms/Sort.h b/src/api/algorithms/Sort.h
new file mode 100644
index 0000000..f9d2daf
--- /dev/null
+++ b/src/api/algorithms/Sort.h
@@ -0,0 +1,203 @@
+// ***************************************************************************
+// Sort.h (c) 2009 Derek Barnett
+// Marth Lab, Department of Biology, Boston College
+// All rights reserved.
+// ---------------------------------------------------------------------------
+// Last modified: 29 September 2011 (DB)
+// ---------------------------------------------------------------------------
+// Provides sorting functionality.
+// ***************************************************************************
+
+#ifndef ALGORITHMS_SORT_H
+#define ALGORITHMS_SORT_H
+
+#include <api/api_global.h>
+#include <api/BamAlignment.h>
+#include <api/BamReader.h>
+#include <api/BamMultiReader.h>
+#include <algorithm>
+#include <cassert>
+#include <functional>
+#include <string>
+#include <vector>
+
+namespace BamTools {
+namespace Algorithms {
+
+struct API_EXPORT Sort {
+
+    enum Order { AscendingOrder = 0
+               , DescendingOrder
+               };
+
+    template<typename ElemType>
+    static inline bool sort_helper(const Sort::Order& order,
+                                   const ElemType& lhs,
+                                   const ElemType& rhs)
+    {
+        switch ( order ) {
+            case ( Sort::AscendingOrder  ) : { std::less<ElemType> comp;    return comp(lhs, rhs); }
+            case ( Sort::DescendingOrder ) : { std::greater<ElemType> comp; return comp(lhs, rhs); }
+            default : assert(false);
+        }
+        return false; // <-- unreachable
+    }
+
+    typedef std::binary_function<BamAlignment, BamAlignment, bool> AlignmentSortBase;
+
+    // compare alignments by name
+    struct ByName : public AlignmentSortBase {
+
+        ByName(const Sort::Order& order = Sort::AscendingOrder)
+            : m_order(order)
+        { }
+
+        bool operator()(const BamTools::BamAlignment& lhs,
+                        const BamTools::BamAlignment& rhs)
+        {
+            return sort_helper(m_order, lhs.Name, rhs.Name);
+        }
+
+        static inline bool UsesCharData(void) { return true; }
+
+        private:
+            const Sort::Order& m_order;
+    };
+
+    // compare alignments by position
+    struct ByPosition : public AlignmentSortBase {
+
+        ByPosition(const Sort::Order& order = Sort::AscendingOrder)
+            : m_order(order)
+        { }
+
+        bool operator()(const BamTools::BamAlignment& lhs,
+                        const BamTools::BamAlignment& rhs)
+        {
+            // force unmapped aligmnents to end
+            if ( lhs.RefID == -1 ) return false;
+            if ( rhs.RefID == -1 ) return true;
+
+            // if on same reference, sort on position
+            if ( lhs.RefID == rhs.RefID )
+                return sort_helper(m_order, lhs.Position, rhs.Position);
+
+            // otherwise sort on reference ID
+            return sort_helper(m_order, lhs.RefID, rhs.RefID);
+        }
+
+        static inline bool UsesCharData(void) { return false; }
+
+        private:
+            Sort::Order m_order;
+    };
+
+    // compare alignments by tag value
+    template<typename T>
+    struct ByTag : public AlignmentSortBase {
+
+        ByTag(const std::string& tag,
+              const Sort::Order& order = Sort::AscendingOrder)
+            : m_tag(tag)
+            , m_order(order)
+        { }
+
+        bool operator()(const BamTools::BamAlignment& lhs,
+                        const BamTools::BamAlignment& rhs)
+        {
+            // force alignments without tag to end
+            T lhsTagValue;
+            T rhsTagValue;
+            if ( !lhs.GetTag(m_tag, lhsTagValue) ) return false;
+            if ( !rhs.GetTag(m_tag, rhsTagValue) ) return true;
+
+            // otherwise compare on tag values
+            return sort_helper(m_order, lhsTagValue, rhsTagValue);
+        }
+
+        static inline bool UsesCharData(void) { return true; }
+
+        private:
+            std::string m_tag;
+            Sort::Order m_order;
+    };
+
+    // essentially a placeholder comparison object, ignores the alignments' data
+    // N.B. - returning false tends to retain insertion order
+    struct Unsorted : public AlignmentSortBase {
+        inline bool operator()(const BamTools::BamAlignment& /*lhs*/,
+                               const BamTools::BamAlignment& /*rhs*/)
+        {
+            return false;
+        }
+
+        static inline bool UsesCharData(void) { return false; }
+    };
+};
+
+// in-place sorting of alignment vector
+template<typename Compare>
+inline
+void SortAlignments(std::vector<BamAlignment>& data,
+                    const Compare& comp = Compare())
+{
+    std::sort(data.begin(), data.end(), comp);
+}
+
+// returns sorted copy of input alignment vector
+template<typename Compare>
+inline
+std::vector<BamAlignment> SortAlignmentsCopy(const std::vector<BamAlignment>& input,
+                                             const Compare& comp = Compare())
+{
+    std::vector<BamAlignment> output(input);
+    SortAlignments(output, comp);
+    return output;
+}
+
+// pulls a region from a position-sorted BAM file
+// returns the alignments sorted by user-defined Compare type
+template<typename Compare>
+std::vector<BamAlignment> GetSortedRegion(BamReader& reader,
+                                          const BamRegion& region,
+                                          const Compare& comp = Compare())
+{
+    // return empty container if unable to find region
+    if ( !reader.IsOpen() )          return std::vector<BamAlignment>();
+    if ( !reader.SetRegion(region) ) return std::vector<BamAlignment>();
+
+    // iterate through region, grabbing alignments
+    BamAlignment al;
+    std::vector<BamAlignment> results;
+    while ( reader.GetNextAlignmentCore(al) )
+        results.push_back(al);
+
+    // sort & return alignments
+    SortAlignments(results, comp);
+    return results;
+}
+
+template<typename Compare>
+std::vector<BamAlignment> GetSortedRegion(BamMultiReader& reader,
+                                          const BamRegion& region,
+                                          const Compare& comp = Compare())
+{
+    // return empty container if unable to find region
+    if ( !reader.HasOpenReaders() )  return std::vector<BamAlignment>();
+    if ( !reader.SetRegion(region) ) return std::vector<BamAlignment>();
+
+    // iterate through region, grabbing alignments
+    BamAlignment al;
+    std::vector<BamAlignment> results;
+    while ( reader.GetNextAlignmentCore(al) )
+        results.push_back(al);
+
+    // sort & return alignments
+    SortAlignments(results, comp);
+    return results;
+}
+
+} // namespace Algorithms
+} // namespace BamTools
+
+#endif // ALGORITHMS_SORT_H
diff --git a/src/api/internal/BamMultiMerger_p.h b/src/api/internal/BamMultiMerger_p.h
index ae67eea..bd596ef 100644
--- a/src/api/internal/BamMultiMerger_p.h
+++ b/src/api/internal/BamMultiMerger_p.h
@@ -3,7 +3,6 @@
 // Marth Lab, Department of Biology, Boston College
 // All rights reserved.
 // ---------------------------------------------------------------------------
-// Last modified: 18 March 2011 (DB)
 // ---------------------------------------------------------------------------
 // Provides merging functionality for BamMultiReader.  At this point, supports
 // sorting results by (refId, position) or by read name.
@@ -24,242 +23,207 @@
 
 #include <api/BamAlignment.h>
 #include <api/BamReader.h>
-#include <map>
-#include <queue>
+#include <api/algorithms/Sort.h>
+#include <deque>
+#include <functional>
+#include <set>
 #include <string>
-#include <utility>
 
 namespace BamTools {
 namespace Internal {
 
-typedef std::pair<BamReader*, BamAlignment*> ReaderAlignment;
+struct MergeItem {
 
-// generic MultiMerger interface
-class IBamMultiMerger {
+    // data members
+    BamReader*    Reader;
+    BamAlignment* Alignment;
 
-    public:
-        IBamMultiMerger(void) { }
-        virtual ~IBamMultiMerger(void) { }
+    // ctors & dtor
+    MergeItem(BamReader* reader = 0,
+              BamAlignment* alignment = 0)
+        : Reader(reader)
+        , Alignment(alignment)
+    { }
 
