isl_basic_set_sample: don't project out lineality space up front

[isl.git] / isl_sample.c

#include "isl_sample.h"
#include "isl_sample_piplib.h"
#include "isl_vec.h"
#include "isl_mat.h"
#include "isl_seq.h"
#include "isl_map_private.h"
#include "isl_equalities.h"
#include "isl_tab.h"
#include "isl_basis_reduction.h"

static struct isl_vec *empty_sample(struct isl_basic_set *bset)
{
        struct isl_vec *vec;

        vec = isl_vec_alloc(bset->ctx, 0);
        isl_basic_set_free(bset);
        return vec;
}

/* Construct a zero sample of the same dimension as bset.
 * As a special case, if bset is zero-dimensional, this
 * function creates a zero-dimensional sample point.
 */
static struct isl_vec *zero_sample(struct isl_basic_set *bset)
{
        unsigned dim;
        struct isl_vec *sample;

        dim = isl_basic_set_total_dim(bset);
        sample = isl_vec_alloc(bset->ctx, 1 + dim);
        if (sample) {
                isl_int_set_si(sample->el[0], 1);
                isl_seq_clr(sample->el + 1, dim);
        }
        isl_basic_set_free(bset);
        return sample;
}

static struct isl_vec *interval_sample(struct isl_basic_set *bset)
{
        int i;
        isl_int t;
        struct isl_vec *sample;

        bset = isl_basic_set_simplify(bset);
        if (!bset)
                return NULL;
        if (isl_basic_set_fast_is_empty(bset))
                return empty_sample(bset);
        if (bset->n_eq == 0 && bset->n_ineq == 0)
                return zero_sample(bset);

        sample = isl_vec_alloc(bset->ctx, 2);
        isl_int_set_si(sample->block.data[0], 1);

        if (bset->n_eq > 0) {
                isl_assert(bset->ctx, bset->n_eq == 1, goto error);
                isl_assert(bset->ctx, bset->n_ineq == 0, goto error);
                if (isl_int_is_one(bset->eq[0][1]))
                        isl_int_neg(sample->el[1], bset->eq[0][0]);
                else {
                        isl_assert(bset->ctx, isl_int_is_negone(bset->eq[0][1]),
                                   goto error);
                        isl_int_set(sample->el[1], bset->eq[0][0]);
                }
                isl_basic_set_free(bset);
                return sample;
        }

        isl_int_init(t);
        if (isl_int_is_one(bset->ineq[0][1]))
                isl_int_neg(sample->block.data[1], bset->ineq[0][0]);
        else
                isl_int_set(sample->block.data[1], bset->ineq[0][0]);
        for (i = 1; i < bset->n_ineq; ++i) {
                isl_seq_inner_product(sample->block.data,
                                        bset->ineq[i], 2, &t);
                if (isl_int_is_neg(t))
                        break;
        }
        isl_int_clear(t);
        if (i < bset->n_ineq) {
                isl_vec_free(sample);
                return empty_sample(bset);
        }

        isl_basic_set_free(bset);
        return sample;
error:
        isl_basic_set_free(bset);
        isl_vec_free(sample);
        return NULL;
}

static struct isl_mat *independent_bounds(struct isl_ctx *ctx,
        struct isl_basic_set *bset)
{
        int i, j, n;
        struct isl_mat *dirs = NULL;
        struct isl_mat *bounds = NULL;
        unsigned dim;

        if (!bset)
                return NULL;

        dim = isl_basic_set_n_dim(bset);
        bounds = isl_mat_alloc(ctx, 1+dim, 1+dim);
        if (!bounds)
                return NULL;

        isl_int_set_si(bounds->row[0][0], 1);
        isl_seq_clr(bounds->row[0]+1, dim);
        bounds->n_row = 1;

        if (bset->n_ineq == 0)
                return bounds;

        dirs = isl_mat_alloc(ctx, dim, dim);
        if (!dirs) {
                isl_mat_free(ctx, bounds);
                return NULL;
        }
        isl_seq_cpy(dirs->row[0], bset->ineq[0]+1, dirs->n_col);
        isl_seq_cpy(bounds->row[1], bset->ineq[0], bounds->n_col);
        for (j = 1, n = 1; n < dim && j < bset->n_ineq; ++j) {
                int pos;

