Pretty printing for expressions (#206)

* Pretty printing for linear expressions

* Pretty printing for constraint expressions

* Add release notes entry for pretty printing

Flips.Tests/Tests.fs

@@ -255,6 +255,103 @@ module Types =
             LinearExpression.Empty.GetHashCode() |> ignore
             (LinearExpression.OfFloat 1.0).GetHashCode() |> ignore
 
+        [<Fact>]
+        let ``Pretty print zero expression yields zero`` () =
+            let actual = LinearExpression.Zero.PrettyPrint()
+            Assert.Equal("0", actual)
+
+        [<Fact>]
+        let ``Pretty print positive constant expression yields constant without sign`` () =
+            let actual = 23.0 + LinearExpression.Zero |> _.PrettyPrint()
+            Assert.Equal("23", actual)
+
+        [<Fact>]
+        let ``Pretty print negative constant expression yields constant with minus`` () =
+            let actual = -23.0 + LinearExpression.Zero |> _.PrettyPrint()
+            Assert.Equal("-23", actual)
+
+        [<Fact>]
+        let ``Pretty print constant addition yields constants`` () =
+            let actual = 10.0 + 13.0 + LinearExpression.Zero |> _.PrettyPrint()
+            Assert.Equal("23", actual)
+
+        [<Fact>]
+        let ``Pretty print constant close to zero yields zero`` () =
+            let actual = 1e-8 + LinearExpression.Zero |> _.PrettyPrint()
+            Assert.Equal("0", actual)
+
+        [<Fact>]
+        let ``Pretty print linear combination with constant yields normalized expression`` () =
+            let x = Decision.createContinuous "x" 0.0 1.0
+            let y = Decision.createContinuous "y" 0.0 1.0
+            let z = Decision.createContinuous "z" 0.0 1.0
+            let actual = 8.0 * x + 7.0 * y - 2.0 * z + 25.0 |> _.PrettyPrint()
+            Assert.Equal("25 + 8 * x + 7 * y - 2 * z", actual)
+
+        [<Fact>]
+        let ``Pretty print linear combination without constants yields normalized expression`` () =
+            let x = Decision.createContinuous "x" 0.0 1.0
+            let y = Decision.createContinuous "y" 0.0 1.0
+            let z = Decision.createContinuous "z" 0.0 1.0
+            let actual = 8.0 * x + 7.0 * y - 2.0 * z |> _.PrettyPrint()
+            Assert.Equal("8 * x + 7 * y - 2 * z", actual)
+
+        [<Fact>]
+        let ``Pretty print linear combination with leading negative coefficient yields normalized expression`` () =
+            let x = Decision.createContinuous "x" 0.0 1.0
+            let y = Decision.createContinuous "y" 0.0 1.0
+            let z = Decision.createContinuous "z" 0.0 1.0
+            let actual = -8.0 * x + 7.0 * y - 2.0 * z |> _.PrettyPrint()
+            Assert.Equal("-8 * x + 7 * y - 2 * z", actual)
+
+        [<Fact>]
+        let ``Pretty print linear combination with leading minus one coefficient omits one`` () =
+            let x = Decision.createContinuous "x" 0.0 1.0
+            let y = Decision.createContinuous "y" 0.0 1.0
+            let z = Decision.createContinuous "z" 0.0 1.0
+            let actual = -1.0 * x + 7.0 * y - 2.0 * z |> _.PrettyPrint()
+            Assert.Equal("-x + 7 * y - 2 * z", actual)
+
+        [<Fact>]
+        let ``Pretty print linear combination with middle coefficient one omits one`` () =
+            let x = Decision.createContinuous "x" 0.0 1.0
+            let y = Decision.createContinuous "y" 0.0 1.0
+            let z = Decision.createContinuous "z" 0.0 1.0
+            let actual = -8.0 * x + 1.0 * y - 2.0 * z |> _.PrettyPrint()
+            Assert.Equal("-8 * x + y - 2 * z", actual)
+
+        [<Fact>]
+        let ``Pretty print linear combination with coefficient close to one omits one`` () =
+            let x = Decision.createContinuous "x" 0.0 1.0
+            let y = Decision.createContinuous "y" 0.0 1.0
+            let z = Decision.createContinuous "z" 0.0 1.0
+            let actual = -8.0 * x + 1.0000001 * y - 2.0 * z |> _.PrettyPrint()
+            Assert.Equal("-8 * x + y - 2 * z", actual)
+
+    module ConstraintExpression =
+        [<Fact>]
+        let ``Pretty print equality uses equal sign`` () =
+            let x = Decision.createContinuous "x" 0.0 1.0
+            let y = Decision.createContinuous "y" 0.0 1.0
+            let z = Decision.createContinuous "z" 0.0 1.0
+            let actual = -2.0 + y == x + 2.0 * z + 10.0 |> _.PrettyPrint()
+            Assert.Equal("-2 + y = 10 + x + 2 * z", actual)
+
+        [<Fact>]
+        let ``Pretty print greater-or-equality uses greater-or-equal sign`` () =
+            let x = Decision.createContinuous "x" 0.0 1.0
+            let y = Decision.createContinuous "y" 0.0 1.0
+            let z = Decision.createContinuous "z" 0.0 1.0
+            let actual = -2.0 + y >== x + 2.0 * z + 10.0 |> _.PrettyPrint()
+            Assert.Equal("-2 + y >= 10 + x + 2 * z", actual)
+
+        [<Fact>]
+        let ``Pretty print less-or-equality uses less-or-equal sign`` () =
+            let x = Decision.createContinuous "x" 0.0 1.0
+            let y = Decision.createContinuous "y" 0.0 1.0
+            let z = Decision.createContinuous "z" 0.0 1.0
+            let actual = -2.0 + y <== x + 2.0 * z + 10.0 |> _.PrettyPrint()
+            Assert.Equal("-2 + y <= 10 + x + 2 * z", actual)
 
