--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/com/tmate/hgkit/fs/DataAccessProvider.java	Sat Dec 25 21:50:12 2010 +0100
@@ -0,0 +1,191 @@
+/*
+ * Copyright (c) 2010 Artem Tikhomirov 
+ */
+package com.tmate.hgkit.fs;
+
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.nio.MappedByteBuffer;
+import java.nio.channels.FileChannel;
+
+/**
+ *
+ * @author artem
+ */
+public class DataAccessProvider {
+
+	private final int mapioMagicBoundary;
+	private final int bufferSize;
+
+	public DataAccessProvider() {
+		mapioMagicBoundary = 100 * 1024;
+		bufferSize = 8 * 1024;
+	}
+
+	public DataAccess create(File f) {
+		if (!f.exists()) {
+			return new DataAccess();
+		}
+		try {
+			FileChannel fc = new FileInputStream(f).getChannel();
+			if (fc.size() > mapioMagicBoundary) {
+				return new MemoryMapFileAccess(fc, fc.size());
+			} else {
+				// XXX once implementation is more or less stable,
+				// may want to try ByteBuffer.allocateDirect() to see
+				// if there's any performance gain. 
+				boolean useDirectBuffer = false;
+				return new FileAccess(fc, fc.size(), bufferSize, useDirectBuffer);
+			}
+		} catch (IOException ex) {
+			// unlikely to happen, we've made sure file exists.
+			ex.printStackTrace(); // FIXME log error
+		}
+		return new DataAccess(); // non-null, empty.
+	}
+
+	// DOESN'T WORK YET 
+	private static class MemoryMapFileAccess extends DataAccess {
+		private FileChannel fileChannel;
+		private final long size;
+		private long position = 0;
+
+		public MemoryMapFileAccess(FileChannel fc, long channelSize) {
+			fileChannel = fc;
+			size = channelSize;
+		}
+
+		@Override
+		public void seek(long offset) {
+			position = offset;
+		}
+
+		@Override
+		public void skip(int bytes) throws IOException {
+			position += bytes;
+		}
+
+		private boolean fill() throws IOException {
+			final int BUFFER_SIZE = 8 * 1024;
+			long left = size - position;
+			MappedByteBuffer rv = fileChannel.map(FileChannel.MapMode.READ_ONLY, position, left < BUFFER_SIZE ? left : BUFFER_SIZE);
+			position += rv.capacity();
+			return rv.hasRemaining();
+		}
+
+		@Override
+		public void done() {
+			if (fileChannel != null) {
+				try {
+					fileChannel.close();
+				} catch (IOException ex) {
+					ex.printStackTrace(); // log debug
+				}
+				fileChannel = null;
+			}
+		}
+	}
+
+	// (almost) regular file access - FileChannel and buffers.
+	private static class FileAccess extends DataAccess {
+		private FileChannel fileChannel;
+		private final long size;
+		private ByteBuffer buffer;
+		private long bufferStartInFile = 0; // offset of this.buffer in the file.
+
+		public FileAccess(FileChannel fc, long channelSize, int bufferSizeHint, boolean useDirect) {
+			fileChannel = fc;
+			size = channelSize;
+			final int capacity = size < bufferSizeHint ? (int) size : bufferSizeHint;
+			buffer = useDirect ? ByteBuffer.allocateDirect(capacity) : ByteBuffer.allocate(capacity);
+			buffer.flip(); // or .limit(0) to indicate it's empty
+		}
+		
+		@Override
+		public boolean isEmpty() {
+			return bufferStartInFile + buffer.position() >= size;
+		}
+		
+		@Override
+		public void seek(long offset) throws IOException {
+			if (offset < bufferStartInFile + buffer.limit() && offset >= bufferStartInFile) {
+				buffer.position((int) (offset - bufferStartInFile));
+			} else {
+				// out of current buffer, invalidate it (force re-read) 
+				// XXX or ever re-read it right away?
+				bufferStartInFile = offset;
+				buffer.clear();
+				buffer.limit(0); // or .flip() to indicate we switch to reading
+				fileChannel.position(offset);
+			}
+		}
+
+		@Override
+		public void skip(int bytes) throws IOException {
+			final int newPos = buffer.position() + bytes;
+			if (newPos >= 0 && newPos < buffer.limit()) {
+				// no need to move file pointer, just rewind/seek buffer 
+				buffer.position(newPos);
+			} else {
+				//
+				seek(fileChannel.position()+ bytes);
+			}
+		}
+
+		private boolean fill() throws IOException {
+			if (!buffer.hasRemaining()) {
+				bufferStartInFile += buffer.limit();
+				buffer.clear();
+				if (bufferStartInFile < size) { // just in case there'd be any exception on EOF, not -1 
+					fileChannel.read(buffer);
+					// may return -1 when EOF, but empty will reflect this, hence no explicit support here   
+				}
+				buffer.flip();
+			}
+			return buffer.hasRemaining();
+		}
+
+		@Override
+		public void readBytes(byte[] buf, int offset, int length) throws IOException {
+			final int tail = buffer.remaining();
+			if (tail >= length) {
+				buffer.get(buf, offset, length);
+			} else {
+				buffer.get(buf, offset, tail);
+				if (fill()) {
+					buffer.get(buf, offset + tail, length - tail);
+				} else {
+					throw new IOException(); // shall not happen provided stream contains expected data and no attempts to read past nonEmpty() == false are made. 
+				}
+			}
+		}
+
+		@Override
+		public byte readByte() throws IOException {
+			if (buffer.hasRemaining()) {
+				return buffer.get();
+			}
+			if (fill()) {
+				return buffer.get();
+			}
+			throw new IOException();
+		}
+
+		@Override
+		public void done() {
+			if (buffer != null) {
+				buffer = null;
+			}
+			if (fileChannel != null) {
+				try {
+					fileChannel.close();
+				} catch (IOException ex) {
+					ex.printStackTrace(); // log debug
+				}
+				fileChannel = null;
+			}
+		}
+	}
+}