fix(prettify): render fractions and separators correctly

ChangeLog.md

@@ -2,6 +2,7 @@
 
 ## Unreleased changes
 
+- Correct fractional coefficients in pretty-printed expressions and remove trailing plus separators. ([#19](https://github.com/rasheedja/simplex-method/pull/19))
 - Share shift and split coefficient-map transformations between objectives and constraints, and simplify access to their fields. ([#23](https://github.com/rasheedja/simplex-method/pull/23))
   - Avoid ambiguous constraint record updates using the existing field lenses, group the substitution helpers together, and test them directly.
 - Use standard Data.Map selection and deletion operations for solver variable filtering. ([#20](https://github.com/rasheedja/simplex-method/pull/20))

src/Linear/Simplex/Prettify.hs

@@ -13,23 +13,24 @@
 module Linear.Simplex.Prettify where
 
 import Data.Generics.Labels ()
+import Data.List (intercalate)
 import Data.Map qualified as M
 import Data.Ratio (denominator, numerator)
 import Linear.Simplex.Types (ObjectiveFunction (..), PolyConstraint (..), VarLitMapSum)
 
 -- | Convert a 'VarLitMapSum' into a human-readable 'String'
 prettyShowVarLitMapSum :: VarLitMapSum -> String
-prettyShowVarLitMapSum = aux . M.toList
+prettyShowVarLitMapSum = intercalate " + " . map prettyShowTerm . M.toList
   where
-    aux [] = ""
-    aux ((vName, vCoeff) : vs) = prettyShowRational vCoeff ++ " * " ++ show vName ++ " + " ++ aux vs
+    prettyShowTerm (vName, vCoeff) = prettyShowRational vCoeff ++ " * " ++ show vName
+
+    prettyShowRational r =
+      if r < 0 then "(" ++ rendered ++ ")" else rendered
       where
-        prettyShowRational r =
-          if r < 0
-            then "(" ++ r' ++ ")"
-            else r'
-          where
-            r' = if denominator r == 1 then show (numerator r) else show (numerator r) ++ " / " ++ show (numerator r)
+        rendered =
+          if denominator r == 1
+            then show (numerator r)
+            else show (numerator r) ++ " / " ++ show (denominator r)
 
 -- | Convert a 'PolyConstraint' into a human-readable 'String'
 prettyShowPolyConstraint :: PolyConstraint -> String

test/Linear/Simplex/Solver/TwoPhaseSpec.hs

@@ -2677,7 +2677,7 @@
             domainMap = VarDomainMap $ M.fromList [(1, lowerBoundOnly (-5))]
         let ([newObj], newConstraints, transforms) = preprocess [obj] domainMap constraints
         length transforms `shouldBe` 1
         case head transforms of
           Shift {..} -> do
             originalVar `shouldBe` 1
             shiftBy `shouldBe` (-5)
@@ -2689,7 +2689,7 @@
             domainMap = VarDomainMap $ M.fromList [(1, unbounded)]
         let (_, _, transforms) = preprocess [obj] domainMap constraints
         length transforms `shouldBe` 1
         case head transforms of
           Split {..} -> originalVar `shouldBe` 1
           _ -> expectationFailure "Expected Split transform"
 
@@ -3120,31 +3120,35 @@
       prettyShowVarLitMapSum M.empty `shouldBe` ""
 
     it "shows single positive coefficient" $ do
-      prettyShowVarLitMapSum (M.fromList [(1, 3)]) `shouldBe` "3 * 1 + "
+      prettyShowVarLitMapSum (M.fromList [(1, 3)]) `shouldBe` "3 * 1"
 
     it "shows single negative coefficient with parentheses" $ do
-      prettyShowVarLitMapSum (M.fromList [(1, -2)]) `shouldBe` "(-2) * 1 + "
+      prettyShowVarLitMapSum (M.fromList [(1, -2)]) `shouldBe` "(-2) * 1"
+
+    it "shows positive and negative fractional coefficients exactly" $ do
+      prettyShowVarLitMapSum (M.fromList [(1, 1 % 2), (2, (-3) % 4)])
+        `shouldBe` "1 / 2 * 1 + (-3 / 4) * 2"
 
     it "shows multiple coefficients" $ do
       let result = prettyShowVarLitMapSum (M.fromList [(1, 2), (2, 3)])
-      result `shouldBe` "2 * 1 + 3 * 2 + "
+      result `shouldBe` "2 * 1 + 3 * 2"
 
   describe "prettyShowPolyConstraint" $ do
     it "shows LEQ constraint" $ do
-      prettyShowPolyConstraint (LEQ (M.fromList [(1, 2)]) 10) `shouldBe` "2 * 1 + " ++ " <= " ++ show (10 :: Rational)
+      prettyShowPolyConstraint (LEQ (M.fromList [(1, 2)]) 10) `shouldBe` "2 * 1" ++ " <= " ++ show (10 :: Rational)
 
     it "shows GEQ constraint" $ do
-      prettyShowPolyConstraint (GEQ (M.fromList [(1, 1)]) 5) `shouldBe` "1 * 1 + " ++ " >= " ++ show (5 :: Rational)
+      prettyShowPolyConstraint (GEQ (M.fromList [(1, 1)]) 5) `shouldBe` "1 * 1" ++ " >= " ++ show (5 :: Rational)
 
     it "shows EQ constraint" $ do
-      prettyShowPolyConstraint (EQ (M.fromList [(1, 1)]) 3) `shouldBe` "1 * 1 + " ++ " == " ++ show (3 :: Rational)
+      prettyShowPolyConstraint (EQ (M.fromList [(1, 1)]) 3) `shouldBe` "1 * 1" ++ " == " ++ show (3 :: Rational)
 
   describe "prettyShowObjectiveFunction" $ do
     it "shows Max objective" $ do
-      prettyShowObjectiveFunction (Max (M.fromList [(1, 2)])) `shouldBe` "max: 2 * 1 + "
+      prettyShowObjectiveFunction (Max (M.fromList [(1, 2)])) `shouldBe` "max: 2 * 1"
 
     it "shows Min objective" $ do
-      prettyShowObjectiveFunction (Min (M.fromList [(1, 5)])) `shouldBe` "min: 5 * 1 + "
+      prettyShowObjectiveFunction (Min (M.fromList [(1, 5)])) `shouldBe` "min: 5 * 1"
 
   describe "twoPhaseSimplex with multiple objectives" $ do
     it "optimizes two objectives over the same feasible region" $ do