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Switched to separate-tag encoding of forward-looking CRCs

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Previously forward-looking CRCs was just two new CRC types, one for
commits with forward-looking CRCs, one without. These both contained the
CRC needed to complete the current commit (note that the commit CRC
must come last!).

         [--   32   --|--   32   --|--   32   --|--   32   --]
with:    [  crc3 tag  | nprog size |  nprog crc | commit crc ]
without: [  crc2 tag  | commit crc ]

This meant there had to be several checks for the two possible structure
sizes, messying up the implementation.

         [--   32   --|--   32   --|--   32   --|--   32   --|--   32   --]
with:    [nprogcrc tag| nprog size |  nprog crc | commit tag | commit crc ]
without: [ commit tag | commit crc ]

But we already have a mechanism for storing optional metadata! The
different metadata tags! So why not use a separate tage for the
forward-looking CRC, separate from the commit CRC?

I wasn't sure this would actually help that much, there are still
necessary conditions for wether or not a forward-looking CRC is there,
but in the end it simplified the code quite nicely, and resulted in a ~200 byte
code-cost saving.
This commit is contained in:
Christopher Haster
2020-12-06 23:54:55 -06:00
parent 7535795a44
commit 97b5d04bf4
3 changed files with 82 additions and 80 deletions
Download Patch File Download Diff File Expand all files Collapse all files

30
scripts/readmdir.py
View File

@@ -24,6 +24,7 @@ TAG_TYPES = {
'gstate': (0x700, 0x700),
'movestate': (0x7ff, 0x7ff),
'crc': (0x700, 0x500),
'nprogcrc': (0x7ff, 0x5ff),
}
class Tag:
@@ -125,9 +126,10 @@ class Tag:
return ntag
def typerepr(self):
if self.is_('crc') and getattr(self, 'crc', 0xffffffff) != 0xffffffff:
if (self.is_('crc') and not self.is_('nprogcrc') and
getattr(self, 'crc', 0xffffffff) != 0xffffffff):
crc_status = ' (bad)'
elif self.is_('crc') and getattr(self, 'erased', False):
elif self.is_('nprogcrc') and getattr(self, 'erased', False):
crc_status = ' (era)'
else:
crc_status = ''
@@ -186,6 +188,8 @@ class MetadataPair:
self.rev, = struct.unpack('<I', block[0:4])
crc = binascii.crc32(block[0:4])
etag = None
estate = None
# parse tags
corrupt = False
@@ -199,9 +203,7 @@ class MetadataPair:
tag = Tag((int(tag) ^ ntag) & 0x7fffffff)
tag.off = off + 4
tag.data = block[off+4:off+tag.dsize]
if tag.is_('crc 0x3'):
crc = binascii.crc32(block[off:off+4*4], crc)
elif tag.is_('crc'):
if tag.is_('crc') and not tag.is_('nprogcrc'):
crc = binascii.crc32(block[off:off+2*4], crc)
else:
crc = binascii.crc32(block[off:off+tag.dsize], crc)
@@ -210,25 +212,29 @@ class MetadataPair:
self.all_.append(tag)
if tag.is_('crc'):
if tag.is_('nprogcrc') and len(tag.data) == 8:
etag = tag
estate = struct.unpack('<II', tag.data)
elif tag.is_('crc'):
# is valid commit?
if crc != 0xffffffff:
corrupt = True
if not corrupt:
self.log = self.all_.copy()
# end of commit?
if tag.is_('crc 0x3'):
esize, ecrc = struct.unpack('<II', tag.data[:8])
if estate:
esize, ecrc = estate
dcrc = 0xffffffff ^ binascii.crc32(block[off:off+esize])
if ecrc == dcrc:
tag.erased = True
etag.erased = True
corrupt = True
elif tag.is_('crc 0x2'):
elif not (tag.is_('crc 0x0') or tag.is_('crc 0x1')):
corrupt = True
# reset tag parsing
crc = 0
etag = None
estate = None
# find active ids
self.ids = list(it.takewhile(
@@ -305,7 +311,7 @@ class MetadataPair:
f.write('\n')
for tag in tags:
f.write("%08x: %08x %-13s %4s %4s" % (
f.write("%08x: %08x %-14s %3s %4s" % (
tag.off, tag,
tag.typerepr(), tag.idrepr(), tag.sizerepr()))
if truncate: