{-# LANGUAGE RecordWildCards, NamedFieldPuns #-}
module UnitTests.Distribution.Client.VCS (tests) where

import Distribution.Client.Compat.Prelude
import Distribution.Client.VCS
import Distribution.Client.RebuildMonad
         ( execRebuild )
import Distribution.Simple.Program
import Distribution.Verbosity as Verbosity
import Distribution.Client.SourceRepo (SourceRepositoryPackage (..), SourceRepoProxy)

import Data.List
import Data.Tuple
import qualified Data.Map as Map
import qualified Data.Set as Set

import qualified Control.Monad.State as State
import Control.Monad.State (StateT, liftIO, execStateT)
import Control.Exception
import Control.Concurrent (threadDelay)

import System.IO
import System.FilePath
import System.Directory
import System.Random

import Test.Tasty
import Test.Tasty.QuickCheck
import UnitTests.Distribution.Client.ArbitraryInstances
import UnitTests.TempTestDir (withTestDir, removeDirectoryRecursiveHack)


-- | These tests take the following approach: we generate a pure representation
-- of a repository plus a corresponding real repository, and then run various
-- test operations and compare the actual working state with the expected
-- working state.
--
-- The first test simply checks that the test infrastructure works. It
-- constructs a repository on disk and then checks out every tag or commmit
-- and checks that the working state is the same as the pure representation.
--
-- The second test works in a similar way but tests 'syncSourceRepos'. It
-- uses an arbitrary source repo and a set of (initially empty) destination
-- directories. It picks a number of tags or commits from the source repo and
-- synchronises the destination directories to those target states, and then
-- checks that the working state is as expected (given the pure representation).
--
tests :: MTimeChange -> [TestTree]
tests mtimeChange =
  [ testGroup "check VCS test framework" $
    [ testProperty "git"    prop_framework_git
    ] ++
    [ testProperty "darcs" (prop_framework_darcs mtimeChange)
    | enableDarcsTests
    ]
  , testGroup "cloneSourceRepo" $
    [ testProperty "git"    prop_cloneRepo_git
    ] ++
    [ testProperty "darcs" (prop_cloneRepo_darcs mtimeChange)
    | enableDarcsTests
    ]
  , testGroup "syncSourceRepos" $
    [ testProperty "git"    prop_syncRepos_git
    ] ++
    [ testProperty "darcs" (prop_syncRepos_darcs mtimeChange)
    | enableDarcsTests
    ]
  ]
  where
    -- for the moment they're not yet working
    enableDarcsTests = False


prop_framework_git :: BranchingRepoRecipe -> Property
prop_framework_git =
    ioProperty
  . prop_framework vcsGit vcsTestDriverGit
  . WithBranchingSupport

prop_framework_darcs :: MTimeChange -> NonBranchingRepoRecipe -> Property
prop_framework_darcs mtimeChange =
    ioProperty
  . prop_framework vcsDarcs (vcsTestDriverDarcs mtimeChange)
  . WithoutBranchingSupport

prop_cloneRepo_git :: BranchingRepoRecipe -> Property
prop_cloneRepo_git =
    ioProperty
  . prop_cloneRepo vcsGit vcsTestDriverGit
  . WithBranchingSupport

prop_cloneRepo_darcs :: MTimeChange
                     -> NonBranchingRepoRecipe -> Property
prop_cloneRepo_darcs mtimeChange =
    ioProperty
  . prop_cloneRepo vcsDarcs (vcsTestDriverDarcs mtimeChange)
  . WithoutBranchingSupport

prop_syncRepos_git :: RepoDirSet -> SyncTargetIterations -> PrngSeed
                   -> BranchingRepoRecipe -> Property
prop_syncRepos_git destRepoDirs syncTargetSetIterations seed =
    ioProperty
  . prop_syncRepos vcsGit vcsTestDriverGit
                   destRepoDirs syncTargetSetIterations seed
  . WithBranchingSupport

prop_syncRepos_darcs :: MTimeChange
                     -> RepoDirSet -> SyncTargetIterations -> PrngSeed
                     -> NonBranchingRepoRecipe -> Property
prop_syncRepos_darcs  mtimeChange destRepoDirs syncTargetSetIterations seed =
    ioProperty
  . prop_syncRepos vcsDarcs (vcsTestDriverDarcs mtimeChange)
                   destRepoDirs syncTargetSetIterations seed
  . WithoutBranchingSupport


