split cost out into its own module

Added a function to insert a new cost into a list, which could be used to
asjust costs after a drag and drop.

Config/Cost.hs

@@ -0,0 +1,79 @@
+{- Remote costs.
+ -
+ - Copyright 2011-2013 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Config.Cost where
+
+{- We use a float for a cost to ensure that there is a cost in
+ - between any two other costs. -}
+type Cost = Float
+
+{- Some predefined default costs.
+ - Users setting costs in config files can be aware of these,
+ - and pick values relative to them. So don't change. -}
+cheapRemoteCost :: Cost
+cheapRemoteCost = 100
+nearlyCheapRemoteCost :: Cost
+nearlyCheapRemoteCost = 110
+semiExpensiveRemoteCost :: Cost
+semiExpensiveRemoteCost = 175
+expensiveRemoteCost :: Cost
+expensiveRemoteCost = 200
+veryExpensiveRemoteCost :: Cost
+veryExpensiveRemoteCost = 1000
+
+{- Adjusts a remote's cost to reflect it being encrypted. -}
+encryptedRemoteCostAdj :: Cost
+encryptedRemoteCostAdj = 50
+
+{- Given an ordered list of costs, and the position of one of the items
+ - the list, inserts a new cost into the list, in between the item
+ - and the item after it.
+ -
+ - If both items have the same cost, one of them will have its cost
+ - adjusted to make room. The costs of other items in the list are left
+ - unchanged.
+ -
+ - To insert the new cost before any other in the list, specify a negative
+ - position. To insert the new cost at the end of the list, specify a
+ - position longer than the list.
+ -}
+insertCostAfter :: [Cost] -> Int -> [Cost]
+insertCostAfter [] _ = error "insertCostAfter: empty list"
+insertCostAfter l pos
+	| pos < 0 = costBetween 0 (l !! 0) : l
+	| nextpos > maxpos = l ++ [1 + l !! maxpos]
+	| item == nextitem =
+		let (_dup:new:l') = insertCostAfter lastsegment 0
+		in firstsegment ++ [costBetween item new, new] ++ l'
+	| otherwise =
+		firstsegment ++ [costBetween item nextitem ] ++ lastsegment
+  where
+  	nextpos = pos + 1
+	maxpos = length l - 1
+	
+	item = l !! pos
+	nextitem = l !! nextpos
+		
+	(firstsegment, lastsegment) = splitAt (pos + 1) l
+
+costBetween :: Cost -> Cost -> Cost
+costBetween x y
+	| x == y = x
+	| x > y = y + (x - y) / 2
+	| otherwise = costBetween y x
+
+{- Make sure the remote cost numbers work out. -}
+prop_cost_sane :: Bool
+prop_cost_sane = False `notElem`
+	[ expensiveRemoteCost > 0
+	, cheapRemoteCost < nearlyCheapRemoteCost
+	, nearlyCheapRemoteCost < semiExpensiveRemoteCost
+	, semiExpensiveRemoteCost < expensiveRemoteCost
+	, cheapRemoteCost + encryptedRemoteCostAdj > nearlyCheapRemoteCost
+	, nearlyCheapRemoteCost + encryptedRemoteCostAdj < semiExpensiveRemoteCost
+	, nearlyCheapRemoteCost + encryptedRemoteCostAdj < expensiveRemoteCost
+	]