{- git-annex location log
 -
 - git-annex keeps track of which repositories have the contents of annexed
 - files.
 -
 - Repositories record their UUID and the date when they --get or --drop
 - a value.
 -
 - A line of the log will look like: "date N UUID"
 - Where N=1 when the repo has the file, and 0 otherwise.
 - 
 - Copyright 2010-2011 Joey Hess <joey@kitenet.net>
 -
 - Licensed under the GNU GPL version 3 or higher.
 -}

module LocationLog (
	LogStatus(..),
	logChange,
	readLog,
	writeLog,
	keyLocations,
	loggedKeys
) where

import Data.Time.Clock.POSIX
import Data.Time
import System.Locale
import System.FilePath
import qualified Data.Map as Map
import Control.Monad (when)
import Data.Maybe
import Control.Monad.State (liftIO)

import qualified GitRepo as Git
import qualified Branch
import UUID
import Types
import Locations

data LogLine = LogLine {
	date :: POSIXTime,
	status :: LogStatus,
	uuid :: UUID
} deriving (Eq)

data LogStatus = ValuePresent | ValueMissing | Undefined
	deriving (Eq)

instance Show LogStatus where
	show ValuePresent = "1"
	show ValueMissing = "0"
	show Undefined = "undefined"

instance Read LogStatus where
	readsPrec _ "1" = [(ValuePresent, "")]
	readsPrec _ "0" = [(ValueMissing, "")]
	readsPrec _ _   = [(Undefined, "")]

instance Show LogLine where
	show (LogLine d s u) = unwords [show d, show s, u]

instance Read LogLine where
	-- This parser is robust in that even unparsable log lines are
	-- read without an exception being thrown.
	-- Such lines have a status of Undefined.
	readsPrec _ string = 
		if length w == 3
			then maybe bad good pdate
			else bad
		where
			w = words string
			s = read $ w !! 1
			u = w !! 2
			pdate :: Maybe UTCTime
			pdate = parseTime defaultTimeLocale "%s%Qs" $ head w

			good v = ret $ LogLine (utcTimeToPOSIXSeconds v) s u
			bad = ret $ LogLine 0 Undefined ""
			ret v = [(v, "")]

{- Log a change in the presence of a key's value in a repository. -}
logChange :: Git.Repo -> Key -> UUID -> LogStatus -> Annex ()
logChange repo key u s = do
	when (null u) $
		error $ "unknown UUID for " ++ Git.repoDescribe repo ++ 
			" (have you run git annex init there?)"
	line <- logNow s u
	let f = logFile key
	ls <- readLog f
	writeLog f (compactLog $ line:ls)

{- Reads a log file.
 - Note that the LogLines returned may be in any order. -}
readLog :: FilePath -> Annex [LogLine]
readLog file = return . parseLog =<< Branch.get file

parseLog :: String -> [LogLine]
parseLog s = filter parsable $ map read $ lines s
	where
		-- some lines may be unparseable, avoid them
		parsable l = status l /= Undefined

{- Stores a set of lines in a log file -}
writeLog :: FilePath -> [LogLine] -> Annex ()
writeLog file ls = Branch.change file (unlines $ map show ls)

{- Generates a new LogLine with the current date. -}
logNow :: LogStatus -> UUID -> Annex LogLine
logNow s u = do
	now <- liftIO $ getPOSIXTime
	return $ LogLine now s u

{- Returns a list of repository UUIDs that, according to the log, have
 - the value of a key. -}
keyLocations :: Key -> Annex [UUID]
keyLocations key = do
	ls <- readLog $ logFile key
	return $ map uuid $ filterPresent ls

{- Filters the list of LogLines to find ones where the value
 - is (or should still be) present. -}
filterPresent :: [LogLine] -> [LogLine]
filterPresent ls = filter (\l -> ValuePresent == status l) $ compactLog ls

type LogMap = Map.Map UUID LogLine

{- Compacts a set of logs, returning a subset that contains the current
 - status. -}
compactLog :: [LogLine] -> [LogLine]
compactLog ls = compactLog' Map.empty ls
compactLog' :: LogMap -> [LogLine] -> [LogLine]
compactLog' m [] = Map.elems m
compactLog' m (l:ls) = compactLog' (mapLog m l) ls

{- Inserts a log into a map of logs, if the log has better (ie, newer)
 - information about a repo than the other logs in the map -}
mapLog :: LogMap -> LogLine -> LogMap
mapLog m l = 
	if better
		then Map.insert u l m
		else m
	where
		better = maybe True (\l' -> date l' <= date l) $ Map.lookup u m
		u = uuid l

{- Finds all keys that have location log information.
 - (There may be duplicate keys in the list.) -}
loggedKeys :: Annex [Key]
loggedKeys =
	return . catMaybes . map (logFileKey . takeFileName) =<< Branch.files