content: {-# LANGUAGE DeriveGeneric #-}
  {-# LANGUAGE FlexibleContexts #-}
  {-# LANGUAGE FlexibleInstances #-}
  
  -----------------------------------------------------------------------------
  -- |
  -- Module      :  Distribution.Simple.PackageIndex
  -- Copyright   :  (c) David Himmelstrup 2005,
  --                    Bjorn Bringert 2007,
  --                    Duncan Coutts 2008-2009
  --
  -- Maintainer  :  cabal-devel@haskell.org
  -- Portability :  portable
  --
  -- An index of packages whose primary key is 'UnitId'.  Public libraries
  -- are additionally indexed by 'PackageName' and 'Version'.
  -- Technically, these are an index of *units* (so we should eventually
  -- rename it to 'UnitIndex'); but in the absence of internal libraries
  -- or Backpack each unit is equivalent to a package.
  --
  -- While 'PackageIndex' is parametric over what it actually records,
  -- it is in fact only ever instantiated with a single element:
  -- The 'InstalledPackageIndex' (defined here) contains a graph of
  -- 'InstalledPackageInfo's representing the packages in a
  -- package database stack.  It is used in a variety of ways:
  --
  --   * The primary use to let Cabal access the same installed
  --     package database which is used by GHC during compilation.
  --     For example, this data structure is used by 'ghc-pkg'
  --     and 'Cabal' to do consistency checks on the database
  --     (are the references closed).
  --
  --   * Given a set of dependencies, we can compute the transitive
  --     closure of dependencies.  This is to check if the versions
  --     of packages are consistent, and also needed by multiple
  --     tools (Haddock must be explicitly told about the every
  --     transitive package to do cross-package linking;
  --     preprocessors must know about the include paths of all
  --     transitive dependencies.)
  --
  -- This 'PackageIndex' is NOT to be confused with
  -- 'Distribution.Client.PackageIndex', which indexes packages only by
  -- 'PackageName' (this makes it suitable for indexing source packages,
  -- for which we don't know 'UnitId's.)
  --
  module Distribution.Simple.PackageIndex (
    -- * Package index data type
    InstalledPackageIndex,
    PackageIndex,
  
    -- * Creating an index
    fromList,
  
    -- * Updates
    merge,
  
    insert,
  
    deleteUnitId,
    deleteSourcePackageId,
    deletePackageName,
  --  deleteDependency,
  
    -- * Queries
  
    -- ** Precise lookups
    lookupUnitId,
    lookupComponentId,
    lookupSourcePackageId,
    lookupPackageId,
    lookupPackageName,
    lookupDependency,
    lookupInternalDependency,
  
    -- ** Case-insensitive searches
    searchByName,
    SearchResult(..),
    searchByNameSubstring,
  
    -- ** Bulk queries
    allPackages,
    allPackagesByName,
    allPackagesBySourcePackageId,
    allPackagesBySourcePackageIdAndLibName,
  
    -- ** Special queries
    brokenPackages,
    dependencyClosure,
    reverseDependencyClosure,
    topologicalOrder,
    reverseTopologicalOrder,
    dependencyInconsistencies,
    dependencyCycles,
    dependencyGraph,
    moduleNameIndex,
  
    -- * Backwards compatibility
    deleteInstalledPackageId,
    lookupInstalledPackageId,
    ) where
  
  import Prelude ()
  import Distribution.Compat.Prelude hiding (lookup)
  import qualified Distribution.Compat.Map.Strict as Map
  
  import Distribution.Package
  import Distribution.Backpack
  import Distribution.ModuleName
  import qualified Distribution.InstalledPackageInfo as IPI
  import Distribution.Version
  import Distribution.Simple.Utils
  import Distribution.Types.Dependency
  import Distribution.Types.UnqualComponentName
  
  import Control.Exception (assert)
  import Data.Array ((!))
  import qualified Data.Array as Array
  import qualified Data.Graph as Graph
  import Data.List as List ( groupBy,  deleteBy, deleteFirstsBy )
  import qualified Data.Tree  as Tree
  import Control.Monad
  import Distribution.Compat.Stack
  
