RMiniFile.cxx

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/// \file RMiniFile.cxx
/// \author Jakob Blomer <jblomer@cern.ch>
/// \date 2019-12-22
 
/*************************************************************************
 * Copyright (C) 1995-2019, Rene Brun and Fons Rademakers.               *
 * All rights reserved.                                                  *
 *                                                                       *
 * For the licensing terms see $ROOTSYS/LICENSE.                         *
 * For the list of contributors see $ROOTSYS/README/CREDITS.             *
 *************************************************************************/
 
#include "Rtypes.h"
#include <ROOT/RConfig.hxx>
#include <ROOT/RError.hxx>
#include <ROOT/RMiniFile.hxx>
#include <ROOT/RRawFile.hxx>
#include <ROOT/RNTupleUtils.hxx>
#include <ROOT/RNTupleZip.hxx>
#include <ROOT/RNTupleSerialize.hxx>
#include <ROOT/RNTupleWriteOptions.hxx>
#include <ROOT/RFile.hxx>
 
#include <Byteswap.h>
#include <TBufferFile.h>
#include <TDirectory.h>
#include <TError.h>
#include <TFile.h>
#include <TKey.h>
#include <TObjString.h>
#include <TUUID.h>
#include <TStreamerInfo.h>
 
#include <xxhash.h>
 
#include <algorithm>
#include <cassert>
#include <cerrno>
#include <cstdio>
#include <cstring>
#include <memory>
#include <string>
#include <chrono>
 
#ifdef R__LINUX
#include <fcntl.h>
#endif
 
#ifndef R__LITTLE_ENDIAN
#ifdef R__BYTESWAP
// `R__BYTESWAP` is defined in RConfig.hxx for little-endian architectures; undefined otherwise
#define R__LITTLE_ENDIAN 1
#else
#define R__LITTLE_ENDIAN 0
#endif
#endif /* R__LITTLE_ENDIAN */
 
namespace {
 
// The following types are used to read and write the TFile binary format
 
/// Big-endian 16-bit unsigned integer
class RUInt16BE {
private:
   std::uint16_t fValBE = 0;
   static std::uint16_t Swap(std::uint16_t val)
   {
#if R__LITTLE_ENDIAN == 1
      return RByteSwap<sizeof(val)>::bswap(val);
#else
      return val;
#endif
   }
 
public:
   RUInt16BE() = default;
   explicit RUInt16BE(const std::uint16_t val) : fValBE(Swap(val)) {}
   operator std::uint16_t() const { return Swap(fValBE); }
   RUInt16BE &operator=(const std::uint16_t val)
   {
      fValBE = Swap(val);
      return *this;
   }
};
 
/// Big-endian 32-bit unsigned integer
class RUInt32BE {
private:
   std::uint32_t fValBE = 0;
   static std::uint32_t Swap(std::uint32_t val)
   {
#if R__LITTLE_ENDIAN == 1
      return RByteSwap<sizeof(val)>::bswap(val);
#else
      return val;
#endif
   }
 
public:
   RUInt32BE() = default;
   explicit RUInt32BE(const std::uint32_t val) : fValBE(Swap(val)) {}
   operator std::uint32_t() const { return Swap(fValBE); }
   RUInt32BE &operator=(const std::uint32_t val)
   {
      fValBE = Swap(val);
      return *this;
   }
};
 
/// Big-endian 32-bit signed integer
class RInt32BE {
private:
   std::int32_t fValBE = 0;
   static std::int32_t Swap(std::int32_t val)
   {
#if R__LITTLE_ENDIAN == 1
      return RByteSwap<sizeof(val)>::bswap(val);
#else
      return val;
#endif
   }
 
public:
   RInt32BE() = default;
   explicit RInt32BE(const std::int32_t val) : fValBE(Swap(val)) {}
   operator std::int32_t() const { return Swap(fValBE); }
   RInt32BE &operator=(const std::int32_t val)
   {
      fValBE = Swap(val);
      return *this;
   }
};
 
/// Big-endian 64-bit unsigned integer
class RUInt64BE {
private:
   std::uint64_t fValBE = 0;
   static std::uint64_t Swap(std::uint64_t val)
   {
#if R__LITTLE_ENDIAN == 1
      return RByteSwap<sizeof(val)>::bswap(val);
#else
      return val;
#endif
   }
 
public:
   RUInt64BE() = default;
   explicit RUInt64BE(const std::uint64_t val) : fValBE(Swap(val)) {}
   operator std::uint64_t() const { return Swap(fValBE); }
   RUInt64BE &operator=(const std::uint64_t val)
   {
      fValBE = Swap(val);
      return *this;
   }
};
 
#pragma pack(push, 1)
/// A name (type, identifier, ...) in the TFile binary format
struct RTFString {
   unsigned char fLName{0};
   char fData[255];
   RTFString() = default;
   RTFString(const std::string &str)
   {
      // The length of strings with 255 characters and longer are encoded with a 32-bit integer following the first
      // byte. This is currently not handled.
      R__ASSERT(str.length() < 255);
      fLName = static_cast<unsigned char>(str.length());
      memcpy(fData, str.data(), fLName);
   }
   std::size_t GetSize() const
   {
      // A length of 255 is special and means that the first byte is followed by a 32-bit integer with the actual
      // length.
      R__ASSERT(fLName != 255);
      return 1 + fLName;
   }
};
 
/// The timestamp format used in TFile; the default constructor initializes with the current time
struct RTFDatetime {
   RUInt32BE fDatetime;
   RTFDatetime()
   {
      auto now = std::chrono::system_clock::now();
      auto tt = std::chrono::system_clock::to_time_t(now);
      auto tm = *localtime(&tt);
      fDatetime = (tm.tm_year + 1900 - 1995) << 26 | (tm.tm_mon + 1) << 22 | tm.tm_mday << 17 | tm.tm_hour << 12 |
                  tm.tm_min << 6 | tm.tm_sec;
   }
   explicit RTFDatetime(RUInt32BE val) : fDatetime(val) {}
};
 
/// The key part of a TFile record excluding the class, object, and title names
struct RTFKey {
   static constexpr unsigned kBigKeyVersion = 1000;
 
   RInt32BE fNbytes{0};
   RUInt16BE fVersion{4};
   RUInt32BE fObjLen{0};
   RTFDatetime fDatetime;
   RUInt16BE fKeyLen{0};
   RUInt16BE fCycle{1};
   union {
      struct {
         RUInt32BE fSeekKey{0};
         RUInt32BE fSeekPdir{0};
      } fInfoShort;
      struct {
         RUInt64BE fSeekKey{0};
         RUInt64BE fSeekPdir{0};
      } fInfoLong;
   };
 
   RTFKey() : fInfoLong() {}
   RTFKey(std::uint64_t seekKey, std::uint64_t seekPdir, const RTFString &clName, const RTFString &objName,
          const RTFString &titleName, std::size_t szObjInMem, std::size_t szObjOnDisk = 0)
   {
      R__ASSERT(szObjInMem <= std::numeric_limits<std::uint32_t>::max());
      R__ASSERT(szObjOnDisk <= std::numeric_limits<std::uint32_t>::max());
      // For writing, we always produce "big" keys with 64-bit SeekKey and SeekPdir.
      fVersion = fVersion + kBigKeyVersion;
      fObjLen = szObjInMem;
      fKeyLen = static_cast<RUInt16BE>(GetHeaderSize() + clName.GetSize() + objName.GetSize() + titleName.GetSize());
      fInfoLong.fSeekKey = seekKey;
      fInfoLong.fSeekPdir = seekPdir;
      // Depends on fKeyLen being set
      fNbytes = fKeyLen + ((szObjOnDisk == 0) ? szObjInMem : szObjOnDisk);
   }
 
   std::uint32_t GetSize() const
   {
      // Negative size indicates a gap in the file
      if (fNbytes < 0)
         return -fNbytes;
      return fNbytes;
   }
 
   std::uint32_t GetHeaderSize() const
   {
      if (fVersion >= kBigKeyVersion)
         return 18 + sizeof(fInfoLong);
      return 18 + sizeof(fInfoShort);
   }
 
   std::uint64_t GetSeekKey() const
   {
      if (fVersion >= kBigKeyVersion)
         return fInfoLong.fSeekKey;
      return fInfoShort.fSeekKey;
   }
};
 
/// The TFile global header
struct RTFHeader {
   static constexpr unsigned kBEGIN = 100;
   static constexpr unsigned kBigHeaderVersion = 1000000;
 
   char fMagic[4]{'r', 'o', 'o', 't'};
   RUInt32BE fVersion{(ROOT_VERSION_CODE >> 16) * 10000 + ((ROOT_VERSION_CODE & 0xFF00) >> 8) * 100 +
                      (ROOT_VERSION_CODE & 0xFF)};
   RUInt32BE fBEGIN{kBEGIN};
   union {
      struct {
         RUInt32BE fEND{0};
         RUInt32BE fSeekFree{0};
         RUInt32BE fNbytesFree{0};
         RUInt32BE fNfree{1};
         RUInt32BE fNbytesName{0};
         unsigned char fUnits{4};
         RUInt32BE fCompress{0};
         RUInt32BE fSeekInfo{0};
         RUInt32BE fNbytesInfo{0};
      } fInfoShort;
      struct {
         RUInt64BE fEND{0};
         RUInt64BE fSeekFree{0};
         RUInt32BE fNbytesFree{0};
         RUInt32BE fNfree{1};
         RUInt32BE fNbytesName{0};
         unsigned char fUnits{8};
         RUInt32BE fCompress{0};
         RUInt64BE fSeekInfo{0};
         RUInt32BE fNbytesInfo{0};
      } fInfoLong;
   };
 
   RTFHeader() : fInfoShort() {}
   RTFHeader(int compression) : fInfoShort() { fInfoShort.fCompress = compression; }
 
   void SetBigFile()
   {
      if (fVersion >= kBigHeaderVersion)
         return;
 