                isl_seq_cpy(dirs->row[n], bset->ineq[j]+1, dirs->n_col);

                pos = isl_seq_first_non_zero(dirs->row[n], dirs->n_col);
                if (pos < 0)
                        continue;
                for (i = 0; i < n; ++i) {
                        int pos_i;
                        pos_i = isl_seq_first_non_zero(dirs->row[i], dirs->n_col);
                        if (pos_i < pos)
                                continue;
                        if (pos_i > pos)
                                break;
                        isl_seq_elim(dirs->row[n], dirs->row[i], pos,
                                        dirs->n_col, NULL);
                        pos = isl_seq_first_non_zero(dirs->row[n], dirs->n_col);
                        if (pos < 0)
                                break;
                }
                if (pos < 0)
                        continue;
                if (i < n) {
                        int k;
                        isl_int *t = dirs->row[n];
                        for (k = n; k > i; --k)
                                dirs->row[k] = dirs->row[k-1];
                        dirs->row[i] = t;
                }
                ++n;
                isl_seq_cpy(bounds->row[n], bset->ineq[j], bounds->n_col);
        }
        isl_mat_free(ctx, dirs);
        bounds->n_row = 1+n;
        return bounds;
}

static void swap_inequality(struct isl_basic_set *bset, int a, int b)
{
        isl_int *t = bset->ineq[a];
        bset->ineq[a] = bset->ineq[b];
        bset->ineq[b] = t;
}

/* Skew into positive orthant and project out lineality space.
 *
 * We perform a unimodular transformation that turns a selected
 * maximal set of linearly independent bounds into constraints
 * on the first dimensions that impose that these first dimensions
 * are non-negative.  In particular, the constraint matrix is lower
 * triangular with positive entries on the diagonal and negative
 * entries below.
 * If "bset" has a lineality space then these constraints (and therefore
 * all constraints in bset) only involve the first dimensions.
 * The remaining dimensions then do not appear in any constraints and
 * we can select any value for them, say zero.  We therefore project
 * out this final dimensions and plug in the value zero later.  This
 * is accomplished by simply dropping the final columns of
 * the unimodular transformation.
 */
static struct isl_basic_set *isl_basic_set_skew_to_positive_orthant(
        struct isl_basic_set *bset, struct isl_mat **T)
{
        struct isl_mat *U = NULL;
        struct isl_mat *bounds = NULL;
        int i, j;
        unsigned old_dim, new_dim;
        struct isl_ctx *ctx;

        *T = NULL;
        if (!bset)
                return NULL;

        ctx = bset->ctx;
        isl_assert(ctx, isl_basic_set_n_param(bset) == 0, goto error);
        isl_assert(ctx, bset->n_div == 0, goto error);
        isl_assert(ctx, bset->n_eq == 0, goto error);
        
        old_dim = isl_basic_set_n_dim(bset);
        /* Try to move (multiples of) unit rows up. */
        for (i = 0, j = 0; i < bset->n_ineq; ++i) {
                int pos = isl_seq_first_non_zero(bset->ineq[i]+1, old_dim);
                if (pos < 0)
                        continue;
                if (isl_seq_first_non_zero(bset->ineq[i]+1+pos+1,
                                                old_dim-pos-1) >= 0)
                        continue;
                if (i != j)
                        swap_inequality(bset, i, j);
                ++j;
        }
        bounds = independent_bounds(ctx, bset);
        if (!bounds)
                goto error;
        new_dim = bounds->n_row - 1;
        bounds = isl_mat_left_hermite(ctx, bounds, 1, &U, NULL);
        if (!bounds)
                goto error;
        U = isl_mat_drop_cols(ctx, U, 1 + new_dim, old_dim - new_dim);
        bset = isl_basic_set_preimage(bset, isl_mat_copy(ctx, U));
        if (!bset)
                goto error;
        *T = U;
        isl_mat_free(ctx, bounds);
        return bset;
error:
        isl_mat_free(ctx, bounds);
        isl_mat_free(ctx, U);
        isl_basic_set_free(bset);
        return NULL;
}

