{-# LANGUAGE DeriveDataTypeable #-}
{-# 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
  ) where

import Prelude ()
import Distribution.Compat.Prelude hiding (lookup)
import qualified Data.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.LibraryName

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.List.NonEmpty as NE
import qualified Data.Tree  as Tree
import Control.Monad
import Distribution.Compat.Stack

import qualified Prelude (foldr1)

-- | 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, LibraryName) (Map Version [a]))

  } deriving (Eq, Generic, Show, Read, Typeable)

instance Binary a => Binary (PackageIndex a)
instance Structured a => Structured (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 = Prelude.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, LibraryName)
                      (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 ((fmap . fmap) toList pnames)
  where
    pids      = Map.fromList [ (installedUnitId pkg, pkg) | pkg <- pkgs ]
    pnames    =
      Map.fromList
        [ (liftM2 (,) packageName IPI.sourceLibName (NE.head pkgsN), pvers)
        | pkgsN <- NE.groupBy (equating  (liftM2 (,) packageName IPI.sourceLibName))
                 . sortBy  (comparing (liftM3 (,,) packageName IPI.sourceLibName packageVersion))
                 $ pkgs
        , let pvers =
                Map.fromList
                [ (packageVersion (NE.head pkgsNV),
                   NE.nubBy (equating installedUnitId) (NE.reverse pkgsNV))
                | pkgsNV <- NE.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

-- | 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, LMainLibName) 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, LMainLibName)

    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, LMainLibName) pnames of
    Nothing     -> original
    Just pvers  -> mkPackageIndex
                     (foldl' (flip (Map.delete . installedUnitId)) pids
                             (concat (Map.elems pvers)))
                     (Map.delete (name, LMainLibName) 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, LMainLibName), 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)
  | ((_, LMainLibName), 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, LibraryName), [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)

-- | 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, LMainLibName) (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, LMainLibName) (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 -> PackageName -> VersionRange
                 -> [(Version, [IPI.InstalledPackageInfo])]
lookupDependency index pn vr =
    -- Yes, a little bit of a misnomer here!
    lookupInternalDependency index pn vr LMainLibName

-- | 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 -> PackageName -> VersionRange
                 -> LibraryName
                 -> [(Version, [IPI.InstalledPackageInfo])]
lookupInternalDependency index 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, LMainLibName),_) <- 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, LMainLibName), 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, LibraryName, 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!