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    , tryChangeGroup

--

-- /Note:/ this function should correctly execute both intra-group

-- evacuations (in all modes) and inter-group evacuations (in the

-- 'ChangeAll' mode). Of course, this requires that the correct list

-- of target nodes is passed.

-- | Change-group IAllocator mode main function.

--

-- This is very similar to 'tryNodeEvac', the only difference is that

-- we don't choose as target group the current instance group, but

-- instead:

--

--   1. at the start of the function, we compute which are the target

--   groups; either no groups were passed in, in which case we choose

--   all groups out of which we don't evacuate instance, or there were

--   some groups passed, in which case we use those

--

--   2. for each instance, we use 'findBestAllocGroup' to choose the

--   best group to hold the instance, and then we do what

--   'tryNodeEvac' does, except for this group instead of the current

--   instance group.

--

-- Note that the correct behaviour of this function relies on the

-- function 'nodeEvacInstance' to be able to do correctly both

-- intra-group and inter-group moves when passed the 'ChangeAll' mode.

tryChangeGroup :: Group.List    -- ^ The cluster groups

               -> Node.List     -- ^ The node list (cluster-wide)

               -> Instance.List -- ^ Instance list (cluster-wide)

               -> [Gdx]         -- ^ Target groups; if empty, any

                                -- groups not being evacuated

               -> [Idx]         -- ^ List of instance (indices) to be evacuated

               -> Result EvacSolution

tryChangeGroup gl ini_nl ini_il gdxs idxs =

    let evac_gdxs = nub $ map (instancePriGroup ini_nl .

                               flip Container.find ini_il) idxs

        target_gdxs = (if null gdxs

                       then Container.keys gl

                       else gdxs) \\ evac_gdxs

        offline = map Node.idx . filter Node.offline $ Container.elems ini_nl

        excl_ndx = foldl' (flip IntSet.insert) IntSet.empty offline

        group_ndx = map (\(gdx, (nl, _)) -> (gdx, map Node.idx

                                             (Container.elems nl))) $

                      splitCluster ini_nl ini_il

        (_, _, esol) =

            foldl' (\state@(nl, il, _) inst ->

                        let solution = do

                              let ncnt = Instance.requiredNodes $

                                         Instance.diskTemplate inst

                              (gdx, _, _) <- findBestAllocGroup gl nl il

                                             (Just target_gdxs) inst ncnt

                              av_nodes <- availableGroupNodes group_ndx

                                          excl_ndx gdx

                              nodeEvacInstance nl il ChangeAll inst av_nodes

                        in updateEvacSolution state inst solution

                   )

            (ini_nl, ini_il, emptyEvacSolution)

            (map (`Container.find` ini_il) idxs)

    in return $ reverseEvacSolution esol