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deltas: Add a compression size heuristic for endianness detection 

I see when analyzing a delta here that due to byteswapping a negative
compression ratio of 540%, 66%, and 28%.  Let's arbitrarily pick 20%
as a threshold for detecting byetswapping.
This commit is contained in:
Colin Walters 2016-02-25 11:07:30 -05:00
parent 7fdf072710
commit 5345573470
1 changed files with 29 additions and 7 deletions
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22
src/libostree/ostree-repo-static-delta-core.c
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@ -705,6 +705,7 @@ _ostree_delta_get_endianness (GVariant *superblock,
{ g_autoptr(GVariant) meta_entries = NULL;
guint n_parts;
guint i;
gboolean is_byteswapped = FALSE;
g_variant_get_child (superblock, 6, "@a" OSTREE_STATIC_DELTA_META_ENTRY_FORMAT, &meta_entries);
n_parts = g_variant_n_children (meta_entries);
@ -721,8 +722,23 @@ _ostree_delta_get_endianness (GVariant *superblock,
total_objects += n_objects;
total_size += size;
total_usize += usize;
if (size > usize)
{
double ratio = ((double)size)/((double)usize);
/* This should really never happen where compressing things makes it more than 50% bigger.
*/
if (ratio > 1.2)
{
is_byteswapped = TRUE;
break;
}
}
}
if (!is_byteswapped)
{
/* If the average object size is greater than 4GiB, let's assume
* we're dealing with opposite endianness. I'm fairly confident
* no one is going to be shipping peta- or exa- byte size ostree
@ -730,6 +746,12 @@ _ostree_delta_get_endianness (GVariant *superblock,
* bittorrent or something.
*/
if ((total_size / total_objects) > G_MAXUINT32)
{
is_byteswapped = TRUE;
}
}
if (is_byteswapped)
{
switch (G_BYTE_ORDER)
{