Solution for day 15

day15/Cargo.toml

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+[package]
+name = "day15"
+version = "0.1.0"
+authors = ["inga-lovinde <52715130+inga-lovinde@users.noreply.github.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

day15/src/main.rs

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+use std::collections::HashMap;
+use std::io::{self, BufRead};
+
+fn main() {
+    let stdin = io::stdin();
+    let mut stdin_lines = stdin.lock().lines();
+    let seed: Vec<usize> = stdin_lines.next().unwrap().unwrap().split(",")
+        .map(|entry| entry.parse().unwrap())
+        .collect();
+
+    let mut last_occurrences = HashMap::new();
+    for i in 0..seed.len()-1 {
+        last_occurrences.insert(seed[i], i+1);
+    }
+
+    let mut last_number = seed[seed.len()-1];
+    for turn in seed.len()+1.. {
+        let next_number = match last_occurrences.get(&last_number) {
+            Some(&last_occurrence) => (turn-1) - last_occurrence,
+            None => 0,
+        };
+
+        last_occurrences.insert(last_number, turn-1);
+        last_number = next_number;
+        if turn == 2020 || (turn % 1_000_000) == 0 {
+            println!("{}: {}", turn, last_number);
+        }
+    }
+}