-    public:
-        virtual void Add(const ReaderAlignment& value) =0;
-        virtual void Clear(void) =0;
-        virtual const ReaderAlignment& First(void) const =0;
-        virtual bool IsEmpty(void) const =0;
-        virtual void Remove(BamReader* reader) =0;
-        virtual int Size(void) const =0;
-        virtual ReaderAlignment TakeFirst(void) =0;
-};
+    MergeItem(const MergeItem& other)
+        : Reader(other.Reader)
+        , Alignment(other.Alignment)
+    { }
 
-// IBamMultiMerger implementation - sorted on BamAlignment: (RefId, Position)
-class PositionMultiMerger : public IBamMultiMerger {
+    ~MergeItem(void) { }
+};
 
-    public:
-        PositionMultiMerger(void) : IBamMultiMerger() { }
-        ~PositionMultiMerger(void) { }
+template<typename Compare>
+struct MergeItemSorter : public std::binary_function<MergeItem, MergeItem, bool> {
 
     public:
-        void Add(const ReaderAlignment& value);
-        void Clear(void);
-        const ReaderAlignment& First(void) const;
-        bool IsEmpty(void) const;
-        void Remove(BamReader* reader);
-        int Size(void) const;
-        ReaderAlignment TakeFirst(void);
+        MergeItemSorter(const Compare& comp = Compare())
+            : m_comp(comp)
+        { }
+
+        bool operator()(const MergeItem& lhs, const MergeItem& rhs) {
+            const BamAlignment& l = *lhs.Alignment;
+            const BamAlignment& r = *rhs.Alignment;
+            return m_comp(l,r);
+        }
 
     private:
-        typedef std::pair<int, int>           KeyType;
-        typedef ReaderAlignment               ValueType;
-        typedef std::pair<KeyType, ValueType> ElementType;
-
-        typedef std::multimap<KeyType, ValueType> ContainerType;
-        typedef ContainerType::iterator           DataIterator;
-        typedef ContainerType::const_iterator     DataConstIterator;
-
-        ContainerType m_data;
+        Compare m_comp;
 };
 
-// IBamMultiMerger implementation - sorted on BamAlignment: Name
-class ReadNameMultiMerger : public IBamMultiMerger {
+// pure ABC so we can just work polymorphically with any specific merger implementation
+class IMultiMerger {
 
     public:
-        ReadNameMultiMerger(void) : IBamMultiMerger() { }
-        ~ReadNameMultiMerger(void) { }
-
+        IMultiMerger(void) { }
+        virtual ~IMultiMerger(void) { }
     public:
-        void Add(const ReaderAlignment& value);
-        void Clear(void);
-        const ReaderAlignment& First(void) const;
-        bool IsEmpty(void) const;
-        void Remove(BamReader* reader);
-        int Size(void) const;
-        ReaderAlignment TakeFirst(void);
-
-    private:
-        typedef std::string                   KeyType;
-        typedef ReaderAlignment               ValueType;
-        typedef std::pair<KeyType, ValueType> ElementType;
-
-        typedef std::multimap<KeyType, ValueType> ContainerType;
-        typedef ContainerType::iterator           DataIterator;
-        typedef ContainerType::const_iterator     DataConstIterator;
-
-        ContainerType m_data;
+        virtual void Add(MergeItem item) =0;
+        virtual void Clear(void) =0;
+        virtual const MergeItem& First(void) const =0;
+        virtual bool IsEmpty(void) const =0;
+        virtual void Remove(BamReader* reader) =0;
+        virtual int Size(void) const =0;
+        virtual MergeItem TakeFirst(void) =0;
 };
 
-// IBamMultiMerger implementation - unsorted BAM file(s)
-class UnsortedMultiMerger : public IBamMultiMerger {
+// general merger
+template<typename Compare>
+class MultiMerger : public IMultiMerger {
 
     public:
-        UnsortedMultiMerger(void) : IBamMultiMerger() { }
-        ~UnsortedMultiMerger(void) { }
+        typedef Compare                      CompareType;
+        typedef MergeItemSorter<CompareType> MergeType;
 
     public:
-        void Add(const ReaderAlignment& value);
+        explicit MultiMerger(const Compare& comp = Compare())
+            : IMultiMerger()
+            , m_data( MergeType(comp) )
+        { }
+        ~MultiMerger(void) { }
+
+    public:
+        void Add(MergeItem item);
         void Clear(void);
-        const ReaderAlignment& First(void) const;
+        const MergeItem& First(void) const;
         bool IsEmpty(void) const;
         void Remove(BamReader* reader);
         int Size(void) const;
-        ReaderAlignment TakeFirst(void);
+        MergeItem TakeFirst(void);
 
     private:
-        typedef ReaderAlignment ElementType;
-        typedef std::vector<ReaderAlignment>  ContainerType;
-        typedef ContainerType::iterator       DataIterator;
-        typedef ContainerType::const_iterator DataConstIterator;
-
+        typedef MergeItem                              ValueType;
+        typedef std::multiset<ValueType, MergeType>    ContainerType;
+        typedef typename ContainerType::iterator       DataIterator;
+        typedef typename ContainerType::const_iterator DataConstIterator;
         ContainerType m_data;
 };
 
-// ------------------------------------------
-// PositionMultiMerger implementation
-
-inline void PositionMultiMerger::Add(const ReaderAlignment& value) {
-    const KeyType key( value.second->RefID, value.second->Position );
-    m_data.insert( ElementType(key, value) );
+template <typename Compare>
+inline void MultiMerger<Compare>::Add(MergeItem item) {
+    if ( CompareType::UsesCharData() )
+        item.Alignment->BuildCharData();
+    m_data.insert(item);
 }
 
-inline void PositionMultiMerger::Clear(void) {
+template <typename Compare>
+inline void MultiMerger<Compare>::Clear(void) {
     m_data.clear();
 }
 
-inline const ReaderAlignment& PositionMultiMerger::First(void) const {
-    const ElementType& entry = (*m_data.begin());
-    return entry.second;
+template <typename Compare>
+inline const MergeItem& MultiMerger<Compare>::First(void) const {
+    const ValueType& entry = (*m_data.begin());
+    return entry;
 }
 
-inline bool PositionMultiMerger::IsEmpty(void) const {
+template <typename Compare>
+inline bool MultiMerger<Compare>::IsEmpty(void) const {
     return m_data.empty();
 }
-
-inline void PositionMultiMerger::Remove(BamReader* reader) {
+template <typename Compare>
+inline void MultiMerger<Compare>::Remove(BamReader* reader) {
 
     if ( reader == 0 ) return;
-    const std::string filenameToRemove = reader->GetFilename();
+    const std::string& filenameToRemove = reader->GetFilename();
 
     // iterate over readers in cache
     DataIterator dataIter = m_data.begin();
     DataIterator dataEnd  = m_data.end();
     for ( ; dataIter != dataEnd; ++dataIter ) {
-        const ValueType& entry = (*dataIter).second;
-        const BamReader* entryReader = entry.first;
-        if ( entryReader == 0 ) continue;
+        const MergeItem& item = (*dataIter);
+        const BamReader* itemReader = item.Reader;
+        if ( itemReader == 0 ) continue;
 
         // remove iterator on match
-        if ( entryReader->GetFilename() == filenameToRemove ) {
+        if ( itemReader->GetFilename() == filenameToRemove ) {
             m_data.erase(dataIter);
             return;
         }
     }
 }
-
-inline int PositionMultiMerger::Size(void) const {
+template <typename Compare>
+inline int MultiMerger<Compare>::Size(void) const {
     return m_data.size();
 }
 
-inline ReaderAlignment PositionMultiMerger::TakeFirst(void) {
-    DataIterator first = m_data.begin();
-    ReaderAlignment next = (*first).second;
-    m_data.erase(first);
-    return next;
-}
-
-// ------------------------------------------
-// ReadNameMultiMerger implementation
-
-inline void ReadNameMultiMerger::Add(const ReaderAlignment& value) {
-    const KeyType key(value.second->Name);
-    m_data.insert( ElementType(key, value) );
-}
-
-inline void ReadNameMultiMerger::Clear(void) {
-    m_data.clear();
+template <typename Compare>
+inline MergeItem MultiMerger<Compare>::TakeFirst(void) {
+    DataIterator firstIter = m_data.begin();
+    MergeItem    firstItem = (*firstIter);
+    m_data.erase(firstIter);
+    return firstItem;
 }
 
