Merge pull request #2066 from Jake-Moss/monomial_deflation_div_by_zero

Fix division by zero in `monomials_deflate.c`.

Author: fredrik-johansson

src/fmpz_mpoly/test/t-inflate_deflate.c

@@ -196,5 +196,65 @@ TEST_FUNCTION_START(fmpz_mpoly_inflate_deflate, state)
         fmpz_mpoly_ctx_clear(ctx);
     }
 
+    /* Check deflate with zero-stride does not divide by zero */
+    for (i = 0; i < 40 * flint_test_multiplier(); i++)
+    {
+        fmpz_mpoly_ctx_t ctx;
+        fmpz_mpoly_t x, res;
+        fmpz *stride, *shift;
+
+        fmpz_mpoly_ctx_init(ctx, 1, ORD_LEX);
+        fmpz_mpoly_init(x, ctx);
+        fmpz_mpoly_init(res, ctx);
+
+        stride = flint_malloc(ctx->minfo->nvars * sizeof(fmpz));
+        shift = flint_malloc(ctx->minfo->nvars * sizeof(fmpz));
+        fmpz_init(stride + 0);
+        fmpz_init(shift + 0);
+
+        /* Set x to just the generator */
+        fmpz_mpoly_gen(x, 0, ctx);
+
+        /* --- With zero shift --- */
+        fmpz_set_ui(shift + 0, 0);
+
+        /* Attempt to deflate x to 1 with a shift and stride 0. */
+        /* That is 1 -> (1 - 0) / 0 */
+        /* Division by zero should not be raised here */
+        fmpz_mpoly_deflate(res, x, shift, stride, ctx);
+
+        if (!fmpz_mpoly_equal_ui(res, 1, ctx))
+        {
+            printf("FAIL\n");
+            flint_printf("Check deflate with zero-shift and zero-stride\n");
+            fflush(stdout);
+            flint_abort();
+        }
+
+        /* --- With non-zero shift --- */
+        fmpz_set_ui(shift + 0, 1);
+
+        /* Attempt to deflate x to 1 with a shift of 1 and stride 0. */
+        /* That is 1 -> (1 - 1) / 0 */
+        /* Division by zero should not be raised here */
+        fmpz_mpoly_deflate(res, x, shift, stride, ctx);
+
+        if (!fmpz_mpoly_equal_ui(res, 1, ctx))
+        {
+            printf("FAIL\n");
+            flint_printf("Check deflate with non-zero shift and zero-stride\n");
+            fflush(stdout);
+            flint_abort();
+        }
+
+        fmpz_clear(stride + 0);
+        fmpz_clear(shift + 0);
+        flint_free(stride);
+        flint_free(shift);
+
+        fmpz_mpoly_clear(x, ctx);
+        fmpz_mpoly_ctx_clear(ctx);
+    }
+
     TEST_FUNCTION_END(state);
 }

src/mpoly/monomials_deflate.c

@@ -36,12 +36,18 @@ void mpoly_monomials_deflate(ulong * Aexps, flint_bitcnt_t Abits,
         for (j = 0; j < nvars; j++)
         {
             fmpz_sub(exps + j, exps + j, shift + j);
-            /* stride + j is allowed to be zero */
-            if (!fmpz_is_zero(exps + j))
+            /* stride + j is allowed to be zero, if it is then exps + j is
+               assumed to be zero, and the quotient is defined as zero */
+            if (!fmpz_is_zero(exps + j) && !fmpz_is_zero(stride + j))
             {
                 FLINT_ASSERT(fmpz_divisible(exps + j, stride + j));
                 fmpz_divexact(exps + j, exps + j, stride + j);
             }
+            else
+            {
+                fmpz_zero(exps + j);
+            }
+
         }
         FLINT_ASSERT(Abits >= mpoly_exp_bits_required_ffmpz(exps, mctx));
         mpoly_set_monomial_ffmpz(Aexps + NA*i, exps, Abits, mctx);