diff --git a/sortbin.cpp b/sortbin.cpp
index 17478c1..bc2c7a5 100644
--- a/sortbin.cpp
+++ b/sortbin.cpp
@@ -62,6 +62,7 @@
 
 
 // TODO : use a background thread for file I/O
+// TODO : use fadvise to drop used data from system cache
 
 
 /* Maximum amount of RAM to use (in MBytes). */
@@ -498,8 +499,6 @@ public:
         m_bufpos += m_record_size;
 
         if (m_bufpos == m_bufend) {
-            m_bufpos = m_buffer.data();
-            m_bufend = m_buffer.data();
             refill_buffer();
         }
     }
@@ -514,13 +513,15 @@ public:
         assert(m_bufpos == m_bufend);
 
         uint64_t file_size = m_input_file.size();
-        assert(m_block_stride < file_size - m_block_offset);
+        assert(m_block_offset <= file_size);
 
-        m_block_offset += m_block_stride;
         m_file_offset = m_block_offset;
         m_block_remaining =
             std::min(m_block_size, file_size - m_block_offset);
 
+        m_block_offset += std::min(m_block_stride,
+                                   file_size - m_block_offset);
+
         refill_buffer();
     }
 
@@ -535,6 +536,7 @@ private:
             m_input_file.read(m_buffer.data(), m_file_offset, transfer_size);
             m_file_offset += transfer_size;
             m_block_remaining -= transfer_size;
+            m_bufpos = m_buffer.data();
             m_bufend = m_buffer.data() + transfer_size;
         }
     }
@@ -577,9 +579,11 @@ public:
       : m_output_file(output_file),
         m_record_size(record_size),
         m_file_offset(file_offset),
-        m_bufpos(0),
         m_buffer(buffer_size)
-    { }
+    {
+        m_bufpos = m_buffer.data();
+        m_bufend = m_buffer.data() + m_buffer.size();
+    }
 
     // Prevent copying and assignment.
     RecordOutputStream(const RecordOutputStream&) = delete;
@@ -588,19 +592,28 @@ public:
     /** Append a record to the output stream. */
     inline void put(const unsigned char *record)
     {
-        if (m_record_size > m_buffer.size() - m_bufpos) {
+        if (m_record_size > m_bufend - m_bufpos) {
             flush();
         }
-        memcpy(m_buffer.data() + m_bufpos, record, m_record_size);
+        memcpy(m_bufpos, record, m_record_size);
+        m_bufpos += m_record_size;
+    }
+
+    /** Return the current file offset. Flush before calling this function. */
+    inline uint64_t file_offset() const
+    {
+        assert(m_bufpos == m_buffer.data());
+        return m_file_offset;
     }
 
     /** Flush buffered records to the output file. */
     void flush()
     {
-        if (m_bufpos > 0) {
-            m_output_file.write(m_buffer.data(), m_file_offset, m_bufpos);
-            m_file_offset += m_bufpos;
-            m_bufpos = 0;
+        size_t flush_size = m_bufpos - m_buffer.data();
+        if (flush_size > 0) {
+            m_output_file.write(m_buffer.data(), m_file_offset, flush_size);
+            m_file_offset += flush_size;
+            m_bufpos = m_buffer.data();
         }
     }
 
@@ -608,7 +621,8 @@ private:
     BinaryFile&         m_output_file;
     const unsigned int  m_record_size;
     uint64_t            m_file_offset;
-    size_t              m_bufpos;
+    unsigned char *     m_bufpos;
+    unsigned char *     m_bufend;
     std::vector<unsigned char>  m_buffer;
 };
 
@@ -1343,7 +1357,7 @@ void merge_n_blocks(
         const unsigned char * rec;
         RecordInputStream * instream;
         std::tie(rec, instream) = heap[0];
-        std::pop_heap(heap.begin(), heap.end());
+        std::pop_heap(heap.begin(), heap.end(), cmp_heap_elem);
 
         if (filter_dupl) {
 
@@ -1372,7 +1386,7 @@ void merge_n_blocks(
         } else {
             // Push next record from the stream into the heap.
             heap.back() = std::make_tuple(instream->record(), instream);
-            std::push_heap(heap.begin(), heap.end());
+            std::push_heap(heap.begin(), heap.end(), cmp_heap_elem);
         }
     }
 }
@@ -1510,6 +1524,14 @@ void merge_pass(
     // Flush output stream buffers.
     output_stream.flush();
 
+    // Shrink output file if duplicate records were removed.
+    if (filter_dupl) {
+        uint64_t output_size = output_stream.file_offset();
+        uint64_t num_output_records = output_size / ctx.record_size;
+        log(ctx, "found %zu unique records\n", num_output_records);
+        output_file.truncate_file(output_size);
+    }
+
     timer.stop();
     log(ctx, "  t = %.3f seconds\n", timer.value());
 }
@@ -1740,7 +1762,7 @@ int main(int argc, char **argv)
     std::string tempdir = get_default_tmpdir();
     int opt;
 
-    while ((opt = getopt_long(argc, argv, "uv", longopts, NULL)) != -1) {
+    while ((opt = getopt_long(argc, argv, "s:T:uv", longopts, NULL)) != -1) {
         switch (opt) {
             case 's':
                 record_size = atoi(optarg);