  -- | The collection of information about packages from one or more 'PackageDB's.
  -- These packages generally should have an instance of 'PackageInstalled'
  --
  -- Packages are uniquely identified in by their 'UnitId', they can
  -- also be efficiently looked up by package name or by name and version.
  --
  data PackageIndex a = PackageIndex {
    -- The primary index. Each InstalledPackageInfo record is uniquely identified
    -- by its UnitId.
    --
    unitIdIndex :: !(Map UnitId a),
  
    -- This auxiliary index maps package names (case-sensitively) to all the
    -- versions and instances of that package. This allows us to find all
    -- versions satisfying a dependency.
    --
    -- It is a three-level index. The first level is the package name,
    -- the second is the package version and the final level is instances
    -- of the same package version. These are unique by UnitId
    -- and are kept in preference order.
    --
    -- FIXME: Clarify what "preference order" means. Check that this invariant is
    -- preserved. See #1463 for discussion.
    packageIdIndex :: !(Map (PackageName, Maybe UnqualComponentName) (Map Version [a]))
  
    } deriving (Eq, Generic, Show, Read)
  
  instance Binary a => Binary (PackageIndex a)
  
  -- | The default package index which contains 'InstalledPackageInfo'.  Normally
  -- use this.
  type InstalledPackageIndex = PackageIndex IPI.InstalledPackageInfo
  
  instance Monoid (PackageIndex IPI.InstalledPackageInfo) where
    mempty  = PackageIndex Map.empty Map.empty
    mappend = (<>)
    --save one mappend with empty in the common case:
    mconcat [] = mempty
    mconcat xs = foldr1 mappend xs
  
  instance Semigroup (PackageIndex IPI.InstalledPackageInfo) where
    (<>) = merge
  
  {-# NOINLINE invariant #-}
  invariant :: WithCallStack (InstalledPackageIndex -> Bool)
  invariant (PackageIndex pids pnames) =
    -- trace (show pids' ++ "\n" ++ show pnames') $
    pids' == pnames'
   where
    pids' = map installedUnitId (Map.elems pids)
    pnames' = sort
       [ assert pinstOk (installedUnitId pinst)
       | ((pname, plib), pvers)  <- Map.toList pnames
       , let pversOk = not (Map.null pvers)
       , (pver,  pinsts) <- assert pversOk $ Map.toList pvers
       , let pinsts'  = sortBy (comparing installedUnitId) pinsts
             pinstsOk = all (\g -> length g == 1)
                            (groupBy (equating installedUnitId) pinsts')
       , pinst           <- assert pinstsOk $ pinsts'
       , let pinstOk = packageName    pinst == pname
                    && packageVersion pinst == pver
                    && IPI.sourceLibName  pinst == plib
       ]
    -- If you see this invariant failing (ie the assert in mkPackageIndex below)
    -- then one thing to check is if it is happening in fromList. Check if the
    -- second list above (the sort [...] bit) is ending up with duplicates. This
    -- has been observed in practice once due to a messed up ghc-pkg db. How/why
    -- it became messed up was not discovered.
  
  
  --
  -- * Internal helpers
  --
  
  mkPackageIndex :: WithCallStack (Map UnitId IPI.InstalledPackageInfo
                 -> Map (PackageName, Maybe UnqualComponentName)
                        (Map Version [IPI.InstalledPackageInfo])
                 -> InstalledPackageIndex)
  mkPackageIndex pids pnames = assert (invariant index) index
    where index = PackageIndex pids pnames
  
  
  --
  -- * Construction
  --
  
  -- | Build an index out of a bunch of packages.
  --
  -- If there are duplicates by 'UnitId' then later ones mask earlier
  -- ones.
  --
  fromList :: [IPI.InstalledPackageInfo] -> InstalledPackageIndex
  fromList pkgs = mkPackageIndex pids pnames
    where
      pids      = Map.fromList [ (installedUnitId pkg, pkg) | pkg <- pkgs ]
      pnames    =
        Map.fromList
          [ (liftM2 (,) packageName IPI.sourceLibName (head pkgsN), pvers)
          | pkgsN <- groupBy (equating  (liftM2 (,) packageName IPI.sourceLibName))
                   . sortBy  (comparing (liftM3 (,,) packageName IPI.sourceLibName packageVersion))
                   $ pkgs
          , let pvers =
                  Map.fromList
                  [ (packageVersion (head pkgsNV),
                     nubBy (equating installedUnitId) (reverse pkgsNV))
                  | pkgsNV <- groupBy (equating packageVersion) pkgsN
                  ]
          ]
  
