tech: Java code to tail -N a text file

The technique is to simply map the file into memory using java.nio.channels.FileChannel.map in the Java NIO library and manipulate the file data using memory techniques.

For the "tail" function, we walk back the mapped bytes counting newlines. The MMapIterator class conveniently provides a way to iterate over lines once we find the starting line.

There is a point where care must be taken in the MMapIterator.next() implementation. That is making sure that bytes are converted to the appropriate string encoding. We use "UTF-8" but if you are dealing with a different encoding in the input file, this should be changed.

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