#883 - Intersection of two polygons does not detect tighter constraints correctly

src/concrete_intersection.jl

@@ -127,26 +127,46 @@ one.
 function intersection(P1::AbstractHPolygon{N},
                       P2::AbstractHPolygon{N}
                      )::Union{HPolygon{N}, EmptySet{N}} where N<:Real
-    @inline function choose_first_diff_dir(i, i1, i2, c1, c2)
+    # all constraints of one polygon are processed; now add the other polygon's
+    # constraints
+    @inline function add_remaining_constraints!(c, i, c1, i1, duplicates)
+        c[i+1:length(c)-duplicates] = c1[i1:length(c1)]
+        return true
+    end
+
+    # choose the constraint of c1; the directions are different
+    @inline function choose_first_diff_dir!(c, i, i1, i2, c1, c2, duplicates)
         c[i] = c1[i1]
         if i1 == length(c1)
-            c[i+1:length(c)] = c2[i2:length(c2)]
-            return true
+            return add_remaining_constraints!(c, i, c2, i2, duplicates)
         end
         return false
     end
-    @inline function choose_first_same_dir(i, i1, i2, c1, c2)
+
+    # choose the constraint of c1; the directions are equivalent (i.e., linearly
+    # dependent)
+    @inline function choose_first_same_dir!(c, i, i1, i2, c1, c2, duplicates)
         c[i] = c1[i1]
         if i1 == length(c1)
             if i2 < length(c2)
-                c[i+1:length(c)] = c2[i2+1:length(c2)]
+                return add_remaining_constraints!(c, i, c2, i2+1, duplicates)
             end
             return true
+        elseif i2 == length(c2)
+            return add_remaining_constraints!(c, i, c1, i1+1, duplicates)
         end
         return false
     end
-    @inline function is_first_constraint_tighter(lc1, lc2)
-        return lc1.a[1]/lc1.b <= lc2.a[1]/lc2.b
+
+    # check if the first of two constraint with equivalent direction is tighter
+    @inline function is_first_constraint_tighter(lc1::LinearConstraint{N},
+                                                 lc2::LinearConstraint{N}
+                                                ) where {N<:Real}
+        if lc1.a[1] == zero(N)
+            @assert lc2.a[1] == zero(N)
+            return lc1.b <= lc1.a[2]/lc2.a[2] * lc2.b
+        end
+        return lc1.b <= lc1.a[1]/lc2.a[1] * lc2.b
     end
 
     c1 = constraints_list(P1)
@@ -167,27 +187,27 @@ function intersection(P1::AbstractHPolygon{N},
                 # constraints have the same normal vector: take the tighter one
                 if is_first_constraint_tighter(c1[i1], c2[i2])
                     # first constraint is tighter
-                    if choose_first_same_dir(i, i1, i2, c1, c2)
+                    if choose_first_same_dir!(c, i, i1, i2, c1, c2, duplicates)
                         break
                     end
                 else
                     # second constraint is tighter
-                    if choose_first_same_dir(i, i2, i1, c2, c1)
+                    if choose_first_same_dir!(c, i, i2, i1, c2, c1, duplicates)
                         break
                     end
                 end
                 i1 += 1
                 i2 += 1
             else
                 # first constraint comes first
-                if choose_first_diff_dir(i, i1, i2, c1, c2)
+                if choose_first_diff_dir!(c, i, i1, i2, c1, c2, duplicates)
                     break
                 end
                 i1 += 1
             end
         else
             # second constraint comes first
-            if choose_first_diff_dir(i, i2, i1, c2, c1)
+            if choose_first_diff_dir!(c, i, i2, i1, c2, c1, duplicates)
                 break
             end
             i2 += 1

test/unit_Polygon.jl

@@ -122,6 +122,28 @@ for N in [Float64, Float32, Rational{Int}]
     p3 = intersection(p, p2)
     @test length(constraints_list(p3)) == 4
 
+    # check that tighter constraints are used in intersection (#883)
+    h1 = HalfSpace([N(1), N(0)], N(3))
+    h2 = HalfSpace([N(-1), N(0)], N(-3))
+    h3 = HalfSpace([N(0), N(1)], N(7))
+    h4 = HalfSpace([N(0), N(-1)], N(-5))
+    h5 = HalfSpace([N(0), N(1)], N(6))
+    h6 = HalfSpace([N(0), N(-1)], N(-4))
+    p1 = HPolygon([h1, h3, h2, h4])
+    p2 = HPolygon([h1, h5, h2, h6])
+    c = intersection(p1, p2).constraints
+    @test c == [h1, h5, h2, h4]
+    h1 = HalfSpace([N(1), N(1)], N(1))
+    h2 = HalfSpace([N(-1), N(1)], N(1))
+    h3 = HalfSpace([N(0), N(-1)], N(0))
+    p1 = HPolygon([h1, h2, h3])
+    h4 = HalfSpace([N(0), N(1)], N(1))
+    h5 = HalfSpace([N(-1), N(-1)], N(0))
+    h6 = HalfSpace([N(1), N(-1)], N(0))
+    p2 = HPolygon([h4, h5, h6])
+    c = intersection(p1, p2).constraints
+    @test c == [h1, h4, h2, h5, h3, h6]
+
     # is intersection empty
     p3 = convert(HPolygon, Hyperrectangle(low=N[-1, -1], high=N[1, 1]))
     I1 = Interval(N(0.9), N(1.1))