Add tests for compression blocks

Author: hahnjo

compression/README.md

@@ -5,3 +5,4 @@ Therefore all tests in this category write a single `Int64` field with type `std
 The entries have ascending values and the reference `.json` files only contain the sum of all elements.
 
  * [`algorithms`](algorithms): `zlib`, `lzma`, `lz4`, `zstd`
+ * [`block`](block): big and short compression blocks

compression/block/README.md

@@ -0,0 +1,4 @@
+# Compression Blocks
+
+ * [`big`](big): big compression blocks, larger than 16 MiB
+ * [`short`](short): a "short" compression that is actually uncompressed

compression/block/big/read.C

@@ -0,0 +1,6 @@
+#include "../../read_compression.hxx"
+
+void read(std::string_view input = "compression.block.big.root",
+          std::string_view output = "compression.block.big.json") {
+  read_compression(input, output);
+}

compression/block/big/write.C

@@ -0,0 +1,38 @@
+#include <ROOT/RNTupleModel.hxx>
+#include <ROOT/RNTupleUtil.hxx>
+#include <ROOT/RNTupleWriteOptions.hxx>
+#include <ROOT/RNTupleWriter.hxx>
+
+using ROOT::Experimental::EColumnType;
+using ROOT::Experimental::RNTupleModel;
+using ROOT::Experimental::RNTupleWriteOptions;
+using ROOT::Experimental::RNTupleWriter;
+
+#include <cstdint>
+#include <memory>
+#include <string_view>
+
+void write(std::string_view filename = "compression.block.big.root") {
+  auto model = RNTupleModel::Create();
+
+  auto Int64 = model->MakeField<std::int64_t>("Int64");
+  model->GetMutableField("Int64").SetColumnRepresentatives(
+      {{EColumnType::kSplitInt64}});
+
+  RNTupleWriteOptions options;
+  // Crank up the zstd compression level to reduce the output file size by
+  // approximately a factor 6 (from 76K with 505 to 12K).
+  options.SetCompression(509);
+  // Increase the maximum unzipped page size to make it bigger than the maximum
+  // size of a compression block, which is 16 MiB.
+  options.SetMaxUnzippedPageSize(128 * 1024 * 1024);
+  auto writer =
+      RNTupleWriter::Recreate(std::move(model), "ntpl", filename, options);
+
+  // Write 40 MiB of entries that will be split into three compression blocks.
+  static constexpr int Entries = 40 * 1024 * 1024 / sizeof(std::int64_t);
+  for (int i = 0; i < Entries; i++) {
+    *Int64 = i;
+    writer->Fill();
+  }
+}

compression/block/short/read.C

@@ -0,0 +1,6 @@
+#include "../../read_compression.hxx"
+
+void read(std::string_view input = "compression.block.short.root",
+          std::string_view output = "compression.block.short.json") {
+  read_compression(input, output);
+}

compression/block/short/write.C

@@ -0,0 +1,33 @@
+#include <ROOT/RNTupleModel.hxx>
+#include <ROOT/RNTupleUtil.hxx>
+#include <ROOT/RNTupleWriteOptions.hxx>
+#include <ROOT/RNTupleWriter.hxx>
+
+using ROOT::Experimental::EColumnType;
+using ROOT::Experimental::RNTupleModel;
+using ROOT::Experimental::RNTupleWriteOptions;
+using ROOT::Experimental::RNTupleWriter;
+
+#include <cstdint>
+#include <memory>
+#include <string_view>
+
+void write(std::string_view filename = "compression.block.short.root") {
+  auto model = RNTupleModel::Create();
+
+  auto Int64 = model->MakeField<std::int64_t>("Int64");
+  model->GetMutableField("Int64").SetColumnRepresentatives(
+      {{EColumnType::kSplitInt64}});
+
+  RNTupleWriteOptions options;
+  options.SetCompression(505);
+  auto writer =
+      RNTupleWriter::Recreate(std::move(model), "ntpl", filename, options);
+
+  // Write only 2 entries to make sure the compression block is small and
+  // actually stored uncompressed.
+  for (int i = 0; i < 2; i++) {
+    *Int64 = i;
+    writer->Fill();
+  }
+}