/**
   * Read a particular segmentFileName. Note that this may throw an IOException if a commit is in
   * process.
   *
   * @param directory -- directory containing the segments file
   * @param segmentFileName -- segment file to load
   * @throws CorruptIndexException if the index is corrupt
   * @throws IOException if there is a low-level IO error
   */
  public static final SegmentInfos readCommit(Directory directory, String segmentFileName)
      throws IOException {

    long generation = generationFromSegmentsFileName(segmentFileName);
    try (ChecksumIndexInput input = directory.openChecksumInput(segmentFileName, IOContext.READ)) {
      // NOTE: as long as we want to throw indexformattooold (vs corruptindexexception), we need
      // to read the magic ourselves.
      int magic = input.readInt();
      if (magic != CodecUtil.CODEC_MAGIC) {
        throw new IndexFormatTooOldException(
            input, magic, CodecUtil.CODEC_MAGIC, CodecUtil.CODEC_MAGIC);
      }
      // 4.0+
      int format = CodecUtil.checkHeaderNoMagic(input, "segments", VERSION_40, VERSION_CURRENT);
      // 5.0+
      byte id[] = null;
      if (format >= VERSION_50) {
        id = new byte[StringHelper.ID_LENGTH];
        input.readBytes(id, 0, id.length);
        CodecUtil.checkIndexHeaderSuffix(input, Long.toString(generation, Character.MAX_RADIX));
      }

      SegmentInfos infos = new SegmentInfos();
      infos.id = id;
      infos.generation = generation;
      infos.lastGeneration = generation;
      if (format >= VERSION_53) {
        // TODO: in the future (7.0?  sigh) we can use this to throw IndexFormatTooOldException ...
        // or just rely on the
        // minSegmentLuceneVersion check instead:
        infos.luceneVersion =
            Version.fromBits(input.readVInt(), input.readVInt(), input.readVInt());
      } else {
        // else compute the min version down below in the for loop
      }

      infos.version = input.readLong();
      infos.counter = input.readInt();
      int numSegments = input.readInt();
      if (numSegments < 0) {
        throw new CorruptIndexException("invalid segment count: " + numSegments, input);
      }

      if (format >= VERSION_53) {
        if (numSegments > 0) {
          infos.minSegmentLuceneVersion =
              Version.fromBits(input.readVInt(), input.readVInt(), input.readVInt());
          if (infos.minSegmentLuceneVersion.onOrAfter(Version.LUCENE_4_0_0_ALPHA) == false) {
            throw new IndexFormatTooOldException(
                input,
                "this index contains a too-old segment (version: "
                    + infos.minSegmentLuceneVersion
                    + ")");
          }
        } else {
          // else leave as null: no segments
        }
      } else {
        // else we recompute it below as we visit segments; it can't be used for throwing
        // IndexFormatTooOldExc, but consumers of
        // SegmentInfos can maybe still use it for other reasons
      }

