Add day 15

Author: LinAGKar

Day15_input.txt

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+2,0,6,12,1,3

day15a/Cargo.toml

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+[package]
+name = "day15a"
+version = "0.1.0"
+authors = ["Linus Kardell <[email protected]>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

day15a/src/main.rs

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+use std::io;
+
+fn main() {
+    let mut input = String::new();
+    io::stdin().read_line(&mut input).unwrap();
+
+    const ITERATIONS: usize = 2020;
+
+    let numbers: Vec<_> = input.trim().split(',').map(|x| x.parse().unwrap()).collect();
+    let (&last_number, rest) = numbers.split_last().unwrap();
+    let mut number_timestamps = vec![None; ITERATIONS];
+    for (&i, n) in rest.iter().zip(1..) {
+        number_timestamps[i] = Some(n);
+    }
+    
+    let (_, last_number) = (numbers.len()..ITERATIONS).fold(
+        (number_timestamps, last_number),
+        |(mut number_timestamps, last_number), curr_time| {
+            let new_number = if let Some(last_time) = number_timestamps[last_number] {
+                curr_time - last_time
+            } else {
+                0
+            };
+            number_timestamps[last_number] = Some(curr_time);
+            (number_timestamps, new_number)
+        },
+    );
+
+    println!("{}", last_number);
+}

day15b/Cargo.toml

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+[package]
+name = "day15b"
+version = "0.1.0"
+authors = ["Linus Kardell <[email protected]>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

day15b/src/main.rs

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+use std::io;
+
+fn main() {
+    let mut input = String::new();
+    io::stdin().read_line(&mut input).unwrap();
+
+    const ITERATIONS: usize = 30000000;
+
+    let numbers: Vec<_> = input.trim().split(',').map(|x| x.parse().unwrap()).collect();
+    let (&last_number, rest) = numbers.split_last().unwrap();
+    let mut number_timestamps = vec![None; ITERATIONS];
+    for (&i, n) in rest.iter().zip(1..) {
+        number_timestamps[i] = Some(n);
+    }
+    
+    let (_, last_number) = (numbers.len()..ITERATIONS).fold(
+        (number_timestamps, last_number),
+        |(mut number_timestamps, last_number), curr_time| {
+            let new_number = if let Some(last_time) = number_timestamps[last_number] {
+                curr_time - last_time
+            } else {
+                0
+            };
+            number_timestamps[last_number] = Some(curr_time);
+            (number_timestamps, new_number)
+        },
+    );
+
+    println!("{}", last_number);
+}