The function f(x) appears to shift each character in the input string to a different character according to the following mapping:

a->z, b->g, c->e, d->f, e->h, f->j, g->b, h->l, i->r, j->w, k->v, l->k, m->o, n->p, o->u, p->n, q->q, r->m, s->d, t->a, u->x, v->t, w->c, x->i, y->s, z->y

[CODE]:
def f_approx(x):
    char_map = {
        'a': 'z', 'b': 'g', 'c': 'e', 'd': 'f', 'e': 'h', 'f': 'j', 'g': 'b', 'h': 'l', 'i': 'r', 'j': 'w', 'k': 'v', 'l': 'k',
        'm': 'o', 'n': 'p', 'o': 'u', 'p': 'n', 'q': 'q', 'r': 'm', 's': 'd', 't': 'a', 'u': 'x', 'v': 't', 'w': 'c', 'x': 'i',
        'y': 's', 'z': 'y'
    }
    return ''.join([char_map[c] for c in x])