/* Find a sample integer point, if any, in bset, which is known
 * to have equalities.  If bset contains no integer points, then
 * return a zero-length vector.
 * We simply remove the known equalities, compute a sample
 * in the resulting bset, using the specified recurse function,
 * and then transform the sample back to the original space.
 */
static struct isl_vec *sample_eq(struct isl_basic_set *bset,
        struct isl_vec *(*recurse)(struct isl_basic_set *))
{
        struct isl_mat *T;
        struct isl_vec *sample;
        struct isl_ctx *ctx;

        if (!bset)
                return NULL;

        ctx = bset->ctx;
        bset = isl_basic_set_remove_equalities(bset, &T, NULL);
        sample = recurse(bset);
        if (!sample || sample->size == 0)
                isl_mat_free(ctx, T);
        else
                sample = isl_mat_vec_product(ctx, T, sample);
        return sample;
}

/* Given a basic set "bset" and an affine function "f"/"denom",
 * check if bset is bounded and non-empty and if so, return the minimal
 * and maximal value attained by the affine function in "min" and "max".
 * The minimal value is rounded up to the nearest integer, while the
 * maximal value is rounded down.
 * The return value indicates whether the set was empty or unbounded.
 *
 * If we happen to find an integer point while looking for the minimal
 * or maximal value, then we record that value in "bset" and return early.
 */
static enum isl_lp_result basic_set_range(struct isl_basic_set *bset,
        isl_int *f, isl_int denom, isl_int *min, isl_int *max)
{
        unsigned dim;
        struct isl_tab *tab;
        enum isl_lp_result res;

        if (!bset)
                return isl_lp_error;
        if (isl_basic_set_fast_is_empty(bset))
                return isl_lp_empty;

        tab = isl_tab_from_basic_set(bset);
        res = isl_tab_min(bset->ctx, tab, f, denom, min, NULL, 0);
        if (res != isl_lp_ok)
                goto done;

        if (isl_tab_sample_is_integer(bset->ctx, tab)) {
                isl_vec_free(bset->sample);
                bset->sample = isl_tab_get_sample_value(bset->ctx, tab);
                if (!bset->sample)
                        goto error;
                isl_int_set(*max, *min);
                goto done;
        }

        dim = isl_basic_set_total_dim(bset);
        isl_seq_neg(f, f, 1 + dim);
        res = isl_tab_min(bset->ctx, tab, f, denom, max, NULL, 0);
        isl_seq_neg(f, f, 1 + dim);
        isl_int_neg(*max, *max);

        if (isl_tab_sample_is_integer(bset->ctx, tab)) {
                isl_vec_free(bset->sample);
                bset->sample = isl_tab_get_sample_value(bset->ctx, tab);
                if (!bset->sample)
                        goto error;
        }

done:
        isl_tab_free(bset->ctx, tab);
        return res;
error:
        isl_tab_free(bset->ctx, tab);
        return isl_lp_error;
}

/* Perform a basis reduction on "bset" and return the inverse of
 * the new basis, i.e., an affine mapping from the new coordinates to the old,
 * in *T.
 */
static struct isl_basic_set *basic_set_reduced(struct isl_basic_set *bset,
        struct isl_mat **T)
{
        struct isl_ctx *ctx;
        unsigned gbr_only_first;

        *T = NULL;
        if (!bset)
                return NULL;

        ctx = bset->ctx;

        gbr_only_first = ctx->gbr_only_first;
        ctx->gbr_only_first = 1;
        *T = isl_basic_set_reduced_basis(bset);
        ctx->gbr_only_first = gbr_only_first;

        *T = isl_mat_lin_to_aff(bset->ctx, *T);
        *T = isl_mat_right_inverse(bset->ctx, *T);

        bset = isl_basic_set_preimage(bset, isl_mat_copy(bset->ctx, *T));
        if (!bset)
                goto error;

        return bset;
error:
        isl_mat_free(ctx, *T);
        *T = NULL;
        return NULL;
}

static struct isl_vec *sample_bounded(struct isl_basic_set *bset);