-inline const ReaderAlignment& ReadNameMultiMerger::First(void) const {
-    const ElementType& entry = (*m_data.begin());
-    return entry.second;
-}
+// unsorted "merger"
+template<>
+class MultiMerger<Algorithms::Sort::Unsorted> : public IMultiMerger {
 
-inline bool ReadNameMultiMerger::IsEmpty(void) const {
-    return m_data.empty();
-}
-
-inline void ReadNameMultiMerger::Remove(BamReader* reader) {
-
-    if ( reader == 0 ) return;
-    const std::string filenameToRemove = reader->GetFilename();
-
-    // iterate over readers in cache
-    DataIterator dataIter = m_data.begin();
-    DataIterator dataEnd  = m_data.end();
-    for ( ; dataIter != dataEnd; ++dataIter ) {
-        const ValueType& entry = (*dataIter).second;
-        const BamReader* entryReader = entry.first;
-        if ( entryReader == 0 ) continue;
-
-        // remove iterator on match
-        if ( entryReader->GetFilename() == filenameToRemove ) {
-            m_data.erase(dataIter);
-            return;
-        }
-    }
-
-}
-
-inline int ReadNameMultiMerger::Size(void) const {
-    return m_data.size();
-}
+    public:
+        explicit MultiMerger(const Algorithms::Sort::Unsorted& comp = Algorithms::Sort::Unsorted())
+            : IMultiMerger()
+        { }
+        ~MultiMerger(void) { }
 
-inline ReaderAlignment ReadNameMultiMerger::TakeFirst(void) {
-    DataIterator first = m_data.begin();
-    ReaderAlignment next = (*first).second;
-    m_data.erase(first);
-    return next;
-}
+    public:
+        void Add(MergeItem item);
+        void Clear(void);
+        const MergeItem& First(void) const;
+        bool IsEmpty(void) const;
+        void Remove(BamReader* reader);
+        int Size(void) const;
+        MergeItem TakeFirst(void);
 
-// ------------------------------------------
-// UnsortedMultiMerger implementation
+    private:
+        typedef MergeItem                     ValueType;
+        typedef std::deque<ValueType>         ContainerType;
+        typedef ContainerType::iterator       DataIterator;
+        typedef ContainerType::const_iterator DataConstIterator;
+        ContainerType m_data;
+};
 
-inline void UnsortedMultiMerger::Add(const ReaderAlignment& value) {
-    m_data.push_back(value);
+inline
+void MultiMerger<Algorithms::Sort::Unsorted>::Add(MergeItem item) {
+    m_data.push_back(item);
 }
 
-inline void UnsortedMultiMerger::Clear(void) {
-    for (size_t i = 0; i < m_data.size(); ++i )
-        m_data.pop_back();
+inline
+void MultiMerger<Algorithms::Sort::Unsorted>::Clear(void) {
+    m_data.clear();
 }
 
-inline const ReaderAlignment& UnsortedMultiMerger::First(void) const {
+inline
+const MergeItem& MultiMerger<Algorithms::Sort::Unsorted>::First(void) const {
     return m_data.front();
 }
 
-inline bool UnsortedMultiMerger::IsEmpty(void) const {
+inline
+bool MultiMerger<Algorithms::Sort::Unsorted>::IsEmpty(void) const {
     return m_data.empty();
 }
 
-inline void UnsortedMultiMerger::Remove(BamReader* reader) {
+inline
+void MultiMerger<Algorithms::Sort::Unsorted>::Remove(BamReader* reader) {
 
     if ( reader == 0 ) return;
     const std::string filenameToRemove = reader->GetFilename();
@@ -268,25 +232,28 @@ inline void UnsortedMultiMerger::Remove(BamReader* reader) {
     DataIterator dataIter = m_data.begin();
     DataIterator dataEnd  = m_data.end();
     for ( ; dataIter != dataEnd; ++dataIter ) {
-        const BamReader* entryReader = (*dataIter).first;
-        if ( entryReader == 0 ) continue;
+        const MergeItem& item = (*dataIter);
+        const BamReader* itemReader = item.Reader;
+        if ( itemReader == 0 ) continue;
 
         // remove iterator on match
-        if ( entryReader->GetFilename() == filenameToRemove ) {
+        if ( itemReader->GetFilename() == filenameToRemove ) {
             m_data.erase(dataIter);
             return;
         }
     }
 }
 
-inline int UnsortedMultiMerger::Size(void) const {
+inline
+int MultiMerger<Algorithms::Sort::Unsorted>::Size(void) const {
     return m_data.size();
 }
 
-inline ReaderAlignment UnsortedMultiMerger::TakeFirst(void) {
-    ReaderAlignment first = m_data.front();
-    m_data.erase( m_data.begin() );
-    return first;
+inline
+MergeItem MultiMerger<Algorithms::Sort::Unsorted>::TakeFirst(void) {
+    MergeItem firstItem = m_data.front();
+    m_data.pop_front();
+    return firstItem;
 }
 
 } // namespace Internal
diff --git a/src/api/internal/BamMultiReader_p.cpp b/src/api/internal/BamMultiReader_p.cpp
index e789d06..a5c7ca6 100644
--- a/src/api/internal/BamMultiReader_p.cpp
+++ b/src/api/internal/BamMultiReader_p.cpp
@@ -1,16 +1,16 @@
 // ***************************************************************************
 // BamMultiReader_p.cpp (c) 2010 Derek Barnett, Erik Garrison
 // Marth Lab, Department of Biology, Boston College
-// All rights reserved.
 // ---------------------------------------------------------------------------
-// Last modified: 5 April 2011 (DB)
+// Last modified: 3 October 2011 (DB)
 // ---------------------------------------------------------------------------
 // Functionality for simultaneously reading multiple BAM files
 // *************************************************************************
 
 #include <api/BamAlignment.h>
 #include <api/BamMultiReader.h>
-#include <api/internal/BamMultiMerger_p.h>
+#include <api/SamConstants.h>
+#include <api/algorithms/Sort.h>
 #include <api/internal/BamMultiReader_p.h>
 using namespace BamTools;
 using namespace BamTools::Internal;
@@ -24,20 +24,14 @@ using namespace std;
 
 // ctor
 BamMultiReaderPrivate::BamMultiReaderPrivate(void)
-    : m_alignments(0)
-    , m_isCoreMode(false)
-    , m_sortOrder(BamMultiReader::SortedByPosition)
+    : m_alignmentCache(0)
 { }
 
 // dtor
 BamMultiReaderPrivate::~BamMultiReaderPrivate(void) {
 
-    // close all open BAM readers
+    // close all open BAM readers (& clean up cache)
     Close();
-
-    // clean up alignment cache
-    delete m_alignments;
-    m_alignments = 0;
 }
 
 // close all BAM files
@@ -46,7 +40,7 @@ void BamMultiReaderPrivate::Close(void) {
 }
 
 // close requested BAM file
-void BamMultiReaderPrivate::CloseFile(const string& filename) {    
+void BamMultiReaderPrivate::CloseFile(const string& filename) {
     vector<string> filenames(1, filename);
     CloseFiles(filenames);
 }
@@ -62,41 +56,45 @@ void BamMultiReaderPrivate::CloseFiles(const vector<string>& filenames) {
         if ( filename.empty() ) continue;
 
         // iterate over readers
-        vector<ReaderAlignment>::iterator readerIter = m_readers.begin();
-        vector<ReaderAlignment>::iterator readerEnd  = m_readers.end();
+        vector<MergeItem>::iterator readerIter = m_readers.begin();
+        vector<MergeItem>::iterator readerEnd  = m_readers.end();
         for ( ; readerIter != readerEnd; ++readerIter ) {
-            BamReader* reader = (*readerIter).first;
+            MergeItem& item = (*readerIter);
+            BamReader* reader = item.Reader;
             if ( reader == 0 ) continue;
 
             // if reader matches requested filename
             if ( reader->GetFilename() == filename ) {
 
-                // remove reader/alignment from alignment cache
-                m_alignments->Remove(reader);
+                // remove reader's entry from alignment cache
+                m_alignmentCache->Remove(reader);
 
-                // close & delete reader
+                // clean up reader & its alignment
                 reader->Close();
                 delete reader;
                 reader = 0;
 
