unit-test the newly introduced 'proportionallyTo'

Getting this one wrong would be quite bad :s
cardano-foundation · Jul 6, 2020 · 547b130 · 547b130
1 parent 12b28f7
commit 547b130
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diff --git a/lib/core/src/Cardano/Wallet/Primitive/CoinSelection.hs b/lib/core/src/Cardano/Wallet/Primitive/CoinSelection.hs
@@ -20,6 +20,7 @@ module Cardano.Wallet.Primitive.CoinSelection
     , outputBalance
     , changeBalance
     , feeBalance
+    , proportionallyTo
     , ErrCoinSelection (..)
     , CoinSelectionOptions (..)
     ) where
@@ -30,6 +31,8 @@ import Cardano.Wallet.Primitive.Types
     ( Coin (..), TxIn, TxOut (..) )
 import Data.List
     ( foldl' )
+import Data.Ratio
+    ( Ratio, denominator, numerator )
 import Data.Word
     ( Word64, Word8 )
 import Fmt
@@ -123,6 +126,25 @@ addTxOut total = addCoin total . coin
 addCoin :: Integral a => a -> Coin -> a
 addCoin total c = total + (fromIntegral (getCoin c))
 
+-- | Compute the fraction of the first input to match the given ratio, rounded
+-- down.
+--
+-- /invariant:/ The ratio mustn't be greater than 1 without risk to overflow.
+--
+-- >>> 10 `proportionallyTo` 1%1
+-- 10
+--
+-- >>> 10 `proportionallyTo` 1%2
+-- 5
+--
+-- >>> 10 `proportionallyTo` 1%3
+-- 3
+proportionallyTo :: Integral a => a -> Ratio a -> a
+proportionallyTo n r
+    | r > 1 = error "proportionallyTo: ratio is greater than 1!"
+    | otherwise = fromIntegral $
+        toInteger n * toInteger (numerator r) `div` toInteger (denominator r)
+
 data ErrCoinSelection e
     = ErrNotEnoughMoney Word64 Word64
     -- ^ UTxO exhausted during input selection

diff --git a/lib/core/src/Cardano/Wallet/Primitive/CoinSelection/Random.hs b/lib/core/src/Cardano/Wallet/Primitive/CoinSelection/Random.hs
@@ -18,7 +18,11 @@ module Cardano.Wallet.Primitive.CoinSelection.Random
 import Prelude
 
 import Cardano.Wallet.Primitive.CoinSelection
-    ( CoinSelection (..), CoinSelectionOptions (..), ErrCoinSelection (..) )
+    ( CoinSelection (..)
+    , CoinSelectionOptions (..)
+    , ErrCoinSelection (..)
+    , proportionallyTo
+    )
 import Cardano.Wallet.Primitive.CoinSelection.LargestFirst
     ( largestFirst )
 import Cardano.Wallet.Primitive.Types
@@ -51,10 +55,10 @@ import Data.Ord
     ( comparing )
 import Data.Quantity
     ( Quantity (..) )
-import Data.Ratio
-    ( Ratio, denominator, numerator, (%) )
 import Data.Word
     ( Word64 )
+import Data.Ratio
+    ( (%) )
 
 import qualified Data.List as L
 import qualified Data.List.NonEmpty as NE
@@ -278,18 +282,3 @@ mkChange (TxOut _ (Coin out)) inps =
                 []
             c ->
                 [ Coin c ]
-
--- | Compute the fraction of the first input to match the given ratio, rounded
--- down.
---
--- >>> 10 `proportionallyTo` 1%1
--- 10
---
--- >>> 10 `proportionallyTo` 1%2
--- 5
---
--- >>> 10 `proportionallyTo` 1%3
--- 3
-proportionallyTo :: Integral a => a -> Ratio a -> a
-proportionallyTo n r = fromIntegral $
-    toInteger n * toInteger (numerator r) `div` toInteger (denominator r)
diff --git a/lib/core/test/unit/Cardano/Wallet/Primitive/CoinSelectionSpec.hs b/lib/core/test/unit/Cardano/Wallet/Primitive/CoinSelectionSpec.hs
@@ -28,7 +28,11 @@ import Prelude
 import Cardano.Wallet.Api.Server
     ( assignMigrationAddresses )
 import Cardano.Wallet.Primitive.CoinSelection
-    ( CoinSelection (..), CoinSelectionOptions (..), ErrCoinSelection (..) )
+    ( CoinSelection (..)
+    , CoinSelectionOptions (..)
+    , ErrCoinSelection (..)
+    , proportionallyTo
+    )
 import Cardano.Wallet.Primitive.Types
     ( Address (..)
     , Coin (..)
@@ -47,6 +51,8 @@ import Data.Maybe
     ( catMaybes )
 import Data.Quantity
     ( Quantity (..) )
+import Data.Ratio
+    ( Ratio, (%) )
 import Data.Vector.Shuffle
     ( shuffle )
 import Data.Word
@@ -65,13 +71,15 @@ import Test.QuickCheck
     , applyArbitrary2
     , checkCoverageWith
     , choose
+    , conjoin
     , counterexample
     , cover
     , elements
     , generate
     , scale
     , vector
     , (===)
+    , (==>)
     )
 import Test.QuickCheck.Monadic
     ( monadicIO )
@@ -101,6 +109,16 @@ spec = do
         prop "All inputs are used" prop_allInputsAreUsed
         prop "All inputs are used per transaction" prop_allInputsAreUsedPerTx
         prop "Addresses are recycled fairly" prop_fairAddressesRecycled
+
+    describe "proportionallyTo" $ do
+        prop "proportionallyTo behaves as expected" prop_proportionallyTo
+        it "10 `proportionallyTo` 1%1 == 10" $
+            (10 `proportionallyTo` (1%1)) `shouldBe` (10 :: Integer)
+        it "10 `proportionallyTo` 1%2 == 5" $
+            (10 `proportionallyTo` (1%2)) `shouldBe` (5 :: Integer)
+        it "10 `proportionallyTo` 1%3 == 3" $
+            (10 `proportionallyTo` (1%3)) `shouldBe` (3 :: Integer)
+
   where
     lowerConfidence :: Confidence
     lowerConfidence = Confidence (10^(6 :: Integer)) 0.75
@@ -109,6 +127,15 @@ spec = do
                                  Properties
 -------------------------------------------------------------------------------}
 
+prop_proportionallyTo
+    :: Word8
+    -> Ratio Word8
+    -> Property
+prop_proportionallyTo n r = conjoin
+    [ r <= 1 ==> n `proportionallyTo` r <= n
+    , n `proportionallyTo` 1 === n
+    ]
+
 prop_utxoToListOrderDeterministic
     :: UTxO
     -> Property