/* Given a basic set "bset" whose first coordinate ranges between
 * "min" and "max", step through all values from min to max, until
 * the slice of bset with the first coordinate fixed to one of these
 * values contains an integer point.  If such a point is found, return it.
 * If none of the slices contains any integer point, then bset itself
 * doesn't contain any integer point and an empty sample is returned.
 */
static struct isl_vec *sample_scan(struct isl_basic_set *bset,
        isl_int min, isl_int max)
{
        unsigned total;
        struct isl_basic_set *slice = NULL;
        struct isl_vec *sample = NULL;
        isl_int tmp;

        total = isl_basic_set_total_dim(bset);

        isl_int_init(tmp);
        for (isl_int_set(tmp, min); isl_int_le(tmp, max);
             isl_int_add_ui(tmp, tmp, 1)) {
                int k;

                slice = isl_basic_set_copy(bset);
                slice = isl_basic_set_cow(slice);
                slice = isl_basic_set_extend_constraints(slice, 1, 0);
                k = isl_basic_set_alloc_equality(slice);
                if (k < 0)
                        goto error;
                isl_int_set(slice->eq[k][0], tmp);
                isl_int_set_si(slice->eq[k][1], -1);
                isl_seq_clr(slice->eq[k] + 2, total - 1);
                slice = isl_basic_set_simplify(slice);
                sample = sample_bounded(slice);
                slice = NULL;
                if (!sample)
                        goto error;
                if (sample->size > 0)
                        break;
                isl_vec_free(sample);
                sample = NULL;
        }
        if (!sample)
                sample = empty_sample(bset);
        else
                isl_basic_set_free(bset);
        isl_int_clear(tmp);
        return sample;
error:
        isl_basic_set_free(bset);
        isl_basic_set_free(slice);
        isl_int_clear(tmp);
        return NULL;
}

/* Given a basic set that is known to be bounded, find and return
 * an integer point in the basic set, if there is any.
 *
 * After handling some trivial cases, we check the range of the
 * first coordinate.  If this coordinate can only attain one integer
 * value, we are happy.  Otherwise, we perform basis reduction and
 * determine the new range.
 *
 * Then we step through all possible values in the range in sample_scan.
 *
 * If any basis reduction was performed, the sample value found, if any,
 * is transformed back to the original space.
 */ 
static struct isl_vec *sample_bounded(struct isl_basic_set *bset)
{
        unsigned dim;
        struct isl_ctx *ctx;
        struct isl_vec *sample;
        struct isl_vec *obj = NULL;
        struct isl_mat *T = NULL;
        isl_int min, max;
        enum isl_lp_result res;

        if (!bset)
                return NULL;

        if (isl_basic_set_fast_is_empty(bset))
                return empty_sample(bset);

        ctx = bset->ctx;
        dim = isl_basic_set_total_dim(bset);
        if (dim == 0)
                return zero_sample(bset);
        if (dim == 1)
                return interval_sample(bset);
        if (bset->n_eq > 0)
                return sample_eq(bset, sample_bounded);

        isl_int_init(min);
        isl_int_init(max);
        obj = isl_vec_alloc(bset->ctx, 1 + dim);
        if (!obj)
                goto error;
        isl_seq_clr(obj->el, 1+ dim);
        isl_int_set_si(obj->el[1], 1);

        res = basic_set_range(bset, obj->el, bset->ctx->one, &min, &max);
        if (res == isl_lp_error)
                goto error;
        isl_assert(bset->ctx, res != isl_lp_unbounded, goto error);
        if (bset->sample) {
                sample = isl_vec_copy(bset->sample);
                isl_basic_set_free(bset);
                goto out;
        }
        if (res == isl_lp_empty || isl_int_lt(max, min)) {
                sample = empty_sample(bset);
                goto out;
        }

        if (isl_int_ne(min, max)) {
                bset = basic_set_reduced(bset, &T);
                if (!bset)
                        goto error;

