Fix ZZMatrix modular reduction (issue #2947) (#1564)

src/flint/FlintTypes.jl

@@ -5449,8 +5449,31 @@ mutable struct ZZModMatrix <: MatElem{ZZModRingElem}
     end
     return z
   end
+
+  function ZZModMatrix(n::UInt, b::ZZMatrix)
+    return ZZModMatrix(ZZ(n), b)
+  end
+
+  function ZZModMatrix(n::Int, b::ZZMatrix)
+    return ZZModMatrix(ZZ(n), b)
+  end
+
+  function ZZModMatrix(n::ZZRingElem, b::ZZMatrix)
+    (n < 2) && error("Modulus must be >= 2")
+    z = new()
+    ccall((:fmpz_mod_mat_init, libflint), Nothing,
+            (Ref{ZZModMatrix}, Int, Int, Ref{ZZRingElem}), z, b.r, b.c, n)
+    finalizer(_fmpz_mod_mat_clear_fn, z)
+    for i = 1:b.r
+       for j = 1:b.c
+         setindex_raw!(z, mod(ZZRingElem(b[i,j]), n), i, j)
+       end
+    end
+    return z
+  end
 end
 
+
 function _fmpz_mod_mat_clear_fn(mat::ZZModMatrix)
   ccall((:fmpz_mod_mat_clear, libflint), Nothing, (Ref{ZZModMatrix}, ), mat)
 end

src/flint/fmpz_mod_mat.jl

@@ -835,6 +835,13 @@ function (a::ZZModMatrixSpace)(arr::AbstractVector{ZZModRingElem})
   return z
 end
 
+function (a::ZZModMatrixSpace)(b::ZZMatrix)
+  (ncols(a) != b.c || nrows(a) != b.r) && error("Dimensions do not fit")
+  z = ZZModMatrix(modulus(base_ring(a)), b)
+  z.base_ring = a.base_ring
+  return z
+end
+
 ###############################################################################
 #
 #   Matrix constructor

test/generic/Matrix-test.jl

@@ -233,6 +233,29 @@ end
     @test floor(ZZMatrix, M) == ZZMatrix(map(x->floor(Int,x), M))
 end
 
+@testset "Modular reduction of ZZMatrix" begin
+  S0 = matrix_space(ZZ,2,2)
+  M0 = S0(1)
+  M0[1,2] = 99
+  M0[2,1] = 199
+  M0[2,2] = -499
+  # First call the ZZModMatrix constructor directly: (3 cases)
+  ignore = ZZModMatrix(UInt(81), M0)
+  ignore = ZZModMatrix(Int(81), M0)
+  ignore = ZZModMatrix(ZZ(81), M0)
+  # Now check conversion via matrix_space: first use a modulus of type Int/UInt
+  m = 81
+  ZZmodm = residue_ring(ZZ,m)
+  S81 = matrix_space(ZZmodm,2,2)
+  M81 = S81(M0)
+  @test M81[1,1] == 1 && M81[1,2] == 18 && M81[2,1] == 37 && M81[2,2] == -13;
+  # Now use matrix_space with modulus of type ZZRingElem (see GitHub issue #2947)
+  ZZmodM = residue_ring(ZZ,ZZ(m))
+  SZZ81 = matrix_space(ZZmodM,2,2)
+  MZZ81 = SZZ81(M0)
+  @test MZZ81[1,1] == 1 && MZZ81[1,2] == 18 && MZZ81[2,1] == 37 && MZZ81[2,2] == -13;
+end
+
 #=
    TODO: Add tests for the following when there are rings that are not fields
          that have delayed reduction