                 // delete reader's alignment entry
-                BamAlignment* alignment = (*readerIter).second;
+                BamAlignment* alignment = item.Alignment;
                 delete alignment;
                 alignment = 0;
 
-                // remove reader from container
+                // remove reader from reader list
                 m_readers.erase(readerIter);
 
                 // on match, just go on to next filename
-                // (no need to keep looking and iterator is invalid now anyway)
+                // (no need to keep looking and item iterator is invalid now anyway)
                 break;
             }
         }
     }
 
-    // make sure alignment cache is cleared if all readers are now closed
-    if ( m_readers.empty() && m_alignments != 0 )
-        m_alignments->Clear();
+    // make sure alignment cache is cleaned up if all readers closed
+    if ( m_readers.empty() && m_alignmentCache ) {
+        m_alignmentCache->Clear();
+        delete m_alignmentCache;
+        m_alignmentCache = 0;
+    }
 }
 
 // creates index files for BAM files that don't have them
@@ -105,10 +103,11 @@ bool BamMultiReaderPrivate::CreateIndexes(const BamIndex::IndexType& type) {
     bool result = true;
 
     // iterate over readers
-    vector<ReaderAlignment>::iterator readerIter = m_readers.begin();
-    vector<ReaderAlignment>::iterator readerEnd  = m_readers.end();
-    for ( ; readerIter != readerEnd; ++readerIter ) {
-        BamReader* reader = (*readerIter).first;
+    vector<MergeItem>::iterator itemIter = m_readers.begin();
+    vector<MergeItem>::iterator itemEnd  = m_readers.end();
+    for ( ; itemIter != itemEnd; ++itemIter ) {
+        MergeItem& item = (*itemIter);
+        BamReader* reader = item.Reader;
         if ( reader == 0 ) continue;
 
         // if reader doesn't have an index, create one
@@ -119,34 +118,21 @@ bool BamMultiReaderPrivate::CreateIndexes(const BamIndex::IndexType& type) {
     return result;
 }
 
-IBamMultiMerger* BamMultiReaderPrivate::CreateMergerForCurrentSortOrder(void) const {
-    switch ( m_sortOrder ) {
-        case ( BamMultiReader::SortedByPosition ) : return new PositionMultiMerger;
-        case ( BamMultiReader::SortedByReadName ) : return new ReadNameMultiMerger;
-        case ( BamMultiReader::Unsorted )         : return new UnsortedMultiMerger;
-        default :
-            cerr << "BamMultiReader ERROR: requested sort order is unknown" << endl;
-            return 0;
-    }
-}
+IMultiMerger* BamMultiReaderPrivate::CreateAlignmentCache(void) const {
 
-const string BamMultiReaderPrivate::ExtractReadGroup(const string& headerLine) const {
+    // fetch SamHeader
+    SamHeader header = GetHeader();
 
-    string readGroup("");
-    stringstream headerLineSs(headerLine);
-    string part;
+    // if BAM files are sorted by position
+    if ( header.SortOrder == Constants::SAM_HD_SORTORDER_COORDINATE )
+        return new MultiMerger<Algorithms::Sort::ByPosition>();
 
-    // parse @RG header line, looking for the ID: tag
-    while( getline(headerLineSs, part, '\t') ) {
-        stringstream partSs(part);
-        string subtag;
-        getline(partSs, subtag, ':');
-        if ( subtag == "ID" ) {
-            getline(partSs, readGroup, ':');
-            break;
-        }
-    }
-    return readGroup;
+    // if BAM files are sorted by read name
+    if ( header.SortOrder == Constants::SAM_HD_SORTORDER_QUERYNAME )
+        return new MultiMerger<Algorithms::Sort::ByName>();
+
+    // otherwise "unknown" or "unsorted", use unsorted merger and just read in
+    return new MultiMerger<Algorithms::Sort::Unsorted>();
 }
 
 const vector<string> BamMultiReaderPrivate::Filenames(void) const {
@@ -156,16 +142,17 @@ const vector<string> BamMultiReaderPrivate::Filenames(void) const {
     filenames.reserve( m_readers.size() );
 
     // iterate over readers
-    vector<ReaderAlignment>::const_iterator readerIter = m_readers.begin();
-    vector<ReaderAlignment>::const_iterator readerEnd  = m_readers.end();
-    for ( ; readerIter != readerEnd; ++readerIter ) {
-        const BamReader* reader = (*readerIter).first;
+    vector<MergeItem>::const_iterator itemIter = m_readers.begin();
+    vector<MergeItem>::const_iterator itemEnd  = m_readers.end();
+    for ( ; itemIter != itemEnd; ++itemIter ) {
+        const MergeItem& item = (*itemIter);
+        const BamReader* reader = item.Reader;
         if ( reader == 0 ) continue;
 
         // store filename if not empty
-        const string filename = reader->GetFilename();
+        const string& filename = reader->GetFilename();
         if ( !filename.empty() )
-            filenames.push_back( reader->GetFilename() );
+            filenames.push_back(filename);
     }
 
     // return result
@@ -173,102 +160,55 @@ const vector<string> BamMultiReaderPrivate::Filenames(void) const {
 }
 
 SamHeader BamMultiReaderPrivate::GetHeader(void) const {
-    string text = GetHeaderText();
+    const string& text = GetHeaderText();
     return SamHeader(text);
 }
 
 // makes a virtual, unified header for all the bam files in the multireader
 string BamMultiReaderPrivate::GetHeaderText(void) const {
 
-    // TODO: merge SamHeader objects instead of parsing string data (again)
-
-    // if only one reader is open
-    if ( m_readers.size() == 1 ) {
-
-        // just return reader's header text
-        const ReaderAlignment& ra = m_readers.front();
-        const BamReader* reader = ra.first;
-        if ( reader ) return reader->GetHeaderText();
+    // N.B. - right now, simply copies all header data from first BAM,
+    //        and then appends RG's from other BAM files
+    // TODO: make this more intelligent wrt other header lines/fields
 
-        // invalid reader
-        return string();
-    }
+    // if no readers open
+    const size_t numReaders = m_readers.size();
+    if ( numReaders == 0 ) return string();
 
-    string mergedHeader("");
-    map<string, bool> readGroups;
+    // retrieve first reader's header
+    const MergeItem& firstItem = m_readers.front();
+    const BamReader* reader = firstItem.Reader;
+    if ( reader == 0 ) return string();
+    SamHeader mergedHeader = reader->GetHeader();
 
-    // foreach extraction entry (each BAM file)
-    vector<ReaderAlignment>::const_iterator readerBegin = m_readers.begin();
-    vector<ReaderAlignment>::const_iterator readerIter  = readerBegin;
-    vector<ReaderAlignment>::const_iterator readerEnd   = m_readers.end();
-    for ( ; readerIter != readerEnd; ++readerIter ) {
-        const BamReader* reader = (*readerIter).first;
+    // iterate over any remaining readers (skipping the first)
+    for ( size_t i = 1; i < numReaders; ++i ) {
+        const MergeItem& item = m_readers.at(i);
+        const BamReader* reader = item.Reader;
         if ( reader == 0 ) continue;
 
-        // get header from reader
-        string headerText = reader->GetHeaderText();
-        if ( headerText.empty() ) continue;
-
-        // store header text in lines
-        map<string, bool> currentFileReadGroups;
-        const vector<string> lines = SplitHeaderText(headerText);
-
-        // iterate over header lines
-        vector<string>::const_iterator linesIter = lines.begin();
-        vector<string>::const_iterator linesEnd  = lines.end();
-        for ( ; linesIter != linesEnd; ++linesIter ) {
-
-            // get next line from header, skip if empty
-            const string headerLine = (*linesIter);
-            if ( headerLine.empty() ) continue;
-
-            // if first file, save HD & SQ entries
-            // TODO: what if first file has empty header, should just check for empty 'mergedHeader' instead ?
-            if ( readerIter == readerBegin ) {
-                if ( headerLine.find("@HD") == 0 || headerLine.find("@SQ") == 0) {
-                    mergedHeader.append(headerLine.c_str());
-                    mergedHeader.append(1, '\n');
-                }
-            }
+        // retrieve current reader's header
+        const SamHeader currentHeader = reader->GetHeader();
 