     [<Properties(Arbitrary = [| typeof<Types> |] )>]
     module ModelTests =

Flips/Types.fs

@@ -1,4 +1,4 @@
-namespace Flips.Types
+namespace Flips.Types
 
 open System.Collections.Generic
 open System
@@ -349,12 +349,54 @@ and
     static member (>==) (lhs:LinearExpression, rhs:LinearExpression): ConstraintExpression =
         Inequality (lhs, GreaterOrEqual, rhs)
 
+    member this.PrettyPrint() : string =
+        let reduced = LinearExpression.Reduce this
+
+        let ppCoefficient (x: double) =
+            let c =
+                if Math.Abs x - 1.0 < 10e-4 then ""
+                else if x < 0 then (-x).ToString() + " * "
+                else x.ToString() + " * "
+
+            if x < 0 then " - " + c else " + " + c
+
+        let ppSummands =
+            [ for (DecisionName decision, coefficient) in (reduced.Coefficients |> Map.ofDictionary |> Map.toList) ->
+                  ppCoefficient coefficient + decision ]
+
+        let ppConstant =
+            if Math.Abs reduced.Offset < 10e-4 then
+                "0"
+            else
+                reduced.Offset.ToString()
+
+        let includedConstant =
+            if ppSummands.Length = 0 then ppConstant
+            elif ppConstant <> "0" then ppConstant
+            else ""
+
+        String.concat "" (includedConstant :: ppSummands)
+        |> fun output ->
+            if output.StartsWith(" + ") then output[3..]
+            else if output.StartsWith(" - ") then "-" + output[3..]
+            else output
+
 and 
     /// The representation of how two LinearExpressions must relate to one another
     [<NoComparison>]
     ConstraintExpression = 
     | Inequality of LHS:LinearExpression * Inequality * RHS:LinearExpression
     | Equality of LHS:LinearExpression * RHS:LinearExpression
+    member this.PrettyPrint() : string =
+        match this with
+        | Inequality(lhs, relation, rhs) ->
+            let ppRelation =
+                match relation with
+                | GreaterOrEqual -> ">="
+                | LessOrEqual -> "<="
+
+            lhs.PrettyPrint() + " " + ppRelation + " " + rhs.PrettyPrint()
+        | Equality(lhs, rhs) -> lhs.PrettyPrint() + " = " + rhs.PrettyPrint()
 
 /// A unique identified for a Constraint
 type ConstraintName = ConstraintName of string

RELEASE_NOTES.md

@@ -3,6 +3,7 @@
 * Remove x64 restriction
 * Change to FAKE build project
 * Update to latest Google.OrTools package
+* Add methods to pretty print linear and constraint expressions
 
 ### 2.4.9 - Wednesday, October 19th, 2022
 * Fix problem with Units of Measure when multiplying float and LinearExpression