      // clang-format off
      std::uint32_t end        = fInfoShort.fEND;
      std::uint32_t seekFree   = fInfoShort.fSeekFree;
      std::uint32_t nbytesFree = fInfoShort.fNbytesFree;
      std::uint32_t nFree      = fInfoShort.fNfree;
      std::uint32_t nbytesName = fInfoShort.fNbytesName;
      std::uint32_t compress   = fInfoShort.fCompress;
      std::uint32_t seekInfo   = fInfoShort.fSeekInfo;
      std::uint32_t nbytesInfo = fInfoShort.fNbytesInfo;
      fInfoLong.fEND        = end;
      fInfoLong.fSeekFree   = seekFree;
      fInfoLong.fNbytesFree = nbytesFree;
      fInfoLong.fNfree      = nFree;
      fInfoLong.fNbytesName = nbytesName;
      fInfoLong.fUnits      = 8;
      fInfoLong.fCompress   = compress;
      fInfoLong.fSeekInfo   = seekInfo;
      fInfoLong.fNbytesInfo = nbytesInfo;
      fVersion = fVersion + kBigHeaderVersion;
      // clang-format on
   }
 
   bool IsBigFile(std::uint64_t offset = 0) const
   {
      return (fVersion >= kBigHeaderVersion) ||
             (offset > static_cast<unsigned int>(std::numeric_limits<std::int32_t>::max()));
   }
 
   std::uint32_t GetSize() const
   {
      std::uint32_t sizeHead = sizeof(fMagic) + sizeof(fVersion) + sizeof(fBEGIN);
      if (IsBigFile())
         return sizeHead + sizeof(fInfoLong);
      return sizeHead + sizeof(fInfoShort);
   }
 
   std::uint64_t GetEnd() const
   {
      if (IsBigFile())
         return fInfoLong.fEND;
      return fInfoShort.fEND;
   }
 
   void SetEnd(std::uint64_t value)
   {
      if (IsBigFile(value)) {
         SetBigFile();
         fInfoLong.fEND = value;
      } else {
         fInfoShort.fEND = value;
      }
   }
 
   std::uint64_t GetSeekFree() const
   {
      if (IsBigFile())
         return fInfoLong.fSeekFree;
      return fInfoShort.fSeekFree;
   }
 
   void SetSeekFree(std::uint64_t value)
   {
      if (IsBigFile(value)) {
         SetBigFile();
         fInfoLong.fSeekFree = value;
      } else {
         fInfoShort.fSeekFree = value;
      }
   }
 
   void SetNbytesFree(std::uint32_t value)
   {
      if (IsBigFile()) {
         fInfoLong.fNbytesFree = value;
      } else {
         fInfoShort.fNbytesFree = value;
      }
   }
 
   void SetNbytesName(std::uint32_t value)
   {
      if (IsBigFile()) {
         fInfoLong.fNbytesName = value;
      } else {
         fInfoShort.fNbytesName = value;
      }
   }
 
   std::uint64_t GetSeekInfo() const
   {
      if (IsBigFile())
         return fInfoLong.fSeekInfo;
      return fInfoShort.fSeekInfo;
   }
 
   void SetSeekInfo(std::uint64_t value)
   {
      if (IsBigFile(value)) {
         SetBigFile();
         fInfoLong.fSeekInfo = value;
      } else {
         fInfoShort.fSeekInfo = value;
      }
   }
 
   std::uint64_t GetNbytesInfo() const
   {
      if (IsBigFile())
         return fInfoLong.fNbytesInfo;
      return fInfoShort.fNbytesInfo;
   }
 
   void SetNbytesInfo(std::uint32_t value)
   {
      if (IsBigFile()) {
         fInfoLong.fNbytesInfo = value;
      } else {
         fInfoShort.fNbytesInfo = value;
      }
   }
 
   void SetCompression(std::uint32_t value)
   {
      if (IsBigFile()) {
         fInfoLong.fCompress = value;
      } else {
         fInfoShort.fCompress = value;
      }
   }
};
 
/// A reference to an unused byte-range in a TFile
struct RTFFreeEntry {
   static constexpr unsigned kBigFreeEntryVersion = 1000;
 
   RUInt16BE fVersion{1};
   union {
      struct {
         RUInt32BE fFirst{0};
         RUInt32BE fLast{0};
      } fInfoShort;
      struct {
         RUInt64BE fFirst{0};
         RUInt64BE fLast{0};
      } fInfoLong;
   };
 
   RTFFreeEntry() : fInfoShort() {}
   void Set(std::uint64_t first, std::uint64_t last)
   {
      if (last > static_cast<unsigned int>(std::numeric_limits<std::int32_t>::max())) {
         fVersion = fVersion + kBigFreeEntryVersion;
         fInfoLong.fFirst = first;
         fInfoLong.fLast = last;
      } else {
         fInfoShort.fFirst = first;
         fInfoShort.fLast = last;
      }
   }
   std::uint32_t GetSize() { return (fVersion >= kBigFreeEntryVersion) ? 18 : 10; }
};
 
/// The header of the directory key index
struct RTFKeyList {
   RUInt32BE fNKeys;
   std::uint32_t GetSize() const { return sizeof(RTFKeyList); }
   explicit RTFKeyList(std::uint32_t nKeys) : fNKeys(nKeys) {}
};
 
/// A streamed TDirectory (TFile) object
struct RTFDirectory {
   static constexpr unsigned kBigFileVersion = 1000;
 
   RUInt16BE fClassVersion{5};
   RTFDatetime fDateC;
   RTFDatetime fDateM;
   RUInt32BE fNBytesKeys{0};
   RUInt32BE fNBytesName{0};
   // The version of the key has to tell whether offsets are 32bit or 64bit long
   union {
      struct {
         RUInt32BE fSeekDir{RTFHeader::kBEGIN};
         RUInt32BE fSeekParent{0};
         RUInt32BE fSeekKeys{0};
      } fInfoShort;
      struct {
         RUInt64BE fSeekDir{RTFHeader::kBEGIN};
         RUInt64BE fSeekParent{0};
         RUInt64BE fSeekKeys{0};
      } fInfoLong;
   };
 
   RTFDirectory() : fInfoShort() {}
 
   // In case of a short TFile record (<2G), 3 padding ints are written after the UUID
   std::uint32_t GetSize() const
   {
      if (fClassVersion >= kBigFileVersion)
         return sizeof(RTFDirectory);
      return 18 + sizeof(fInfoShort);
   }
 
   std::uint64_t GetSeekKeys() const
   {
      if (fClassVersion >= kBigFileVersion)
         return fInfoLong.fSeekKeys;
      return fInfoShort.fSeekKeys;
   }
 
   void SetSeekKeys(std::uint64_t seekKeys)
   {
      if (seekKeys > static_cast<unsigned int>(std::numeric_limits<std::int32_t>::max())) {
         std::uint32_t seekDir = fInfoShort.fSeekDir;
         std::uint32_t seekParent = fInfoShort.fSeekParent;
         fInfoLong.fSeekDir = seekDir;
         fInfoLong.fSeekParent = seekParent;
         fInfoLong.fSeekKeys = seekKeys;
         fClassVersion = fClassVersion + kBigFileVersion;
      } else {
         fInfoShort.fSeekKeys = seekKeys;
      }
   }
};
 
/// A zero UUID stored at the end of the TFile record
struct RTFUUID {
   RUInt16BE fVersionClass{1};
   unsigned char fUUID[16];
 
   RTFUUID()
   {
      TUUID uuid{TUUID::UUIDv4()};
      char *buffer = reinterpret_cast<char *>(this);
      uuid.FillBuffer(buffer);
      assert(reinterpret_cast<RTFUUID *>(buffer) <= (this + 1));
   }
   std::uint32_t GetSize() const { return sizeof(RTFUUID); }
};
 
/// A streamed RNTuple class
///
/// NOTE: this must be kept in sync with RNTuple.hxx.
/// Aside ensuring consistency between the two classes' members, you need to make sure
/// that fVersionClass matches the class version of RNTuple.
struct RTFNTuple {
   RUInt32BE fByteCount{0x40000000 | (sizeof(RTFNTuple) - sizeof(fByteCount))};
   RUInt16BE fVersionClass{2};
   RUInt16BE fVersionEpoch{0};
   RUInt16BE fVersionMajor{0};
   RUInt16BE fVersionMinor{0};
   RUInt16BE fVersionPatch{0};
   RUInt64BE fSeekHeader{0};
   RUInt64BE fNBytesHeader{0};
   RUInt64BE fLenHeader{0};
   RUInt64BE fSeekFooter{0};
   RUInt64BE fNBytesFooter{0};
   RUInt64BE fLenFooter{0};
   RUInt64BE fMaxKeySize{0};
 
   static constexpr std::uint32_t GetSizePlusChecksum() { return sizeof(RTFNTuple) + sizeof(std::uint64_t); }
 
   RTFNTuple() = default;
   explicit RTFNTuple(const ROOT::RNTuple &inMemoryAnchor)
   {
      fVersionEpoch = inMemoryAnchor.GetVersionEpoch();
      fVersionMajor = inMemoryAnchor.GetVersionMajor();
      fVersionMinor = inMemoryAnchor.GetVersionMinor();
      fVersionPatch = inMemoryAnchor.GetVersionPatch();
      fSeekHeader = inMemoryAnchor.GetSeekHeader();
      fNBytesHeader = inMemoryAnchor.GetNBytesHeader();
      fLenHeader = inMemoryAnchor.GetLenHeader();
      fSeekFooter = inMemoryAnchor.GetSeekFooter();
      fNBytesFooter = inMemoryAnchor.GetNBytesFooter();
      fLenFooter = inMemoryAnchor.GetLenFooter();
      fMaxKeySize = inMemoryAnchor.GetMaxKeySize();
   }
   std::uint32_t GetSize() const { return sizeof(RTFNTuple); }
   // The byte count and class version members are not checksummed
   std::uint32_t GetOffsetCkData() { return sizeof(fByteCount) + sizeof(fVersionClass); }
   std::uint32_t GetSizeCkData() { return GetSize() - GetOffsetCkData(); }
   unsigned char *GetPtrCkData() { return reinterpret_cast<unsigned char *>(this) + GetOffsetCkData(); }
};
 
