#
# This piece of code is in the public domain.
# <zh.jesse@gmail.com>
#

#### the filter ####

def insert_ellipse(lst, ellipse=0):
    '''Insert ellipse where it's not sequential

    An ellipse is represented by @ellipse.

    E.g.:
        [1, 2, 5] -> [1, 2, 0, 5]
        [1, 2, 4] -> [1, 2, 3, 4]

    The last example is that if only one step is missing, just add it instead
    of an ellipse.
    '''

    if len(lst) <= 1:
        return list(lst[:])
    gap = lst[1] - lst[0]
    if gap <= 1:
        return [lst[0]] + insert_ellipse(lst[1:], ellipse)
    elif gap == 2:
        return [lst[0], lst[0] + 1] + insert_ellipse(lst[1:], ellipse)
    else:
        return [lst[0], ellipse] + insert_ellipse(lst[1:], ellipse)

@register.filter
def pick_pages(num_pages, current):
    '''pick pages to display in the pager

    It's like: 1 ... (x-2) (x-1) (x) (x+1) (x+2) ... N.
    '''

    ret = []

    for p in range(current - 2, current):
        if p >= 1:
            ret.append(p)
    ret.append(current)
    for p in range(current + 1, current + 3):
        if p <= num_pages:
            ret.append(p)

    if ret[0] != 1:
        ret.insert(0, 1)
    if ret[-1] != num_pages:
        ret.append(num_pages)
    ret = insert_ellipse(ret)

    return ret

#### template ####

{% if page.paginator.num_pages > 1 %}
<div class="pager">
  {% if page.number > 1 %}
    <a href="{% add_get_param request 'page' page.previous_page_number %}">
      <span class="page-number prev">←prev</span>
    </a>
  {% endif %}

  {% for p in page.paginator.num_pages|pick_pages:page.number %}
    {% if p == page.number %}
      <span class="page-number current">{{p}}</span>
    {% else %}{% if p == 0 %}
      <span class="page-number ellipse">&hellip;</span>
    {% else %}
      <a href="{% add_get_param request 'page' p %}">
        <span class="page-number">{{p}}</span>
      </a>
    {% endif %}
    {% endif %}
  {% endfor %}

  {% if page.number < page.paginator.num_pages %}
    <a href="{% add_get_param request 'page' page.next_page_number %}">
      <span class="page-number next">next→</span>
    </a>
  {% endif %}
</div>
{% endif %}

#### unittest / examples ####

def test_insert_ellipse(self):
    for lst, result in (
            ((0, 1, 3), [0, 1, 2, 3]),
            ((0, 1, 4), [0, 1, None, 4]),
            ):
        self.assertEqual(insert_ellipse(lst, ellipse=None), result)

    for lst, result in (
            ((1,), [1]),
            ((1, 3, 5), [1, 2, 3, 4, 5]),
            ((3, 7), [3, 0, 7]),
            ):
        self.assertEqual(points_utils.insert_ellipse(lst), result)

def test_filter_pick_pages(self):
    for num_pages, current, output in (
            (3, 1, [1, 2, 3]),
            (5, 1, [1, 2, 3, 4, 5]),
            (5, 3, [1, 2, 3, 4, 5]),
            (6, 1, [1, 2, 3, 0, 6]),
            (6, 5, [1, 2, 3, 4, 5, 6]),
            (6, 6, [1, 0, 4, 5, 6]),
            (1, 1, [1]),
            ):
        self.assertEqual(pick_pages(num_pages, current), output)