Johnson's rule throws exception if the problem cannot be reduced into…

… 2-machines problem

src/OperationsResearchModels.jl

@@ -50,7 +50,7 @@ import .CPM: CpmActivity, earliestfinishtime, longestactivity, CpmProblem, CpmRe
 import .CPM: PertActivity, PertProblem, PertResult
 import .Knapsack: KnapsackResult, KnapsackProblem
 import .Latex: latex
-import .Johnsons: JohnsonResult, johnsons
+import .Johnsons: JohnsonResult, johnsons, JohnsonException
 
 export TransportationProblem, TransportationResult, balance, isbalanced, northwestcorner
 export Connection, ShortestPathResult, MaximumFlowResult, nodes
@@ -65,7 +65,7 @@ export KnapsackResult, KnapsackProblem
 export Simplex
 export Utility
 export latex
-export JohnsonResult, johnsons
+export JohnsonResult, johnsons, JohnsonException
 
 export JuMP, HiGHS
 

src/johnsons.jl

@@ -1,14 +1,21 @@
 module Johnsons
 
-export JohnsonResult, johnsons
+export JohnsonResult, JohnsonException, johnsons
 
 # TODO: Add makespan calculation 
 
+struct JohnsonException <: Exception
+    message::String
+end
+
+
 struct JohnsonResult
     permutation::Vector{Int}
     # makespan::Float64
 end
 
+
+
 """
     johnsons(times::Matrix)
 
@@ -26,6 +33,8 @@ at least one of the following conditions must be satisfied:
 - min(A) >= max(B, C)
 - min(D) >= max(B, C)
 
+The function throws a JohnsonException if the problem cannot be reduced to a 2-machine problem.
+
 # Arguments
 
 - `times::Matrix`: a matrix of times
@@ -102,7 +111,9 @@ function johnsons_nmachines(times::Matrix)::JohnsonResult
     minlast = minimum(times[:, end])
     maxothers = maximum(times[:, 2:(end-1)])
 
-    @assert (minfirst >= maxothers) || (minlast >= maxothers)
+    if !((minfirst >= maxothers) || (minlast >= maxothers))
+        throw(JohnsonException("The problem cannot be reduced to a 2-machine problem: minfirst >= maxothers and/or minlast >= maxothers"))
+    end
 
     Gcollection = times[:, 1:(end-1)]
     Hcollection = times[:, 2:end]

test/testjohnsons.jl

@@ -71,4 +71,19 @@
     end
 
 
+    @testset "Cannot reduce to 2-machines" begin 
+
+            times = Float64[
+                3 3 5 2;
+                8 4 8 3;
+                7 2 10 4;
+                5 1 7 5;
+                2 5 6 6
+            ]
+
+            @test_throws JohnsonException johnsons(times)
+
+    end
+
+
 end