Support compression in VMware sparse disk

[chessbyte]

Much of the comments and understanding in this PR derives from the VMware Virtual Disk Format 5.0 Documentation and this repo

[Fryguy]

I think there's a way to do bit level here, so you can actually give each of these flags an individual name here, and then you don't need the method that maps to a hash.

[Fryguy]

I tried doing this, but all the ways to do it are kind of clunky, so probably ok to go with what's here...

So I thought you could do

'b', :flag1,
'b', :flag2,
'b', :flag3,
...

Unfortunately the way the b identifier works is that it consumes 1 bit, then moves the iterator forward the rest of the byte. So, the only way to read consecutive bits is to do something like...

'b8', :flags

which then gives something like {:flags => "01100100"}.

---

I think BinaryStruct could be enhanced with some sort of bit decoding, but I'm not sure what a nice interface would look like offhand. I'm guessing the caller would just want to deal with bits individually, but under the covers as we build the format string we'd have to understand consecutive b definitions and pack them into a single b<N>.

[Fryguy]

That being said, it might be easier to use the bit decoding and do something like this...

# Instead of
'L',  'flags'

# use this
'b16' 'flags'

# and then immediately after decoding do:

sparseHeader["flags"] = sparseHeader["flags"].chars.map {|c| c == "1"}

# Then perhps something like...

sparseHeader["valid_new_line_detection_test"] = sparseHeader["flags"][0]
sparseHeader["redundant_grain_table_used"]    = sparseHeader["flags"][1]
sparseHeader["zero_grain_gte_used"]           = sparseHeader["flags"][2]
sparseHeader["grains_compressed"]             = sparseHeader["flags"][16]

[chessbyte]

shouldn't this be b32 since we need to access bit 0 as well as bit 16 and bit 17?

[Fryguy]

Oh yes...I typoed it.

[miq-bot]

This pull request has been automatically marked as stale because it has not been updated for at least 3 months.

If these changes are still valid, please remove the stale label, make any changes requested by reviewers (if any), and ensure that this issue is being looked at by the assigned/reviewer(s)

Thank you for all your contributions! More information about the ManageIQ triage process can be found in the triage process documentation.

[miq-bot]

This pull request has been automatically closed because it has not been updated for at least 3 months.

Feel free to reopen this pull request if these changes are still valid.

Thank you for all your contributions! More information about the ManageIQ triage process can be found in the triage process documentation.

[miq-bot]

Checked commits chessbyte/manageiq-smartstate@8521141~...f6e9f29 with ruby 2.6.10, rubocop 1.28.2, haml-lint 0.35.0, and yamllint
1 file checked, 36 offenses detected

lib/disk/modules/VMWareSparseDisk.rb

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