-            // (for all files) append RG entries if they are unique
-            if ( headerLine.find("@RG") == 0 ) {
-
-                // extract read group name from line
-                const string readGroup = ExtractReadGroup(headerLine);
-
-                // make sure not to duplicate @RG entries
-                if ( readGroups.find(readGroup) == readGroups.end() ) {
-                    mergedHeader.append(headerLine.c_str() );
-                    mergedHeader.append(1, '\n');
-                    readGroups[readGroup] = true;
-                    currentFileReadGroups[readGroup] = true;
-                } else {
-                    // warn iff we are reading one file and discover duplicated @RG tags in the header
-                    // otherwise, we emit no warning, as we might be merging multiple BAM files with identical @RG tags
-                    if ( currentFileReadGroups.find(readGroup) != currentFileReadGroups.end() ) {
-                        cerr << "BamMultiReader WARNING: duplicate @RG tag " << readGroup
-                             << " entry in header of " << reader->GetFilename() << endl;
-                    }
-                }
-            }
-        }
+        // append current reader's RG entries to merged header
+        // N.B. - SamReadGroupDictionary handles duplicate-checking
+        mergedHeader.ReadGroups.Add(currentHeader.ReadGroups);
+
+        // TODO: merge anything else??
     }
 
-    // return merged header text
-    return mergedHeader;
+    // return stringified header
+    return mergedHeader.ToString();
 }
 
 // get next alignment among all files
 bool BamMultiReaderPrivate::GetNextAlignment(BamAlignment& al) {
-    m_isCoreMode = false;
-    return LoadNextAlignment(al);
+    return PopNextCachedAlignment(al, true);
 }
 
 // get next alignment among all files without parsing character data from alignments
 bool BamMultiReaderPrivate::GetNextAlignmentCore(BamAlignment& al) {
-    m_isCoreMode = true;
-    return LoadNextAlignment(al);
+    return PopNextCachedAlignment(al, false);
 }
 
 // ---------------------------------------------------------------------------------------
@@ -289,8 +229,8 @@ int BamMultiReaderPrivate::GetReferenceCount(void) const {
         return 0;
 
     // return reference count from first reader
-    const ReaderAlignment& ra = m_readers.front();
-    const BamReader* reader = ra.first;
+    const MergeItem& item = m_readers.front();
+    const BamReader* reader = item.Reader;
     if ( reader ) return reader->GetReferenceCount();
 
     // invalid reader
@@ -305,8 +245,8 @@ const RefVector BamMultiReaderPrivate::GetReferenceData(void) const {
         return RefVector();
 
     // return reference data from first BamReader
-    const ReaderAlignment& ra = m_readers.front();
-    const BamReader* reader = ra.first;
+    const MergeItem& item = m_readers.front();
+    const BamReader* reader = item.Reader;
     if ( reader ) return reader->GetReferenceData();
 
     // invalid reader
@@ -321,8 +261,8 @@ int BamMultiReaderPrivate::GetReferenceID(const string& refName) const {
         return -1;
 
     // return reference ID from first BamReader
-    const ReaderAlignment& ra = m_readers.front();
-    const BamReader* reader = ra.first;
+    const MergeItem& item = m_readers.front();
+    const BamReader* reader = item.Reader;
     if ( reader ) return reader->GetReferenceID(refName);
 
     // invalid reader
@@ -330,13 +270,6 @@ int BamMultiReaderPrivate::GetReferenceID(const string& refName) const {
 }
 // ---------------------------------------------------------------------------------------
 
-// checks if any readers still have alignments
-bool BamMultiReaderPrivate::HasAlignmentData(void) const {
-    if ( m_alignments == 0 )
-        return false;
-    return !m_alignments->IsEmpty();
-}
-
 // returns true if all readers have index data available
 // this is useful to indicate whether Jump() or SetRegion() are possible
 bool BamMultiReaderPrivate::HasIndexes(void) const {
@@ -348,10 +281,11 @@ bool BamMultiReaderPrivate::HasIndexes(void) const {
     bool result = true;
 
     // iterate over readers
-    vector<ReaderAlignment>::const_iterator readerIter = m_readers.begin();
-    vector<ReaderAlignment>::const_iterator readerEnd  = m_readers.end();
+    vector<MergeItem>::const_iterator readerIter = m_readers.begin();
+    vector<MergeItem>::const_iterator readerEnd  = m_readers.end();
     for ( ; readerIter != readerEnd; ++readerIter ) {
-        const BamReader* reader = (*readerIter).first;
+        const MergeItem& item = (*readerIter);
+        const BamReader* reader = item.Reader;
         if ( reader  == 0 ) continue;
 
         // see if current reader has index data
@@ -365,10 +299,11 @@ bool BamMultiReaderPrivate::HasIndexes(void) const {
 bool BamMultiReaderPrivate::HasOpenReaders(void) {
 
     // iterate over readers
-    vector<ReaderAlignment>::const_iterator readerIter = m_readers.begin();
-    vector<ReaderAlignment>::const_iterator readerEnd  = m_readers.end();
+    vector<MergeItem>::const_iterator readerIter = m_readers.begin();
+    vector<MergeItem>::const_iterator readerEnd  = m_readers.end();
     for ( ; readerIter != readerEnd; ++readerIter ) {
-        const BamReader* reader = (*readerIter).first;
+        const MergeItem& item = (*readerIter);
+        const BamReader* reader = item.Reader;
         if ( reader == 0 ) continue;
 
         // return true whenever an open reader is found
@@ -388,10 +323,11 @@ bool BamMultiReaderPrivate::Jump(int refID, int position) {
     // UpdateAlignments(), and continue.
 
     // iterate over readers
-    vector<ReaderAlignment>::iterator readerIter = m_readers.begin();
-    vector<ReaderAlignment>::iterator readerEnd  = m_readers.end();
+    vector<MergeItem>::iterator readerIter = m_readers.begin();
+    vector<MergeItem>::iterator readerEnd  = m_readers.end();
     for ( ; readerIter != readerEnd; ++readerIter ) {
-        BamReader* reader = (*readerIter).first;
+        MergeItem& item = (*readerIter);
+        BamReader* reader = item.Reader;
         if ( reader == 0 ) continue;
 
         // attempt jump() on each
@@ -401,30 +337,8 @@ bool BamMultiReaderPrivate::Jump(int refID, int position) {
         }
     }
 
-    // update alignment cache & return success
-    UpdateAlignmentCache();
-    return true;
-}
-
-bool BamMultiReaderPrivate::LoadNextAlignment(BamAlignment& al) {
-
-    // bail out if no more data to process
-    if ( !HasAlignmentData() )
-        return false;
-
-    // "pop" next alignment and reader
-    ReaderAlignment nextReaderAlignment = m_alignments->TakeFirst();
-    BamReader* reader = nextReaderAlignment.first;
-    BamAlignment* alignment = nextReaderAlignment.second;
-
-    // store cached alignment into destination parameter (by copy)
-    al = *alignment;
-
-    // peek to next alignment & store in cache
-    SaveNextAlignment(reader, alignment);
-
-    // return success
-    return true;
+    // returns status of cache update
+    return UpdateAlignmentCache();
 }
 
 // locate (& load) index files for BAM readers that don't already have one loaded
@@ -433,10 +347,11 @@ bool BamMultiReaderPrivate::LocateIndexes(const BamIndex::IndexType& preferredTy
     bool result = true;
 
     // iterate over readers
-    vector<ReaderAlignment>::iterator readerIter = m_readers.begin();
-    vector<ReaderAlignment>::iterator readerEnd  = m_readers.end();
+    vector<MergeItem>::iterator readerIter = m_readers.begin();
+    vector<MergeItem>::iterator readerEnd  = m_readers.end();
     for ( ; readerIter != readerEnd; ++readerIter ) {
-        BamReader* reader = (*readerIter).first;
+        MergeItem& item = (*readerIter);
+        BamReader* reader = item.Reader;
         if ( reader == 0 ) continue;
 
         // if reader has no index, try to locate one
@@ -450,11 +365,10 @@ bool BamMultiReaderPrivate::LocateIndexes(const BamIndex::IndexType& preferredTy
 // opens BAM files
 bool BamMultiReaderPrivate::Open(const vector<string>& filenames) {
 
-    // create alignment cache if neccessary
-    if ( m_alignments == 0 ) {
-        m_alignments = CreateMergerForCurrentSortOrder();
-        if ( m_alignments == 0 ) return false;
-    }
+    bool openedOk = true;
+
+    // put all current readers back at beginning
+    openedOk &= Rewind();
 
