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    , oFullEvacuation

-- | Compute the result of migrating an instance.

migrate :: Idx -> (Node.List, Instance.List)

           -> OpResult (Node.List, Instance.List)

migrate idx (nl, il) = do

  let inst = Container.find idx il

      pdx = Instance.pNode inst

      sdx = Instance.sNode inst

      pNode = Container.find pdx nl

      sNode = Container.find sdx nl

      pNode' = Node.removePri pNode inst

      sNode' = Node.removeSec sNode inst

  sNode'' <- Node.addPriEx True sNode' inst

  pNode'' <- Node.addSecEx True pNode' inst sdx

  let inst' = Instance.setBoth inst sdx pdx

      nl' = Container.addTwo pdx pNode'' sdx sNode'' nl

      il' = Container.add idx inst' il

  return (nl', il')

-- | Obtain the list of primaries for a given node.

-- This restricts to those instances that have a secondary node.

primaries :: (Node.List, Instance.List) -> Ndx -> [Idx]

primaries (nl, il) = 

  filter (Instance.hasSecondary . flip Container.find  il) 

  . Node.pList . flip Container.find nl

-- | Migrate all instances of a given list of nodes.

-- The list of nodes is repeated as first argument in the result.

migrateOffNodes :: ([Ndx], (Node.List, Instance.List))

                   -> OpResult ([Ndx], (Node.List, Instance.List))

migrateOffNodes (ndxs, conf) = do

  let instances = ndxs >>= primaries conf

  conf' <- foldM (flip migrate) conf instances

  return (ndxs, conf')

-- | Compute the result of replacing the secondary node of an instance.

replaceSecondary :: Idx -> Ndx -> (Node.List, Instance.List)

        -> OpResult (Node.List, Instance.List)

replaceSecondary idx new_ndx (nl, il) = do

  let  new_secondary = Container.find new_ndx nl

       inst = Container.find idx il

       old_ndx = Instance.sNode inst

       pdx = Instance.pNode inst

       old_secondary = Container.find pdx nl

  if pdx == new_ndx then Bad FailInternal else Ok ()

  new_secondary' <- Node.addSecEx True new_secondary inst pdx

  let old_secondary' = Node.removeSec old_secondary inst

      inst' = Instance.setSec inst new_ndx

      nl' = Container.addTwo old_ndx old_secondary' new_ndx new_secondary' nl

      il' = Container.add idx inst' il

  return (nl', il')

-- | Find a suitable secondary node for the given instance from a list of nodes.

findSecondary :: Idx -> [Ndx] -> (Node.List, Instance.List)

                 -> Result (Node.List, Instance.List)

findSecondary idx ndxs conf =

  msum $ map (opToResult . flip (replaceSecondary idx) conf) ndxs

-- | Find suitable secondary nodes from the given nodes for a list of instances.

findSecondaries :: [Idx] -> [Ndx] -> (Node.List, Instance.List) 

                   -> Result (Node.List, Instance.List)

findSecondaries idxs ndxs conf =

  foldM (\ cf idx -> findSecondary idx ndxs cf) conf idxs

-- | Obtain the list of secondaries for a given node.

secondaries :: (Node.List, Instance.List) -> Ndx -> [Idx]

secondaries (nl, _) = Node.sList . flip Container.find nl

-- | Greedily move secondaries away from a list of nodes.

-- Returns a list of nodes that can be cleared simultaneously, 

-- and the configuration after these nodes are cleared.

clearSecondaries :: [Ndx] -> [Ndx] -> (Node.List, Instance.List)

                    -> Result ([Ndx], (Node.List, Instance.List))

clearSecondaries = greedyClearNodes secondaries findSecondaries

-- | Partition a list of nodes into chunks according to the ability to find

-- suitable replacement secondary nodes.

partitionSecondaries :: [Ndx] -> [Ndx] -> (Node.List, Instance.List)

                        -> Result [([Ndx], (Node.List, Instance.List))]

partitionSecondaries = partitionNodes clearSecondaries

-- Do not show instances where only primary and secondary switched their

-- role, as here the instance is not moved in a proper sense.

            -> [(Instance.Instance, (Node.Node, Maybe Node.Node))]

      hasSec = Instance.hasSecondary inst

          || (hasSec && Instance.sNode inst /= Instance.sNode inst')

  guard . not $ Instance.pNode inst' == Instance.sNode inst

                && Instance.sNode inst' == Instance.pNode inst

  return (inst', (Container.find (Instance.pNode inst') nl', 

                  if hasSec

                     then Just $ Container.find (Instance.sNode inst') nl'

                     else Nothing))

  let migrated = mapM migrateOffNodes rebootGroups

  rebootGroups' <- if not . optFullEvacuation $ opts

                      then return rebootGroups

                      else case migrated of

                             Ok migratedGroup -> return migratedGroup

                             Bad _ -> exitErr "Failed to migrate instances\ 

                                              \ off nodes"

  let splitted' = mapM (\(grp, conf) -> partitionSecondaries grp allNdx conf)

                  rebootGroups'

  rebootGroups'' <- if optFullEvacuation opts

                      then case splitted' of

                             Ok splitgroups -> return $ concat splitgroups

                             Bad _ -> exitErr "Not enough capacity to move\

                                              \ secondaries"

                      else return rebootGroups'

      nodesRebootGroups = map (first $ map idToNode

                                       . filter (`IntMap.member` nodes))

                          rebootGroups''

      confToMoveNames =

        map (Instance.name *** (Node.name *** flip (>>=) (return . Node.name)))

        . getMoves (nlf, ilf)

      showMoves :: [(String, (String, Maybe String))] -> IO ()

                    then mapM_ $ putStrLn . \(a,(b,c)) ->

                                                maybe (printf "  %s %s" a b)

                                                      (printf "  %s %s %s" a b)

                                                      c