                res = basic_set_range(bset, obj->el, bset->ctx->one, &min, &max);
                if (res == isl_lp_error)
                        goto error;
                isl_assert(bset->ctx, res != isl_lp_unbounded, goto error);
                if (bset->sample) {
                        sample = isl_vec_copy(bset->sample);
                        isl_basic_set_free(bset);
                        goto out;
                }
                if (res == isl_lp_empty || isl_int_lt(max, min)) {
                        sample = empty_sample(bset);
                        goto out;
                }
        }

        sample = sample_scan(bset, min, max);
out:
        if (T) {
                if (!sample || sample->size == 0)
                        isl_mat_free(ctx, T);
                else
                        sample = isl_mat_vec_product(ctx, T, sample);
        }
        isl_vec_free(obj);
        isl_int_clear(min);
        isl_int_clear(max);
        return sample;
error:
        isl_mat_free(ctx, T);
        isl_basic_set_free(bset);
        isl_vec_free(obj);
        isl_int_clear(min);
        isl_int_clear(max);
        return NULL;
}

/* Given a basic set "bset" and a value "sample" for the first coordinates
 * of bset, plug in these values and drop the corresponding coordinates.
 *
 * We do this by computing the preimage of the transformation
 *
 *           [ 1 0 ]
 *      x =  [ s 0 ] x'
 *           [ 0 I ]
 *
 * where [1 s] is the sample value and I is the identity matrix of the
 * appropriate dimension.
 */
static struct isl_basic_set *plug_in(struct isl_basic_set *bset,
        struct isl_vec *sample)
{
        int i;
        unsigned total;
        struct isl_mat *T;

        if (!bset || !sample)
                goto error;

        total = isl_basic_set_total_dim(bset);
        T = isl_mat_alloc(bset->ctx, 1 + total, 1 + total - (sample->size - 1));
        if (!T)
                goto error;

        for (i = 0; i < sample->size; ++i) {
                isl_int_set(T->row[i][0], sample->el[i]);
                isl_seq_clr(T->row[i] + 1, T->n_col - 1);
        }
        for (i = 0; i < T->n_col - 1; ++i) {
                isl_seq_clr(T->row[sample->size + i], T->n_col);
                isl_int_set_si(T->row[sample->size + i][1 + i], 1);
        }
        isl_vec_free(sample);

        bset = isl_basic_set_preimage(bset, T);
        return bset;
error:
        isl_basic_set_free(bset);
        isl_vec_free(sample);
        return NULL;
}

/* Given a basic set "bset", return any (possibly non-integer) point
 * in the basic set.
 */
static struct isl_vec *rational_sample(struct isl_basic_set *bset)
{
        struct isl_tab *tab;
        struct isl_vec *sample;

        if (!bset)
                return NULL;

        tab = isl_tab_from_basic_set(bset);
        sample = isl_tab_get_sample_value(bset->ctx, tab);
        isl_tab_free(bset->ctx, tab);

        isl_basic_set_free(bset);

        return sample;
}

/* Given a rational vector, with the denominator in the first element
 * of the vector, round up all coordinates.
 */
struct isl_vec *isl_vec_ceil(struct isl_vec *vec)
{
        int i;

        vec = isl_vec_cow(vec);
        if (!vec)
                return NULL;

        isl_seq_cdiv_q(vec->el + 1, vec->el + 1, vec->el[0], vec->size - 1);

        isl_int_set_si(vec->el[0], 1);