-- ------------------------------------------------------------
-- * General test setup
-- ------------------------------------------------------------

testSetup :: VCS Program
          -> (Verbosity -> VCS ConfiguredProgram
                        -> FilePath -> VCSTestDriver)
          -> RepoRecipe
          -> (VCSTestDriver -> FilePath -> RepoState -> IO a)
          -> IO a
testSetup vcs mkVCSTestDriver repoRecipe theTest = do
    -- test setup
    vcs' <- configureVCS verbosity vcs
    withTestDir verbosity "vcstest" $ \tmpdir -> do
      let srcRepoPath = tmpdir </> "src"
          vcsDriver   = mkVCSTestDriver verbosity vcs' srcRepoPath
      repoState <- createRepo vcsDriver repoRecipe

      -- actual test
      result <- theTest vcsDriver tmpdir repoState

      return result
  where
    verbosity = silent

-- ------------------------------------------------------------
-- * Test 1: VCS infrastructure
-- ------------------------------------------------------------

-- | This test simply checks that the test infrastructure works. It constructs
-- a repository on disk and then checks out every tag or commit and checks that
-- the working state is the same as the pure representation.
--
prop_framework :: VCS Program
               -> (Verbosity -> VCS ConfiguredProgram
                             -> FilePath -> VCSTestDriver)
               -> RepoRecipe
               -> IO ()
prop_framework vcs mkVCSTestDriver repoRecipe =
    testSetup vcs mkVCSTestDriver repoRecipe $ \vcsDriver tmpdir repoState ->
      mapM_ (checkAtTag vcsDriver tmpdir) (Map.toList (allTags repoState))
  where
    -- Check for any given tag/commit in the 'RepoState' that the working state
    -- matches the actual working state from the repository at that tag/commit.
    checkAtTag VCSTestDriver {..} tmpdir (tagname, expectedState) =
      case vcsCheckoutTag of
        -- We handle two cases: inplace checkouts for VCSs that support it
        -- (e.g. git) and separate dir otherwise (e.g. darcs)
        Left checkoutInplace -> do
          checkoutInplace tagname
          checkExpectedWorkingState vcsIgnoreFiles vcsRepoRoot expectedState

        Right checkoutCloneTo -> do
          checkoutCloneTo tagname destRepoPath
          checkExpectedWorkingState vcsIgnoreFiles destRepoPath expectedState
          removeDirectoryRecursiveHack silent destRepoPath
        where
          destRepoPath = tmpdir </> "dest"


-- ------------------------------------------------------------
-- * Test 2: 'cloneSourceRepo'
-- ------------------------------------------------------------

prop_cloneRepo :: VCS Program
               -> (Verbosity -> VCS ConfiguredProgram
                             -> FilePath -> VCSTestDriver)
               -> RepoRecipe
               -> IO ()
prop_cloneRepo vcs mkVCSTestDriver repoRecipe =
    testSetup vcs mkVCSTestDriver repoRecipe $ \vcsDriver tmpdir repoState ->
      mapM_ (checkAtTag vcsDriver tmpdir) (Map.toList (allTags repoState))
  where
    checkAtTag VCSTestDriver{..} tmpdir (tagname, expectedState) = do
        cloneSourceRepo verbosity vcsVCS repo destRepoPath
        checkExpectedWorkingState vcsIgnoreFiles destRepoPath expectedState
        removeDirectoryRecursiveHack verbosity destRepoPath
      where
        destRepoPath = tmpdir </> "dest"
        repo = SourceRepositoryPackage
            { srpType     = vcsRepoType vcsVCS
            , srpLocation = vcsRepoRoot
            , srpTag      = Just tagname
            , srpBranch   = Nothing
            , srpSubdir   = []
            }
    verbosity = silent


