Handle constant objectives and constraints containing no variables (#13)

* handle constant objectives and constraints containing no variables

* format

CHANGELOG.md

@@ -6,11 +6,13 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [0.2.0]
+
   - Improve test coverage, error messages, and others [#1](https://github.com/RelationalAI/SolverAPI.jl/pull/1), [#7](https://github.com/RelationalAI/SolverAPI.jl/pull/7),
     [#8](https://github.com/RelationalAI/SolverAPI.jl/pull/8), [#10](https://github.com/RelationalAI/SolverAPI.jl/pull/10)
   - Fix `StackOverflowError` and improve performance [#6](https://github.com/RelationalAI/SolverAPI.jl/pull/6)
   - Return solve time and solver version [#5](https://github.com/RelationalAI/SolverAPI.jl/pull/5)
-  - Add default time limit [#2](https://github.com/RelationalAI/SolverAPI.jl/pull/2) 
+  - Add default time limit [#2](https://github.com/RelationalAI/SolverAPI.jl/pull/2)
 
 ## [0.1.0]
+
   - Initial release

src/json_to_moi.jl

@@ -4,7 +4,7 @@ function add_obj!(
     ::Type{T},
     model::MOI.ModelLike,
     sense::String,
-    a::Union{String,JSON3.Array},
+    a::Any,
     vars_map::Dict,
     ::Dict,
 ) where {T<:Real}
@@ -16,6 +16,9 @@ function add_obj!(
     MOI.set(model, MOI.ObjectiveSense(), moi_sense)
 
     g = canonicalize_SNF(T, json_to_snf(a, vars_map))
+    if !(g isa MOI.AbstractScalarFunction)
+        g = convert(MOI.ScalarAffineFunction{T}, g)
+    end
     g_type = MOI.ObjectiveFunction{typeof(g)}()
     if !MOI.supports(model, g_type)
         msg = "Objective function $(trunc_str(g)) isn't supported by this solver."
@@ -139,7 +142,11 @@ function shift_terms(::Type{T}, args::Vector) where {T<:Real}
     @assert length(args) == 2 # This should never happen.
     g1 = canonicalize_SNF(T, args[1])
     g2 = canonicalize_SNF(T, args[2])
-    return MOI.Utilities.operate(-, T, g1, g2)
+    g = MOI.Utilities.operate(-, T, g1, g2)
+    if !(g isa MOI.AbstractScalarFunction)
+        g = convert(MOI.ScalarAffineFunction{T}, g)
+    end
+    return g
 end
 
 # Convert object to string and truncate string length if too long.

test/all_tests.jl

@@ -69,6 +69,9 @@ end
         "tiny_infeas",
         "simple_lp",
         "n_queens",
+        "min_constant",
+        "cons_true",
+        "cons_false",
     ]
 
     # solve and check output is expected for each input json file

test/inputs/cons_false.json

@@ -0,0 +1 @@
+{"version":"0.1","sense":"feas","variables":["x"],"constraints":[["==",1,0],["Nonneg","x"]],"objectives":[],"options":{"solver":"highs"}}

test/inputs/cons_true.json

@@ -0,0 +1 @@
+{"version":"0.1","sense":"feas","variables":["x_1","x_2"],"constraints":[["==",0,0],["Nonneg","x_1"],["Nonneg","x_2"]],"objectives":[],"options":{"solver":"highs"}}

test/inputs/min_constant.json

@@ -0,0 +1 @@
+{"version":"0.1","sense":"min","variables":["x"],"constraints":[["==","x",1],["Int","x"]],"objectives":[1.5],"options":{"solver":"HiGHS"}}

test/outputs/cons_false.json

@@ -0,0 +1 @@
+{"termination_status":"INFEASIBLE","version":"0.1"}

test/outputs/cons_true.json

@@ -0,0 +1 @@
+{"termination_status":"OPTIMAL","results":[{"names":["\"x_1\"","\"x_2\""],"values":[0.0,0.0],"primal_status":"FEASIBLE_POINT"}],"version":"0.1"}

test/outputs/min_constant.json

@@ -0,0 +1 @@
+{"termination_status":"OPTIMAL","results":[{"names":["\"x\""],"values":[1.0],"primal_status":"FEASIBLE_POINT","objective_value":1.5}],"version":"0.1"}