/// The bare file global header
struct RBareFileHeader {
   char fMagic[7]{'r', 'n', 't', 'u', 'p', 'l', 'e'};
   RUInt32BE fRootVersion{(ROOT_VERSION_CODE >> 16) * 10000 + ((ROOT_VERSION_CODE & 0xFF00) >> 8) * 100 +
                          (ROOT_VERSION_CODE & 0xFF)};
   RUInt32BE fFormatVersion{1};
   RUInt32BE fCompress{0};
   RTFNTuple fNTuple;
   // followed by the ntuple name
};
#pragma pack(pop)
 
/// The artificial class name shown for opaque RNTuple keys (see TBasket)
constexpr char const *kBlobClassName = "RBlob";
/// The class name of the RNTuple anchor
constexpr char const *kNTupleClassName = "ROOT::RNTuple";
 
} // anonymous namespace
 
namespace ROOT {
namespace Internal {
/// If a TFile container is written by a C stream (simple file), on dataset commit, the file header
/// and the TFile record need to be updated
struct RTFileControlBlock {
   RTFHeader fHeader;
   RTFDirectory fFileRecord;
   std::uint64_t fSeekNTuple{0}; // Remember the offset for the keys list
   std::uint64_t fSeekFileRecord{0};
};
 
/// The RKeyBlob writes an invisible key into a TFile.  That is, a key that is not indexed in the list of keys,
/// like a TBasket.
/// NOTE: out of anonymous namespace because otherwise ClassDefInline fails to compile
/// on some platforms.
class RKeyBlob : public TKey { // NOLINT(misc-use-internal-linkage)
public:
   RKeyBlob() = default;
 
   explicit RKeyBlob(TFile *file) : TKey(file)
   {
      fClassName = kBlobClassName;
      fVersion += RTFKey::kBigKeyVersion;
      fKeylen = Sizeof();
   }
 
   /// Register a new key for a data record of size nbytes
   void Reserve(size_t nbytes, std::uint64_t *seekKey)
   {
      Create(nbytes);
      *seekKey = fSeekKey;
   }
 
   bool WasAllocatedInAFreeSlot() const { return fLeft > 0; }
 
   ClassDefInlineOverride(RKeyBlob, 0)
};
 
} // namespace Internal
} // namespace ROOT
 
// Computes how many chunks do we need to fit `nbytes` of payload, considering that the
// first chunk also needs to house the offsets of the other chunks and no chunk can
// be bigger than `maxChunkSize`. When saved to a TFile, each chunk is part of a separate TKey.
static size_t ComputeNumChunks(size_t nbytes, size_t maxChunkSize)
{
   constexpr size_t kChunkOffsetSize = sizeof(std::uint64_t);
 
   assert(nbytes > maxChunkSize);
   size_t nChunks = (nbytes + maxChunkSize - 1) / maxChunkSize;
   assert(nChunks > 1);
   size_t nbytesTail = nbytes % maxChunkSize;
   size_t nbytesExtra = (nbytesTail > 0) * (maxChunkSize - nbytesTail);
   size_t nbytesChunkOffsets = (nChunks - 1) * kChunkOffsetSize;
   if (nbytesChunkOffsets > nbytesExtra) {
      ++nChunks;
      nbytesChunkOffsets += kChunkOffsetSize;
   }
 
   // We don't support having more chunkOffsets than what fits in one chunk.
   // For a reasonable-sized maxKeySize it looks very unlikely that we can have more chunks
   // than we can fit in the first `maxKeySize` bytes. E.g. for maxKeySize = 1GiB we can fit
   // 134217728 chunk offsets, making our multi-key blob's capacity exactly 128 PiB.
   R__ASSERT(nbytesChunkOffsets <= maxChunkSize);
 
   return nChunks;
}
 
ROOT::Internal::RMiniFileReader::RMiniFileReader(ROOT::Internal::RRawFile *rawFile) : fRawFile(rawFile) {}
 
ROOT::RResult<ROOT::RNTuple> ROOT::Internal::RMiniFileReader::GetNTuple(std::string_view ntupleName)
{
   char ident[4];
   ReadBuffer(ident, 4, 0);
   if (std::string(ident, 4) == "root")
      return GetNTupleProper(ntupleName);
   fIsBare = true;
   return GetNTupleBare(ntupleName);
}
 
/// Searches for a key with the given name and type in the key index of the given directory.
/// Return 0 if the key was not found.
std::uint64_t ROOT::Internal::RMiniFileReader::SearchInDirectory(std::uint64_t &offsetDir, std::string_view keyName,
                                                                 std::string_view typeName)
{
   RTFDirectory directory;
   ReadBuffer(&directory, sizeof(directory), offsetDir);
 
   RTFKey key;
   RUInt32BE nKeys;
   std::uint64_t offset = directory.GetSeekKeys();
   ReadBuffer(&key, sizeof(key), offset);
   offset += key.fKeyLen;
   ReadBuffer(&nKeys, sizeof(nKeys), offset);
   offset += sizeof(nKeys);
 
   for (unsigned int i = 0; i < nKeys; ++i) {
      ReadBuffer(&key, sizeof(key), offset);
      auto offsetNextKey = offset + key.fKeyLen;
 
      offset += key.GetHeaderSize();
      RTFString name;
      ReadBuffer(&name, 1, offset);
      ReadBuffer(&name, name.GetSize(), offset);
      if (std::string_view(name.fData, name.fLName) != typeName) {
         offset = offsetNextKey;
         continue;
      }
      offset += name.GetSize();
      ReadBuffer(&name, 1, offset);
      ReadBuffer(&name, name.GetSize(), offset);
      if (std::string_view(name.fData, name.fLName) == keyName) {
         return key.GetSeekKey();
      }
      offset = offsetNextKey;
   }
 
   // Not found
   return 0;
}
 
void ROOT::Internal::RMiniFileReader::LoadStreamerInfo()
{
   if (fIsBare)
      return;
 
   RTFKey key;
   ReadBuffer(&key, sizeof(key), fSeekKeyInfo);
 
   R__ASSERT(fNbytesKeyAndInfo >= key.fKeyLen);
   const std::uint64_t nbytesInfo = fNbytesKeyAndInfo - key.fKeyLen;
   const std::uint64_t seekInfo = fSeekKeyInfo + key.fKeyLen;
   const std::uint32_t uncompLenInfo = key.fObjLen;
   auto streamerInfo = MakeUninitArray<char>(uncompLenInfo);
   if (nbytesInfo == uncompLenInfo) {
      // Uncompressed
      ReadBuffer(streamerInfo.get(), nbytesInfo, seekInfo);
   } else {
      auto buffer = MakeUninitArray<std::byte>(nbytesInfo);
      ReadBuffer(buffer.get(), nbytesInfo, seekInfo);
      RNTupleDecompressor::Unzip(buffer.get(), nbytesInfo, uncompLenInfo, streamerInfo.get());
   }
 
   TBufferFile buffer(TBuffer::kRead, uncompLenInfo, streamerInfo.release());
   // This is necessary to allow the "class tags" inside the StreamerInfo list to refer to the proper offset into
   // the buffer. Normally TFile loads the StreamerInfo via TKey::ReadObjWithBuffer, whose buffer also includes the
   // key itself. Since we dealt with the key above already, we are only passing the payload to TBufferFile so offsets
   // need to be patched up.
   buffer.SetBufferDisplacement(key.fKeyLen);
   TList streamerInfoList;
   streamerInfoList.Streamer(buffer);
   TObjLink *lnk = streamerInfoList.FirstLink();
   while (lnk) {
      auto obj = lnk->GetObject();
      // NOTE: the last element of the streamer info list may be a TList with the IO customization rules, so we need
      // to check before static casting.
      if (obj->IsA() == TStreamerInfo::Class()) {
         auto info = static_cast<TStreamerInfo *>(obj);
         info->BuildCheck();
      }
      lnk = lnk->Next();
   }
}
 
ROOT::RResult<ROOT::RNTuple> ROOT::Internal::RMiniFileReader::GetNTupleProper(std::string_view ntuplePath)
{
   RTFHeader fileHeader;
   ReadBuffer(&fileHeader, sizeof(fileHeader), 0);
 
   fSeekKeyInfo = fileHeader.GetSeekInfo();
   fNbytesKeyAndInfo = fileHeader.GetNbytesInfo();
 
   RTFKey key;
   RTFString name;
   ReadBuffer(&key, sizeof(key), fileHeader.fBEGIN);
   // Skip over the entire key length, including the class name, object name, and title stored in it.
   std::uint64_t offset = fileHeader.fBEGIN + key.fKeyLen;
   // Skip over the name and title of the TNamed preceding the TFile (root TDirectory) entry.
   ReadBuffer(&name, 1, offset);
   offset += name.GetSize();
   ReadBuffer(&name, 1, offset);
   offset += name.GetSize();
 
   // split ntupleName by '/' character to open datasets in subdirectories.
   std::string ntuplePathTail(ntuplePath);
   if (!ntuplePathTail.empty() && ntuplePathTail[0] == '/')
      ntuplePathTail = ntuplePathTail.substr(1);
   auto pos = std::string::npos;
   while ((pos = ntuplePathTail.find('/')) != std::string::npos) {
      auto directoryName = ntuplePathTail.substr(0, pos);
      ntuplePathTail.erase(0, pos + 1);
 
      offset = SearchInDirectory(offset, directoryName, "TDirectory");
      if (offset == 0) {
         return R__FAIL("no directory named '" + std::string(directoryName) + "' in file '" + fRawFile->GetUrl() + "'");
      }
      ReadBuffer(&key, sizeof(key), offset);
      offset = key.GetSeekKey() + key.fKeyLen;
   }
   // no more '/' delimiter in ntuplePath
   auto ntupleName = ntuplePathTail;
 
   offset = SearchInDirectory(offset, ntupleName, kNTupleClassName);
   if (offset == 0) {
      return R__FAIL("no RNTuple named '" + std::string(ntupleName) + "' in file '" + fRawFile->GetUrl() + "'");
   }
 
