    -- |
    --
    -- This modules provides functions for working with both the legacy
    -- "build-tools" field, and its replacement, "build-tool-depends". Prefer using
    -- the functions contained to access those fields directly.
    module Distribution.Simple.BuildToolDepends where

    import           Prelude ()
    import           Distribution.Compat.Prelude

    import qualified Data.Map as Map

    import           Distribution.Package
    import           Distribution.PackageDescription
    import           Distribution.Types.ExeDependency
    import           Distribution.Types.LegacyExeDependency
    import           Distribution.Types.UnqualComponentName

    -- | Desugar a "build-tools" entry into proper a executable dependency if
    -- possible.
    --
    -- An entry can be so desguared in two cases:
    --
    -- 1. The name in build-tools matches a locally defined executable.  The
    --    executable dependency produced is on that exe in the current package.
    --
    -- 2. The name in build-tools matches a hard-coded set of known tools.  For now,
    --    the executable dependency produced is one an executable in a package of
    --    the same, but the hard-coding could just as well be per-key.
    --
    -- The first cases matches first.
    desugarBuildTool :: PackageDescription
                     -> LegacyExeDependency
                     -> Maybe ExeDependency
    desugarBuildTool pkg led =
      if foundLocal
      then Just $ ExeDependency (packageName pkg) toolName reqVer
      else Map.lookup name whiteMap
      where
        LegacyExeDependency name reqVer = led
        toolName = mkUnqualComponentName name
        foundLocal = toolName `elem` map exeName (executables pkg)
        whitelist = [ "hscolour", "haddock", "happy", "alex", "hsc2hs", "c2hs"
                    , "cpphs", "greencard", "hspec-discover"
                    ]
        whiteMap  = Map.fromList $ flip map whitelist $ \n ->
          (n, ExeDependency (mkPackageName n) (mkUnqualComponentName n) reqVer)

    -- | Get everything from "build-tool-depends", along with entries from
    -- "build-tools" that we know how to desugar.
    --
    -- This should almost always be used instead of just accessing the
    -- `buildToolDepends` field directly.
    getAllToolDependencies :: PackageDescription
                           -> BuildInfo
                           -> [ExeDependency]
    getAllToolDependencies pkg bi =
      buildToolDepends bi ++ mapMaybe (desugarBuildTool pkg) (buildTools bi)

    -- | Does the given executable dependency map to this current package?
    --
    -- This is a tiny function, but used in a number of places.
    --
    -- This function is only sound to call on `BuildInfo`s from the given package
    -- description. This is because it just filters the package names of each
    -- dependency, and does not check whether version bounds in fact exclude the
    -- current package, or the referenced components in fact exist in the current
    -- package.
    --
    -- This is OK because when a package is loaded, it is checked (in
    -- `Distribution.Package.Check`) that dependencies matching internal components
    -- do indeed have version bounds accepting the current package, and any
    -- depended-on component in the current package actually exists. In fact this
    -- check is performed by gathering the internal tool dependencies of each
    -- component of the package according to this module, and ensuring those
    -- properties on each so-gathered dependency.
    --
    -- version bounds and components of the package are unchecked. This is because
    -- we sanitize exe deps so that the matching name implies these other
    -- conditions.
    isInternal :: PackageDescription -> ExeDependency -> Bool
    isInternal pkg (ExeDependency n _ _) = n == packageName pkg

    -- | Get internal "build-tool-depends", along with internal "build-tools"
    --
    -- This is a tiny function, but used in a number of places. The same
    -- restrictions that apply to `isInternal` also apply to this function.
    getAllInternalToolDependencies :: PackageDescription
                                   -> BuildInfo
                                   -> [UnqualComponentName]
    getAllInternalToolDependencies pkg bi =
      [ toolname
      | dep@(ExeDependency _ toolname _) <- getAllToolDependencies pkg bi
      , isInternal pkg dep
      ]