add special case for N_GField(p, 1) (oscar-system#391)

src/number/NumberTypes.jl

@@ -241,12 +241,29 @@ mutable struct N_GField <: Field
       if cached && haskey(N_GFieldID, (p, n, S))
          d = N_GFieldID[p, n, S]::N_GField
       else
-         gfinfo = GFInfo(Cint(p), Cint(n), pointer(Base.Vector{UInt8}(string(S)*"\0")))
-         GC.@preserve gfinfo begin
-         ptr = libSingular.nInitChar(libSingular.n_GF, pointer_from_objref(gfinfo))
-         d = new(ptr, n, libSingular.n_SetMap(ZZ.ptr, ptr),
-              libSingular.n_SetMap(ptr, ZZ.ptr), 1, S)
+         # NOTE: degree 1 special case
+         # A N_GField with n = 1 - when passed through Singular and to
+         # create_ring_from_singular_ring - will be recognized as a N_ZpField.
+         # This could cause some identity issue since we have the non identity
+         # N_GField(p, 1, ) != N_ZpField(p). There are two solutions:
+         #  (1) accept this and discourage use of N_GField(p, 1, )
+         #  (2) Get singular to return distinct pointers for these two fields
+         # In any case, Singular as of Singular_jll v402.0.104+0 is still buggy
+         # in its treatment of N_GField(p, 1, ). Namely, if the n == 1
+         # special treatment is removed here, the last test in caller.LibNormal
+         # fails because Singular returns the wrong coefficient field. This
+         # has nothing to do per se with the identity issue, it is a separate
+         # Singular bug.
+         if n == 1
+            ptr = libSingular.nInitChar(libSingular.n_Zp, Ptr{Nothing}(p))
+         else
+            gfinfo = GFInfo(Cint(p), Cint(n), pointer(Base.Vector{UInt8}(string(S)*"\0")))
+            GC.@preserve gfinfo begin
+               ptr = libSingular.nInitChar(libSingular.n_GF, pointer_from_objref(gfinfo))
+            end
          end
+         d = new(ptr, n, libSingular.n_SetMap(ZZ.ptr, ptr),
+                         libSingular.n_SetMap(ptr, ZZ.ptr), 1, S)
          finalizer(_Ring_finalizer, d)
          if cached
             N_GFieldID[p, n, S] = d

src/number/n_GF.jl

@@ -50,7 +50,14 @@ one(R::N_GField) = R(1)
 
 zero(R::N_GField) = R(0)
 
-gen(R::N_GField) = R(libSingular.n_Param(Cint(1), R.ptr))
+function gen(R::N_GField)
+   if degree(R) == 1
+      # NOTE: degree 1 special case
+      return zero(R)
+   else
+      return R(libSingular.n_Param(Cint(1), R.ptr))
+   end
+end
 
 function isone(n::n_GF)
    c = parent(n)
@@ -62,10 +69,6 @@ function iszero(n::n_GF)
    return libSingular.n_IsZero(n.ptr, c.ptr)
 end
 
-@doc Markdown.doc"""
-   isunit(n::n_GF)
-Return `true` if $n$ is a unit in the field, i.e. nonzero.
-"""
 isunit(n::n_GF) = !iszero(n)
 
 ###############################################################################
@@ -93,7 +96,11 @@ end
 function AbstractAlgebra.expressify(a::n_GF; context = nothing)::Any
   F = parent(a)
   i = reinterpret(Int, a.ptr.cpp_object)
-  if 1 < i < characteristic(F)^degree(F)
+  p = Int(characteristic(parent(a)))
+  if degree(F) == 1
+    # NOTE: degree 1 special case
+    return i > p - i ? i - p : i
+  elseif 1 < i < p^degree(F)
     return Expr(:call, :^, F.S, i)
   elseif i == 1
     return F.S