   ReadBuffer(&key, sizeof(key), offset);
   offset = key.GetSeekKey() + key.fKeyLen;
 
   // size of a RTFNTuple version 2 (min supported version); future anchor versions can grow.
   constexpr size_t kMinNTupleSize = 78;
   static_assert(kMinNTupleSize == RTFNTuple::GetSizePlusChecksum());
   if (key.fObjLen < kMinNTupleSize) {
      return R__FAIL("invalid anchor size: " + std::to_string(key.fObjLen) + " < " + std::to_string(sizeof(RTFNTuple)));
   }
 
   const auto objNbytes = key.GetSize() - key.fKeyLen;
   auto res = GetNTupleProperAtOffset(offset, objNbytes, key.fObjLen);
 
   return res;
}
 
ROOT::RResult<ROOT::RNTuple> ROOT::Internal::RMiniFileReader::GetNTupleProperAtOffset(std::uint64_t payloadOffset,
                                                                                      std::uint64_t compSize,
                                                                                      std::uint64_t uncompLen)
{
   // The object length can be smaller than the size of RTFNTuple if it comes from a past RNTuple class version,
   // or larger than it if it comes from a future RNTuple class version.
   auto bufAnchor = MakeUninitArray<unsigned char>(std::max<size_t>(uncompLen, sizeof(RTFNTuple)));
   RTFNTuple *ntuple = new (bufAnchor.get()) RTFNTuple;
 
   if (compSize != uncompLen) {
      // Read into a temporary buffer
      auto unzipBuf = MakeUninitArray<unsigned char>(std::max<size_t>(uncompLen, sizeof(RTFNTuple)));
      ReadBuffer(unzipBuf.get(), compSize, payloadOffset);
      // Unzip into the final buffer
      RNTupleDecompressor::Unzip(unzipBuf.get(), compSize, uncompLen, ntuple);
   } else {
      ReadBuffer(ntuple, compSize, payloadOffset);
   }
 
   // We require that future class versions only append members and store the checksum in the last 8 bytes
   // Checksum calculation: strip byte count, class version, fChecksum member
   const auto lenCkData = uncompLen - ntuple->GetOffsetCkData() - sizeof(uint64_t);
   const auto ckCalc = XXH3_64bits(ntuple->GetPtrCkData(), lenCkData);
   uint64_t ckOnDisk;
 
   RUInt64BE *ckOnDiskPtr = reinterpret_cast<RUInt64BE *>(bufAnchor.get() + uncompLen - sizeof(uint64_t));
   ckOnDisk = static_cast<uint64_t>(*ckOnDiskPtr);
   if (ckCalc != ckOnDisk) {
      return R__FAIL("RNTuple anchor checksum mismatch");
   }
 
   return CreateAnchor(ntuple->fVersionEpoch, ntuple->fVersionMajor, ntuple->fVersionMinor, ntuple->fVersionPatch,
                       ntuple->fSeekHeader, ntuple->fNBytesHeader, ntuple->fLenHeader, ntuple->fSeekFooter,
                       ntuple->fNBytesFooter, ntuple->fLenFooter, ntuple->fMaxKeySize);
}
 
ROOT::RResult<ROOT::RNTuple> ROOT::Internal::RMiniFileReader::GetNTupleBare(std::string_view ntupleName)
{
   RBareFileHeader fileHeader;
   ReadBuffer(&fileHeader, sizeof(fileHeader), 0);
   RTFString name;
   auto offset = sizeof(fileHeader);
   ReadBuffer(&name, 1, offset);
   ReadBuffer(&name, name.GetSize(), offset);
   std::string_view foundName(name.fData, name.fLName);
   if (foundName != ntupleName) {
      return R__FAIL("expected RNTuple named '" + std::string(ntupleName) + "' but instead found '" +
                     std::string(foundName) + "' in file '" + fRawFile->GetUrl() + "'");
   }
   offset += name.GetSize();
 
   RTFNTuple ntuple;
   ReadBuffer(&ntuple, sizeof(ntuple), offset);
   std::uint64_t onDiskChecksum;
   ReadBuffer(&onDiskChecksum, sizeof(onDiskChecksum), offset + sizeof(ntuple));
   auto checksum = XXH3_64bits(ntuple.GetPtrCkData(), ntuple.GetSizeCkData());
   if (checksum != static_cast<uint64_t>(onDiskChecksum))
      return R__FAIL("RNTuple bare file: anchor checksum mismatch");
 
   return CreateAnchor(ntuple.fVersionEpoch, ntuple.fVersionMajor, ntuple.fVersionMinor, ntuple.fVersionPatch,
                       ntuple.fSeekHeader, ntuple.fNBytesHeader, ntuple.fLenHeader, ntuple.fSeekFooter,
                       ntuple.fNBytesFooter, ntuple.fLenFooter, ntuple.fMaxKeySize);
}
 
void ROOT::Internal::RMiniFileReader::ReadBuffer(void *buffer, size_t nbytes, std::uint64_t offset)
{
   TryReadBuffer(buffer, nbytes, offset).ThrowOnError();
}
 
ROOT::RResult<void> ROOT::Internal::RMiniFileReader::TryReadBuffer(void *buffer, size_t nbytes, std::uint64_t offset)
{
   const auto ByteReadErr = [](std::size_t expected, std::size_t nread) {
      return R__FAIL("invalid read (expected bytes: " + std::to_string(expected) + ", read: " + std::to_string(nread) +
                     ")");
   };
 
   size_t nread;
   if (fMaxKeySize == 0 || nbytes <= fMaxKeySize) {
      // Fast path: read single blob
      nread = fRawFile->ReadAt(buffer, nbytes, offset);
   } else {
      // Read chunked blob. See RNTupleFileWriter::WriteBlob() for details.
      const size_t nChunks = ComputeNumChunks(nbytes, fMaxKeySize);
      const size_t nbytesChunkOffsets = (nChunks - 1) * sizeof(std::uint64_t);
      const size_t nbytesFirstChunk = fMaxKeySize - nbytesChunkOffsets;
      uint8_t *bufCur = reinterpret_cast<uint8_t *>(buffer);
 
      // Read first chunk
      nread = fRawFile->ReadAt(bufCur, fMaxKeySize, offset);
      if (nread != fMaxKeySize)
         return ByteReadErr(fMaxKeySize, nread);
 
      // NOTE: we read the entire chunk in `bufCur`, but we only advance the pointer by `nbytesFirstChunk`,
      // since the last part of `bufCur` will later be overwritten by the next chunk's payload.
      // We do this to avoid a second ReadAt to read in the chunk offsets.
      bufCur += nbytesFirstChunk;
      nread -= nbytesChunkOffsets;
 
      const auto chunkOffsets = MakeUninitArray<std::uint64_t>(nChunks - 1);
      memcpy(chunkOffsets.get(), bufCur, nbytesChunkOffsets);
 
      size_t remainingBytes = nbytes - nbytesFirstChunk;
      std::uint64_t *curChunkOffset = &chunkOffsets[0];
 
      do {
         std::uint64_t chunkOffset;
         RNTupleSerializer::DeserializeUInt64(curChunkOffset, chunkOffset);
         ++curChunkOffset;
 
         const size_t bytesToRead = std::min<size_t>(fMaxKeySize, remainingBytes);
         // Ensure we don't read outside of the buffer
         R__ASSERT(static_cast<size_t>(bufCur - reinterpret_cast<uint8_t *>(buffer)) <= nbytes - bytesToRead);
 
         auto nbytesRead = fRawFile->ReadAt(bufCur, bytesToRead, chunkOffset);
         if (nbytesRead != bytesToRead)
            return ByteReadErr(bytesToRead, nbytesRead);
 
         nread += bytesToRead;
         bufCur += bytesToRead;
         remainingBytes -= bytesToRead;
      } while (remainingBytes > 0);
   }
 
   if (nread != nbytes)
      return ByteReadErr(nbytes, nread);
 
   return RResult<void>::Success();
}
 
////////////////////////////////////////////////////////////////////////////////
 
/// Prepare a blob key in the provided buffer, which must provide space for kBlobKeyLen bytes. Note that the array type
/// is purely documentation, the argument is actually just a pointer.
void ROOT::Internal::RNTupleFileWriter::PrepareBlobKey(std::int64_t offset, size_t nbytes, size_t len,
                                                       unsigned char buffer[kBlobKeyLen])
{
   RTFString strClass{kBlobClassName};
   RTFString strObject;
   RTFString strTitle;
   RTFKey keyHeader(offset, RTFHeader::kBEGIN, strClass, strObject, strTitle, len, nbytes);
   R__ASSERT(keyHeader.fKeyLen == kBlobKeyLen);
 
   // Copy structures into the buffer.
   unsigned char *writeBuffer = buffer;
   memcpy(writeBuffer, &keyHeader, keyHeader.GetHeaderSize());
   writeBuffer += keyHeader.GetHeaderSize();
   memcpy(writeBuffer, &strClass, strClass.GetSize());
   writeBuffer += strClass.GetSize();
   memcpy(writeBuffer, &strObject, strObject.GetSize());
   writeBuffer += strObject.GetSize();
   memcpy(writeBuffer, &strTitle, strTitle.GetSize());
   writeBuffer += strTitle.GetSize();
   R__ASSERT(writeBuffer == buffer + kBlobKeyLen);
}
 