-- ------------------------------------------------------------
-- * Test 3: 'syncSourceRepos'
-- ------------------------------------------------------------

newtype RepoDirSet           = RepoDirSet Int           deriving Show
newtype SyncTargetIterations = SyncTargetIterations Int deriving Show
newtype PrngSeed             = PrngSeed Int             deriving Show

prop_syncRepos :: VCS Program
               -> (Verbosity -> VCS ConfiguredProgram
                             -> FilePath -> VCSTestDriver)
               -> RepoDirSet
               -> SyncTargetIterations
               -> PrngSeed
               -> RepoRecipe
               -> IO ()
prop_syncRepos vcs mkVCSTestDriver
               repoDirs syncTargetSetIterations seed repoRecipe =
    testSetup vcs mkVCSTestDriver repoRecipe $ \vcsDriver tmpdir repoState ->
      let srcRepoPath   = vcsRepoRoot vcsDriver
          destRepoPaths = map (tmpdir </>) (getRepoDirs repoDirs)
       in checkSyncRepos verbosity vcsDriver repoState
                         srcRepoPath destRepoPaths
                         syncTargetSetIterations seed
  where
    verbosity = silent

    getRepoDirs :: RepoDirSet -> [FilePath]
    getRepoDirs (RepoDirSet n) =
        [ "dest" ++ show i | i <- [1..n] ]


-- | The purpose of this test is to check that irrespective of the local cached
-- repo dir we can sync it to an arbitrary target state. So we do that by
-- syncing each target dir to a sequence of target states without cleaning it
-- in between.
--
-- One slight complication is that 'syncSourceRepos' takes a whole list of
-- target dirs to sync in one go (to allow for sharing). So we must actually
-- generate and sync to a sequence of list of target repo states.
--
-- So, given a source repo dir, the corresponding 'RepoState' and a number of
-- target repo dirs, pick a sequence of (lists of) sync targets from the
-- 'RepoState' and syncronise the target dirs with those targets, checking for
-- each one that the actual working state matches the expected repo state.
--
checkSyncRepos
  :: Verbosity
  -> VCSTestDriver
  -> RepoState
  -> FilePath
  -> [FilePath]
  -> SyncTargetIterations
  -> PrngSeed
  -> IO ()
checkSyncRepos verbosity VCSTestDriver { vcsVCS = vcs, vcsIgnoreFiles }
               repoState srcRepoPath destRepoPath
               (SyncTargetIterations syncTargetSetIterations) (PrngSeed seed) =
    mapM_ checkSyncTargetSet syncTargetSets
  where
    checkSyncTargetSet :: [(SourceRepoProxy, FilePath, RepoWorkingState)] -> IO ()
    checkSyncTargetSet syncTargets = do
      _ <- execRebuild "root-unused" $
           syncSourceRepos verbosity vcs
                           [ (repo, repoPath)
                           | (repo, repoPath, _) <- syncTargets ]
      sequence_
        [ checkExpectedWorkingState vcsIgnoreFiles repoPath workingState
        | (_, repoPath, workingState) <- syncTargets ]

    syncTargetSets = take syncTargetSetIterations
                   $ pickSyncTargetSets (vcsRepoType vcs) repoState
                                        srcRepoPath destRepoPath
                                        (mkStdGen seed)

pickSyncTargetSets :: RepoType -> RepoState
                   -> FilePath -> [FilePath]
                   -> StdGen
                   -> [[(SourceRepoProxy, FilePath, RepoWorkingState)]]
pickSyncTargetSets repoType repoState srcRepoPath dstReposPath =
    assert (Map.size (allTags repoState) > 0) $
    unfoldr (Just . swap . pickSyncTargetSet)
  where
    pickSyncTargetSet :: Rand [(SourceRepoProxy, FilePath, RepoWorkingState)]
    pickSyncTargetSet = flip (mapAccumL (flip pickSyncTarget)) dstReposPath