test/number/n_GF-test.jl

@@ -12,6 +12,19 @@
    @test F != F2
    @test F1 != F2
 
+   F, x = FiniteField(7, 1, "x")
+   F1, x = FiniteField(7, 1, "x")
+   F2, x = FiniteField(7, 1, "x", cached = false)
+
+   @test isa(x, n_GF)
+
+   @test F isa Singular.Field
+   @test F1 isa Singular.Field
+   @test F2 isa Singular.Field
+   @test F == F1
+   @test F != F2
+   @test F1 != F2
+
    @test_throws DomainError FiniteField(257, 1, "a")
    @test_throws DomainError FiniteField(2, 16, "a")
    @test_throws DomainError FiniteField(2, 64, "a") # errors even if 2^64 == 0
@@ -31,6 +44,21 @@ end
    @test sprint(show, "text/plain", x) == "x"
    @test sprint(show, "text/plain", x^2) == "x^2"
    @test sprint(show, "text/plain", x^11) == "x^11"
+   R, x = FiniteField(5, 1, "x")
+
+   @test string(zero(R)) == "0"
+   @test string(one(R)) == "1"
+   @test string(R(2)) == "2"
+   @test occursin(r"\A\d+\Z", string(x))
+   @test occursin(r"\A\d+\Z", string(x^2))
+   @test occursin(r"\A\d+\Z", string(x^11))
+
+   @test sprint(show, "text/plain", zero(R)) == "0"
+   @test sprint(show, "text/plain", one(R)) == "1"
+   @test sprint(show, "text/plain", R(2)) == "2"
+   @test occursin(r"\A\d+\Z", sprint(show, "text/plain", x))
+   @test occursin(r"\A\d+\Z", sprint(show, "text/plain", x^2))
+   @test occursin(r"\A\d+\Z", sprint(show, "text/plain", x^11))
 end
 
 @testset "n_GF.manipulation" begin
@@ -47,6 +75,20 @@ end
    @test degree(R) == 2
 
    @test deepcopy(R(2)) == R(2)
+
+   R, x = FiniteField(5, 1, "x")
+
+   @test isone(one(R))
+   @test iszero(zero(R))
+   @test isunit(R(1)) && isunit(R(2))
+   @test !isunit(R(0))
+
+   @test gen(R) == x
+
+   @test characteristic(R) == 5
+   @test degree(R) == 1
+
+   @test deepcopy(R(2)) == R(2)
 end
 
 @testset "n_GF.unary_ops" begin
@@ -57,6 +99,14 @@ end
    @test -R(3) == R(2)
    @test -R() == R()
    @test -a == 2x - 1
+
+   R, x = FiniteField(5, 1, "x")
+
+   a = 3x + 1
+
+   @test -R(3) == R(2)
+   @test -R() == R()
+   @test -a == 2x - 1
 end
 
 @testset "n_GF.binary_ops" begin
@@ -68,6 +118,15 @@ end
    @test a + b == R(3)
    @test a - b == -x - 1
    @test a*b == x^19
+
+   R, x = FiniteField(5, 1, "x")
+
+   a = 2x + 1
+   b = 3x + 2
+
+   @test a + b == R(3)
+   @test a - b == -x - 1
+   @test a*b == 6*x^2 + 2*x + 2
 end
 
 @testset "n_GF.comparison" begin
@@ -77,6 +136,13 @@ end
 
    @test a == deepcopy(a)
    @test isequal(a, a)
+
+   R, x = FiniteField(5, 1, "x")
+
+   a = 2x + 3
+
+   @test a == deepcopy(a)
+   @test isequal(a, a)
 end
 
 @testset "n_GF.ad_hoc_comparison" begin
@@ -88,13 +154,27 @@ end
    @test 2 != x
    @test isequal(R(2), 2)
    @test isequal(2, R(2))
+
+   R, x = FiniteField(5, 1, "x")
+
+   @test R(2) == 2
+   @test x != 2 || x == 2
+   @test 2 == R(2)
+   @test 2 != x || 2 == x
+   @test isequal(R(2), 2)
+   @test isequal(2, R(2))
 end
 
 @testset "n_GF.powering" begin
    R, x = FiniteField(5, 2, "x")
 
    @test (x + 1)^2 == x^20
    @test_throws DomainError x^-rand(1:99)
+
+   R, x = FiniteField(5, 1, "x")
+   @test x^3 == x*x*x
+   @test R(2)^2 == 2^2
+   @test R(2)^3 == 2^3
 end
 
 @testset "n_GF.exact_division" begin
@@ -105,6 +185,11 @@ end
 
    @test inv(a) == x^2
    @test divexact(a, b) == x^14
+
+   R, x = FiniteField(5, 1, "x")
+
+   @test 2*inv(R(2)) == 1
+   @test divexact(R(2), R(3)) == R(4)
 end
 
 @testset "n_GF.gcd_lcm" begin
@@ -115,6 +200,11 @@ end
 
    @test gcd(a, b) == R(1)
    @test gcd(R(0), R(0)) == R(0)
+
+   R, x = FiniteField(5, 1, "x")
+
+   @test gcd(R(0), R(0)) == R(0)
+   @test gcd(R(2), R(3)) == R(1)
 end
 
 @testset "n_GF.Polynomials" begin