Fix deflection of monomial (#174)

Author: blegat

src/gcd.jl

@@ -164,11 +164,15 @@ function deflation(p::AbstractPolynomialLike)
     return s, d
 end
 
+function _zero_to_one_exp(defl::AbstractMonomial)
+    # TODO Make it faster by calling something like `mapexponents`.
+    return prod(variables(defl).^map(d -> iszero(d) ? one(d) : d, exponents(defl)))
+end
 function deflate(p::AbstractPolynomialLike, shift, defl)
     if isconstant(shift) && all(d -> isone(d) || iszero(d), exponents(defl))
         return p
     end
-    q = MA.operate(deflate, p, shift, defl)
+    q = MA.operate(deflate, p, shift, _zero_to_one_exp(defl))
     return q
 end
 function inflate(α, shift, defl)
@@ -178,7 +182,7 @@ function inflate(p::AbstractPolynomialLike, shift, defl)
     if isconstant(shift) && all(d -> isone(d) || iszero(d), exponents(defl))
         return p
     end
-    q = MA.operate(inflate, p, shift, defl)
+    q = MA.operate(inflate, p, shift, _zero_to_one_exp(defl))
     return q
 end
 
@@ -193,7 +197,6 @@ end
 
 # Inspired from to `AbstractAlgebra.deflate`
 function MA.operate(op::Union{typeof(deflate), typeof(inflate)}, p::AbstractPolynomialLike, shift, defl)
-    defl = prod(variables(defl).^map(d -> iszero(d) ? one(d) : d, exponents(defl)))
     return polynomial(map(terms(p)) do t
         return term(coefficient(t), MA.operate(op, monomial(t), shift, defl))
     end)

test/division.jl

@@ -143,6 +143,12 @@ function multivariate_gcd_test(::Type{T}, algo=GeneralizedEuclideanAlgorithm())
     sym_test(f3, f1, x, algo)
     triple_test(f1, f2, f3, algo)
 
+    @testset "Issue #173" begin
+        p1 = o*x*y + x
+        p2 = x^2
+        sym_test(p1, p2, x, algo)
+    end
+
     p1 = o*z - z
     p2 = z
     @test gcd(p1, p2, algo) == z