        return vec;
}

/* Given a linear cone "cone" and a rational point "vec",
 * construct a polyhedron with shifted copies of the constraints in "cone",
 * i.e., a polyhedron with "cone" as its recession cone, such that each
 * point x in this polyhedron is such that the unit box positioned at x
 * lies entirely inside the affine cone 'vec + cone'.
 * Any rational point in this polyhedron may therefore be rounded up
 * to yield an integer point that lies inside said affine cone.
 *
 * Denote the constraints of cone by "<a_i, x> >= 0" and the rational
 * point "vec" by v/d.
 * Let b_i = <a_i, v>.  Then the affine cone 'vec + cone' is given
 * by <a_i, x> - b/d >= 0.
 * The polyhedron <a_i, x> - ceil{b/d} >= 0 is a subset of this affine cone.
 * We prefer this polyhedron over the actual affine cone because it doesn't
 * require a scaling of the constraints.
 * If each of the vertices of the unit cube positioned at x lies inside
 * this polyhedron, then the whole unit cube at x lies inside the affine cone.
 * We therefore impose that x' = x + \sum e_i, for any selection of unit
 * vectors lies inside the polyhedron, i.e.,
 *
 *      <a_i, x'> - ceil{b/d} = <a_i, x> + sum a_i - ceil{b/d} >= 0
 *
 * The most stringent of these constraints is the one that selects
 * all negative a_i, so the polyhedron we are looking for has constraints
 *
 *      <a_i, x> + sum_{a_i < 0} a_i - ceil{b/d} >= 0
 *
 * Note that if cone were known to have only non-negative rays
 * (which can be accomplished by a unimodular transformation),
 * then we would only have to check the points x' = x + e_i
 * and we only have to add the smallest negative a_i (if any)
 * instead of the sum of all negative a_i.
 */
static struct isl_basic_set *shift_cone(struct isl_basic_set *cone,
        struct isl_vec *vec)
{
        int i, j, k;
        unsigned total;

        struct isl_basic_set *shift = NULL;

        if (!cone || !vec)
                goto error;

        isl_assert(cone->ctx, cone->n_eq == 0, goto error);

        total = isl_basic_set_total_dim(cone);

        shift = isl_basic_set_alloc_dim(isl_basic_set_get_dim(cone),
                                        0, 0, cone->n_ineq);

        for (i = 0; i < cone->n_ineq; ++i) {
                k = isl_basic_set_alloc_inequality(shift);
                if (k < 0)
                        goto error;
                isl_seq_cpy(shift->ineq[k] + 1, cone->ineq[i] + 1, total);
                isl_seq_inner_product(shift->ineq[k] + 1, vec->el + 1, total,
                                      &shift->ineq[k][0]);
                isl_int_cdiv_q(shift->ineq[k][0],
                               shift->ineq[k][0], vec->el[0]);
                isl_int_neg(shift->ineq[k][0], shift->ineq[k][0]);
                for (j = 0; j < total; ++j) {
                        if (isl_int_is_nonneg(shift->ineq[k][1 + j]))
                                continue;
                        isl_int_add(shift->ineq[k][0],
                                    shift->ineq[k][0], shift->ineq[k][1 + j]);
                }
        }

        isl_basic_set_free(cone);
        isl_vec_free(vec);

        return isl_basic_set_finalize(shift);
error:
        isl_basic_set_free(shift);
        isl_basic_set_free(cone);
        isl_vec_free(vec);
        return NULL;
}

/* Given a rational point vec in a (transformed) basic set,
 * such that cone is the recession cone of the original basic set,
 * "round up" the rational point to an integer point.
 *
 * We first check if the rational point just happens to be integer.
 * If not, we transform the cone in the same way as the basic set,
 * pick a point x in this cone shifted to the rational point such that
 * the whole unit cube at x is also inside this affine cone.
 * Then we simply round up the coordinates of x and return the
 * resulting integer point.
 */
static struct isl_vec *round_up_in_cone(struct isl_vec *vec,
        struct isl_basic_set *cone, struct isl_mat *U)
{
        unsigned total;

        if (!vec || !cone || !U)
                goto error;

        isl_assert(vec->ctx, vec->size != 0, goto error);
        if (isl_int_is_one(vec->el[0])) {
                isl_mat_free(vec->ctx, U);
                isl_basic_set_free(cone);
                return vec;
        }

        total = isl_basic_set_total_dim(cone);
        cone = isl_basic_set_preimage(cone, U);
        cone = isl_basic_set_remove_dims(cone, 0, total - (vec->size - 1));

        cone = shift_cone(cone, vec);

        vec = rational_sample(cone);
        vec = isl_vec_ceil(vec);
        return vec;
error:
        isl_mat_free(vec ? vec->ctx : cone ? cone->ctx : NULL, U);
        isl_vec_free(vec);
        isl_basic_set_free(cone);
        return NULL;
}