    pickSyncTarget :: FilePath -> Rand (SourceRepoProxy, FilePath, RepoWorkingState)
    pickSyncTarget destRepoPath prng =
        (prng', (repo, destRepoPath, workingState))
      where
        repo                = SourceRepositoryPackage
                              { srpType     = repoType
                              , srpLocation = srcRepoPath
                              , srpTag      = Just tag
                              , srpBranch   = Nothing
                              , srpSubdir   = Proxy
                              }
        (tag, workingState) = Map.elemAt tagIdx (allTags repoState)
        (tagIdx, prng')     = randomR (0, Map.size (allTags repoState) - 1) prng

type Rand a = StdGen -> (StdGen, a)

instance Arbitrary RepoDirSet where
  arbitrary =
    sized $ \n -> oneof $ [ RepoDirSet <$> pure 1 ]
                       ++ [ RepoDirSet <$> choose (2,5) | n >= 3 ]
  shrink (RepoDirSet n) =
    [ RepoDirSet i | i <- shrink n, i > 0 ]

instance Arbitrary SyncTargetIterations where
  arbitrary =
    sized $ \n -> SyncTargetIterations <$> elements [ 1 .. min 20 (n + 1) ]
  shrink (SyncTargetIterations n) =
    [ SyncTargetIterations i | i <- shrink n, i > 0 ]

instance Arbitrary PrngSeed where
  arbitrary = PrngSeed <$> arbitraryBoundedRandom


-- ------------------------------------------------------------
-- * Instructions for constructing repositories
-- ------------------------------------------------------------

-- These instructions for constructing a repository can be interpreted in two
-- ways: to make a pure representation of repository state, and to execute
-- VCS commands to make a repository on-disk.

data FileUpdate    = FileUpdate FilePath String        deriving Show
data Commit        = Commit [FileUpdate]               deriving Show
data TaggedCommits = TaggedCommits TagName    [Commit] deriving Show
data BranchCommits = BranchCommits BranchName [Commit] deriving Show

type BranchName    = String
type TagName       = String

-- | Instructions to make a repository without branches, for VCSs that do not
-- support branches (e.g. darcs).
newtype NonBranchingRepoRecipe = NonBranchingRepoRecipe [TaggedCommits]
 deriving Show

-- | Instructions to make a repository with branches, for VCSs that do
-- support branches (e.g. git).
newtype BranchingRepoRecipe = BranchingRepoRecipe
                                [Either TaggedCommits BranchCommits]
 deriving Show

data RepoRecipe = WithBranchingSupport       BranchingRepoRecipe
                | WithoutBranchingSupport NonBranchingRepoRecipe

-- ---------------------------------------------------------------------------
-- Arbitrary instances for them

instance Arbitrary FileUpdate where
  arbitrary = FileUpdate <$> genFileName <*> genFileContent
    where
      genFileName    = (\c -> "file" </> [c]) <$> choose ('A', 'E')
      genFileContent = vectorOf 10 (choose ('#', '~'))

instance Arbitrary Commit where
  arbitrary = Commit <$> shortListOf1 5 arbitrary
  shrink (Commit writes) = Commit <$> filter (not . null) (shrink writes)

instance Arbitrary TaggedCommits where
  arbitrary = TaggedCommits <$> genTagName <*>  shortListOf1 5 arbitrary
    where
      genTagName = ("tag_" ++) <$> shortListOf1 5 (choose ('A', 'Z'))
  shrink (TaggedCommits tag commits) =
    TaggedCommits tag <$> filter (not . null) (shrink commits)

instance Arbitrary BranchCommits where
  arbitrary = BranchCommits <$> genBranchName <*> shortListOf1 5 arbitrary
    where
      genBranchName =
        sized $ \n ->
          (\c -> "branch_" ++ [c]) <$> elements (take (max 1 n) ['A'..'E'])

  shrink (BranchCommits branch commits) =
    BranchCommits branch <$> filter (not . null) (shrink commits)

instance Arbitrary NonBranchingRepoRecipe where
  arbitrary = NonBranchingRepoRecipe <$> shortListOf1 15 arbitrary
  shrink (NonBranchingRepoRecipe xs) =
    NonBranchingRepoRecipe <$> filter (not . null) (shrink xs)

instance Arbitrary BranchingRepoRecipe where
  arbitrary = BranchingRepoRecipe <$> shortListOf1 15 taggedOrBranch
    where
      taggedOrBranch = frequency [ (3, Left  <$> arbitrary)
                                 , (1, Right <$> arbitrary)
                                 ]
  shrink (BranchingRepoRecipe xs) =
    BranchingRepoRecipe <$> filter (not . null) (shrink xs)