////////////////////////////////////////////////////////////////////////////////
 
ROOT::Internal::RNTupleFileWriter::RImplSimple::RImplSimple() = default;
 
void ROOT::Internal::RNTupleFileWriter::RImplSimple::AllocateBuffers(std::size_t bufferSize)
{
   static_assert(kHeaderBlockSize % kBlockAlign == 0, "invalid header block size");
   if (bufferSize % kBlockAlign != 0)
      throw RException(R__FAIL("Buffer size not a multiple of alignment: " + std::to_string(bufferSize)));
 
   assert(!fShared);
 
   fShared = std::make_shared<RSharedData>(nullptr);
 
   fShared->fBlockSize = bufferSize;
   fShared->fControlBlock = std::make_unique<ROOT::Internal::RTFileControlBlock>();
 
   std::align_val_t blockAlign{kBlockAlign};
   fShared->fHeaderBlock = static_cast<unsigned char *>(::operator new[](kHeaderBlockSize, blockAlign));
   memset(fShared->fHeaderBlock, 0, kHeaderBlockSize);
   fShared->fBlock = static_cast<unsigned char *>(::operator new[](fShared->fBlockSize, blockAlign));
   memset(fShared->fBlock, 0, fShared->fBlockSize);
}
 
ROOT::Internal::RNTupleFileWriter::RImplSimple::RSharedData::RSharedData(FILE *file) : fFile(file) {}
 
ROOT::Internal::RNTupleFileWriter::RImplSimple::RSharedData::~RSharedData()
{
   if (fFile)
      fclose(fFile);
 
   std::align_val_t blockAlign{kBlockAlign};
   if (fHeaderBlock)
      ::operator delete[](fHeaderBlock, blockAlign);
   if (fBlock)
      ::operator delete[](fBlock, blockAlign);
}
 
namespace {
int FSeek64(FILE *stream, std::int64_t offset, int origin)
{
#ifdef R__SEEK64
   return fseeko64(stream, offset, origin);
#else
   return fseek(stream, offset, origin);
#endif
}
} // namespace
 
void ROOT::Internal::RNTupleFileWriter::RImplSimple::Flush()
{
   auto &shared = *fShared;
 
   // Write the last partially filled block, which may still need appropriate alignment for Direct I/O.
   // If it is the first block, get the updated header block.
   if (shared.fBlockOffset == 0) {
      std::size_t headerBlockSize = kHeaderBlockSize;
      if (headerBlockSize > shared.fFilePos) {
         headerBlockSize = shared.fFilePos;
      }
      memcpy(shared.fBlock, shared.fHeaderBlock, headerBlockSize);
   }
 
   std::size_t retval = FSeek64(shared.fFile, shared.fBlockOffset, SEEK_SET);
   if (retval)
      throw RException(R__FAIL(std::string("Seek failed: ") + strerror(errno)));
 
   std::size_t lastBlockSize = shared.fFilePos - shared.fBlockOffset;
   R__ASSERT(lastBlockSize <= shared.fBlockSize);
   if (shared.fDirectIO) {
      // Round up to a multiple of kBlockAlign.
      lastBlockSize += kBlockAlign - 1;
      lastBlockSize = (lastBlockSize / kBlockAlign) * kBlockAlign;
      R__ASSERT(lastBlockSize <= shared.fBlockSize);
   }
   retval = fwrite(shared.fBlock, 1, lastBlockSize, shared.fFile);
   if (retval != lastBlockSize)
      throw RException(R__FAIL(std::string("write failed: ") + strerror(errno)));
 
   // Write the (updated) header block, unless it was part of the write above.
   if (shared.fBlockOffset > 0) {
      retval = FSeek64(shared.fFile, 0, SEEK_SET);
      if (retval)
         throw RException(R__FAIL(std::string("Seek failed: ") + strerror(errno)));
 
      retval = fwrite(shared.fHeaderBlock, 1, kHeaderBlockSize, shared.fFile);
      if (retval != RImplSimple::kHeaderBlockSize)
         throw RException(R__FAIL(std::string("write failed: ") + strerror(errno)));
   }
 
   retval = fflush(shared.fFile);
   if (retval)
      throw RException(R__FAIL(std::string("Flush failed: ") + strerror(errno)));
}
 
void ROOT::Internal::RNTupleFileWriter::RImplSimple::Write(const void *buffer, size_t nbytes, std::int64_t offset)
{
   auto &shared = *fShared;
 
   R__ASSERT(shared.fFile);
   size_t retval;
   if ((offset >= 0) && (static_cast<std::uint64_t>(offset) != shared.fFilePos)) {
      shared.fFilePos = offset;
   }
 
   // Keep header block to overwrite on commit.
   if (shared.fFilePos < kHeaderBlockSize) {
      std::size_t headerBytes = nbytes;
      if (shared.fFilePos + headerBytes > kHeaderBlockSize) {
         headerBytes = kHeaderBlockSize - shared.fFilePos;
      }
      memcpy(shared.fHeaderBlock + shared.fFilePos, buffer, headerBytes);
   }
 
   R__ASSERT(shared.fFilePos >= shared.fBlockOffset);
 
   while (nbytes > 0) {
      std::uint64_t posInBlock = shared.fFilePos % shared.fBlockSize;
      std::uint64_t blockOffset = shared.fFilePos - posInBlock;
      if (blockOffset != shared.fBlockOffset) {
         // Write the block.
         retval = FSeek64(shared.fFile, shared.fBlockOffset, SEEK_SET);
         if (retval)
            throw RException(R__FAIL(std::string("Seek failed: ") + strerror(errno)));
 
         retval = fwrite(shared.fBlock, 1, shared.fBlockSize, shared.fFile);
         if (retval != shared.fBlockSize)
            throw RException(R__FAIL(std::string("write failed: ") + strerror(errno)));
 
         // Null the buffer contents for good measure.
         memset(shared.fBlock, 0, shared.fBlockSize);
      }
 
      shared.fBlockOffset = blockOffset;
      std::size_t blockSize = nbytes;
      if (blockSize > shared.fBlockSize - posInBlock) {
         blockSize = shared.fBlockSize - posInBlock;
      }
      memcpy(shared.fBlock + posInBlock, buffer, blockSize);
      buffer = static_cast<const unsigned char *>(buffer) + blockSize;
      nbytes -= blockSize;
      shared.fFilePos += blockSize;
   }
}
 
std::uint64_t
ROOT::Internal::RNTupleFileWriter::RImplSimple::WriteKey(const void *buffer, std::size_t nbytes, std::size_t len,
                                                         std::int64_t offset, std::uint64_t directoryOffset,
                                                         const std::string &className, const std::string &objectName,
                                                         const std::string &title)
{
   auto &shared = *fShared;
 
   if (offset > 0)
      shared.fKeyOffset = offset;
   RTFString strClass{className};
   RTFString strObject{objectName};
   RTFString strTitle{title};
 
   RTFKey key(shared.fKeyOffset, directoryOffset, strClass, strObject, strTitle, len, nbytes);
   Write(&key, key.GetHeaderSize(), shared.fKeyOffset);
   Write(&strClass, strClass.GetSize());
   Write(&strObject, strObject.GetSize());
   Write(&strTitle, strTitle.GetSize());
   auto offsetData = shared.fFilePos;
   // The next key starts after the data.
   shared.fKeyOffset = offsetData + nbytes;
   if (buffer)
      Write(buffer, nbytes);
 
   return offsetData;
}
 
std::uint64_t ROOT::Internal::RNTupleFileWriter::RImplSimple::ReserveBlobKey(std::size_t nbytes, std::size_t len,
                                                                             unsigned char keyBuffer[kBlobKeyLen])
{
   auto &shared = *fShared;
 
   if (keyBuffer) {
      PrepareBlobKey(shared.fKeyOffset, nbytes, len, keyBuffer);
   } else {
      unsigned char localKeyBuffer[kBlobKeyLen];
      PrepareBlobKey(shared.fKeyOffset, nbytes, len, localKeyBuffer);
      Write(localKeyBuffer, kBlobKeyLen, shared.fKeyOffset);
   }
 
   auto offsetData = shared.fKeyOffset + kBlobKeyLen;
   // The next key starts after the data.
   shared.fKeyOffset = offsetData + nbytes;
 
   return offsetData;
}
 
////////////////////////////////////////////////////////////////////////////////
 
template <typename T>
std::uint64_t ROOT::Internal::RNTupleFileWriter::ReserveBlobKey(T &caller, TFile &file, std::size_t nbytes,
                                                                std::size_t len, unsigned char keyBuffer[kBlobKeyLen])
{
   std::uint64_t offsetKey;
   ROOT::Internal::RKeyBlob keyBlob(&file);
   // Since it is unknown beforehand if offsetKey is beyond the 2GB limit or not,
   // RKeyBlob will always reserve space for a big key (version >= 1000)
   keyBlob.Reserve(nbytes, &offsetKey);
 
   if (keyBuffer) {
      PrepareBlobKey(offsetKey, nbytes, len, keyBuffer);
   } else {
      unsigned char localKeyBuffer[kBlobKeyLen];
      PrepareBlobKey(offsetKey, nbytes, len, localKeyBuffer);
      caller.Write(localKeyBuffer, kBlobKeyLen, offsetKey);
   }
 
   if (keyBlob.WasAllocatedInAFreeSlot()) {
      // If the key was allocated in a free slot, the last 4 bytes of its buffer contain the new size
      // of the remaining free slot and we need to write it to disk before the key gets destroyed at the end of the
      // function.
      caller.Write(keyBlob.GetBuffer() + nbytes, sizeof(Int_t), offsetKey + kBlobKeyLen + nbytes);
   }
 
   auto offsetData = offsetKey + kBlobKeyLen;
 
   return offsetData;
}
 
void ROOT::Internal::RNTupleFileWriter::RImplTFile::Write(const void *buffer, size_t nbytes, std::int64_t offset)
{
   fDirectory->GetFile()->Seek(offset);
   bool rv = fDirectory->GetFile()->WriteBuffer((char *)(buffer), nbytes);
   if (rv)
      throw RException(R__FAIL("WriteBuffer failed."));
}
 
std::uint64_t ROOT::Internal::RNTupleFileWriter::RImplTFile::ReserveBlobKey(size_t nbytes, size_t len,
                                                                            unsigned char keyBuffer[kBlobKeyLen])
{
   auto offsetData = RNTupleFileWriter::ReserveBlobKey(*this, *fDirectory->GetFile(), nbytes, len, keyBuffer);
   return offsetData;
}
 
