day9

2 files changed, 58 insertions, 0 deletions

diff --git a/src/day9.rs b/src/day9.rs
new file mode 100644
index 0000000..ac21e4d
--- /dev/null
+++ b/src/day9.rs
@@ -0,0 +1,56 @@
+use core::panic;
+use std::collections::HashSet;
+
+type Pt = (i32, i32);
+/**
+ * how should the tail move to catch up with the head?
+ */ 
+fn catchup(head: Pt, tail: Pt) -> Pt {
+    let (hx, hy) = head;
+    let (tx, ty) = tail;
+    let xdiff = hx - tx;
+    let ydiff = hy - ty;
+    // only move if we're not adjescent (incl diagonally)
+    if xdiff.abs() > 1 || ydiff.abs() > 1 {
+        // only move 1 square (x and or y) in the direction of the head
+        (tail.0 + xdiff.signum(), tail.1 + ydiff.signum())
+    } else {
+        (tail.0, tail.1)
+    }
+    
+}
+pub fn run(input: String) {
+    // series of steps
+    // let mut head: Pt = (0,0);
+    // let mut tail: Pt = (0,0);
+    // let mut htrail: Vec<Pt> = Vec::new();
+    let mut rope: Vec<Pt> = (0..10).map(|_| { (0,0) as Pt}).collect();
+    let mut trail: Vec<Pt> = Vec::new();
+    for line in input.lines() {
+        let mut toks = line.split_whitespace();
+        let dir: &str = toks.next().expect("expected a direction!");
+        let reps: u32 = toks.next().expect("expected a reps!").parse().expect("reps wasn't a number!");
+        for _ in 0..reps {
+            let head = rope[0];
+            let newhead = match dir {
+                "U" => (head.0, head.1+1),
+                "D" => (head.0, head.1-1),
+                "L" => (head.0-1, head.1),
+                "R" => (head.0+1, head.1),
+                dir => panic!("unexpected direction {}", dir),
+            };
+            rope[0] = newhead;
+            for l in 1..rope.len() {
+                rope[l] = catchup(rope[l-1], rope[l]);
+            }
+            let tail = rope.last().unwrap();
+            // htrail.push(head);
+            // tail = catchup(head, tail);
+            trail.push(*tail);
+        }
+    }
+    //println!("htrail {:?}", htrail);
+    //println!("trail {:?}", trail);
+    let trailsz = trail.into_iter().collect::<HashSet<Pt>>().len();
+    println!("Day 9: {}", trailsz);
+}
\ No newline at end of file
diff --git a/src/main.rs b/src/main.rs
index 5a7634b..e9ea587 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -8,6 +8,7 @@ mod day5;
 mod day6;
 mod day7;
 mod day8;
+mod day9;
 
 fn main() {
     day1::run(fs::read_to_string("input/day1.txt").expect("Failed to read input file!"));
@@ -18,4 +19,5 @@ fn main() {
     day6::run(fs::read_to_string("input/day6.txt").expect("Failed to read input file!"));
     day7::run(fs::read_to_string("input/day7.txt").expect("Failed to read input file!"));
     day8::run(fs::read_to_string("input/day8.txt").expect("Failed to read input file!"));
+    day9::run(fs::read_to_string("input/day9.txt").expect("Failed to read input file!"));
 }