Basics: floor and ceiling with bisect

Implement class SortedSet on top of a plain Python list kept in sorted order, using the bisect module:

• SortedSet() starts empty.
• add(x) inserts the integer x. Adding a value that's already there does nothing (it's a set).
• floor(x) returns the largest stored value <= x, or None if there isn't one.
• ceiling(x) returns the smallest stored value >= x, or None if there isn't one.
• __len__() returns how many distinct values are stored.

s = SortedSet()
for v in [10, 3, 7, 3]:
    s.add(v)
len(s)          # 3       (stored: [3, 7, 10])
s.floor(8)      # 7
s.ceiling(8)    # 10
s.floor(7)      # 7       (equal counts)
s.floor(2)      # None    (nothing is <= 2)
s.ceiling(11)   # None

Values may be negative. floor and ceiling must use binary search (O(log n)); the tests make 100,000 queries on a set of 50,000 values. bisect.insort is fine for add.