////////////////////////////////////////////////////////////////////////////////
 
void ROOT::Internal::RNTupleFileWriter::RImplRFile::Write(const void *buffer, size_t nbytes, std::int64_t offset)
{
   auto *file = ROOT::Experimental::Internal::GetRFileTFile(*fFile);
   file->Seek(offset);
   bool rv = file->WriteBuffer((char *)(buffer), nbytes);
   if (rv)
      throw RException(R__FAIL("WriteBuffer failed."));
}
 
std::uint64_t ROOT::Internal::RNTupleFileWriter::RImplRFile::ReserveBlobKey(size_t nbytes, size_t len,
                                                                            unsigned char keyBuffer[kBlobKeyLen])
{
   auto *file = ROOT::Experimental::Internal::GetRFileTFile(*fFile);
   auto offsetData = RNTupleFileWriter::ReserveBlobKey(*this, *file, nbytes, len, keyBuffer);
   return offsetData;
}
 
////////////////////////////////////////////////////////////////////////////////
 
ROOT::Internal::RNTupleFileWriter::RNTupleFileWriter(std::string_view name, std::uint64_t maxKeySize, bool isHidden)
   : fIsHidden(isHidden), fNTupleName(name)
{
   fFile.emplace<RImplSimple>();
   fNTupleAnchor.fMaxKeySize = maxKeySize;
   auto infoRNTuple = RNTuple::Class()->GetStreamerInfo();
   fStreamerInfoMap[infoRNTuple->GetNumber()] = infoRNTuple;
}
 
ROOT::Internal::RNTupleFileWriter::~RNTupleFileWriter() = default;
 
std::unique_ptr<ROOT::Internal::RNTupleFileWriter>
ROOT::Internal::RNTupleFileWriter::Recreate(std::string_view ntupleName, std::string_view path,
                                            EContainerFormat containerFormat, const ROOT::RNTupleWriteOptions &options)
{
   std::string fileName(path);
   size_t idxDirSep = fileName.find_last_of("\\/");
   if (idxDirSep != std::string::npos) {
      fileName.erase(0, idxDirSep + 1);
   }
#ifdef R__LINUX
   int flags = O_WRONLY | O_CREAT | O_TRUNC;
#ifdef O_LARGEFILE
   // Add the equivalent flag that is passed by fopen64.
   flags |= O_LARGEFILE;
#endif
   if (options.GetUseDirectIO()) {
      flags |= O_DIRECT;
   }
   int fd = open(std::string(path).c_str(), flags, 0666);
   if (fd == -1) {
      throw RException(R__FAIL(std::string("open failed for file \"") + std::string(path) + "\": " + strerror(errno)));
   }
   FILE *fileStream = fdopen(fd, "wb");
#else
#ifdef R__SEEK64
   FILE *fileStream = fopen64(std::string(path.data(), path.size()).c_str(), "wb");
#else
   FILE *fileStream = fopen(std::string(path.data(), path.size()).c_str(), "wb");
#endif
#endif
   if (!fileStream) {
      throw RException(R__FAIL(std::string("open failed for file \"") + std::string(path) + "\": " + strerror(errno)));
   }
   // RNTupleFileWriter::RImplSimple does its own buffering, turn off additional buffering from C stdio.
   std::setvbuf(fileStream, nullptr, _IONBF, 0);
 
   auto writer = std::unique_ptr<RNTupleFileWriter>(
      new RNTupleFileWriter(ntupleName, options.GetMaxKeySize(), /*isHidden=*/false));
   RImplSimple &fileSimple = std::get<RImplSimple>(writer->fFile);
   fileSimple.AllocateBuffers(options.GetWriteBufferSize());
   fileSimple.fShared->fFile = fileStream;
   fileSimple.fShared->fDirectIO = options.GetUseDirectIO();
   writer->fFileName = fileName;
 
   int defaultCompression = options.GetCompression();
   switch (containerFormat) {
   case EContainerFormat::kTFile: writer->WriteTFileSkeleton(defaultCompression); break;
   case EContainerFormat::kBare:
      writer->fIsBare = true;
      writer->WriteBareFileSkeleton(defaultCompression);
      break;
   default: R__ASSERT(false && "Internal error: unhandled container format");
   }
 
   return writer;
}
 
std::unique_ptr<ROOT::Internal::RNTupleFileWriter>
ROOT::Internal::RNTupleFileWriter::Append(std::string_view ntupleName, TDirectory &fileOrDirectory,
                                          std::uint64_t maxKeySize, bool hidden)
{
   TFile *file = fileOrDirectory.GetFile();
   if (!file)
      throw RException(R__FAIL("invalid attempt to add an RNTuple to a directory that is not backed by a file"));
   assert(file->IsBinary());
 
   auto writer = std::unique_ptr<RNTupleFileWriter>(new RNTupleFileWriter(ntupleName, maxKeySize, hidden));
   auto &fileProper = writer->fFile.emplace<RImplTFile>();
   fileProper.fDirectory = &fileOrDirectory;
   return writer;
}
 
std::unique_ptr<ROOT::Internal::RNTupleFileWriter>
ROOT::Internal::RNTupleFileWriter::Append(std::string_view ntupleName, ROOT::Experimental::RFile &file,
                                          std::string_view ntupleDir, std::uint64_t maxKeySize)
{
   auto writer = std::unique_ptr<RNTupleFileWriter>(new RNTupleFileWriter(ntupleName, maxKeySize, /*isHidden=*/false));
   auto &rfile = writer->fFile.emplace<RImplRFile>();
   rfile.fFile = &file;
   R__ASSERT(ntupleDir.empty() || ntupleDir[ntupleDir.size() - 1] == '/');
   rfile.fDir = ntupleDir;
   return writer;
}
 
std::unique_ptr<ROOT::Internal::RNTupleFileWriter>
ROOT::Internal::RNTupleFileWriter::CloneAsHidden(std::string_view ntupleName) const
{
   if (auto *tfile = std::get_if<RImplTFile>(&fFile)) {
      return Append(ntupleName, *tfile->fDirectory, fNTupleAnchor.fMaxKeySize, /* isHidden= */ true);
   } else if (auto *file = std::get_if<RImplSimple>(&fFile)) {
      if (fIsBare)
         throw ROOT::RException(R__FAIL("cloning a bare file is currently unsupported"));
 
      auto writer = std::unique_ptr<RNTupleFileWriter>(
         new RNTupleFileWriter(ntupleName, fNTupleAnchor.GetMaxKeySize(), /* isHidden= */ true));
      auto &clonedFile = std::get<RImplSimple>(writer->fFile);
      clonedFile.fShared = file->fShared;
      return writer;
   }
   // TODO: support also RFile-based writers
   throw ROOT::RException(R__FAIL("cannot clone an RFile-based RNTupleFileWriter."));
}
 
void ROOT::Internal::RNTupleFileWriter::Seek(std::uint64_t offset)
{
   RImplSimple *fileSimple = std::get_if<RImplSimple>(&fFile);
   if (!fileSimple)
      throw RException(R__FAIL("invalid attempt to seek non-simple writer"));
 
   fileSimple->fShared->fFilePos = offset;
   fileSimple->fShared->fKeyOffset = offset;
   // The next Write() will Flush() if necessary.
}
 
void ROOT::Internal::RNTupleFileWriter::UpdateStreamerInfos(const RNTupleSerializer::StreamerInfoMap_t &streamerInfos)
{
   fStreamerInfoMap.insert(streamerInfos.cbegin(), streamerInfos.cend());
}
 
ROOT::Internal::RNTupleLink ROOT::Internal::RNTupleFileWriter::Commit(int compression)
{
   const auto WriteStreamerInfoToFile = [&](TFile *file) {
      // Make sure the streamer info records used in the RNTuple are written to the file
      TBufferFile buf(TBuffer::kWrite);
      buf.SetParent(file);
      for (auto [_, info] : fStreamerInfoMap)
         buf.TagStreamerInfo(info);
   };
 
   ROOT::Internal::RNTupleLink anchorInfo;
   // NOTE: checksum length is included in the uncompressed len
   anchorInfo.fLength = RTFNTuple{}.GetSize() + sizeof(std::uint64_t);
   anchorInfo.fLocator.SetType(RNTupleLocator::kTypeFile);
 
   if (auto fileProper = std::get_if<RImplTFile>(&fFile)) {
      // Easy case, the ROOT file header and the RNTuple streaming is taken care of by TFile
      fileProper->fDirectory->WriteObject(&fNTupleAnchor, fNTupleName.c_str());
      WriteStreamerInfoToFile(fileProper->fDirectory->GetFile());
      auto key = static_cast<TKey *>(fileProper->fDirectory->GetListOfKeys()->FindObject(fNTupleName.c_str()));
      R__ASSERT(key);
      anchorInfo.fLocator.SetPosition(key->GetSeekKey() + key->GetKeylen());
      anchorInfo.fLocator.SetNBytesOnStorage(key->GetNbytes() - key->GetKeylen());
      // NOTE: this must happen after FindObject(), otherwise some TFile implementations, such as TBufferMergerFile,
      // may reset the keys list upon write.
      fileProper->fDirectory->GetFile()->Write();
 
      if (fIsHidden) {
         // Remove the anchor's key from the directory's KeysList to disallow retrieving directly the
         // attribute RNTuple from the TFile.
         fileProper->fDirectory->GetListOfKeys()->Remove(key);
      }
   } else if (auto fileRFile = std::get_if<RImplRFile>(&fFile)) {
      // Same as the case above but handled via RFile
      fileRFile->fFile->Put(fileRFile->fDir + fNTupleName, fNTupleAnchor);
      WriteStreamerInfoToFile(ROOT::Experimental::Internal::GetRFileTFile(*fileRFile->fFile));
      auto key = fileRFile->fFile->GetKeyInfo(fNTupleName);
      R__ASSERT(key);
      anchorInfo.fLocator.SetPosition(key->GetSeekKey() + key->GetNBytesKey());
      anchorInfo.fLocator.SetNBytesOnStorage(key->GetNBytesObj());
      fileRFile->fFile->Flush();
   } else {
      // Writing by C file stream: prepare the container format header and stream the RNTuple anchor object
      auto &fileSimple = std::get<RImplSimple>(fFile);
      auto &shared = *fileSimple.fShared;
 
      if (fIsBare) {
         RTFNTuple ntupleOnDisk(fNTupleAnchor);
 
