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use std::io::Read;
struct FlawedPattern {
base: Vec<i8>,
repeat: usize,
pos: usize,
}
impl FlawedPattern {
fn new(base: Vec<i8>, repeat: usize) -> FlawedPattern {
FlawedPattern {
base,
repeat,
pos: 0,
}
}
}
impl Iterator for FlawedPattern {
type Item = i8;
fn next(&mut self) -> Option<Self::Item> {
let ret = self.base[(self.pos / (self.repeat + 1)) % self.base.len()];
self.pos += 1;
Some(ret)
}
}
fn read_input() -> Vec<i8> {
let stdin = std::io::stdin();
stdin.lock().bytes().map(|byte| {
match byte {
Ok(b) => if b >= b'0' && b <= b'9' {
(b-b'0') as i8
} else if b == b'\n' || b == b'\r' {
-1
} else {
panic!("Invalid digit: {:?}", b);
},
_ => panic!("Invalid indata. {:?}", byte),
}
}).filter(|n| *n >= 0).map(|n| n as i8).collect()
}
fn calculate_digits(input: &Vec<i8>) -> Vec<i8> {
input.iter().rev()
.scan(0, |s, n| { *s += *n; *s %= 10; Some(*s)})
.collect::<Vec<i8>>().into_iter()
.rev().map(|n| n.abs() % 10)
.collect()
}
fn calculate_digit(input: &Vec<i8>, n: usize) -> i8 {
let base_pattern = vec![0, 1, 0, -1];
let fft = FlawedPattern::new(base_pattern, n);
let factors: Vec<i8> = input.iter()
.zip(fft.skip(1).take(input.len()).collect::<Vec<i8>>())
.map(|(n, m)| (*n)*m).collect();
(factors.iter().map(|n| i64::from(*n)).sum::<i64>().abs() % 10) as i8
}
fn main() {
let input = read_input();
let mut num = input.clone();
for _ in 0..100 {
num = (0..num.len())
.map(|n| calculate_digit(&num, n))
.collect();
}
let part1: usize = num.iter().take(8).fold(0, |r, n| r*10 + (*n as usize));
println!("part1: {}", part1);
num = input;
let offset = num.iter().take(7).fold(0, |r, n| r*10 + (*n as usize));
let repeat = 10000;
let len = num.len();
num = num.into_iter().cycle().take(len*repeat).skip(offset).collect();
for _ in 0..100 { num = calculate_digits(&num); }
let part2: usize = num.iter().take(8).fold(0, |r, n| r*10 + (*n as usize));
println!("part2: {}", part2);
}
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