Source code for titan_pylib.algorithm.arithmetic_progression


[docs]
 1class ArithmeticProgression:
 2
 3    def __init__(self, a: int, d: int) -> None:
 4        """
 5        Args:
 6            a (int): 初項です。
 7            d (int): 公差です。
 8        """
 9        self.a = a
10        self.d = d
11

[docs]
12    def __getitem__(self, k: int) -> int:
13        """0-indexed."""
14        assert k >= 0
15        return self.a + k * self.d

16

[docs]
17    def index(self, v: int) -> int:
18        """vが先頭から何番目か(0-indexed)"""
19        v -= self.a
20        assert v % self.d == 0
21        i = v // self.d
22        assert i >= 0
23        return i

24

[docs]
25    def pref(self, n: int) -> int:
26        """先頭n項の和."""
27        if n < 0:
28            return 0
29        return (self[0] + self[n - 1]) // 2 * n

30

[docs]
31    def sum(self, l: int, r: int) -> int:
32        """[l, r)"""
33        return self.pref(r) - self.pref(l - 1)