-- ------------------------------------------------------------
-- * A pure model of repository state
-- ------------------------------------------------------------

-- | The full state of a repository. In particular it records the full working
-- state for every tag.
--
-- This is also the interpreter state for executing a 'RepoRecipe'.
--
-- This allows us to compare expected working states with the actual files in
-- the working directory of a repository. See 'checkExpectedWorkingState'.
--
data RepoState =
     RepoState {
       currentBranch  :: BranchName,
       currentWorking :: RepoWorkingState,
       allTags        :: Map TagOrCommitId RepoWorkingState,
       allBranches    :: Map BranchName RepoWorkingState
     }
  deriving Show

type RepoWorkingState = Map FilePath String
type CommitId         = String
type TagOrCommitId    = String


------------------------------------------------------------------------------
-- Functions used to interpret instructions for constructing repositories

initialRepoState :: RepoState
initialRepoState =
    RepoState {
      currentBranch  = "branch_master",
      currentWorking = Map.empty,
      allTags        = Map.empty,
      allBranches    = Map.empty
    }

updateFile :: FilePath -> String -> RepoState -> RepoState
updateFile filename content state@RepoState{currentWorking} =
  state { currentWorking = Map.insert filename content currentWorking }

addTagOrCommit :: TagOrCommitId -> RepoState -> RepoState
addTagOrCommit commit state@RepoState{currentWorking, allTags} =
  state { allTags = Map.insert commit currentWorking allTags }

switchBranch :: BranchName -> RepoState -> RepoState
switchBranch branch state@RepoState{currentWorking, currentBranch, allBranches} =
  -- Use updated allBranches to cover case of switching to the same branch
  let allBranches' = Map.insert currentBranch currentWorking allBranches in
  state {
    currentBranch  = branch,
    currentWorking = case Map.lookup branch allBranches' of
                       Just working -> working
                       -- otherwise we're creating a new branch, which starts
                       -- from our current branch state
                       Nothing      -> currentWorking,
    allBranches    = allBranches'
  }


-- ------------------------------------------------------------
-- * Comparing on-disk with expected 'RepoWorkingState'
-- ------------------------------------------------------------

-- | Compare expected working states with the actual files in
-- the working directory of a repository.
--
checkExpectedWorkingState :: Set FilePath
                          -> FilePath -> RepoWorkingState -> IO ()
checkExpectedWorkingState ignore repoPath expectedState = do
    currentState <- getCurrentWorkingState ignore repoPath
    unless (currentState == expectedState) $
      throwIO (WorkingStateMismatch expectedState currentState)

data WorkingStateMismatch =
     WorkingStateMismatch RepoWorkingState -- expected
                          RepoWorkingState -- actual
  deriving Show

instance Exception WorkingStateMismatch

getCurrentWorkingState :: Set FilePath -> FilePath -> IO RepoWorkingState
getCurrentWorkingState ignore repoRoot = do
    entries <- getDirectoryContentsRecursive ignore repoRoot ""
    Map.fromList <$> mapM getFileEntry
                          [ file | (file, isDir) <- entries, not isDir ]
  where
   getFileEntry name =
     withBinaryFile (repoRoot </> name) ReadMode $ \h -> do
       str <- hGetContents h
       _   <- evaluate (length str)
       return (name, str)

getDirectoryContentsRecursive :: Set FilePath -> FilePath -> FilePath
                              -> IO [(FilePath, Bool)]
getDirectoryContentsRecursive ignore dir0 dir = do
    entries  <- getDirectoryContents (dir0 </> dir)
    entries' <- sequence
                  [ do isdir <- doesDirectoryExist (dir0 </> dir </> entry)
                       return (dir </> entry, isdir)
                  | entry <- entries
                  , not (isPrefixOf "." entry)
                  , (dir </> entry) `Set.notMember` ignore
                  ]
    let subdirs = [ d | (d, True)  <- entries' ]
    subdirEntries <- mapM (getDirectoryContentsRecursive ignore dir0) subdirs
    return (concat (entries' : subdirEntries))


