15

1 files changed, 52 insertions, 0 deletions

diff --git a/15/src/main.rs b/15/src/main.rs
new file mode 100644
index 0000000..12bf0fa
--- /dev/null
+++ b/15/src/main.rs
@@ -0,0 +1,52 @@
+use advent_lib::prelude::*;
+
+use std::collections::BTreeMap;
+
+
+
+fn main() -> Result<()> {
+  let args = std::env::args();
+  if args.len() != 1 {
+    eprintln!("Usage: advent");
+  }
+
+  let starting_numbers = vec![2, 15, 0, 9, 1, 20];
+
+  let mut history: BTreeMap<usize, usize> = BTreeMap::new();
+  let mut output = 0;
+  let mut next_output = 0;
+
+  for i in 0 .. starting_numbers.len() {
+    output = starting_numbers[i];
+    next_output = match history.get(&output) {
+      Some(previous) => i - previous,
+      None => 0,
+    };
+    history.insert(output, i);
+  }
+
+  for i in starting_numbers.len() .. 2020 {
+    output = next_output;
+    next_output = match history.get(&output) {
+      Some(previous) => i - previous,
+      None => 0,
+    };
+    history.insert(output, i);
+  }
+
+  println!("{}", output);
+
+  for i in 2020 .. 30000000 {
+    output = next_output;
+    next_output = match history.get(&output) {
+      Some(previous) => i - previous,
+      None => 0,
+    };
+    history.insert(output, i);
+  }
+
+  println!("{}", output);
+
+  Ok(())
+}
+