  --
  -- * Updates
  --
  
  -- | Merge two indexes.
  --
  -- Packages from the second mask packages from the first if they have the exact
  -- same 'UnitId'.
  --
  -- For packages with the same source 'PackageId', packages from the second are
  -- \"preferred\" over those from the first. Being preferred means they are top
  -- result when we do a lookup by source 'PackageId'. This is the mechanism we
  -- use to prefer user packages over global packages.
  --
  merge :: InstalledPackageIndex -> InstalledPackageIndex
        -> InstalledPackageIndex
  merge (PackageIndex pids1 pnames1) (PackageIndex pids2 pnames2) =
    mkPackageIndex (Map.unionWith (\_ y -> y) pids1 pids2)
                   (Map.unionWith (Map.unionWith mergeBuckets) pnames1 pnames2)
    where
      -- Packages in the second list mask those in the first, however preferred
      -- packages go first in the list.
      mergeBuckets xs ys = ys ++ (xs \\ ys)
      (\\) = deleteFirstsBy (equating installedUnitId)
  
  
  -- | Inserts a single package into the index.
  --
  -- This is equivalent to (but slightly quicker than) using 'mappend' or
  -- 'merge' with a singleton index.
  --
  insert :: IPI.InstalledPackageInfo -> InstalledPackageIndex -> InstalledPackageIndex
  insert pkg (PackageIndex pids pnames) =
      mkPackageIndex pids' pnames'
  
    where
      pids'   = Map.insert (installedUnitId pkg) pkg pids
      pnames' = insertPackageName pnames
      insertPackageName =
        Map.insertWith (\_ -> insertPackageVersion)
                       (packageName pkg, IPI.sourceLibName pkg)
                       (Map.singleton (packageVersion pkg) [pkg])
  
      insertPackageVersion =
        Map.insertWith (\_ -> insertPackageInstance)
                       (packageVersion pkg) [pkg]
  
      insertPackageInstance pkgs =
        pkg : deleteBy (equating installedUnitId) pkg pkgs
  
  
  -- | Removes a single installed package from the index.
  --
  deleteUnitId :: UnitId -> InstalledPackageIndex
               -> InstalledPackageIndex
  deleteUnitId ipkgid original@(PackageIndex pids pnames) =
    case Map.updateLookupWithKey (\_ _ -> Nothing) ipkgid pids of
      (Nothing,     _)     -> original
      (Just spkgid, pids') -> mkPackageIndex pids'
                                            (deletePkgName spkgid pnames)
  
    where
      deletePkgName spkgid =
        Map.update (deletePkgVersion spkgid) (packageName spkgid, IPI.sourceLibName spkgid)
  
      deletePkgVersion spkgid =
          (\m -> if Map.null m then Nothing else Just m)
        . Map.update deletePkgInstance (packageVersion spkgid)
  
      deletePkgInstance =
          (\xs -> if null xs then Nothing else Just xs)
        . List.deleteBy (\_ pkg -> installedUnitId pkg == ipkgid) undefined
  
  -- | Backwards compatibility wrapper for Cabal pre-1.24.
  {-# DEPRECATED deleteInstalledPackageId "Use deleteUnitId instead" #-}
  deleteInstalledPackageId :: UnitId -> InstalledPackageIndex
                           -> InstalledPackageIndex
  deleteInstalledPackageId = deleteUnitId
  
  -- | Removes all packages with this source 'PackageId' from the index.
  --
  deleteSourcePackageId :: PackageId -> InstalledPackageIndex
                        -> InstalledPackageIndex
  deleteSourcePackageId pkgid original@(PackageIndex pids pnames) =
    -- NB: Doesn't delete internal packages
    case Map.lookup (packageName pkgid, Nothing) pnames of
      Nothing     -> original
      Just pvers  -> case Map.lookup (packageVersion pkgid) pvers of
        Nothing   -> original
        Just pkgs -> mkPackageIndex
                       (foldl' (flip (Map.delete . installedUnitId)) pids pkgs)
                       (deletePkgName pnames)
    where
      deletePkgName =
        Map.update deletePkgVersion (packageName pkgid, Nothing)
  
      deletePkgVersion =
          (\m -> if Map.null m then Nothing else Just m)
        . Map.delete (packageVersion pkgid)
  
