projection of Line2D

src/Sets/Line2D.jl

@@ -368,3 +368,29 @@ function translate(L::Line2D, v::AbstractVector; share::Bool=false)
     b = L.b + dot(L.a, v)
     return Line2D(a, b)
 end
+
+function project(L::Line2D{N}, block::AbstractVector{Int}) where {N}
+    m = length(block)
+    if m == 2
+        @assert ispermutation(block, 1:2) "invalid dimensions $block for projection"
+        return L  # no projection
+    elseif m == 1
+        # projection to dimension i
+        cdims = constrained_dimensions(L)
+        if length(cdims) == 1
+            @inbounds if cdims[1] == block[1]
+                # L: aᵢxᵢ = b where aᵢ ≠ 0
+                return Singleton([L.b / L.a[cdims[1]]])
+            else
+                # L: aⱼxⱼ = b where i ≠ j
+                return Universe{N}(1)
+            end
+        else
+            # L is constrained in both dimensions
+            @assert length(cdims) == 2
+            return Universe{N}(1)
+        end
+    else
+        throw(ArgumentError("cannot project a two-dimensional line to $m dimensions"))
+    end
+end

test/unit_Line2D.jl

@@ -91,6 +91,15 @@ for N in [Float64, Rational{Int}, Float32]
     @test lm isa Line2D{N, Vector{N}}
     @test lm.a ≈ N[1/2, -2] && lm.b ≈ N(0)
 
+    # projection
+    L = Line2D(N[1, -1], N(0))  # x = y
+    @test project(L, [1]) == project(L, [2]) == Universe{N}(1)
+    @test project(L, [1, 2]) == L
+    L = Line2D(N[2, 0], N(4))  # x = 2
+    @test project(L, [1]) == Singleton(N[2])
+    @test project(L, [2]) == Universe{N}(1)
+    @test project(L, [1, 2]) == L
+
     # translation
     @test translate(l1, N[1, 2]) == Line2D(a1, N(3))
 end