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UMassCTF 2021 - Chains: Dumping the first 90000000 values for Collatz's function
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use std::fs::File; | |
use std::io::prelude::*; | |
use std::io::BufWriter; | |
/* Needs lru = "0.6.5" in Cargo.toml */ | |
use lru::LruCache; | |
const CACHE_SIZE: usize = 1024 * 2000; | |
fn collatz(mut n: u32, cache: &mut LruCache<u32, i32>) -> i32 { | |
let mut i: i32 = 0; | |
while n != 1 { | |
if let Some(e) = cache.get(&n) { | |
return *e + i; | |
} | |
if n & 1 == 0 { | |
n = n >> 1; | |
} else { | |
n = n * 3 + 1; | |
} | |
i += 1; | |
} | |
return i; | |
} | |
fn main() -> std::io::Result<()> { | |
let mut cache = LruCache::new(CACHE_SIZE); | |
let file = File::create("collatz.out")?; | |
let mut writer = BufWriter::new(file); | |
let mut res: i32; | |
let desired_outputs = [105, 110, 113, 116, 136, 160, 181, 185, 199, 202, 211, 247, 266, 271, 273, 303, 318, 341, 352] ; | |
for i in 1..900_000_001 { | |
/* Show progress */ | |
if (i % 100_000) == 0 { | |
print!("{}\r", i); | |
} | |
/* Run collatz and update cache */ | |
res = collatz(i, &mut cache); | |
cache.put(i, res); | |
/* Update file if collatz's output is found in `desired_outputs` */ | |
if let Ok(_) = desired_outputs.binary_search(&res) { | |
writer.write(format!("{} {}\n", i, res).as_bytes())?; | |
} | |
} | |
println!("Cache entries used: {}/{}", cache.len(), CACHE_SIZE); | |
Ok(()) | |
} |
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