  
  -- | Removes all packages with this (case-sensitive) name from the index.
  --
  -- NB: Does NOT delete internal libraries from this package.
  --
  deletePackageName :: PackageName -> InstalledPackageIndex
                    -> InstalledPackageIndex
  deletePackageName name original@(PackageIndex pids pnames) =
    case Map.lookup (name, Nothing) pnames of
      Nothing     -> original
      Just pvers  -> mkPackageIndex
                       (foldl' (flip (Map.delete . installedUnitId)) pids
                               (concat (Map.elems pvers)))
                       (Map.delete (name, Nothing) pnames)
  
  {-
  -- | Removes all packages satisfying this dependency from the index.
  --
  deleteDependency :: Dependency -> PackageIndex -> PackageIndex
  deleteDependency (Dependency name verstionRange) =
    delete' name (\pkg -> packageVersion pkg `withinRange` verstionRange)
  -}
  
  --
  -- * Bulk queries
  --
  
  -- | Get all the packages from the index.
  --
  allPackages :: PackageIndex a -> [a]
  allPackages = Map.elems . unitIdIndex
  
  -- | Get all the packages from the index.
  --
  -- They are grouped by package name (case-sensitively).
  --
  -- (Doesn't include private libraries.)
  --
  allPackagesByName :: PackageIndex a -> [(PackageName, [a])]
  allPackagesByName index =
    [ (pkgname, concat (Map.elems pvers))
    | ((pkgname, Nothing), pvers) <- Map.toList (packageIdIndex index) ]
  
  -- | Get all the packages from the index.
  --
  -- They are grouped by source package id (package name and version).
  --
  -- (Doesn't include private libraries)
  --
  allPackagesBySourcePackageId :: HasUnitId a => PackageIndex a
                               -> [(PackageId, [a])]
  allPackagesBySourcePackageId index =
    [ (packageId ipkg, ipkgs)
    | ((_, Nothing), pvers) <- Map.toList (packageIdIndex index)
    , ipkgs@(ipkg:_) <- Map.elems pvers ]
  
  -- | Get all the packages from the index.
  --
  -- They are grouped by source package id and library name.
  --
  -- This DOES include internal libraries.
  allPackagesBySourcePackageIdAndLibName :: HasUnitId a => PackageIndex a
                               -> [((PackageId, Maybe UnqualComponentName), [a])]
  allPackagesBySourcePackageIdAndLibName index =
    [ ((packageId ipkg, ln), ipkgs)
    | ((_, ln), pvers) <- Map.toList (packageIdIndex index)
    , ipkgs@(ipkg:_) <- Map.elems pvers ]
  
  --
  -- * Lookups
  --
  
  -- | Does a lookup by unit identifier.
  --
  -- Since multiple package DBs mask each other by 'UnitId',
  -- then we get back at most one package.
  --
  lookupUnitId :: PackageIndex a -> UnitId
               -> Maybe a
  lookupUnitId index uid = Map.lookup uid (unitIdIndex index)
  
  -- | Does a lookup by component identifier.  In the absence
  -- of Backpack, this is just a 'lookupUnitId'.
  --
  lookupComponentId :: PackageIndex a -> ComponentId
                    -> Maybe a
  lookupComponentId index cid =
      Map.lookup (newSimpleUnitId cid) (unitIdIndex index)
  
  -- | Backwards compatibility for Cabal pre-1.24.
  {-# DEPRECATED lookupInstalledPackageId "Use lookupUnitId instead" #-}
  lookupInstalledPackageId :: PackageIndex a -> UnitId
                           -> Maybe a
  lookupInstalledPackageId = lookupUnitId
  
