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+{-|
+
+Budget planning in the Kingdom of Malodivo.
+
+-}
+module Malodivo.Budget
+  ( DistrictFunds
+  , manyToOne
+  ) where
+
+import qualified Data.HashMap.Strict as HM
+
+import qualified Malodivo.Types.Bill as B
+import qualified Malodivo.Types.District as D
+
+-- | Convenient type.
+type DistrictFunds = HM.HashMap D.District Integer
+
+{-|
+
+Trivial case: many districts, one bill, no contraints (wishes,
+limits).  We assume that, with no explicit wishes, each district
+wants to contribute all its funds.
+
+>>> :set -XOverloadedStrings
+>>> import qualified Data.HashMap.Strict as HM
+>>> import qualified Malodivo.Types.Bill as B
+>>> import qualified Malodivo.Types.District as D
+>>> import qualified Malodivo.Types.Ministry as M
+
+>>> let medium = B.Bill { B.amount = 30, B.name = "A medium bill", B.ministry = M.Science }
+>>> let one = B.Bill { B.amount = 1, B.name = "A trivial bill", B.ministry = M.Welfare }
+
+
+If any district can pay the bill, take funds in proportion.  We use
+'HM.lookup', because 'show' of 'HM.HashMap' is not determinate,
+and the test can occasionally fail:
+
+>>> let funds = HM.fromList [(D.Palolene, 100), (D.Lakos, 200)]
+>>> let contribution = manyToOne funds medium
+>>> HM.lookup D.Palolene contribution
+Just 10
+>>> HM.lookup D.Lakos contribution
+Just 20
+
+>>> let funds = HM.fromList [(D.Palolene, 30), (D.Lakos, 30)]
+>>> HM.elems $ manyToOne funds medium
+[15,15]
+
+
+It works with a single district:
+
+>>> let funds = HM.fromList [(D.SouthernPalolene, 500)]
+>>> HM.elems $ manyToOne funds medium
+[30]
+>>> HM.elems $ manyToOne funds one
+[1]
+
+
+__TODO__ It /should/ not have rounding issues. In particular,
+when the bill's amount is bigger than the number of districts,
+/each/ district would contribute some. This problem is known as
+<https://en.wikipedia.org/wiki/Partition_(number_theory) integer partition>.
+
+>>> let funds = HM.fromList [(D.Palolene, 10000), (D.Lakos, 1)]
+>>> B.amount medium > fromIntegral (HM.size funds)
+True
+>>> let contribution = manyToOne funds medium
+>>> let taken = HM.foldl' (+) 0 contribution
+
+
+But at the moment we make use of some freedom coming from the fact
+that \"it is possible that a bill will receive less funds than the
+Parliament decides\", and round down contribution of each district.
+/Thus these two tests show 'False', while should show 'True':/
+
+>>> HM.null $ HM.filter (== 0) contribution
+False
+>>> taken == B.amount medium
+False
+
+
+If all districts together can't pay the bill, take all their money.
+Note that due to the principle of proportionality it is impossible
+that some districts can pay their shares and others can't:
+
+>>> let low = HM.fromList [(D.Palolene, 10), (D.Lakos, 15)]
+>>> manyToOne low medium == low
+True
+
+-}
+manyToOne ::
+     DistrictFunds -- ^ Amounts of available funds per district.
+  -> B.Bill -- ^ A bill requiring funding.
+  -> DistrictFunds -- ^ Contribution of each district.
+manyToOne funds bill = HM.map takeMoney funds
+  where
+    needed = B.amount bill
+    available = sum $ HM.elems funds
+    requested = min needed available
+    takeMoney m = requested * m `div` available