         // Compute the checksum
         std::uint64_t checksum = XXH3_64bits(ntupleOnDisk.GetPtrCkData(), ntupleOnDisk.GetSizeCkData());
         memcpy(shared.fHeaderBlock + shared.fControlBlock->fSeekNTuple, &ntupleOnDisk, ntupleOnDisk.GetSize());
         memcpy(shared.fHeaderBlock + shared.fControlBlock->fSeekNTuple + ntupleOnDisk.GetSize(), &checksum,
                sizeof(checksum));
         fileSimple.Flush();
 
         anchorInfo.fLocator.SetPosition(shared.fControlBlock->fSeekNTuple);
         anchorInfo.fLocator.SetNBytesOnStorage(ntupleOnDisk.GetSize());
      } else {
         anchorInfo = WriteTFileNTupleKey(compression);
         if (!fIsHidden) {
            WriteTFileKeysList(anchorInfo.fLocator.GetNBytesOnStorage()); // NOTE: this is written uncompressed
            WriteTFileStreamerInfo(compression);
            WriteTFileFreeList(); // NOTE: this is written uncompressed
 
            // Update header and TFile record
            memcpy(shared.fHeaderBlock, &shared.fControlBlock->fHeader, shared.fControlBlock->fHeader.GetSize());
            R__ASSERT(shared.fControlBlock->fSeekFileRecord + shared.fControlBlock->fFileRecord.GetSize() <
                      RImplSimple::kHeaderBlockSize);
            memcpy(shared.fHeaderBlock + shared.fControlBlock->fSeekFileRecord, &shared.fControlBlock->fFileRecord,
                   shared.fControlBlock->fFileRecord.GetSize());
         }
 
         fileSimple.Flush();
      }
   }
 
   return anchorInfo;
}
 
std::uint64_t ROOT::Internal::RNTupleFileWriter::WriteBlob(const void *data, size_t nbytes, size_t len)
{
   auto writeKey = [this](const void *payload, size_t nBytes, size_t length) {
      std::uint64_t offset = ReserveBlob(nBytes, length);
      WriteIntoReservedBlob(payload, nBytes, offset);
      return offset;
   };
 
   const std::uint64_t maxKeySize = fNTupleAnchor.fMaxKeySize;
   R__ASSERT(maxKeySize > 0);
   // We don't need the object length except for seeing compression ratios in TFile::Map()
   // Make sure that the on-disk object length fits into the TKey header.
   if (static_cast<std::uint64_t>(len) > static_cast<std::uint64_t>(std::numeric_limits<std::uint32_t>::max()))
      len = nbytes;
 
   if (nbytes <= maxKeySize) {
      // Fast path: only write 1 key.
      return writeKey(data, nbytes, len);
   }
 
   /**
    * Writing a key bigger than the max allowed size. In this case we split the payload
    * into multiple keys, reserving the end of the first key payload for pointers to the
    * next ones. E.g. if a key needs to be split into 3 chunks, the first chunk will have
    * the format:
    *  +--------------------+
    *  |                    |
    *  |        Data        |
    *  |--------------------|
    *  | pointer to chunk 2 |
    *  | pointer to chunk 3 |
    *  +--------------------+
    */
   const size_t nChunks = ComputeNumChunks(nbytes, maxKeySize);
   const size_t nbytesChunkOffsets = (nChunks - 1) * sizeof(std::uint64_t);
   const size_t nbytesFirstChunk = maxKeySize - nbytesChunkOffsets;
   // Skip writing the first chunk, it will be written last (in the file) below.
 
   const uint8_t *chunkData = reinterpret_cast<const uint8_t *>(data) + nbytesFirstChunk;
   size_t remainingBytes = nbytes - nbytesFirstChunk;
 
   const auto chunkOffsetsToWrite = MakeUninitArray<std::uint64_t>(nChunks - 1);
   std::uint64_t chunkOffsetIdx = 0;
 
   do {
      const size_t bytesNextChunk = std::min<size_t>(remainingBytes, maxKeySize);
      const std::uint64_t offset = writeKey(chunkData, bytesNextChunk, bytesNextChunk);
 
      RNTupleSerializer::SerializeUInt64(offset, &chunkOffsetsToWrite[chunkOffsetIdx]);
      ++chunkOffsetIdx;
 
      remainingBytes -= bytesNextChunk;
      chunkData += bytesNextChunk;
 
   } while (remainingBytes > 0);
 
   // Write the first key, with part of the data and the pointers to (logically) following keys appended.
   const std::uint64_t firstOffset = ReserveBlob(maxKeySize, maxKeySize);
   WriteIntoReservedBlob(data, nbytesFirstChunk, firstOffset);
   const std::uint64_t chunkOffsetsOffset = firstOffset + nbytesFirstChunk;
   WriteIntoReservedBlob(chunkOffsetsToWrite.get(), nbytesChunkOffsets, chunkOffsetsOffset);
 
   return firstOffset;
}
 
std::uint64_t
ROOT::Internal::RNTupleFileWriter::ReserveBlob(size_t nbytes, size_t len, unsigned char keyBuffer[kBlobKeyLen])
{
   // ReserveBlob cannot be used to reserve a multi-key blob
   R__ASSERT(nbytes <= fNTupleAnchor.GetMaxKeySize());
 
   std::uint64_t offset;
   if (auto *fileSimple = std::get_if<RImplSimple>(&fFile)) {
      if (fIsBare) {
         offset = fileSimple->fShared->fKeyOffset;
         fileSimple->fShared->fKeyOffset += nbytes;
      } else {
         offset = fileSimple->ReserveBlobKey(nbytes, len, keyBuffer);
      }
   } else if (auto *fileProper = std::get_if<RImplTFile>(&fFile)) {
      offset = fileProper->ReserveBlobKey(nbytes, len, keyBuffer);
   } else {
      auto &fileRFile = std::get<RImplRFile>(fFile);
      offset = fileRFile.ReserveBlobKey(nbytes, len, keyBuffer);
   }
   return offset;
}
 
void ROOT::Internal::RNTupleFileWriter::WriteIntoReservedBlob(const void *buffer, size_t nbytes, std::int64_t offset)
{
   if (auto *fileSimple = std::get_if<RImplSimple>(&fFile)) {
      fileSimple->Write(buffer, nbytes, offset);
   } else if (auto *fileProper = std::get_if<RImplTFile>(&fFile)) {
      fileProper->Write(buffer, nbytes, offset);
   } else {
      auto &fileRFile = std::get<RImplRFile>(fFile);
      fileRFile.Write(buffer, nbytes, offset);
   }
}
 
std::uint64_t ROOT::Internal::RNTupleFileWriter::WriteNTupleHeader(const void *data, size_t nbytes, size_t lenHeader)
{
   auto offset = WriteBlob(data, nbytes, lenHeader);
   fNTupleAnchor.fLenHeader = lenHeader;
   fNTupleAnchor.fNBytesHeader = nbytes;
   fNTupleAnchor.fSeekHeader = offset;
   return offset;
}
 
std::uint64_t ROOT::Internal::RNTupleFileWriter::WriteNTupleFooter(const void *data, size_t nbytes, size_t lenFooter)
{
   auto offset = WriteBlob(data, nbytes, lenFooter);
   fNTupleAnchor.fLenFooter = lenFooter;
   fNTupleAnchor.fNBytesFooter = nbytes;
   fNTupleAnchor.fSeekFooter = offset;
   return offset;
}
 
void ROOT::Internal::RNTupleFileWriter::WriteBareFileSkeleton(int defaultCompression)
{
   RBareFileHeader bareHeader;
   bareHeader.fCompress = defaultCompression;
   auto &fileSimple = std::get<RImplSimple>(fFile);
   fileSimple.Write(&bareHeader, sizeof(bareHeader), 0);
   RTFString ntupleName{fNTupleName};
   fileSimple.Write(&ntupleName, ntupleName.GetSize());
 
   // Write zero-initialized ntuple to reserve the space; will be overwritten on commit
   RTFNTuple ntupleOnDisk;
   fileSimple.fShared->fControlBlock->fSeekNTuple = fileSimple.fShared->fFilePos;
   fileSimple.Write(&ntupleOnDisk, ntupleOnDisk.GetSize());
   std::uint64_t checksum = 0;
   fileSimple.Write(&checksum, sizeof(checksum));
   fileSimple.fShared->fKeyOffset = fileSimple.fShared->fFilePos;
}
 
void ROOT::Internal::RNTupleFileWriter::WriteTFileStreamerInfo(int compression)
{
   // The streamer info record is a TList of TStreamerInfo object.  We cannot use
   // RNTupleSerializer::SerializeStreamerInfos because that uses TBufferIO::WriteObject.
   // This would prepend the streamed TList with self-decription information.
   // The streamer info record is just the streamed TList.
 
   TList streamerInfoList;
   for (auto [_, info] : fStreamerInfoMap) {
      streamerInfoList.Add(info);
   }
 
   // We will stream the list with a TBufferFile. When reading the streamer info records back,
   // the read buffer includes the key and the streamed list.  Therefore, we need to start streaming
   // with an offset of the key length.  Otherwise, the offset for referencing duplicate objects in the
   // buffer will point to the wrong places.
 