     // iterate over filenames
     vector<string>::const_iterator filenameIter = filenames.begin();
@@ -463,22 +377,30 @@ bool BamMultiReaderPrivate::Open(const vector<string>& filenames) {
         const string& filename = (*filenameIter);
         if ( filename.empty() ) continue;
 
-        // attempt to open BamReader on filename
-        BamReader* reader = OpenReader(filename);
-        if ( reader == 0 ) continue;
+        // attempt to open BamReader
+        BamReader* reader = new BamReader;
+        const bool readerOpened = reader->Open(filename);
 
-        // store reader with new alignment
-        m_readers.push_back( make_pair(reader, new BamAlignment) );
-    }
+        // if opened OK, store it
+        if ( readerOpened )
+            m_readers.push_back( MergeItem(reader, new BamAlignment) );
+
+        // otherwise clean up invalid reader
+        else delete reader;
 
-    // validate & rewind any opened readers, also refreshes alignment cache
-    if ( !m_readers.empty() ) {
-        ValidateReaders();
-        Rewind();
+        // update method return status
+        openedOk &= readerOpened;
     }
 
+    // if more than one reader open, check for consistency
+    if ( m_readers.size() > 1 )
+        openedOk &= ValidateReaders();
+
+    // update alignment cache
+    openedOk &= UpdateAlignmentCache();
+
     // return success
-    return true;
+    return openedOk;
 }
 
 bool BamMultiReaderPrivate::OpenFile(const std::string& filename) {
@@ -493,7 +415,7 @@ bool BamMultiReaderPrivate::OpenIndexes(const vector<string>& indexFilenames) {
     //       first reader without an index?
 
     // make sure same number of index filenames as readers
-    if ( m_readers.size() != indexFilenames.size() || !indexFilenames.empty() )
+    if ( m_readers.size() != indexFilenames.size() )
         return false;
 
     // init result flag
@@ -502,10 +424,11 @@ bool BamMultiReaderPrivate::OpenIndexes(const vector<string>& indexFilenames) {
     // iterate over BamReaders
     vector<string>::const_iterator indexFilenameIter = indexFilenames.begin();
     vector<string>::const_iterator indexFilenameEnd  = indexFilenames.end();
-    vector<ReaderAlignment>::iterator readerIter = m_readers.begin();
-    vector<ReaderAlignment>::iterator readerEnd  = m_readers.end();
+    vector<MergeItem>::iterator readerIter = m_readers.begin();
+    vector<MergeItem>::iterator readerEnd  = m_readers.end();
     for ( ; readerIter != readerEnd; ++readerIter ) {
-        BamReader* reader = (*readerIter).first;
+        MergeItem& item = (*readerIter);
+        BamReader* reader = item.Reader;
 
         // open index filename on reader
         if ( reader ) {
@@ -518,69 +441,52 @@ bool BamMultiReaderPrivate::OpenIndexes(const vector<string>& indexFilenames) {
             break;
     }
 
-    // TODO: validation ??
+    // TODO: any validation needed here??
 
     // return success/fail
     return result;
 }
 
-BamReader* BamMultiReaderPrivate::OpenReader(const std::string& filename) {
+bool BamMultiReaderPrivate::PopNextCachedAlignment(BamAlignment& al, const bool needCharData) {
 
-    // create new BamReader
-    BamReader* reader = new BamReader;
-
-    // if reader opens OK
-    if ( reader->Open(filename) ) {
+    // skip if no alignments available
+    if ( m_alignmentCache == 0 || m_alignmentCache->IsEmpty() )
+        return false;
 
-        // attempt to read first alignment (sanity check)
-        // if ok, then return BamReader pointer
-        BamAlignment al;
-        if ( reader->GetNextAlignmentCore(al) )
-            return reader;
+    // pop next merge item entry from cache
+    MergeItem item = m_alignmentCache->TakeFirst();
+    BamReader* reader = item.Reader;
+    BamAlignment* alignment = item.Alignment;
+    if ( reader == 0 || alignment == 0 )
+        return false;
 
-        // could not read alignment
-        else {
-            cerr << "BamMultiReader WARNING: Could not read first alignment from "
-                 << filename << ", ignoring file" << endl;
-        }
+    // set char data if requested
+    if ( needCharData ) {
+        alignment->BuildCharData();
+        alignment->Filename = reader->GetFilename();
     }
 
-    // reader could not open
-    else {
-        cerr << "BamMultiReader WARNING: Could not open "
-              << filename << ", ignoring file" << endl;
-    }
+    // store cached alignment into destination parameter (by copy)
+    al = *alignment;
 
-    // if we get here, there was a problem with this BAM file (opening or reading)
-    // clean up memory allocation & return null pointer
-    delete reader;
-    return 0;
-}
+    // load next alignment from reader & store in cache
+    SaveNextAlignment(reader, alignment);
 
-// print associated filenames to stdout
-void BamMultiReaderPrivate::PrintFilenames(void) const {
-    const vector<string>& filenames = Filenames();
-    vector<string>::const_iterator filenameIter = filenames.begin();
-    vector<string>::const_iterator filenameEnd  = filenames.end();
-    for ( ; filenameIter != filenameEnd; ++filenameIter )
-        cout << (*filenameIter) << endl;
+    // return success
+    return true;
 }
 
 // returns BAM file pointers to beginning of alignment data & resets alignment cache
 bool BamMultiReaderPrivate::Rewind(void) {
 
-    // clear out alignment cache
-    m_alignments->Clear();
-
     // attempt to rewind files
     if ( !RewindReaders() ) {
         cerr << "BamMultiReader ERROR: could not rewind file(s) successfully";
         return false;
     }
 
-    // reset cache & return success
-    UpdateAlignmentCache();
-    return true;
+    // return status of cache update
+    return UpdateAlignmentCache();
 }
 
 // returns BAM file pointers to beginning of alignment data
@@ -589,10 +495,11 @@ bool BamMultiReaderPrivate::RewindReaders(void) {
     bool result = true;
 
     // iterate over readers
-    vector<ReaderAlignment>::iterator readerIter = m_readers.begin();
-    vector<ReaderAlignment>::iterator readerEnd  = m_readers.end();
+    vector<MergeItem>::iterator readerIter = m_readers.begin();
+    vector<MergeItem>::iterator readerEnd  = m_readers.end();
     for ( ; readerIter != readerEnd; ++readerIter ) {
-        BamReader* reader = (*readerIter).first;
+        MergeItem& item = (*readerIter);
+        BamReader* reader = item.Reader;
         if ( reader == 0 ) continue;
 
         // attempt rewind on BamReader
@@ -604,27 +511,22 @@ bool BamMultiReaderPrivate::RewindReaders(void) {
 
 void BamMultiReaderPrivate::SaveNextAlignment(BamReader* reader, BamAlignment* alignment) {
 
-    // must be in core mode && NOT sorting by read name to call GNACore()
-    if ( m_isCoreMode && m_sortOrder != BamMultiReader::SortedByReadName ) {
-        if ( reader->GetNextAlignmentCore(*alignment) )
-            m_alignments->Add( make_pair(reader, alignment) );
-    }
-
-    // not in core mode and/or sorting by readname, must call GNA()
-    else {
-        if ( reader->GetNextAlignment(*alignment) )
-            m_alignments->Add( make_pair(reader, alignment) );
-    }
+    // if can read alignment from reader, store in cache
+    // N.B. - lazy building of alignment's char data,
+    // only populated on demand by sorting merger or client call to GetNextAlignment()
+    if ( reader->GetNextAlignmentCore(*alignment) )
+        m_alignmentCache->Add(MergeItem(reader, alignment));
 }
 
 // sets the index caching mode on the readers
 void BamMultiReaderPrivate::SetIndexCacheMode(const BamIndex::IndexCacheMode mode) {
 
     // iterate over readers
-    vector<ReaderAlignment>::iterator readerIter = m_readers.begin();
-    vector<ReaderAlignment>::iterator readerEnd  = m_readers.end();
+    vector<MergeItem>::iterator readerIter = m_readers.begin();
+    vector<MergeItem>::iterator readerEnd  = m_readers.end();
     for ( ; readerIter != readerEnd; ++readerIter ) {
-        BamReader* reader = (*readerIter).first;
+        MergeItem& item = (*readerIter);
+        BamReader* reader = item.Reader;
         if ( reader == 0 ) continue;
 
         // set reader's index cache mode
@@ -640,10 +542,11 @@ bool BamMultiReaderPrivate::SetRegion(const BamRegion& region) {
     // UpdateAlignments(), and continue.
 
