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  <record name="LCIORandomAccess">
     There are two types of LCIORandomAccess records
       file record -- one per file, always first record on file
       index record -- one or more per file, points to associated LCIOIndex record
     <block name="LCIORandomAccess" major="1" minor="0">
         <data type="int" name="runMin"/>
         <data type="int" name="eventMin"/>
         <data type="int" name="runMax"/>
         <data type="int" name="eventMax"/>
         <data type="int" name="nRunHeaders"/>
         <data type="int" name="nEvents"/>
         <data type="int" name="recordsAreInOrder"/>
         <data type="long" name="indexLocation"/>>
            Location in file off associated index. Always null for file record.
         </data>
         <data type="long" name="prevLocation"/>>
            For file record location of first index record in file
            For index record location of previous index record (or null if first)
         </data>
         <data type="long" name="nextLocation"/>>
            For file record location of last index record in file
            For index record location of next index record (or null if last)
         </data>
     </block>
  </record>

  <record name="LCIOIndex">
     <block name="LCIOIndex" major="1" minor="0">
         <data type="int" name="controlWord"/>>
            Bit 0 = single Run
            Bit 1 = long offset required
         </data>
         <data type="int" name="runMin"/>
         <data type="long" name="baseOffset"/>
         <data type="int" name="size"/>
         <repeat count="size">
             <if condition="(controlWord&amp;1)==0">
                <data type="int" name="runOffset"/>>
                    Relative to runMin
                </data>
             </if>
             <data type="int" name="eventNumber"/>>
                Event number, or -1 for run header records
             </data>
             <if condition="(controlWord&amp;2)==1">
                <data type="long" name="locationOffset"/>>
                    Relative to baseOffset
                </data>
             <else/>
                <data type="int" name="locationOffset"/>>
                    Relative to baseOffset
                </data>
             </if>
         </repeat>
     </block>
  </record>

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If the file is written with support for random access enabled then the first record in the file will be a LCIORandomAccess record which describes the entire file. This block will then point to one or more LCIORandomAccess records elsewhere in the file, each of which will have an associated LCIOIndex record. The LCIOIndex record contains the location of each record EventHeader/RunHeader record in the file.

Implementation notes

• The purpose of having two types of records is that the (small) LCIORandomAccess records can be written uncompressed. This allows it them to be read quickly, and updated after it is they are written using the SIO random access mechanism described below. The larger LCIOIndex record records can be written with compression turned on since it they never needs need to be updated once it is they are written.
• The reason for having multiple LCIOIndex records in a file is to prevent the problem of having to store a potentially infinitely large index blocks in memory. By storing something like 10-100k records per index block performance to access records is still good, without huge memory usage even for very large (chains) of files. This also makes the task of appending to an existing files much easier.
• Older LCIO readers should ignore/skip the LCIOIndex/LCIORandomAccess records.