Enhance calculation of significant unallocated space (#499202)

Many file systems report differing percentages of unallocated space over
a range of sizes, as well differing figures using their own specific
tools or using statvfs() system call when mounted.

File systems reporting intrinsic unallocated space using their specific
tools are: jfs, lvm2 pv and ntfs.  LVM2 PV has the largest amount of
unallocated space with its default Physical Extent size of 4 MiB.  For a
100 MiB partition it has 4.0% unallocated space.

File systems reporting intrinsic unallocated space using the statvfs()
system call when mounted are: ext2/3/4, fat16/32, hfs, jfs, nilfs2,
ntfs, reiserfs, and xfs.  Xfs has the worst identified unallocated space
of ~4.7% in a 100 MiB partition.  Ext2/3 exhibit unusual behaviour by
reporting unallocated space of ~4.6% in a 100 MiB partition falling to a
constant percentage of ~1.8% for sizes of 1 GiB and above.

Update the calculation for used to estimate the maximum size of
intrinsic unallocated space.  Limit is now 5% for partitions smaller
than 100 MiB, 2% for partitions larger than 1 GiB and linear scaling of
the percentage between.  Will still get false unallocated space warnings
for mounted xfs file systems and lvm2 pvs smaller than 100 MiB.

Also add a short note and worked example calculation of unallocated
space to the HACKING file.

Bug #499202 - gparted does not see the difference if partition size
              differs from filesystem size

HACKING

@@ -17,3 +17,45 @@ even when it's only one character. =)
 
 Thanks,
 Curtis
+
+
+
+partition usage and unallocated space
+=====================================
+
+After addressing bug #499202 GParted now also displays unallocated space
+as well as used and unused space for recognised file systems.  The
+unallocated space is shown graphically and the numeric figure is shown
+in the Information Dialog if it is greater than zero.  Additionally a
+warning is reported if the unallocated space is considered too much.
+See the code and commit messages for more details.
+
+Worked example of partition and file system usage for a newly created
+jfs file system in a 200 MiB partition reporting a small amount of
+unallocated space:
+
+# sfdisk -s /dev/sda9
+204800
+
+# echo dm | jfs_debugfs /dev/sda9 | egrep 'Block Size:|dn_mapsize:|dn_nfree:'
+Aggregate Block Size: 4096
+[1] dn_mapsize:		0x0000000c6cb	[9] dn_agheigth:	0
+[2] dn_nfree:		0x0000000c6a3	[10] dn_agwidth:	1
+
+Partition size = 204800  (1K blocks) = 409600 (512 byte sectors)
+FS size        = 0x0c6cb (4K blocks)
+               = 50891   (4K blocks) = 407128 (512 byte sectors)
+FS free        = 0x0c6a3 (4K blocks)
+               = 50851   (4K blocks) = 406808 (512 byte sectors)
+
+Calculation:
+size        = ptn_size           = 409600                 = 200.00 MiB
+used        = fs_size - fs_free  = 407128 - 406808 =  320 = 160.00 KiB
+unused      = fs_free            = 406808                 = 198.64 MiB
+unallocated = ptn_size - fs_size = 409600 - 407128 = 2472 =   1.21 MiB
+
+Figures displayed in the Information dialog:
+Size:         200.00 MiB
+Used:         160.00 KiB  ( 0% )
+Unused:       198.64 MiB  ( 99% )
+Unallocated:  1.21 MiB    ( 1% )

src/Partition.cc

@@ -21,8 +21,6 @@
 namespace GParted
 {
 
-#define SIGNIFICANT_UNALLOCATED_FRACTION 0.05
-
 Partition::Partition()
 {
 	Reset() ;
@@ -293,13 +291,38 @@ bool Partition::compare_paths( const Glib::ustring & A, const Glib::ustring & B
 }
 
 //Return threshold of sectors which is considered above the intrinsic
-//  level for a file system which "fills" the partition.
+//  level for a file system which "fills" the partition.  Calculation
+//  is:
+//      %age of partition size           , when
+//      5%                               ,           ptn size <= 100 MiB
+//      linear scaling from 5% down to 2%, 100 MiB < ptn size <= 1 GiB
+//      2%                               , 1 GiB   < ptn size
 Sector Partition::get_significant_unallocated_sectors() const
 {
-	Sector length = get_sector_length() ;
-	if ( length >= 0 )
-		return Utils::round( length * SIGNIFICANT_UNALLOCATED_FRACTION ) ;
-	return 0 ;
+	const double HIGHER_UNALLOCATED_FRACTION = 0.05 ;
+	const double LOWER_UNALLOCATED_FRACTION  = 0.02 ;
+	Sector     length   = get_sector_length() ;
+	Byte_Value byte_len = length * sector_size ;
+	if ( byte_len <= 0 )
+	{
+		return 0 ;
+	}
+	else if ( byte_len <= 100 * MEBIBYTE )
+	{
+		return Utils::round( length * HIGHER_UNALLOCATED_FRACTION ) ;
+	}
+	else if ( byte_len <= 1 * GIBIBYTE )
+	{
+		double fraction = ( HIGHER_UNALLOCATED_FRACTION - LOWER_UNALLOCATED_FRACTION ) -
+		                  ( byte_len - 100 * MEBIBYTE ) * ( HIGHER_UNALLOCATED_FRACTION - LOWER_UNALLOCATED_FRACTION ) /
+		                  ( 1 * GIBIBYTE - 100 * MEBIBYTE ) +
+		                  LOWER_UNALLOCATED_FRACTION ;
+		return Utils::round( length * fraction ) ;
+	}
+	else
+	{
+		return Utils::round( length * LOWER_UNALLOCATED_FRACTION ) ;
+	}
 }
 
 Partition::~Partition()