     // iterate over alignments
-    vector<ReaderAlignment>::iterator readerIter = m_readers.begin();
-    vector<ReaderAlignment>::iterator readerEnd  = m_readers.end();
+    vector<MergeItem>::iterator readerIter = m_readers.begin();
+    vector<MergeItem>::iterator readerEnd  = m_readers.end();
     for ( ; readerIter != readerEnd; ++readerIter ) {
-        BamReader* reader = (*readerIter).first;
+        MergeItem& item = (*readerIter);
+        BamReader* reader = item.Reader;
         if ( reader == 0 ) continue;
 
         // attempt to set BamReader's region of interest
@@ -654,98 +557,93 @@ bool BamMultiReaderPrivate::SetRegion(const BamRegion& region) {
         }
     }
 
-    // update alignment cache & return success
-    UpdateAlignmentCache();
-    return true;
-}
-
-void BamMultiReaderPrivate::SetSortOrder(const BamMultiReader::SortOrder& order) {
-
-    // skip if no change needed
-    if ( m_sortOrder == order ) return;
-
-    // set new sort order
-    m_sortOrder = order;
-
-    // create new alignment cache based on sort order
-    IBamMultiMerger* newAlignmentCache = CreateMergerForCurrentSortOrder();
-    if ( newAlignmentCache == 0 ) return; // print error?
-
-    // copy old cache contents to new cache
-    while ( m_alignments->Size() > 0 ) {
-        ReaderAlignment value = m_alignments->TakeFirst(); // retrieves & 'pops'
-        newAlignmentCache->Add(value);
-    }
-
-    // remove old cache structure & point to new cache
-    delete m_alignments;
-    m_alignments = newAlignmentCache;
-}
-
-// splits the entire header into a list of strings
-const vector<string> BamMultiReaderPrivate::SplitHeaderText(const string& headerText) const {
-
-    stringstream header(headerText);
-    string item;
-
-    vector<string> lines;
-    while ( getline(header, item) )
-        lines.push_back(item);
-    return lines;
+    // return status of cache update
+    return UpdateAlignmentCache();
 }
 
 // updates our alignment cache
-void BamMultiReaderPrivate::UpdateAlignmentCache(void) {
-
-    // skip if invalid alignment cache
-    if ( m_alignments == 0 ) return;
-
-    // clear the cache
-    m_alignments->Clear();
+bool BamMultiReaderPrivate::UpdateAlignmentCache(void) {
+
+    // create alignment cache if not created yet
+    if ( m_alignmentCache == 0 ) {
+        m_alignmentCache = CreateAlignmentCache();
+        if ( m_alignmentCache == 0 ) {
+            // set error string
+            return false;
+        }
+    }
 
-    // seed cache with fully-populated alignments
-    // further updates will fill with full/core-only as requested
-    m_isCoreMode = false;
+    // clear any prior cache data
+    m_alignmentCache->Clear();
 
     // iterate over readers
-    vector<ReaderAlignment>::iterator readerIter = m_readers.begin();
-    vector<ReaderAlignment>::iterator readerEnd  = m_readers.end();
+    vector<MergeItem>::iterator readerIter = m_readers.begin();
+    vector<MergeItem>::iterator readerEnd  = m_readers.end();
     for ( ; readerIter != readerEnd; ++readerIter ) {
-        BamReader* reader = (*readerIter).first;
-        BamAlignment* alignment = (*readerIter).second;
+        MergeItem& item = (*readerIter);
+        BamReader* reader = item.Reader;
+        BamAlignment* alignment = item.Alignment;
         if ( reader == 0 || alignment == 0 ) continue;
 
         // save next alignment from each reader in cache
         SaveNextAlignment(reader, alignment);
     }
+
+    // if we get here, ok
+    return true;
 }
 
 // ValidateReaders checks that all the readers point to BAM files representing
 // alignments against the same set of reference sequences, and that the
 // sequences are identically ordered.  If these checks fail the operation of
 // the multireader is undefined, so we force program exit.
-void BamMultiReaderPrivate::ValidateReaders(void) const {
+bool BamMultiReaderPrivate::ValidateReaders(void) const {
+
+    // skip if no readers opened
+    if ( m_readers.empty() )
+        return true;
 
-    // retrieve first reader data
-    const BamReader* firstReader = m_readers.front().first;
-    if ( firstReader == 0 ) return;
-    const RefVector firstReaderRefData = firstReader->GetReferenceData();
+    // retrieve first reader
+    const MergeItem& firstItem = m_readers.front();
+    const BamReader* firstReader = firstItem.Reader;
+    if ( firstReader == 0 ) return false;
+
+    // retrieve first reader's header data
+    const SamHeader& firstReaderHeader = firstReader->GetHeader();
+    const string& firstReaderSortOrder = firstReaderHeader.SortOrder;
+
+    // retrieve first reader's reference data
+    const RefVector& firstReaderRefData = firstReader->GetReferenceData();
     const int firstReaderRefCount = firstReader->GetReferenceCount();
     const int firstReaderRefSize = firstReaderRefData.size();
 
     // iterate over all readers
-    vector<ReaderAlignment>::const_iterator readerIter = m_readers.begin();
-    vector<ReaderAlignment>::const_iterator readerEnd  = m_readers.end();
+    vector<MergeItem>::const_iterator readerIter = m_readers.begin();
+    vector<MergeItem>::const_iterator readerEnd  = m_readers.end();
     for ( ; readerIter != readerEnd; ++readerIter ) {
-
-        // get current reader data
-        BamReader* reader = (*readerIter).first;
+        const MergeItem& item = (*readerIter);
+        BamReader* reader = item.Reader;
         if ( reader == 0 ) continue;
+
+        // get current reader's header data
+        const SamHeader& currentReaderHeader = reader->GetHeader();
+        const string& currentReaderSortOrder = currentReaderHeader.SortOrder;
+
+        // check compatible sort order
+        if ( currentReaderSortOrder != firstReaderSortOrder ) {
+            // error string
+            cerr << "BamMultiReader ERROR: mismatched sort order in " << reader->GetFilename()
+                 << ", expected "  << firstReaderSortOrder
+                 << ", but found " << currentReaderSortOrder << endl;
+            return false;
+        }
+
+        // get current reader's reference data
         const RefVector currentReaderRefData = reader->GetReferenceData();
         const int currentReaderRefCount = reader->GetReferenceCount();
         const int currentReaderRefSize  = currentReaderRefData.size();
 
-        // init container iterators
+        // init reference data iterators
         RefVector::const_iterator firstRefIter   = firstReaderRefData.begin();
         RefVector::const_iterator firstRefEnd    = firstReaderRefData.end();
         RefVector::const_iterator currentRefIter = currentReaderRefData.begin();
@@ -757,7 +655,7 @@ void BamMultiReaderPrivate::ValidateReaders(void) const {
             cerr << "BamMultiReader ERROR: mismatched number of references in " << reader->GetFilename()
                  << " expected " << firstReaderRefCount
                  << " reference sequences but only found " << currentReaderRefCount << endl;
-            exit(1);
+            return false;
         }
 
         // this will be ok; we just checked above that we have identically-sized sets of references
@@ -791,7 +689,7 @@ void BamMultiReaderPrivate::ValidateReaders(void) const {
                     cerr << entry.RefName << " " << entry.RefLength << endl;
                 }
 
-                exit(1);
+                return false;
             }
 
             // update iterators
@@ -799,4 +697,7 @@ void BamMultiReaderPrivate::ValidateReaders(void) const {
             ++currentRefIter;
         }
     }
+
+    // if we get here, everything checks out
+    return true;
 }
diff --git a/src/api/internal/BamMultiReader_p.h b/src/api/internal/BamMultiReader_p.h
index b34fb0c..70640a3 100644
--- a/src/api/internal/BamMultiReader_p.h
+++ b/src/api/internal/BamMultiReader_p.h
@@ -1,9 +1,8 @@
 // ***************************************************************************
 // BamMultiReader_p.h (c) 2010 Derek Barnett
 // Marth Lab, Department of Biology, Boston College
-// All rights reserved.
 // ---------------------------------------------------------------------------
-// Last modified: 13 March 2011 (DB)
+// Last modified: 3 October 2011 (DB)
 // ---------------------------------------------------------------------------
 // Functionality for simultaneously reading multiple BAM files
 // *************************************************************************
@@ -23,14 +22,13 @@
 
