Merge pull request #2439 from rburing/gr_mpoly_normalise

Add `_gr_mpoly_normalise` to remove zero coefficients

Author: fredrik-johansson

doc/source/gr_mpoly.rst

@@ -134,6 +134,12 @@ Memory management
 Basic manipulation
 -------------------------------------------------------------------------------
 
+.. function:: void _gr_mpoly_normalise(gr_mpoly_t A, gr_mpoly_ctx_t ctx)
+
+    Removes provably zero coefficients from ``A`` and updates the length.
+    If all coefficients are zero, the length is set to zero.  This function
+    is mainly used internally, as all functions guarantee normalisation.
+
 .. function:: void gr_mpoly_swap(gr_mpoly_t A, gr_mpoly_t B, gr_mpoly_ctx_t ctx)
 
     Swaps *A* and *B* efficiently.

src/gr_mpoly.h

@@ -151,6 +151,8 @@ gr_mpoly_length(const gr_mpoly_t x, gr_mpoly_ctx_t ctx)
 
 /* Basic manipulation */
 
+void _gr_mpoly_normalise(gr_mpoly_t A, gr_mpoly_ctx_t ctx);
+
 GR_MPOLY_INLINE
 void gr_mpoly_swap(gr_mpoly_t A, gr_mpoly_t B, gr_mpoly_ctx_t ctx)
 {

src/gr_mpoly/normalise.c

@@ -0,0 +1,50 @@
+/*
+    Copyright (C) 2025 Ricardo Buring
+
+    This file is part of FLINT.
+
+    FLINT is free software: you can redistribute it and/or modify it under
+    the terms of the GNU Lesser General Public License (LGPL) as published
+    by the Free Software Foundation; either version 3 of the License, or
+    (at your option) any later version.  See <https://www.gnu.org/licenses/>.
+*/
+
+#include "mpoly.h"
+#include "gr_mpoly.h"
+
+/*
+    remove zero coefficients
+*/
+void
+_gr_mpoly_normalise(gr_mpoly_t A, gr_mpoly_ctx_t ctx)
+{
+    mpoly_ctx_struct * mctx = GR_MPOLY_MCTX(ctx);
+    gr_ctx_struct * cctx = GR_MPOLY_CCTX(ctx);
+    gr_method_swap_op swap = GR_SWAP_OP(cctx, SWAP);
+    gr_method_unary_predicate is_zero = GR_UNARY_PREDICATE(cctx, IS_ZERO);
+    slong in, out, N;
+    slong sz = cctx->sizeof_elem;
+
+    N = mpoly_words_per_exp(A->bits, mctx);
+
+    out = -WORD(1);
+
+    for (in = 0; in < A->length; in++)
+    {
+        FLINT_ASSERT(in > out);
+
+        if (out < 0 || is_zero(GR_ENTRY(A->coeffs, out, sz), cctx) != T_TRUE)
+            out++;
+
+        if (out != in)
+        {
+            mpoly_monomial_set(A->exps + N*out, A->exps + N*in, N);
+            swap(GR_ENTRY(A->coeffs, out, sz), GR_ENTRY(A->coeffs, in, sz), cctx);
+        }
+    }
+
+    if (out < 0 || is_zero(GR_ENTRY(A->coeffs, out, sz), cctx) != T_TRUE)
+        out++;
+
+    A->length = out;
+}

src/gr_mpoly/set_other.c

@@ -99,7 +99,7 @@ _gr_mpoly_set_gr_mpoly_other(gr_mpoly_t res, const gr_mpoly_t A, gr_mpoly_ctx_t
             /* there may be zero coefficients; remove them
                (todo: do inline in first loop) */
             /* if (cctx != A_cctx) */
-            status = gr_mpoly_combine_like_terms(res, ctx);
+            _gr_mpoly_normalise(res, ctx);
         }
         else
         {
@@ -188,7 +188,7 @@ _gr_mpoly_set_gr_mpoly_other(gr_mpoly_t res, const gr_mpoly_t A, gr_mpoly_ctx_t
                     /* term order may be different */
                     gr_mpoly_sort_terms(res, ctx);
                     /* todo: combine zero checks with main loop */
-                    status |= gr_mpoly_combine_like_terms(res, ctx);
+                    _gr_mpoly_normalise(res, ctx);
 
                     flint_free(A_exps);
                     flint_free(exps);