  
  -- | Does a lookup by source package id (name & version).
  --
  -- There can be multiple installed packages with the same source 'PackageId'
  -- but different 'UnitId'. They are returned in order of
  -- preference, with the most preferred first.
  --
  lookupSourcePackageId :: PackageIndex a -> PackageId -> [a]
  lookupSourcePackageId index pkgid =
    -- Do not lookup internal libraries
    case Map.lookup (packageName pkgid, Nothing) (packageIdIndex index) of
      Nothing     -> []
      Just pvers  -> case Map.lookup (packageVersion pkgid) pvers of
        Nothing   -> []
        Just pkgs -> pkgs -- in preference order
  
  -- | Convenient alias of 'lookupSourcePackageId', but assuming only
  -- one package per package ID.
  lookupPackageId :: PackageIndex a -> PackageId -> Maybe a
  lookupPackageId index pkgid = case lookupSourcePackageId index pkgid  of
      []    -> Nothing
      [pkg] -> Just pkg
      _     -> error "Distribution.Simple.PackageIndex: multiple matches found"
  
  -- | Does a lookup by source package name.
  --
  lookupPackageName :: PackageIndex a -> PackageName
                    -> [(Version, [a])]
  lookupPackageName index name =
    -- Do not match internal libraries
    case Map.lookup (name, Nothing) (packageIdIndex index) of
      Nothing     -> []
      Just pvers  -> Map.toList pvers
  
  
  -- | Does a lookup by source package name and a range of versions.
  --
  -- We get back any number of versions of the specified package name, all
  -- satisfying the version range constraint.
  --
  -- This does NOT work for internal dependencies, DO NOT use this
  -- function on those; use 'lookupInternalDependency' instead.
  --
  -- INVARIANT: List of eligible 'IPI.InstalledPackageInfo' is non-empty.
  --
  lookupDependency :: InstalledPackageIndex -> Dependency
                   -> [(Version, [IPI.InstalledPackageInfo])]
  lookupDependency index dep =
      -- Yes, a little bit of a misnomer here!
      lookupInternalDependency index dep Nothing
  
  -- | Does a lookup by source package name and a range of versions.
  --
  -- We get back any number of versions of the specified package name, all
  -- satisfying the version range constraint.
  --
  -- INVARIANT: List of eligible 'IPI.InstalledPackageInfo' is non-empty.
  --
  lookupInternalDependency :: InstalledPackageIndex -> Dependency
                   -> Maybe UnqualComponentName
                   -> [(Version, [IPI.InstalledPackageInfo])]
  lookupInternalDependency index (Dependency name versionRange) libn =
    case Map.lookup (name, libn) (packageIdIndex index) of
      Nothing    -> []
      Just pvers -> [ (ver, pkgs')
                    | (ver, pkgs) <- Map.toList pvers
                    , ver `withinRange` versionRange
                    , let pkgs' = filter eligible pkgs
                    -- Enforce the invariant
                    , not (null pkgs')
                    ]
   where
    -- When we select for dependencies, we ONLY want to pick up indefinite
    -- packages, or packages with no instantiations.  We'll do mix-in
    -- linking to improve any such package into an instantiated one
    -- later.
    eligible pkg = IPI.indefinite pkg || null (IPI.instantiatedWith pkg)
  
  
  --
  -- * Case insensitive name lookups
  --
  
  -- | Does a case-insensitive search by package name.
  --
  -- If there is only one package that compares case-insensitively to this name
  -- then the search is unambiguous and we get back all versions of that package.
  -- If several match case-insensitively but one matches exactly then it is also
  -- unambiguous.
  --
  -- If however several match case-insensitively and none match exactly then we
  -- have an ambiguous result, and we get back all the versions of all the
  -- packages. The list of ambiguous results is split by exact package name. So
  -- it is a non-empty list of non-empty lists.
  --
  searchByName :: PackageIndex a -> String -> SearchResult [a]
  searchByName index name =
    -- Don't match internal packages
    case [ pkgs | pkgs@((pname, Nothing),_) <- Map.toList (packageIdIndex index)
                , lowercase (unPackageName pname) == lname ] of
      []               -> None
      [(_,pvers)]      -> Unambiguous (concat (Map.elems pvers))
      pkgss            -> case find ((mkPackageName name ==) . fst . fst) pkgss of
        Just (_,pvers) -> Unambiguous (concat (Map.elems pvers))
        Nothing        -> Ambiguous (map (concat . Map.elems . snd) pkgss)
    where lname = lowercase name
  
  data SearchResult a = None | Unambiguous a | Ambiguous [a]
  
