module Hakyll.Core.DependencyAnalyzer
    ( DependencyAnalyzer (..)
    , Signal (..)
    , makeDependencyAnalyzer
    , step
    , stepAll
    ) where

import Prelude hiding (reverse)
import qualified Prelude as P (reverse)
import Control.Arrow (first)
import Data.Set (Set)
import qualified Data.Set as S
import Data.Monoid (Monoid, mappend, mempty)

import Hakyll.Core.DirectedGraph

-- | This data structure represents the state of the dependency analyzer. It
-- holds a complete graph in 'analyzerGraph', which always contains all items,
-- whether they are to be compiled or not.
-- The 'analyzerRemains' fields holds the items that still need to be compiled,
-- and 'analyzerDone' holds the items which are already compiled. This means
-- that initally, 'analyzerDone' is empty and 'analyzerRemains' contains the
-- items which are out-of-date (or items which have out-of-date dependencies).
-- We also hold the dependency graph from the previous run because we need it
-- when we want to determine when an item is out-of-date. An item is out-of-date
-- when:
-- * the resource from which it compiles is out-of-date, or;
-- * any of it's dependencies is out-of-date, or;
-- * it's set of dependencies has changed since the previous run.
data DependencyAnalyzer a = DependencyAnalyzer
    { -- | The complete dependency graph
      analyzerGraph         :: DirectedGraph a
    , -- | A set of items yet to be compiled
      analyzerRemains       :: Set a
    , -- | A set of items already compiled
      analyzerDone          :: Set a
    , -- | The dependency graph from the previous run
      analyzerPreviousGraph :: DirectedGraph a
    } deriving (Show)

data Signal a = Build a
              | Cycle [a]
              | Done
              deriving (Show)

instance (Ord a, Show a) => Monoid (DependencyAnalyzer a) where
    mempty = DependencyAnalyzer mempty mempty mempty mempty
    mappend x y = growRemains $ DependencyAnalyzer
        (analyzerGraph x `mappend` analyzerGraph y)
        (analyzerRemains x `mappend` analyzerRemains y)
        (analyzerDone x `mappend` analyzerDone y)
        (analyzerPreviousGraph x `mappend` analyzerPreviousGraph y)

-- | Construct a dependency analyzer
makeDependencyAnalyzer :: (Ord a, Show a)
                       => DirectedGraph a       -- ^ The dependency graph
                       -> (a -> Bool)           -- ^ Is an item out-of-date?
                       -> DirectedGraph a       -- ^ The old dependency graph
                       -> DependencyAnalyzer a  -- ^ Resulting analyzer
makeDependencyAnalyzer graph isOutOfDate prev =
    growRemains $ DependencyAnalyzer graph remains S.empty prev
    -- Construct the remains set by filtering using the given predicate
    remains = S.fromList $ filter isOutOfDate $ map fst $ toList graph

-- | The 'analyzerRemains' field of a 'DependencyAnalyzer' is supposed to
-- contain all out-of-date items, including the items with out-of-date
-- dependencies. However, it is easier to just set up the directly out-of-date
-- items initially -- and then grow the remains fields.
-- This function assumes the 'analyzerRemains' fields in incomplete, and tries
-- to correct it. Running it when the field is complete has no effect -- but it
-- is a pretty expensive function, and it should be used with care.
growRemains :: (Ord a, Show a) => DependencyAnalyzer a -> DependencyAnalyzer a
growRemains (DependencyAnalyzer graph remains done prev) =
    (DependencyAnalyzer graph remains' done prev)
    -- Grow the remains set using the indirect and changedDeps values, then
    -- filter out the items already done
    remains' = S.filter (`S.notMember` done) indirect

    -- Select the nodes which are reachable from the remaining nodes in the
    -- reversed dependency graph: these are the indirectly out-of-date items
    indirect = reachableNodes (remains `S.union` changedDeps) $ reverse graph

    -- For all nodes in the graph, check which items have a different dependency
    -- set compared to the previous run
    changedDeps = S.fromList $ map fst $
        filter (uncurry (/=) . first (`neighbours` prev)) $ toList graph

-- | Step a dependency analyzer
step :: (Ord a, Show a) => DependencyAnalyzer a -> (Signal a, DependencyAnalyzer a)
step analyzer@(DependencyAnalyzer graph remains done prev)
    -- No remaining items
    | S.null remains = (Done, analyzer)
    -- An item remains, let's find a ready item
    | otherwise =
        let item = S.findMin remains
        in case findReady analyzer item of
            Done        -> (Done, analyzer)
            Cycle c     -> (Cycle c, analyzer)
            -- A ready item was found, signal a build
            Build build ->
                let remains' = S.delete build remains
                    done'    = S.insert build done
                in (Build build, DependencyAnalyzer graph remains' done' prev)

-- | Step until done, creating a set of items we need to build -- mostly used
-- for debugging purposes
stepAll :: (Ord a, Show a) => DependencyAnalyzer a -> Maybe (Set a)
stepAll = stepAll' S.empty
    stepAll' xs analyzer = case step analyzer of
        (Build x, analyzer') -> stepAll' (S.insert x xs) analyzer'
        (Done, _)            -> Just xs
        (Cycle _, _)         -> Nothing

-- | Find an item ready to be compiled
findReady :: (Ord a, Show a) => DependencyAnalyzer a -> a -> Signal a
findReady analyzer = findReady' [] S.empty
    -- The dependency graph
    graph = analyzerGraph analyzer

    -- Items to do
    todo = analyzerRemains analyzer `S.difference` analyzerDone analyzer

    -- Worker
    findReady' stack visited item
        -- We already visited this item, the cycle is the reversed stack
        | item `S.member` visited = Cycle $ P.reverse stack'
        -- Look at the neighbours we to do
        | otherwise = case filter (`S.member` todo) neighbours' of
            -- No neighbours available to be done: it's ready!
            []      -> Build item
            -- At least one neighbour is available, search for that one
            (x : _) -> findReady' stack' visited' x
        -- Our neighbours
        neighbours' = S.toList $ neighbours item graph

        -- The new visited stack/set
        stack' = item : stack
        visited' = S.insert item visited