Ping Pong

ping_pong.py

def contains_digit(n, k):
    if n == 0:
        return False
    return True if n % 10 == k else contains_digit(n // 10, k)

def ping_pong(n, i=1, d=1, r=0):
    if i > n:
        return r
    turn = i % 7 == 0 or contains_digit(i, 7)
    return ping_pong(n, i + 1, -d if turn else d, r + d)

assert ping_pong(1) == 1
assert ping_pong(8) == 6
assert ping_pong(22) == 0
assert ping_pong(68) == 2
assert ping_pong(100) == 2

ping_pong_gen.py

from functools import reduce

def contains_digit(n, k):
    if n == 0:
        return False
    if n % 10 == k:
        return True
    else:
        return contains_digit(n // 10, k)

def scan(func, iterable, initial=None):
    acc = next(iterable) if initial is None else initial
    yield acc
    for e in iterable:
        acc = func(acc, e)
        yield acc

def last(iterable):
    return reduce(lambda acc, e: e, iterable)

def pingpong(n):
    k = 7
    turns = map(lambda i: i % k == 0 or contains_digit(i, k), range(1, n))
    directions = scan(lambda direction, turn: -direction if turn else direction, turns, 1)
    points = scan(lambda point, direction: point + direction, directions)
    return last(points)

print(pingpong(1))
print(pingpong(8))
print(pingpong(22))
print(pingpong(68))
print(pingpong(100))
print(pingpong(100000))

PingPongTest.kt

class PingPongTest {
    fun containsDigit(n: Int, k: Int): Boolean = if (n == 0) false else if (n % 10 == k) true else containsDigit(n / 10, k)

    fun pingPong(n: Int) = Observable.range(1, n - 1)
            .map { it % 7 == 0 || containsDigit(it, 7) }
            .scan(1, { direction, turn -> direction * if (turn) -1 else 1 })
            .scan { acc, direction -> acc + direction }
            .last()
            .toBlocking()
            .single()

    @Test fun examples() {
        assertTrue(pingPong(8) == 6)
        assertTrue(pingPong(22) == 0)
        assertTrue(pingPong(68) == 2)
        assertTrue(pingPong(100) == 2)
    }
}