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/*
Copyright (C) 2022 Fredrik Johansson
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 "perm.h"
#include "gr.h"
#include "gr_mat.h"
int
gr_mat_rref_lu(slong * res_rank, gr_mat_t R, const gr_mat_t A, gr_ctx_t ctx)
{
slong i, j, k, n, rank;
slong *pivots;
slong *nonpivots;
slong *P;
gr_mat_t U, V;
int status = GR_SUCCESS;
slong sz = ctx->sizeof_elem;
if (gr_mat_is_zero(A, ctx) == T_TRUE)
{
*res_rank = 0;
return GR_SUCCESS;
}
/* Todo: fast path for nrows == 1 */
n = A->c;
P = _perm_init(gr_mat_nrows(A, ctx));
status = gr_mat_lu(&rank, P, R, A, 0, ctx);
_perm_clear(P);
if (status != GR_SUCCESS)
return status;
if (rank == 0)
{
*res_rank = 0;
return GR_SUCCESS;
}
/* Clear L */
for (i = 0; i < A->r; i++)
for (j = 0; j < FLINT_MIN(i, rank); j++)
status |= gr_zero(GR_MAT_ENTRY(R, i, j, sz), ctx);
/* We now reorder U to proper upper triangular form U | V
with U full-rank triangular, set V = U^(-1) V, and then
put the column back in the original order.
An improvement for some matrices would be to compress V by
discarding columns containing nothing but zeros. */
gr_mat_init(U, rank, rank, ctx);
gr_mat_init(V, rank, n - rank, ctx);
pivots = flint_malloc(sizeof(slong) * rank);
nonpivots = flint_malloc(sizeof(slong) * (n - rank));
for (i = j = k = 0; i < rank; i++)
{
while (1)
{
/* Todo: this should not be T_UNKNOWN. Should we save
the pivot data in the lu algorithm? */
truth_t is_zero = gr_is_zero(GR_MAT_ENTRY(R, i, j, sz), ctx);
if (is_zero == T_FALSE)
{
break;
}
else if (is_zero == T_TRUE)
{
nonpivots[k] = j;
k++;
j++;
}
else
{
status = GR_UNABLE;
goto cleanup1;
}
}
pivots[i] = j;
j++;
}
while (k < n - rank)
{
nonpivots[k] = j;
k++;
j++;
}
for (i = 0; i < rank; i++)
for (j = 0; j <= i; j++)
status |= gr_set(GR_MAT_ENTRY(U, j, i, sz), GR_MAT_ENTRY(R, j, pivots[i], sz), ctx);
for (i = 0; i < n - rank; i++)
for (j = 0; j < rank; j++)
status |= gr_set(GR_MAT_ENTRY(V, j, i, sz), GR_MAT_ENTRY(R, j, nonpivots[i], sz), ctx);
status |= gr_mat_nonsingular_solve_triu(V, U, V, 0, ctx);
/* Clear pivot columns */
for (i = 0; i < rank; i++)
{
for (j = 0; j <= i; j++)
{
if (i == j)
status |= gr_one(GR_MAT_ENTRY(R, j, pivots[i], sz), ctx);
else
status |= gr_zero(GR_MAT_ENTRY(R, j, pivots[i], sz), ctx);
}
}
/* Write back the actual content */
for (i = 0; i < n - rank; i++)
for (j = 0; j < rank; j++)
status |= gr_set(GR_MAT_ENTRY(R, j, nonpivots[i], sz), GR_MAT_ENTRY(V, j, i, sz), ctx);
cleanup1:
gr_mat_clear(U, ctx);
gr_mat_clear(V, ctx);
flint_free(pivots);
flint_free(nonpivots);
*res_rank = rank;
return status;
}