-- ------------------------------------------------------------
-- * Executing instructions to make on-disk VCS repos
-- ------------------------------------------------------------

-- | Execute the instructions in a 'RepoRecipe' using the given 'VCSTestDriver'
-- to make an on-disk repository.
--
-- This also returns a 'RepoState'. This is done as part of construction to
-- support VCSs like git that have commit ids, so that those commit ids can be
-- included in the 'RepoState's 'allTags' set.
--
createRepo :: VCSTestDriver -> RepoRecipe -> IO RepoState
createRepo vcsDriver@VCSTestDriver{vcsRepoRoot, vcsInit} recipe = do
    createDirectory vcsRepoRoot
    createDirectory (vcsRepoRoot </> "file")
    vcsInit
    execStateT createRepoAction initialRepoState
  where
    createRepoAction :: StateT RepoState IO ()
    createRepoAction = case recipe of
      WithoutBranchingSupport r -> execNonBranchingRepoRecipe vcsDriver r
      WithBranchingSupport    r -> execBranchingRepoRecipe    vcsDriver r

type CreateRepoAction a = VCSTestDriver -> a -> StateT RepoState IO ()

execNonBranchingRepoRecipe :: CreateRepoAction NonBranchingRepoRecipe
execNonBranchingRepoRecipe vcsDriver (NonBranchingRepoRecipe taggedCommits) =
    mapM_ (execTaggdCommits vcsDriver) taggedCommits

execBranchingRepoRecipe :: CreateRepoAction BranchingRepoRecipe
execBranchingRepoRecipe vcsDriver (BranchingRepoRecipe taggedCommits) =
    mapM_ (either (execTaggdCommits  vcsDriver)
                  (execBranchCommits vcsDriver))
          taggedCommits

execBranchCommits :: CreateRepoAction BranchCommits
execBranchCommits vcsDriver@VCSTestDriver{vcsSwitchBranch}
                  (BranchCommits branch commits) = do
    mapM_ (execCommit vcsDriver) commits
    -- add commits and then switch branch
    State.modify (switchBranch branch)
    state <- State.get -- repo state after the commits and branch switch
    liftIO $ vcsSwitchBranch state branch

    -- It may seem odd that we add commits on the existing branch and then
    -- switch branch. In part this is because git cannot branch from an empty
    -- repo state, it complains that the master branch doesn't exist yet.

execTaggdCommits :: CreateRepoAction TaggedCommits
execTaggdCommits vcsDriver@VCSTestDriver{vcsTagState}
                 (TaggedCommits tagname commits) = do
    mapM_ (execCommit vcsDriver) commits
    -- add commits then tag
    state <- State.get -- repo state after the commits
    liftIO $ vcsTagState state tagname
    State.modify (addTagOrCommit tagname)

execCommit :: CreateRepoAction Commit
execCommit vcsDriver@VCSTestDriver{..} (Commit fileUpdates) = do
    mapM_ (execFileUpdate vcsDriver) fileUpdates
    state <- State.get -- existing state, not updated
    mcommit <- liftIO $ vcsCommitChanges state
    State.modify (maybe id addTagOrCommit mcommit)

execFileUpdate :: CreateRepoAction FileUpdate
execFileUpdate VCSTestDriver{..} (FileUpdate filename content) = do
    liftIO $ writeFile (vcsRepoRoot </> filename) content
    state <- State.get -- existing state, not updated
    liftIO $ vcsAddFile state filename
    State.modify (updateFile filename content)


-- ------------------------------------------------------------
-- * VCSTestDriver for various VCSs
-- ------------------------------------------------------------