/* Concatenate two integer vectors, i.e., two vectors with denominator
 * (stored in element 0) equal to 1.
 */
static struct isl_vec *vec_concat(struct isl_vec *vec1, struct isl_vec *vec2)
{
        struct isl_vec *vec;

        if (!vec1 || !vec2)
                goto error;
        isl_assert(vec1->ctx, vec1->size > 0, goto error);
        isl_assert(vec2->ctx, vec2->size > 0, goto error);
        isl_assert(vec1->ctx, isl_int_is_one(vec1->el[0]), goto error);
        isl_assert(vec2->ctx, isl_int_is_one(vec2->el[0]), goto error);

        vec = isl_vec_alloc(vec1->ctx, vec1->size + vec2->size - 1);
        if (!vec)
                goto error;

        isl_seq_cpy(vec->el, vec1->el, vec1->size);
        isl_seq_cpy(vec->el + vec1->size, vec2->el + 1, vec2->size - 1);

        isl_vec_free(vec1);
        isl_vec_free(vec2);

        return vec;
error:
        isl_vec_free(vec1);
        isl_vec_free(vec2);
        return NULL;
}

/* Drop all constraints in bset that involve any of the dimensions
 * first to first+n-1.
 */
static struct isl_basic_set *drop_constraints_involving
        (struct isl_basic_set *bset, unsigned first, unsigned n)
{
        int i;

        if (!bset)
                return NULL;

        bset = isl_basic_set_cow(bset);

        for (i = bset->n_ineq - 1; i >= 0; --i) {
                if (isl_seq_first_non_zero(bset->ineq[i] + 1 + first, n) == -1)
                        continue;
                isl_basic_set_drop_inequality(bset, i);
        }

        return bset;
}

/* Give a basic set "bset" with recession cone "cone", compute and
 * return an integer point in bset, if any.
 *
 * If the recession cone is full-dimensional, then we know that
 * bset contains an infinite number of integer points and it is
 * fairly easy to pick one of them.
 * If the recession cone is not full-dimensional, then we first
 * transform bset such that the bounded directions appear as
 * the first dimensions of the transformed basic set.
 * We do this by using a unimodular transformation that transforms
 * the equalities in the recession cone to equalities on the first
 * dimensions.
 *
 * The transformed set is then projected onto its bounded dimensions.
 * Note that to compute this projection, we can simply drop all constraints
 * involving any of the unbounded dimensions since these constraints
 * cannot be combined to produce a constraint on the bounded dimensions.
 * To see this, assume that there is such a combination of constraints
 * that produces a constraint on the bounded dimensions.  This means
 * that some combination of the unbounded dimensions has both an upper
 * bound and a lower bound in terms of the bounded dimensions, but then
 * this combination would be a bounded direction too and would have been
 * transformed into a bounded dimensions.
 *
 * We then compute a sample value in the bounded dimensions.
 * If no such value can be found, then the original set did not contain
 * any integer points and we are done.
 * Otherwise, we plug in the value we found in the bounded dimensions,
 * project out these bounded dimensions and end up with a set with
 * a full-dimensional recession cone.
 * A sample point in this set is computed by "rounding up" any
 * rational point in the set.
 *
 * The sample points in the bounded and unbounded dimensions are
 * then combined into a single sample point and transformed back
 * to the original space.
 */
static struct isl_vec *sample_with_cone(struct isl_basic_set *bset,
        struct isl_basic_set *cone)
{
        unsigned total;
        unsigned cone_dim;
        struct isl_mat *M, *U;
        struct isl_vec *sample;
        struct isl_vec *cone_sample;
        struct isl_ctx *ctx;
        struct isl_basic_set *bounded;

        if (!bset || !cone)
                goto error;

        ctx = bset->ctx;
        total = isl_basic_set_total_dim(cone);
        cone_dim = total - cone->n_eq;

