Patch Index in Darcs (formerly known as file cache)

Why do we need a patch index?

The following is simplified for presentation!

PrimitivePatch = Move FilePath FilePath | Hunk Line String String | AddFile FilePath | RmFile FilePath | .. – e.g. Move oldName newName, Hunk l old new, …

Patch = Patch PatchInfo [PrimitivePatch]

In Darcs, the repository is (roughly) of type [Patch]. This view is required for merging, conflict handling, cherry-picking, … .

Some operations are OK using this representation:

*Alice*: [p1,..,pn] <-- pull --> *Bob*: [p1,..,pn,pn+1,..,pn+k]
*Alice*: [p1,..,pn] unpull pk

Others are very slow (mostly query operations):

  • annotate "Makefile" => has to apply all patches (backwards from the current state) back to the creation of the “Makefile”
  • Same problem:

    • changes "Makefile"
    • show contents Makefile -p "Some old patch"

How to solve this?

Keep [Patch] representation, extract some data that speeds up these operations

annotate "Makefile"
find out which patches touch “Makefile” => ignore all other patches

Need function: 'touchingPatches :: DatedFilePath -> (Set PatchId)'

Have to handle renames, directory moves:

p01 Add   A/Foo                                  {A/Foo:"",..}
p02 Hunk  A/Foo ..                               {A/Foo:"some text",..}
p03 MV    A      B                               {B/Foo:"some text",..}
p10 Add   A/Foo                                  {B/Foo:"some text",A/Foo:"",..}
p11 RM    A/Foo                                  {B/Foo:"some text",..}
p12 MV    B/Foo  A/Foo                           {A/Foo:"some text",..}

Unique (local) identifier for each file. Identifier of file should be constant, name and content can change.

We use: creation name and counter as unique identifier for a file:

FileId = FileId {creationName :: ByteString, counter :: Int}

For example, there are 2 files in the repo above A/Foo#1 and A/Foo#2.

Can refer to “old filenames” that are interpreted relative to an old revision of the repository.

FileRelativeRevision = FileRelativeRevision {filePath :: FilePath, revision::PatchId}

A span starting with pstart to (maybe) pend where the file identified by fid had a certain name.

FileIdSpan = FileIdSpan {fid::FileID, pstart::PatchId, pend::Maybe PatchId}

The map that returns all spans for a certain name, we can use it to look up which FileId the name refers to at a certain revision.

SpanMap = Map FilePath [FileIdSpan]

The we just store all patches that touch a file.

PatchesMap = Map FileId (Set PatchId)
CreationNameMap = Map FileId (FilePath)

can use SpanMap to lookup FileId for FileRelativeRevision then use PatchesMap

Patch Index Structure


  • It is a unique identifier for a patch.
  • Look at Patch Hash on this page
  • The name consists of three segments:
    • timestamp (ISO8601-compatible yyyymmmddHHMMSS, UTC)
    • SHA1 hash of the author
    • SHA1 hash of the patch name, author, date, log, and "inverted" flag.
  • Patch Index makes an ordering of the patches based on order of application on the repo.
  • if patch p1 depends on patch p2 then p1 > p2
  • if p1 < p2 then p1 cannot depend on p2


  • The filename is insufficient to track a file as it can change
  • FileId uniquely identifies each file with it’s creation name, and a counter
  • Patch-index carefully tracks the FileName of each FileId, and FileId of each FileName.

Map FileId (Set PatchId)

  • Used to ascertain the patches that modified a file.

Map FileName [(FileId, PatchId, PatchId)]

  • Let (fn, (fi, ps, px)) be an entry
    • Applying all the patches < than ps in the repo, you will have fn /= fi
    • Applying all the patches ≥ ps and ≤ to px in the repo, you will have fn == fi


You can try out changes using patch-index with this The command is darcs show patch-index-fileid ./Path_to_file_wrt_repo. Patch-index changes seems to be 10x faster that the existing changes.

Old Results

Seems to make annotate bearable for large repos (GHC-sized).

Best case for patch-index: README in the ghc repo - touched by 29 files of 21345

$ time darcs changes --patch-index README  > /dev/null

real    0m0.669s
user    0m0.581s
sys     0m0.083s

without “–patch-index”: ran out of patience after 30 minutes

annotate still takes about 15 seconds with “–patch-index” => bottleneck is now reading inventories