  -- | Does a case-insensitive substring search by package name.
  --
  -- That is, all packages that contain the given string in their name.
  --
  searchByNameSubstring :: PackageIndex a -> String -> [a]
  searchByNameSubstring index searchterm =
    [ pkg
    -- Don't match internal packages
    | ((pname, Nothing), pvers) <- Map.toList (packageIdIndex index)
    , lsearchterm `isInfixOf` lowercase (unPackageName pname)
    , pkgs <- Map.elems pvers
    , pkg <- pkgs ]
    where lsearchterm = lowercase searchterm
  
  
  --
  -- * Special queries
  --
  
  -- None of the stuff below depends on the internal representation of the index.
  --
  
  -- | Find if there are any cycles in the dependency graph. If there are no
  -- cycles the result is @[]@.
  --
  -- This actually computes the strongly connected components. So it gives us a
  -- list of groups of packages where within each group they all depend on each
  -- other, directly or indirectly.
  --
  dependencyCycles :: PackageInstalled a => PackageIndex a -> [[a]]
  dependencyCycles index =
    [ vs | Graph.CyclicSCC vs <- Graph.stronglyConnComp adjacencyList ]
    where
      adjacencyList = [ (pkg, installedUnitId pkg, installedDepends pkg)
                      | pkg <- allPackages index ]
  
  
  -- | All packages that have immediate dependencies that are not in the index.
  --
  -- Returns such packages along with the dependencies that they're missing.
  --
  brokenPackages :: PackageInstalled a => PackageIndex a
                 -> [(a, [UnitId])]
  brokenPackages index =
    [ (pkg, missing)
    | pkg  <- allPackages index
    , let missing = [ pkg' | pkg' <- installedDepends pkg
                           , isNothing (lookupUnitId index pkg') ]
    , not (null missing) ]
  
  -- | Tries to take the transitive closure of the package dependencies.
  --
  -- If the transitive closure is complete then it returns that subset of the
  -- index. Otherwise it returns the broken packages as in 'brokenPackages'.
  --
  -- * Note that if the result is @Right []@ it is because at least one of
  -- the original given 'PackageId's do not occur in the index.
  --
  dependencyClosure :: InstalledPackageIndex
                    -> [UnitId]
                    -> Either (InstalledPackageIndex)
                              [(IPI.InstalledPackageInfo, [UnitId])]
  dependencyClosure index pkgids0 = case closure mempty [] pkgids0 of
    (completed, []) -> Left completed
    (completed, _)  -> Right (brokenPackages completed)
   where
      closure completed failed []             = (completed, failed)
      closure completed failed (pkgid:pkgids) = case lookupUnitId index pkgid of
        Nothing   -> closure completed (pkgid:failed) pkgids
        Just pkg  -> case lookupUnitId completed (installedUnitId pkg) of
          Just _  -> closure completed  failed pkgids
          Nothing -> closure completed' failed pkgids'
            where completed' = insert pkg completed
                  pkgids'    = installedDepends pkg ++ pkgids
  
  -- | Takes the transitive closure of the packages reverse dependencies.
  --
  -- * The given 'PackageId's must be in the index.
  --
  reverseDependencyClosure :: PackageInstalled a => PackageIndex a
                           -> [UnitId]
                           -> [a]
  reverseDependencyClosure index =
      map vertexToPkg
    . concatMap Tree.flatten
    . Graph.dfs reverseDepGraph
    . map (fromMaybe noSuchPkgId . pkgIdToVertex)
  
    where
      (depGraph, vertexToPkg, pkgIdToVertex) = dependencyGraph index
      reverseDepGraph = Graph.transposeG depGraph
      noSuchPkgId = error "reverseDependencyClosure: package is not in the graph"
  
  topologicalOrder :: PackageInstalled a => PackageIndex a -> [a]
  topologicalOrder index = map toPkgId
                         . Graph.topSort
                         $ graph
    where (graph, toPkgId, _) = dependencyGraph index
  
  reverseTopologicalOrder :: PackageInstalled a => PackageIndex a -> [a]
  reverseTopologicalOrder index = map toPkgId
                                . Graph.topSort
                                . Graph.transposeG
                                $ graph
    where (graph, toPkgId, _) = dependencyGraph index
  