        M = isl_mat_sub_alloc(bset->ctx, cone->eq, 0, cone->n_eq, 1, total);
        M = isl_mat_left_hermite(bset->ctx, M, 0, &U, NULL);
        if (!M)
                goto error;
        isl_mat_free(bset->ctx, M);

        U = isl_mat_lin_to_aff(bset->ctx, U);
        bset = isl_basic_set_preimage(bset, isl_mat_copy(bset->ctx, U));

        bounded = isl_basic_set_copy(bset);
        bounded = drop_constraints_involving(bounded, total - cone_dim, cone_dim);
        bounded = isl_basic_set_drop_dims(bounded, total - cone_dim, cone_dim);
        sample = sample_bounded(bounded);
        if (!sample || sample->size == 0) {
                isl_basic_set_free(bset);
                isl_basic_set_free(cone);
                isl_mat_free(ctx, U);
                return sample;
        }
        bset = plug_in(bset, isl_vec_copy(sample));
        cone_sample = rational_sample(bset);
        cone_sample = round_up_in_cone(cone_sample, cone, isl_mat_copy(ctx, U));
        sample = vec_concat(sample, cone_sample);
        sample = isl_mat_vec_product(ctx, U, sample);
        return sample;
error:
        isl_basic_set_free(cone);
        isl_basic_set_free(bset);
        return NULL;
}

/* Compute and return a sample point in bset using generalized basis
 * reduction.  We first check if the input set has a non-trivial
 * recession cone.  If so, we perform some extra preprocessing in
 * sample_with_cone.  Otherwise, we directly perform generalized basis
 * reduction.
 */
static struct isl_vec *gbr_sample(struct isl_basic_set *bset)
{
        unsigned dim;
        struct isl_basic_set *cone;

        dim = isl_basic_set_total_dim(bset);

        cone = isl_basic_set_recession_cone(isl_basic_set_copy(bset));

        if (cone->n_eq < dim)
                return sample_with_cone(bset, cone);

        isl_basic_set_free(cone);
        return sample_bounded(bset);
}

static struct isl_vec *pip_sample(struct isl_basic_set *bset)
{
        struct isl_mat *T;
        struct isl_ctx *ctx;
        struct isl_vec *sample;

        bset = isl_basic_set_skew_to_positive_orthant(bset, &T);
        if (!bset)
                return NULL;

        ctx = bset->ctx;
        sample = isl_pip_basic_set_sample(bset);

        if (sample && sample->size != 0)
                sample = isl_mat_vec_product(ctx, T, sample);
        else
                isl_mat_free(ctx, T);

        return sample;
}

struct isl_vec *isl_basic_set_sample(struct isl_basic_set *bset)
{
        struct isl_ctx *ctx;
        unsigned dim;
        if (!bset)
                return NULL;

        ctx = bset->ctx;
        if (isl_basic_set_fast_is_empty(bset))
                return empty_sample(bset);

        dim = isl_basic_set_n_dim(bset);
        isl_assert(ctx, isl_basic_set_n_param(bset) == 0, goto error);
        isl_assert(ctx, bset->n_div == 0, goto error);

        if (bset->sample && bset->sample->size == 1 + dim) {
                int contains = isl_basic_set_contains(bset, bset->sample);
                if (contains < 0)
                        goto error;
                if (contains) {
                        struct isl_vec *sample = isl_vec_copy(bset->sample);
                        isl_basic_set_free(bset);
                        return sample;
                }
        }
        isl_vec_free(bset->sample);
        bset->sample = NULL;

        if (bset->n_eq > 0)
                return sample_eq(bset, isl_basic_set_sample);
        if (dim == 0)
                return zero_sample(bset);
        if (dim == 1)
                return interval_sample(bset);

        switch (bset->ctx->ilp_solver) {
        case ISL_ILP_PIP:
                return pip_sample(bset);
        case ISL_ILP_GBR:
                return gbr_sample(bset);
        }
        isl_assert(bset->ctx, 0, );
error:
        isl_basic_set_free(bset);
        return NULL;
}