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+{- git SHA stuff
+ -
+ - Copyright 2011 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Git.Sha where
+
+import Common
+import Git.Types
+
+{- Runs an action that causes a git subcommand to emit a Sha, and strips
+ - any trailing newline, returning the sha. -}
+getSha :: String -> IO String -> IO Sha
+getSha subcommand a = maybe bad return =<< extractSha <$> a
+ where
+ bad = error $ "failed to read sha from git " ++ subcommand
+
+{- Extracts the Sha from a string. There can be a trailing newline after
+ - it, but nothing else. -}
+extractSha :: String -> Maybe Sha
+extractSha s
+ | len == shaSize = val s
+ | len == shaSize + 1 && length s' == shaSize = val s'
+ | otherwise = Nothing
+ where
+ len = length s
+ s' = firstLine s
+ val v
+ | all (`elem` "1234567890ABCDEFabcdef") v = Just $ Ref v
+ | otherwise = Nothing
+
+{- Size of a git sha. -}
+shaSize :: Int
+shaSize = 40
+
+nullSha :: Ref
+nullSha = Ref $ replicate shaSize '0'