  -- | Builds a graph of the package dependencies.
  --
  -- Dependencies on other packages that are not in the index are discarded.
  -- You can check if there are any such dependencies with 'brokenPackages'.
  --
  dependencyGraph :: PackageInstalled a => PackageIndex a
                  -> (Graph.Graph,
                      Graph.Vertex -> a,
                      UnitId -> Maybe Graph.Vertex)
  dependencyGraph index = (graph, vertex_to_pkg, id_to_vertex)
    where
      graph = Array.listArray bounds
                [ [ v | Just v <- map id_to_vertex (installedDepends pkg) ]
                | pkg <- pkgs ]
  
      pkgs             = sortBy (comparing packageId) (allPackages index)
      vertices         = zip (map installedUnitId pkgs) [0..]
      vertex_map       = Map.fromList vertices
      id_to_vertex pid = Map.lookup pid vertex_map
  
      vertex_to_pkg vertex = pkgTable ! vertex
  
      pkgTable   = Array.listArray bounds pkgs
      topBound = length pkgs - 1
      bounds = (0, topBound)
  
  -- | We maintain the invariant that, for any 'DepUniqueKey', there
  -- is only one instance of the package in our database.
  type DepUniqueKey = (PackageName, Maybe UnqualComponentName, Map ModuleName OpenModule)
  
  -- | Given a package index where we assume we want to use all the packages
  -- (use 'dependencyClosure' if you need to get such a index subset) find out
  -- if the dependencies within it use consistent versions of each package.
  -- Return all cases where multiple packages depend on different versions of
  -- some other package.
  --
  -- Each element in the result is a package name along with the packages that
  -- depend on it and the versions they require. These are guaranteed to be
  -- distinct.
  --
  dependencyInconsistencies :: InstalledPackageIndex
                               -- At DepUniqueKey...
                            -> [(DepUniqueKey,
                                 -- There were multiple packages (BAD!)
                                 [(UnitId,
                                   -- And here are the packages which
                                   -- immediately depended on it
                                   [IPI.InstalledPackageInfo])])]
  dependencyInconsistencies index = do
      (dep_key, insts_map) <- Map.toList inverseIndex
      let insts = Map.toList insts_map
      guard (length insts >= 2)
      return (dep_key, insts)
    where
      inverseIndex :: Map DepUniqueKey (Map UnitId [IPI.InstalledPackageInfo])
      inverseIndex = Map.fromListWith (Map.unionWith (++)) $ do
          pkg <- allPackages index
          dep_ipid <- installedDepends pkg
          Just dep <- [lookupUnitId index dep_ipid]
          let dep_key = (packageName dep, IPI.sourceLibName dep,
                         Map.fromList (IPI.instantiatedWith dep))
          return (dep_key, Map.singleton dep_ipid [pkg])
  
  -- | A rough approximation of GHC's module finder, takes a
  -- 'InstalledPackageIndex' and turns it into a map from module names to their
  -- source packages.  It's used to initialize the @build-deps@ field in @cabal
  -- init@.
  moduleNameIndex :: InstalledPackageIndex -> Map ModuleName [IPI.InstalledPackageInfo]
  moduleNameIndex index =
    Map.fromListWith (++) $ do
      pkg <- allPackages index
      IPI.ExposedModule m reexport <- IPI.exposedModules pkg
      case reexport of
          Nothing -> return (m, [pkg])
          Just (OpenModuleVar _) -> []
          Just (OpenModule _ m') | m == m'   -> []
                                  | otherwise -> return (m', [pkg])
          -- The heuristic is this: we want to prefer the original package
          -- which originally exported a module.  However, if a reexport
          -- also *renamed* the module (m /= m'), then we have to use the
          -- downstream package, since the upstream package has the wrong
          -- module name!
  