Day 16

2 files changed, 51 insertions, 0 deletions

diff --git a/input/day16.txt b/input/day16.txt
new file mode 100644
index 0000000..013b5e3
--- /dev/null
+++ b/input/day16.txt
@@ -0,0 +1 @@
+10111100110001111
\ No newline at end of file
diff --git a/src/bin/day16.rs b/src/bin/day16.rs
new file mode 100644
index 0000000..21648b2
--- /dev/null
+++ b/src/bin/day16.rs
@@ -0,0 +1,50 @@
+use std::io::{self, Read};
+
+fn fill_disk(initial_state: &str, len: usize) -> String {
+    let mut state = String::from(initial_state);
+    while state.len() < len {
+        let mut b: Vec<u8> = state.bytes()
+            .map(|b| {
+                match b {
+                    b'0' => b'1',
+                    b'1' => b'0',
+                    _ => b,
+                }
+            })
+            .collect();
+        b.reverse();
+        let b = String::from_utf8(b).unwrap();
+        state.push('0');
+        state.push_str(&b);
+    }
+    state.truncate(len);
+    state
+}
+
+fn checksum(data: &str) -> String {
+    let mut sum = String::from(data);
+    while sum.len() % 2 == 0 {
+        sum = sum.as_bytes()
+            .chunks(2)
+            .map(|c| if c[0] == c[1] { '1' } else { '0' })
+            .collect();
+    }
+    sum
+}
+
+fn solve(len: usize, initial_state: &str) -> String {
+    let data = fill_disk(initial_state, len);
+    checksum(&data)
+}
+
+fn main() {
+    let mut input = String::new();
+    io::stdin().read_to_string(&mut input).unwrap();
+
+    println!("Part 1: {}", solve(272, input.trim()));
+}
+
+#[test]
+fn part1() {
+    assert_eq!("01100", solve(20, "10000"));
+}