   // Figure out key length
   RTFString strTList{"TList"};
   RTFString strStreamerInfo{"StreamerInfo"};
   RTFString strStreamerTitle{"Doubly linked list"};
   auto &fileSimple = std::get<RImplSimple>(fFile);
   fileSimple.fShared->fControlBlock->fHeader.SetSeekInfo(fileSimple.fShared->fKeyOffset);
   auto keyLen = RTFKey(fileSimple.fShared->fControlBlock->fHeader.GetSeekInfo(), RTFHeader::kBEGIN, strTList,
                        strStreamerInfo, strStreamerTitle, 0)
                    .fKeyLen;
 
   TBufferFile buffer(TBuffer::kWrite, keyLen + 1);
   buffer.SetBufferOffset(keyLen);
   streamerInfoList.Streamer(buffer);
   assert(buffer.Length() > keyLen);
   const auto bufPayload = buffer.Buffer() + keyLen;
   const auto lenPayload = buffer.Length() - keyLen;
 
   auto zipStreamerInfos = MakeUninitArray<unsigned char>(lenPayload);
   auto szZipStreamerInfos = RNTupleCompressor::Zip(bufPayload, lenPayload, compression, zipStreamerInfos.get());
 
   fileSimple.WriteKey(zipStreamerInfos.get(), szZipStreamerInfos, lenPayload,
                       fileSimple.fShared->fControlBlock->fHeader.GetSeekInfo(), RTFHeader::kBEGIN, "TList",
                       "StreamerInfo", "Doubly linked list");
   fileSimple.fShared->fControlBlock->fHeader.SetNbytesInfo(fileSimple.fShared->fFilePos -
                                                            fileSimple.fShared->fControlBlock->fHeader.GetSeekInfo());
}
 
void ROOT::Internal::RNTupleFileWriter::WriteTFileKeysList(std::uint64_t anchorSize)
{
   RTFString strEmpty;
   RTFString strRNTupleClass{"ROOT::RNTuple"};
   RTFString strRNTupleName{fNTupleName};
   RTFString strFileName{fFileName};
 
   auto &fileSimple = std::get<RImplSimple>(fFile);
   RTFKey keyRNTuple(fileSimple.fShared->fControlBlock->fSeekNTuple, RTFHeader::kBEGIN, strRNTupleClass, strRNTupleName,
                     strEmpty, RTFNTuple::GetSizePlusChecksum(), anchorSize);
 
   auto &fileShared = *fileSimple.fShared;
   fileSimple.fShared->fControlBlock->fFileRecord.SetSeekKeys(fileShared.fKeyOffset);
   RTFKeyList keyList{1};
   RTFKey keyKeyList(fileSimple.fShared->fControlBlock->fFileRecord.GetSeekKeys(), RTFHeader::kBEGIN, strEmpty,
                     strFileName, strEmpty, keyList.GetSize() + keyRNTuple.fKeyLen);
   fileSimple.Write(&keyKeyList, keyKeyList.GetHeaderSize(),
                    fileSimple.fShared->fControlBlock->fFileRecord.GetSeekKeys());
   fileSimple.Write(&strEmpty, strEmpty.GetSize());
   fileSimple.Write(&strFileName, strFileName.GetSize());
   fileSimple.Write(&strEmpty, strEmpty.GetSize());
   fileSimple.Write(&keyList, keyList.GetSize());
   fileSimple.Write(&keyRNTuple, keyRNTuple.GetHeaderSize());
   // Write class name, object name, and title for this key.
   fileSimple.Write(&strRNTupleClass, strRNTupleClass.GetSize());
   fileSimple.Write(&strRNTupleName, strRNTupleName.GetSize());
   fileSimple.Write(&strEmpty, strEmpty.GetSize());
   fileSimple.fShared->fControlBlock->fFileRecord.fNBytesKeys =
      fileShared.fFilePos - fileSimple.fShared->fControlBlock->fFileRecord.GetSeekKeys();
   fileShared.fKeyOffset = fileShared.fFilePos;
}
 
void ROOT::Internal::RNTupleFileWriter::WriteTFileFreeList()
{
   auto &fileSimple = std::get<RImplSimple>(fFile);
   auto &fileShared = *fileSimple.fShared;
   fileSimple.fShared->fControlBlock->fHeader.SetSeekFree(fileShared.fKeyOffset);
   RTFString strEmpty;
   RTFString strFileName{fFileName};
   RTFFreeEntry freeEntry;
   RTFKey keyFreeList(fileSimple.fShared->fControlBlock->fHeader.GetSeekFree(), RTFHeader::kBEGIN, strEmpty,
                      strFileName, strEmpty, freeEntry.GetSize());
   std::uint64_t firstFree = fileSimple.fShared->fControlBlock->fHeader.GetSeekFree() + keyFreeList.GetSize();
   freeEntry.Set(firstFree, std::max(2000000000ULL, ((firstFree / 1000000000ULL) + 1) * 1000000000ULL));
   fileSimple.WriteKey(&freeEntry, freeEntry.GetSize(), freeEntry.GetSize(),
                       fileSimple.fShared->fControlBlock->fHeader.GetSeekFree(), RTFHeader::kBEGIN, "", fFileName, "");
   fileSimple.fShared->fControlBlock->fHeader.SetNbytesFree(fileShared.fFilePos -
                                                            fileSimple.fShared->fControlBlock->fHeader.GetSeekFree());
   fileSimple.fShared->fControlBlock->fHeader.SetEnd(fileShared.fFilePos);
}
 
ROOT::Internal::RNTupleLink ROOT::Internal::RNTupleFileWriter::WriteTFileNTupleKey(int compression)
{
   RTFString strRNTupleClass{"ROOT::RNTuple"};
   RTFString strRNTupleName{fNTupleName};
   RTFString strEmpty;
 
   RTFNTuple ntupleOnDisk(fNTupleAnchor);
   RUInt64BE checksum{XXH3_64bits(ntupleOnDisk.GetPtrCkData(), ntupleOnDisk.GetSizeCkData())};
   auto &fileSimple = std::get<RImplSimple>(fFile);
   fileSimple.fShared->fControlBlock->fSeekNTuple = fileSimple.fShared->fKeyOffset;
 
   char keyBuf[RTFNTuple::GetSizePlusChecksum()];
 
   // concatenate the RNTuple anchor with its checksum
   memcpy(keyBuf, &ntupleOnDisk, sizeof(RTFNTuple));
   memcpy(keyBuf + sizeof(RTFNTuple), &checksum, sizeof(checksum));
 
   const auto sizeAnchor = sizeof(RTFNTuple) + sizeof(checksum);
   char zipAnchor[RTFNTuple::GetSizePlusChecksum()];
   auto szZipAnchor = RNTupleCompressor::Zip(keyBuf, sizeAnchor, compression, zipAnchor);
 
   ROOT::Internal::RNTupleLink anchorLink;
   auto anchorOffset =
      fileSimple.WriteKey(zipAnchor, szZipAnchor, sizeof(keyBuf), fileSimple.fShared->fControlBlock->fSeekNTuple,
                          RTFHeader::kBEGIN, "ROOT::RNTuple", fNTupleName, "");
 
   assert(szZipAnchor < std::numeric_limits<decltype(anchorLink.fLength)>::max());
   anchorLink.fLength = sizeof(keyBuf);
   anchorLink.fLocator.SetPosition(anchorOffset);
   anchorLink.fLocator.SetNBytesOnStorage(szZipAnchor);
   return anchorLink;
}
 
void ROOT::Internal::RNTupleFileWriter::WriteTFileSkeleton(int defaultCompression)
{
   RTFString strTFile{"TFile"};
   RTFString strFileName{fFileName};
   RTFString strEmpty;
 
   auto &fileSimple = std::get<RImplSimple>(fFile);
   fileSimple.fShared->fControlBlock->fHeader = RTFHeader(defaultCompression);
 
   RTFUUID uuid;
 
   // First record of the file: the TFile object at offset kBEGIN (= 100)
   RTFKey keyRoot(RTFHeader::kBEGIN, 0, strTFile, strFileName, strEmpty,
                  sizeof(RTFDirectory) + strFileName.GetSize() + strEmpty.GetSize() + uuid.GetSize());
   std::uint32_t nbytesName = keyRoot.fKeyLen + strFileName.GetSize() + 1;
   fileSimple.fShared->fControlBlock->fFileRecord.fNBytesName = nbytesName;
   fileSimple.fShared->fControlBlock->fHeader.SetNbytesName(nbytesName);
 
   fileSimple.Write(&keyRoot, keyRoot.GetHeaderSize(), RTFHeader::kBEGIN);
   // Write class name, object name, and title for the TFile key.
   fileSimple.Write(&strTFile, strTFile.GetSize());
   fileSimple.Write(&strFileName, strFileName.GetSize());
   fileSimple.Write(&strEmpty, strEmpty.GetSize());
   // Write the name and title of the TNamed preceding the TFile entry.
   fileSimple.Write(&strFileName, strFileName.GetSize());
   fileSimple.Write(&strEmpty, strEmpty.GetSize());
   // Will be overwritten on commit
   fileSimple.fShared->fControlBlock->fSeekFileRecord = fileSimple.fShared->fFilePos;
   fileSimple.Write(&fileSimple.fShared->fControlBlock->fFileRecord,
                    fileSimple.fShared->fControlBlock->fFileRecord.GetSize());
   fileSimple.Write(&uuid, uuid.GetSize());
 
   // Padding bytes to allow the TFile record to grow for a big file
   RUInt32BE padding{0};
   for (int i = 0; i < 3; ++i)
      fileSimple.Write(&padding, sizeof(padding));
   fileSimple.fShared->fKeyOffset = fileSimple.fShared->fFilePos;
}