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use std::ops::{AddAssign, Sub};
pub struct FenwickTree<T> {
n: usize,
data: Vec<T>,
init: T,
}
impl<T: Copy + AddAssign + Sub<Output = T>> FenwickTree<T> {
pub fn new(size: usize, init: T) -> FenwickTree<T> {
FenwickTree {
n: size + 1,
data: vec![init; size + 1],
init: init,
}
}
pub fn add(&mut self, k: usize, value: T) {
let mut x = k;
while x < self.n {
self.data[x] += value;
x |= x + 1;
}
}
pub fn sum(&self, l: usize, r: usize) -> T {
self.sum_one(r) - self.sum_one(l)
}
pub fn sum_one(&self, k: usize) -> T {
if k >= self.n {
panic!("");
}
let mut result = self.init;
let mut x = k as i32 - 1;
while x >= 0 {
result += self.data[x as usize];
x = (x & (x + 1)) - 1;
}
result
}
}
#[cfg(test)]
mod test {
extern crate rand;
use self::rand::{thread_rng, Rng};
use super::*;
#[test]
fn random_array() {
let n = 1000;
let mut bit = FenwickTree::new(n, 0);
let mut v = vec![0; n];
for _ in 0..10000 {
let value = thread_rng().gen_range(0, 1000);
let k = thread_rng().gen_range(0, n);
v[k] += value;
bit.add(k, value);
let mut sum = 0;
for i in 0..n {
sum += v[i];
assert_eq!(sum, bit.sum(0, i + 1));
}
}
}
}