initial work toward correctly merging deeper import histories

Pure code is tested working, including with even histories that merge
several lines of development. Needs to be hooked up to git histories
next.

Annex/Import.hs

@@ -179,6 +179,51 @@ buildImportCommit remote importtreeconfig importcommitconfig importable =
 data History t = History t (S.Set (History t))
 	deriving (Show, Eq, Ord)
 
+historyDepth :: History t -> Integer
+historyDepth (History _ s)
+	| S.null s = 1
+	| otherwise = 1 + maximum (map historyDepth (S.toList s))
+
+truncateHistoryToDepth :: Ord t => Integer -> History t -> History t
+truncateHistoryToDepth n (History t s) = History t (go 1 s)
+  where
+	go depth s
+		| depth >= n = S.empty
+		| otherwise = 
+			let depth' = succ depth
+			in flip S.map s $ \(History t' s') ->
+				History t' (go depth' s')
+
+{- Finds the part of the History of git trees that is new and should be
+ - committed on top of the basecommit, skipping parts that have
+ - already been committed.
+ -
+ - Will be Nothing if the basecommit already matches the top of the History.
+ -
+ - In the simple case where there is only one level of History,
+ - if the basecommit matches it, it's nothing and otherwise it should be
+ - committed on top of the basecommit.
+ -
+ - In the complex case where there are several levels of History, finds
+ - the point where it first starts matching up with the trees from the
+ - basecommit.
+ -}
+skipOldHistory :: Maybe Sha -> History Sha -> Annex (Maybe (History Sha))
+skipOldHistory basecommit importedhistory = undefined
+
+{- The knownhistory does not need to be complete; it can be 
+ - truncated to the same depth as the importedhistory to avoid reading
+ - in a lot of past history.
+ -}
+skipOldHistory' :: Ord t => History t -> History t -> Maybe (History t)
+skipOldHistory' knownhistory importedhistory@(History top rest)
+	| sametodepth importedhistory knownhistory = Nothing
+	| otherwise = Just $ 
+		History top $ S.fromList $ catMaybes $
+			map (skipOldHistory' knownhistory) (S.toList rest)
+  where
+	sametodepth a b = a == truncateHistoryToDepth (historyDepth a) b
+
 {- Builds a history of git trees reflecting the ImportableContents.
  -
  - When a subdir is provided, imported tree is grafted into the basetree at

doc/todo/import_tree.mdwn

@@ -18,6 +18,8 @@ and `git annex sync --content` can be configured to use it.
   when importing from a versioned S3 remote,
   although the head of it does reflect the current state of the remote.
 
+  Note: Merge commits may complicate this.
+
 * S3 buckets can be set up to allow reads and listing by an anonymous user.
   That should allow importing from such a bucket, but the S3 remote
   will need changes, since it currently avoids using the S3 API when