Add tests for compression algorithms

Author: hahnjo

compression/README.md

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+# Compression
+
+In this category, all tests write a single `Int64` field with type `std::int64_t`.
+The entries have ascending values and the reference `.json` files only contain the sum of all elements.
+
+ * [`algorithms`](algorithms): `zlib`, `lzma`, `lz4`, `zstd`

compression/algorithms/README.md

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+# Compression Algorithms
+
+ * [`zlib`](zlib): compression settings `101`
+ * [`lzma`](lzma): level 7 (`207`)
+ * [`lz4`](lz4): level 4 (`404`)
+ * [`zstd`](zstd): level 5 (`505`)

compression/algorithms/lz4/read.C

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+#include "../../read_compression.hxx"
+
+void read(std::string_view input = "compression.algorithms.lz4.root",
+          std::string_view output = "compression.algorithms.lz4.json") {
+  read_compression(input, output);
+}

compression/algorithms/lz4/write.C

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+#include "../write_algorithm.hxx"
+
+void write(std::string_view filename = "compression.algorithms.lz4.root") {
+  write_algorithm(filename, 404);
+}

compression/algorithms/lzma/read.C

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+#include "../../read_compression.hxx"
+
+void read(std::string_view input = "compression.algorithms.lzma.root",
+          std::string_view output = "compression.algorithms.lzma.json") {
+  read_compression(input, output);
+}

compression/algorithms/lzma/write.C

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+#include "../write_algorithm.hxx"
+
+void write(std::string_view filename = "compression.algorithms.lzma.root") {
+  write_algorithm(filename, 207);
+}

compression/algorithms/write_algorithm.hxx

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+#include <ROOT/RNTupleModel.hxx>
+#include <ROOT/RNTupleWriteOptions.hxx>
+#include <ROOT/RNTupleWriter.hxx>
+
+using ROOT::Experimental::RNTupleModel;
+using ROOT::Experimental::RNTupleWriteOptions;
+using ROOT::Experimental::RNTupleWriter;
+
+#include <cstdint>
+#include <memory>
+#include <string_view>
+
+void write_algorithm(std::string_view filename, std::uint32_t compression) {
+  auto model = RNTupleModel::Create();
+
+  auto Int64 = model->MakeField<std::int64_t>("Int64");
+
+  RNTupleWriteOptions options;
+  options.SetCompression(compression);
+  auto writer =
+      RNTupleWriter::Recreate(std::move(model), "ntpl", filename, options);
+
+  // Write 32 entries to make sure the compression block is not too small.
+  for (int i = 0; i < 32; i++) {
+    *Int64 = i;
+    writer->Fill();
+  }
+}

compression/algorithms/zlib/read.C

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+#include "../../read_compression.hxx"
+
+void read(std::string_view input = "compression.algorithms.zlib.root",
+          std::string_view output = "compression.algorithms.zlib.json") {
+  read_compression(input, output);
+}

compression/algorithms/zlib/write.C

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+#include "../write_algorithm.hxx"
+
+void write(std::string_view filename = "compression.algorithms.zlib.root") {
+  write_algorithm(filename, 101);
+}

compression/algorithms/zstd/read.C

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+#include "../../read_compression.hxx"
+
+void read(std::string_view input = "compression.algorithms.zstd.root",
+          std::string_view output = "compression.algorithms.zstd.json") {
+  read_compression(input, output);
+}

compression/algorithms/zstd/write.C

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+#include "../write_algorithm.hxx"
+
+void write(std::string_view filename = "compression.algorithms.zstd.root") {
+  write_algorithm(filename, 505);
+}

compression/read_compression.hxx

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+#include <ROOT/REntry.hxx>
+#include <ROOT/RNTupleReader.hxx>
+
+using ROOT::Experimental::REntry;
+using ROOT::Experimental::RNTupleReader;
+
+#include <cstdint>
+#include <fstream>
+#include <ostream>
+#include <string>
+#include <string_view>
+
+void read_compression(std::string_view input, std::string_view output) {
+  auto reader = RNTupleReader::Open("ntpl", input);
+  auto Int64 =
+      reader->GetModel().GetDefaultEntry().GetPtr<std::int64_t>("Int64");
+  std::int64_t sum = 0;
+  for (auto index : *reader) {
+    reader->LoadEntry(index);
+    sum += *Int64;
+  }
+
+  std::ofstream os(std::string{output});
+  os << "{\n";
+  os << "  \"Int64\": " << sum << "\n";
+  os << "}\n";
+}