 #include <api/SamHeader.h>
 #include <api/BamMultiReader.h>
+#include <api/internal/BamMultiMerger_p.h>
 #include <string>
 #include <vector>
 
 namespace BamTools {
 namespace Internal {
 
-class IBamMultiMerger;
-
 class BamMultiReaderPrivate {
 
     // constructor / destructor
@@ -49,7 +47,6 @@ class BamMultiReaderPrivate {
         bool Jump(int refID, int position = 0);
         bool Open(const std::vector<std::string>& filenames);
         bool OpenFile(const std::string& filename);
-        void PrintFilenames(void) const;
         bool Rewind(void);
         bool SetRegion(const BamRegion& region);
 
@@ -57,7 +54,6 @@ class BamMultiReaderPrivate {
         bool GetNextAlignment(BamAlignment& al);
         bool GetNextAlignmentCore(BamAlignment& al);
         bool HasOpenReaders(void);
-        void SetSortOrder(const BamMultiReader::SortOrder& order);
 
         // access auxiliary data
         SamHeader GetHeader(void) const;
@@ -75,25 +71,17 @@ class BamMultiReaderPrivate {
 
     // 'internal' methods
     public:
-        IBamMultiMerger* CreateMergerForCurrentSortOrder(void) const;
-        const std::string ExtractReadGroup(const std::string& headerLine) const;
-        bool HasAlignmentData(void) const;
-        bool LoadNextAlignment(BamAlignment& al);
-        BamTools::BamReader* OpenReader(const std::string& filename);
+        IMultiMerger* CreateAlignmentCache(void) const;
+        bool PopNextCachedAlignment(BamAlignment& al, const bool needCharData);
         bool RewindReaders(void);
-        void SaveNextAlignment(BamTools::BamReader* reader, BamTools::BamAlignment* alignment);
-        const std::vector<std::string> SplitHeaderText(const std::string& headerText) const;
-        void UpdateAlignmentCache(void);
-        void ValidateReaders(void) const;
+        void SaveNextAlignment(BamReader* reader, BamAlignment* alignment);
+        bool UpdateAlignmentCache(void);
+        bool ValidateReaders(void) const;
 
     // data members
     public:
-        typedef std::pair<BamReader*, BamAlignment*> ReaderAlignment;
-        std::vector<ReaderAlignment> m_readers;
-
-        IBamMultiMerger* m_alignments;
-        bool m_isCoreMode;
-        BamMultiReader::SortOrder m_sortOrder;
+        std::vector<MergeItem> m_readers;
+        IMultiMerger* m_alignmentCache;
 };
 
 } // namespace Internal
diff --git a/src/toolkit/bamtools_count.cpp b/src/toolkit/bamtools_count.cpp
index dfd1fa6..d438eab 100644
--- a/src/toolkit/bamtools_count.cpp
+++ b/src/toolkit/bamtools_count.cpp
@@ -112,12 +112,14 @@ bool CountTool::CountToolPrivate::Run(void) {
                 alignments.push_back(al);
         }
 
+        using namespace BamTools::Algorithms;
+
         cerr << endl
              << "------------------------------" << endl
              << "Unsorted Alignments" << endl
              << "------------------------------" << endl
              << endl;
-        std::stable_sort(alignments.begin(), alignments.end(), Algorithms::Unsorted());
+        std::stable_sort(alignments.begin(), alignments.end(), Sort::Unsorted());
         printAlignments(alignments);
         cerr << "------------------------------" << endl
              << endl;
@@ -127,7 +129,8 @@ bool CountTool::CountToolPrivate::Run(void) {
              << "Sorted Alignments (by name)" << endl
              << "------------------------------" << endl
              << endl;
-        std::sort(alignments.begin(), alignments.end(), Algorithms::SortByName<>());
+//        std::sort(alignments.begin(), alignments.end(), Sort::ByName());
+        Algorithms::SortAlignments(alignments, Sort::ByName());
         printAlignments(alignments);
         cerr << endl
              << "------------------------------" << endl
@@ -138,7 +141,8 @@ bool CountTool::CountToolPrivate::Run(void) {
              << "Sorted Alignments (by tag Aq)" << endl
              << "------------------------------" << endl
              << endl;
-        std::sort(alignments.begin(), alignments.end(), Algorithms::SortByTag<int>("Aq"));
+//        std::sort(alignments.begin(), alignments.end(), Sort::ByTag<int>("Aq"));
+        Algorithms::SortAlignments(alignments, Sort::ByTag<int>("Aq"));
         printAlignments(alignments);
         cerr << endl
              << "------------------------------" << endl
@@ -149,7 +153,7 @@ bool CountTool::CountToolPrivate::Run(void) {
              << "Sorted Alignments (by tag Aq) desc" << endl
              << "------------------------------" << endl
              << endl;
-        std::sort(alignments.begin(), alignments.end(), Algorithms::SortByTag<int, std::greater>("Aq"));
+        std::sort(alignments.begin(), alignments.end(), Sort::ByTag<int>("Aq", Sort::DescendingOrder));
         printAlignments(alignments);
         cerr << endl
              << "------------------------------" << endl
@@ -179,16 +183,17 @@ bool CountTool::CountToolPrivate::Run(void) {
             reader.LocateIndexes();
 
             // if index data available for all BAM files, we can use SetRegion
-            if ( reader.IsIndexLoaded() ) {
+            if ( reader.HasIndexes() ) {
 
                 vector<BamAlignment> alignments;
+                using namespace BamTools::Algorithms;
 
-                alignments = Algorithms::SortReaderRegion(reader, region, Algorithms::SortByName<>() );
+                alignments = GetSortedRegion(reader, region, Sort::ByName() );
                 printAlignments(alignments);
 
                 cerr << "################################" << endl;
 
-                alignments = Algorithms::SortReaderRegion(reader, region, Algorithms::SortByTag<int>("Aq"));
+                alignments = GetSortedRegion(reader, region, Sort::ByTag<int>("Aq"));
                 printAlignments(alignments);
 
 //                // attempt to set region on reader
diff --git a/src/toolkit/bamtools_sort.cpp b/src/toolkit/bamtools_sort.cpp
index dd6d951..37a74e5 100644
--- a/src/toolkit/bamtools_sort.cpp
+++ b/src/toolkit/bamtools_sort.cpp
@@ -3,7 +3,7 @@
 // Marth Lab, Department of Biology, Boston College
 // All rights reserved.
 // ---------------------------------------------------------------------------
-// Last modified: 14 June 2011 (DB)
+// Last modified: 3 October 2011 (DB)
 // ---------------------------------------------------------------------------
 // Sorts an input BAM file
 // ***************************************************************************
@@ -161,7 +161,7 @@ bool SortTool::SortToolPrivate::GenerateSortedRuns(void) {
     vector<BamAlignment> buffer;
     buffer.reserve( (size_t)(m_settings->MaxBufferCount*1.1) );
     bool bufferFull = false;
-    
+
     // if sorting by name, we need to generate full char data
     // so can't use GetNextAlignmentCore()
     if ( m_settings->IsSortingByName ) {
@@ -268,12 +268,6 @@ bool SortTool::SortToolPrivate::MergeSortedRuns(void) {
         return false;
     }
 
-    // set sort order for merge
-    if ( m_settings->IsSortingByName )
-        multiReader.SetSortOrder(BamMultiReader::SortedByReadName);
-    else
-        multiReader.SetSortOrder(BamMultiReader::SortedByPosition);
-    
     // open writer for our completely sorted output BAM file
     BamWriter mergedWriter;
     if ( !mergedWriter.Open(m_settings->OutputBamFilename, m_headerText, m_references) ) {
@@ -288,7 +282,7 @@ bool SortTool::SortToolPrivate::MergeSortedRuns(void) {
     while ( multiReader.GetNextAlignmentCore(al) )
         mergedWriter.SaveAlignment(al);
   
-    // close readers
+    // close files
     multiReader.Close();
     mergedWriter.Close();
     
@@ -300,6 +294,7 @@ bool SortTool::SortToolPrivate::MergeSortedRuns(void) {
         remove(tempFilename.c_str());
     }
   
+    // return success
     return true;
 }