add ls-tree interface

This parser should be fast. I hope.

Git/LsTree.hs

@@ -0,0 +1,48 @@
+{- git ls-tree interface
+ -
+ - Copyright 2011 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Git.LsTree (
+	lsTree
+) where
+
+import Numeric
+import Control.Applicative
+import Data.Char
+
+import Git
+import Utility.SafeCommand
+
+type Treeish = String
+
+data TreeItem = TreeItem
+	{ mode :: Int
+	, objtype :: String
+	, sha :: String
+	, file :: FilePath
+	} deriving Show
+
+{- Lists the contents of a Treeish -}
+lsTree :: Repo -> Treeish -> IO [TreeItem]
+lsTree repo t = map parseLsTree <$>
+	pipeNullSplit repo [Params "ls-tree --full-tree -z -r --", File t]
+
+{- Parses a line of ls-tree output.
+ - (The --long format is not currently supported.) -}
+parseLsTree :: String -> TreeItem
+parseLsTree l = TreeItem m o s f
+	where
+		-- l = <mode> SP <type> SP <sha> TAB <file>
+		-- Since everything until the file is fixed-width,
+		-- do not need to split on words.
+		(m, past_m) = head $ readOct l
+		(o, past_o) = splitAt 4 $ space past_m
+		(s, past_s) = splitAt shaSize $ space past_o
+		f = decodeGitFile $ space past_s
+		space s@(sp:rest)
+			| isSpace sp = rest
+			| otherwise = error $
+				"ls-tree parse error at '" ++ s ++ "' in " ++ l