[Jnode-svn-commits] SF.net SVN: jnode:[5968] trunk/fs/src/fs/org/jnode/fs/ntfs/ CompressedDataRun.java

[<ga...@us...>]

Revision: 5968
          http://jnode.svn.sourceforge.net/jnode/?rev=5968&view=rev
Author:   galatnm
Date:     2013-02-20 12:01:01 +0000 (Wed, 20 Feb 2013)
Log Message:
-----------
Better implementation of mapVcnToLcn() for compressed data runs. (Luke Quinane)

Modified Paths:
--------------
    trunk/fs/src/fs/org/jnode/fs/ntfs/CompressedDataRun.java

Modified: trunk/fs/src/fs/org/jnode/fs/ntfs/CompressedDataRun.java
===================================================================
--- trunk/fs/src/fs/org/jnode/fs/ntfs/CompressedDataRun.java	2013-02-20 11:56:05 UTC (rev 5967)
+++ trunk/fs/src/fs/org/jnode/fs/ntfs/CompressedDataRun.java	2013-02-20 12:01:01 UTC (rev 5968)
@@ -224,7 +224,19 @@
 
 	@Override
 	public long mapVcnToLcn(long vcn) {
-		return compressedRun.mapVcnToLcn(vcn);
+        // This is the actual number of stored clusters after compression.
+        // If the number of stored clusters is the same as the compression unit size,
+        // then the data can be read directly without decompressing it.
+        final int compClusters = compressedRun.getLength();
+        if (compClusters == compressionUnitSize) {
+            return compressedRun.mapVcnToLcn(vcn);
+        }
+
+        // Now we know the data is compressed. Map the VCN to the corresponding compressed block...
+        final long actFirstVcn = Math.max(compressedRun.getFirstVcn(), vcn);
+        final int vcnOffsetWithinUnit = (int) (actFirstVcn % compressionUnitSize);
+        final long compFirstVcn = actFirstVcn - vcnOffsetWithinUnit;
+        return compressedRun.mapVcnToLcn(compFirstVcn);
 	}
 
 	/**

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