Fixes for chunk table in no-point LAZ files. (#151)

* Allow empty chunk table when reading LAZ file with no points.

* Fix broken chunk table output for no-point LAZ.

cpp/lazperf/readers.cpp

@@ -306,6 +306,14 @@ void basic_file::Private::parseChunkTable()
     if (chunk_table_header.version != 0)
         throw error("Bad chunk table. Invalid version.");
 
+    if (chunk_table_header.chunk_count == 0)
+    {
+        if (pointCount() != 0)
+            throw error("Missing chunk table.");
+
+        return;
+    }
+
     // Allocate enough room for the chunk table plus one because of the crazy way that
     // the chunk table is written. Once it is fixed up, we resize back to the correct size.
     chunks.resize(chunk_table_header.chunk_count + 1);

cpp/lazperf/writers.cpp

@@ -111,8 +111,9 @@ bool basic_file::Private::open(std::ostream& out, const header12& h, uint32_t cs
 
     if (compressed())
     {
-        // Seek past the chunk table offset.
-        out.seekp(sizeof(uint64_t), std::ios_base::cur);
+        // Reserve 8 bytes for the chunk table offset.
+        const uint64_t dummy = 0;
+        out.write(reinterpret_cast<const char*>(&dummy), sizeof(uint64_t));
     }
     stream.reset(new OutFileStream(out));
     return true;

cpp/test/io_tests.cpp

@@ -742,4 +742,25 @@ TEST(io_tests, can_read_1_1)
   }
 }
 
+TEST(io_tests, can_open_no_points_file)
+{
+    for (const std::string filename : { "no-points-1.3.las", "no-points-1.3.laz" })
+    {
+        reader::named_file f(testFile(filename));
+        EXPECT_EQ(f.header().point_count, 0);
+    }
+}
+
+TEST(io_tests, write_laz_with_no_points_and_read_back)
+{
+    const auto file_name = makeTempFileName();
+
+    const writer::named_file::config c({ 0.01, 0.01, 0.01 }, { 0.0, 0.0, 0.0 });
+    writer::named_file out(file_name, c);
+    out.close();
+
+    reader::named_file f(file_name);
+    EXPECT_EQ(f.header().point_count, 0);
+}
+
 } // namespace lazperf