add git annex view command

(And a vpop command, which is still a bit buggy.)

Still need to do vadd and vrm, though this also adds their documentation.

Currently not very happy with the view log data serialization. I had to
lose the TDFA regexps temporarily, so I can have Read/Show instances of
View. I expect the view log format will change in some incompatable way
later, probably adding last known refs for the parent branch to View
or something like that.

Anyway, it basically works, although it's a bit slow looking up the
metadata. The actual git branch construction is about as fast as it can be
using the current git plumbing.

This commit was sponsored by Peter Hogg.

Logs/View.hs

@@ -0,0 +1,92 @@
+{- git-annex recent views log
+ -
+ - The most recently accessed view comes first.
+ -
+ - This file is stored locally in .git/annex/, not in the git-annex branch.
+ -
+ - Copyright 2014 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Logs.View (
+	currentView,
+	setView,
+	recentViews,
+	branchView,
+	prop_branchView_legal,
+) where
+
+import Common.Annex
+import Types.View
+import Types.MetaData
+import qualified Git
+import qualified Git.Branch
+import qualified Git.Ref
+import Utility.Tmp
+
+import qualified Data.Set as S
+import Data.Char
+
+showLog :: View -> String
+showLog (View branch components) = show branch ++ " " ++ show components
+
+parseLog :: String -> Maybe View
+parseLog s = 
+	let (branch, components) = separate (== ' ') s
+	in View
+		<$> pure (Git.Ref branch)
+		<*> readish components
+
+setView :: View -> Annex ()
+setView v = do
+	l <- take 99 . filter (/= v) <$> recentViews
+	f <- fromRepo gitAnnexViewLog
+	liftIO $ viaTmp writeFile f $ unlines $ map showLog (v : l)
+
+recentViews :: Annex [View]
+recentViews = do
+	f <- fromRepo gitAnnexViewLog
+	liftIO $ mapMaybe parseLog . lines <$> catchDefaultIO [] (readFile f)
+
+{- Gets the currently checked out view, if there is one. -}
+currentView :: Annex (Maybe View)
+currentView = do
+	vs <- recentViews
+	maybe Nothing (go vs) <$> inRepo Git.Branch.current
+  where
+  	go [] _ = Nothing
+	go (v:vs) b
+		| branchView v == b = Just v
+		| otherwise = go vs b
+
+{- Generates a git branch name for a View.
+ - 
+ - There is no guarantee that each view gets a unique branch name,
+ - but the branch name is used to express the view as well as possible.
+ -}
+branchView :: View -> Git.Branch
+branchView view
+	| null name = Git.Ref "refs/heads/views"
+	| otherwise = Git.Ref $ "refs/heads/views/" ++ name
+  where
+	name = intercalate ";" $ map branchcomp (viewComponents view)
+	branchcomp c
+		| multiValue (viewFilter c) = branchcomp' c
+		| otherwise = "(" ++ branchcomp' c ++ ")"
+	branchcomp' (ViewComponent metafield viewfilter)
+		| metafield == tagMetaField = branchvals viewfilter
+		| otherwise = concat
+			[ forcelegal (fromMetaField metafield)
+			, "="
+			, branchvals viewfilter
+			]
+	branchvals (FilterValues set) = forcelegal $
+		intercalate "," $ map fromMetaValue $ S.toList set
+	branchvals (FilterGlob glob) = forcelegal glob
+	forcelegal s
+		| Git.Ref.legal True s = s
+		| otherwise = map (\c -> if isAlphaNum c then c else '_') s
+
+prop_branchView_legal :: View -> Bool
+prop_branchView_legal = Git.Ref.legal False . show . branchView