      long totalDocs = 0;
      for (int seg = 0; seg < numSegments; seg++) {
        String segName = input.readString();
        final byte segmentID[];
        if (format >= VERSION_50) {
          byte hasID = input.readByte();
          if (hasID == 1) {
            segmentID = new byte[StringHelper.ID_LENGTH];
            input.readBytes(segmentID, 0, segmentID.length);
          } else if (hasID == 0) {
            segmentID = null; // 4.x segment, doesn't have an ID
          } else {
            throw new CorruptIndexException("invalid hasID byte, got: " + hasID, input);
          }
        } else {
          segmentID = null;
        }
        Codec codec = readCodec(input, format < VERSION_53);
        SegmentInfo info =
            codec.segmentInfoFormat().read(directory, segName, segmentID, IOContext.READ);
        info.setCodec(codec);
        totalDocs += info.maxDoc();
        long delGen = input.readLong();
        int delCount = input.readInt();
        if (delCount < 0 || delCount > info.maxDoc()) {
          throw new CorruptIndexException(
              "invalid deletion count: " + delCount + " vs maxDoc=" + info.maxDoc(), input);
        }
        long fieldInfosGen = -1;
        if (format >= VERSION_46) {
          fieldInfosGen = input.readLong();
        }
        long dvGen = -1;
        if (format >= VERSION_49) {
          dvGen = input.readLong();
        } else {
          dvGen = fieldInfosGen;
        }
        SegmentCommitInfo siPerCommit =
            new SegmentCommitInfo(info, delCount, delGen, fieldInfosGen, dvGen);
        if (format >= VERSION_46) {
          if (format < VERSION_49) {
            // Recorded per-generation files, which were buggy (see
            // LUCENE-5636). We need to read and keep them so we continue to
            // reference those files. Unfortunately it means that the files will
            // be referenced even if the fields are updated again, until the
            // segment is merged.
            final int numGensUpdatesFiles = input.readInt();
            final Map<Long, Set<String>> genUpdatesFiles;
            if (numGensUpdatesFiles == 0) {
              genUpdatesFiles = Collections.emptyMap();
            } else {
              genUpdatesFiles = new HashMap<>(numGensUpdatesFiles);
              for (int i = 0; i < numGensUpdatesFiles; i++) {
                genUpdatesFiles.put(input.readLong(), input.readStringSet());
              }
            }
            siPerCommit.setGenUpdatesFiles(genUpdatesFiles);
          } else {
            if (format >= VERSION_51) {
              siPerCommit.setFieldInfosFiles(input.readSetOfStrings());
            } else {
              siPerCommit.setFieldInfosFiles(Collections.unmodifiableSet(input.readStringSet()));
            }
            final Map<Integer, Set<String>> dvUpdateFiles;
            final int numDVFields = input.readInt();
            if (numDVFields == 0) {
              dvUpdateFiles = Collections.emptyMap();
            } else {
              Map<Integer, Set<String>> map = new HashMap<>(numDVFields);
              for (int i = 0; i < numDVFields; i++) {
                if (format >= VERSION_51) {
                  map.put(input.readInt(), input.readSetOfStrings());
                } else {
                  map.put(input.readInt(), Collections.unmodifiableSet(input.readStringSet()));
                }
              }
              dvUpdateFiles = Collections.unmodifiableMap(map);
            }
            siPerCommit.setDocValuesUpdatesFiles(dvUpdateFiles);
          }
        }
        infos.add(siPerCommit);

        Version segmentVersion = info.getVersion();
        if (format < VERSION_53) {
          if (infos.minSegmentLuceneVersion == null
              || segmentVersion.onOrAfter(infos.minSegmentLuceneVersion) == false) {
            infos.minSegmentLuceneVersion = segmentVersion;
          }
        } else if (segmentVersion.onOrAfter(infos.minSegmentLuceneVersion) == false) {
          throw new CorruptIndexException(
              "segments file recorded minSegmentLuceneVersion="
                  + infos.minSegmentLuceneVersion
                  + " but segment="
                  + info
                  + " has older version="
                  + segmentVersion,
              input);
        }
      }

      if (format >= VERSION_51) {
        infos.userData = input.readMapOfStrings();
      } else {
        infos.userData = Collections.unmodifiableMap(input.readStringStringMap());
      }

      if (format >= VERSION_48) {
        CodecUtil.checkFooter(input);
      } else {
        final long checksumNow = input.getChecksum();
        final long checksumThen = input.readLong();
        if (checksumNow != checksumThen) {
          throw new CorruptIndexException(
              "checksum failed (hardware problem?) : expected="
                  + Long.toHexString(checksumThen)
                  + " actual="
                  + Long.toHexString(checksumNow),
              input);
        }
        CodecUtil.checkEOF(input);
      }

      // LUCENE-6299: check we are in bounds
      if (totalDocs > IndexWriter.getActualMaxDocs()) {
        throw new CorruptIndexException(
            "Too many documents: an index cannot exceed "
                + IndexWriter.getActualMaxDocs()
                + " but readers have total maxDoc="
                + totalDocs,
            input);
      }

      return infos;
    }
  }