Add solution for 2020 day 15

Dependencies

• [2] B527MN66 Solve day 1 for 2020

Change contents

• file addition: day15.rs (----------)
  [2.17]
  + #![feature(iter_advance_by)]

  + use std::collections::HashMap;

  + use std::env;

  +

  + struct ElfGame {

  + history: HashMap<usize, usize>,

  + last: usize,

  + len: usize,

  + }

  +

  + impl ElfGame {

  + fn new() -> Self {

  + ElfGame {

  + history: HashMap::new(),

  + last: 0,

  + len: 0,

  + }

  + }

  +

  + fn push(&mut self, num: usize) {

  + self.history.insert(self.last, self.len);

  + self.len += 1;

  + self.last = num;

  + }

  + }

  +

  + impl Iterator for ElfGame {

  + type Item = usize;

  +

  + fn next(&mut self) -> Option<Self::Item> {

  + let nxt = self.history.get(&self.last)

  + .map_or_else(|| 0, |p| self.len - p);

  + self.push(nxt);

  + Some(nxt)

  + }

  + }

  +

  + fn main() {

  + use std::io::BufReader;

  + use std::io::BufRead;

  + use std::fs::File;

  + use std::path::Path;

  + use std::str::FromStr;

  + let mut eg = ElfGame::new();

  + let args: Vec<_> = env::args().collect();

  + let file = BufReader::new(File::open(<String as AsRef<Path>>::as_ref(&args[2])).unwrap());

  + for strnum in file.split(b',') {

  + match <usize as FromStr>::from_str(String::from_utf8(strnum.unwrap_or(vec![])).unwrap_or("".to_string()).trim()) {

  + Ok(num) => {

  + eg.push(num);

  + }

  + Err(_) => ()

  + }

  + }

  + match eg.advance_by(<usize as FromStr>::from_str(args[1].as_ref()).unwrap() - eg.len - 1) {

  + Ok(_) => println!("{}", eg.next().unwrap()),

  + Err(err) => eprintln!("{:?}", err),

  + }

  + }