-- | Extends 'VCS' with extra methods to construct a repository. Used by
-- 'createRepo'.
--
-- Several of the methods are allowed to rely on the current 'RepoState'
-- because some VCSs need different commands for initial vs later actions
-- (like adding a file to the tracked set, or creating a new branch).
--
-- The driver instance knows the particular repo directory.
--
data VCSTestDriver = VCSTestDriver {
       vcsVCS           :: VCS ConfiguredProgram,
       vcsRepoRoot      :: FilePath,
       vcsIgnoreFiles   :: Set FilePath,
       vcsInit          :: IO (),
       vcsAddFile       :: RepoState -> FilePath -> IO (),
       vcsCommitChanges :: RepoState -> IO (Maybe CommitId),
       vcsTagState      :: RepoState -> TagName -> IO (),
       vcsSwitchBranch  :: RepoState -> BranchName -> IO (),
       vcsCheckoutTag   :: Either (TagName -> IO ())
                                  (TagName -> FilePath -> IO ())
     }


vcsTestDriverGit :: Verbosity -> VCS ConfiguredProgram
                 -> FilePath -> VCSTestDriver
vcsTestDriverGit verbosity vcs repoRoot =
    VCSTestDriver {
      vcsVCS = vcs

    , vcsRepoRoot = repoRoot

    , vcsIgnoreFiles = Set.empty

    , vcsInit =
        git $ ["init"]  ++ verboseArg

    , vcsAddFile = \_ filename ->
        git ["add", filename]

    , vcsCommitChanges = \_state -> do
        git $ [ "-c", "user.name=A", "-c", "user.email=a@example.com"
              , "commit", "--all", "--message=a patch"
              , "--author=A <a@example.com>"
              ] ++ verboseArg
        commit <- git' ["log", "--format=%H", "-1"]
        let commit' = takeWhile (not . isSpace) commit
        return (Just commit')

    , vcsTagState = \_ tagname ->
        git ["tag", "--force", "--no-sign", tagname]

    , vcsSwitchBranch = \RepoState{allBranches} branchname -> do
        unless (branchname `Map.member` allBranches) $
          git ["branch", branchname]
        git $ ["checkout", branchname] ++ verboseArg

    , vcsCheckoutTag = Left $ \tagname ->
        git $ ["checkout", "--detach", "--force", tagname] ++ verboseArg
    }
  where
    gitInvocation args = (programInvocation (vcsProgram vcs) args) {
                           progInvokeCwd = Just repoRoot
                         }
    git  = runProgramInvocation       verbosity . gitInvocation
    git' = getProgramInvocationOutput verbosity . gitInvocation
    verboseArg = [ "--quiet" | verbosity < Verbosity.normal ]


type MTimeChange = Int

vcsTestDriverDarcs :: MTimeChange -> Verbosity -> VCS ConfiguredProgram
                   -> FilePath -> VCSTestDriver
vcsTestDriverDarcs mtimeChange verbosity vcs repoRoot =
    VCSTestDriver {
      vcsVCS = vcs

    , vcsRepoRoot = repoRoot

    , vcsIgnoreFiles = Set.singleton "_darcs"

    , vcsInit =
        darcs ["initialize"]

    , vcsAddFile = \state filename -> do
        threadDelay mtimeChange
        unless (filename `Map.member` currentWorking state) $
          darcs ["add", filename]
        -- Darcs's file change tracking relies on mtime changes,
        -- so we have to be careful with doing stuff too quickly:

    , vcsCommitChanges = \_state -> do
        threadDelay mtimeChange
        darcs ["record", "--all", "--author=author", "--name=a patch"]
        return Nothing

    , vcsTagState = \_ tagname ->
        darcs ["tag", "--author=author", tagname]

    , vcsSwitchBranch = \_ _ ->
        fail "vcsSwitchBranch: darcs does not support branches within a repo"

    , vcsCheckoutTag = Right $ \tagname dest ->
        darcs ["clone", "--lazy", "--tag=^" ++ tagname ++ "$", ".", dest]
    }
  where
    darcsInvocation args = (programInvocation (vcsProgram vcs) args) {
                               progInvokeCwd = Just repoRoot
                           }
    darcs = runProgramInvocation verbosity . darcsInvocation